From 2a23d86e32a34773c179fcd5425585dc848ced21 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 09:47:39 -0400
Subject: [PATCH 01/46] searchAlongK produces strict results.

---
 src/SES/Myers.hs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index c5d02570f..6827a5ef1 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -67,12 +67,12 @@ runSES eq (EditGraph as bs)
         searchUpToD (Distance d) = do
           v <- get
           let extents = searchAlongK v . Diagonal <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ]
-          put (Map.fromList extents)
-          pure . fmap (snd . snd) $! find isComplete extents
-          where isComplete (k, (x, _)) = x >= n && (x - k) >= m
+          put (Map.fromList ((k' &&& (x' &&& script')) <$> extents))
+          pure . fmap script' $! find isComplete extents
+          where isComplete (Extent k x _) = x >= n && (x - k) >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
-                searchAlongK v (Diagonal k) = (,) k . (x &&& script) . slideFrom $!
+                searchAlongK v (Diagonal k) = toExtent k . slideFrom $!
                   if d == 0 || k < negate m || k > n then
                     -- The top-left corner, or otherwise out-of-bounds.
                     Endpoint 0 0 []
@@ -98,6 +98,8 @@ runSES eq (EditGraph as bs)
                 -- | Move rightward from a given vertex, deleting the element for the corresponding column.
                 moveRightFrom (Endpoint x y script) = Endpoint (succ x) y (if x < n then This (as ! x) : script else script)
 
+                toExtent k (Endpoint x _ script) = Extent k x script
+
                 -- | Slide down any diagonal edges from a given vertex.
                 slideFrom (Endpoint x y script)
                   | x >= 0, x < n
@@ -108,6 +110,8 @@ runSES eq (EditGraph as bs)
                   | otherwise =           (Endpoint       x        y               script)
 
 
+data Extent a b = Extent { k' :: {-# UNPACK #-} !Int, x' :: {-# UNPACK #-} !Int, script' :: !(EditScript a b) }
+
 -- Implementation details
 
 -- | The state stored by Myers’ algorithm; an array of m + n + 1 values indicating the maximum x-index reached and path taken along each diagonal.

From 9308e7f64acdff8ee79e4ed8107c2b03c04bb5ed Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 10:07:59 -0400
Subject: [PATCH 02/46] Search edit envelopes tail-recursively.

---
 src/SES/Myers.hs | 36 +++++++++++-------------------------
 1 file changed, 11 insertions(+), 25 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 6827a5ef1..c93bdf9d0 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -1,9 +1,8 @@
 {-# LANGUAGE GADTs, ImplicitParams, MultiParamTypeClasses, ScopedTypeVariables #-}
+{-# OPTIONS_GHC -Wno-incomplete-patterns #-}
 module SES.Myers
 ( EditScript
-, Myers
 , EditGraph(..)
-, Distance(..)
 , Diagonal(..)
 , Endpoint(..)
 , ses
@@ -22,8 +21,6 @@ import Prologue hiding (error)
 -- | An edit script, i.e. a sequence of changes/copies of elements.
 type EditScript a b = [These a b]
 
-type Myers a b = State (MyersState a b)
-
 -- | Notionally the cartesian product of two sequences, represented as a simple wrapper around those arrays holding those sequences’ elements for O(1) lookups.
 data EditGraph a b = EditGraph { as :: !(Array.Array Int a), bs :: !(Array.Array Int b) }
   deriving (Eq, Show)
@@ -32,10 +29,6 @@ data EditGraph a b = EditGraph { as :: !(Array.Array Int a), bs :: !(Array.Array
 makeEditGraph :: (Foldable t, Foldable u) => t a -> u b -> EditGraph a b
 makeEditGraph as bs = EditGraph (Array.listArray (0, pred (length as)) (toList as)) (Array.listArray (0, pred (length bs)) (toList bs))
 
--- | An edit distance, i.e. a cardinal number of changes.
-newtype Distance = Distance { unDistance :: Int }
-  deriving (Eq, Show)
-
 -- | A diagonal in the edit graph of lists of lengths n and m, numbered from -m to n.
 newtype Diagonal = Diagonal { unDiagonal :: Int }
   deriving (Eq, Ix, Ord, Show)
@@ -48,27 +41,25 @@ data Endpoint a b = Endpoint { x :: {-# UNPACK #-} !Int, y :: {-# UNPACK #-} !In
 
 -- | Compute the shortest edit script using Myers’ algorithm.
 ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript a b
-ses eq as bs = let graph = makeEditGraph as bs in evalState (runSES eq graph) (emptyStateForGraph graph)
+ses eq as bs = let graph = makeEditGraph as bs in runSES eq graph
 
 
 -- Evaluation
 
 -- | Compute the shortest edit script (diff) of an edit graph.
-runSES :: (a -> b -> Bool) -> EditGraph a b -> Myers a b (EditScript a b)
+runSES :: (a -> b -> Bool) -> EditGraph a b -> EditScript a b
 runSES eq (EditGraph as bs)
-  | null bs = return (This <$> toList as)
-  | null as = return (That <$> toList bs)
-  | otherwise = do
-    Just script <- asum <$> for [0..(n + m)] (searchUpToD . Distance)
-    return (reverse script)
+  | null bs = This <$> toList as
+  | null as = That <$> toList bs
+  | otherwise = reverse (searchUpToD [0..(n + m)] (Map.singleton 0 (0, [])))
   where (n, m) = (length as, length bs)
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
-        searchUpToD (Distance d) = do
-          v <- get
-          let extents = searchAlongK v . Diagonal <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ]
-          put (Map.fromList ((k' &&& (x' &&& script')) <$> extents))
-          pure . fmap script' $! find isComplete extents
+        searchUpToD (d:ds) v =
+          let extents = searchAlongK v . Diagonal <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
+          case find isComplete extents of
+            Just (Extent _ _ script) -> script
+            _ -> searchUpToD ds (Map.fromList ((k' &&& (x' &&& script')) <$> extents))
           where isComplete (Extent k x _) = x >= n && (x - k) >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
@@ -117,11 +108,6 @@ data Extent a b = Extent { k' :: {-# UNPACK #-} !Int, x' :: {-# UNPACK #-} !Int,
 -- | The state stored by Myers’ algorithm; an array of m + n + 1 values indicating the maximum x-index reached and path taken along each diagonal.
 type MyersState a b = Map.IntMap (Int, EditScript a b)
 
--- | Compute the empty state of length m + n + 1 for a given edit graph.
-emptyStateForGraph :: EditGraph a b -> MyersState a b
-emptyStateForGraph _ =
-  Map.singleton 0 (0, [])
-
 
 -- | Lifted showing of arrays.
 liftShowsVector :: Show i => (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Array.Array i a -> ShowS

From 105cd7b1ec559f1c53e79ab1869d925eff1cdfeb Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:07:53 -0400
Subject: [PATCH 03/46] Replace Extent with Endpoint.

---
 src/SES/Myers.hs | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index c93bdf9d0..0e89eedd4 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -58,12 +58,12 @@ runSES eq (EditGraph as bs)
         searchUpToD (d:ds) v =
           let extents = searchAlongK v . Diagonal <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
           case find isComplete extents of
-            Just (Extent _ _ script) -> script
-            _ -> searchUpToD ds (Map.fromList ((k' &&& (x' &&& script')) <$> extents))
-          where isComplete (Extent k x _) = x >= n && (x - k) >= m
+            Just (Endpoint _ _ script) -> script
+            _ -> searchUpToD ds (Map.fromList ((\ (Endpoint x y script) -> (x - y, (x, script))) <$> extents))
+          where isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
-                searchAlongK v (Diagonal k) = toExtent k . slideFrom $!
+                searchAlongK v (Diagonal k) = slideFrom $!
                   if d == 0 || k < negate m || k > n then
                     -- The top-left corner, or otherwise out-of-bounds.
                     Endpoint 0 0 []
@@ -89,8 +89,6 @@ runSES eq (EditGraph as bs)
                 -- | Move rightward from a given vertex, deleting the element for the corresponding column.
                 moveRightFrom (Endpoint x y script) = Endpoint (succ x) y (if x < n then This (as ! x) : script else script)
 
-                toExtent k (Endpoint x _ script) = Extent k x script
-
                 -- | Slide down any diagonal edges from a given vertex.
                 slideFrom (Endpoint x y script)
                   | x >= 0, x < n
@@ -101,8 +99,6 @@ runSES eq (EditGraph as bs)
                   | otherwise =           (Endpoint       x        y               script)
 
 
-data Extent a b = Extent { k' :: {-# UNPACK #-} !Int, x' :: {-# UNPACK #-} !Int, script' :: !(EditScript a b) }
-
 -- Implementation details
 
 -- | The state stored by Myers’ algorithm; an array of m + n + 1 values indicating the maximum x-index reached and path taken along each diagonal.

From b1fff9bf52a9a72509be649f336fcd6acaad7407 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:08:17 -0400
Subject: [PATCH 04/46] Rename extents to endpoints.

---
 src/SES/Myers.hs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 0e89eedd4..abe10f01b 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -56,10 +56,10 @@ runSES eq (EditGraph as bs)
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD (d:ds) v =
-          let extents = searchAlongK v . Diagonal <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
-          case find isComplete extents of
+          let endpoints = searchAlongK v . Diagonal <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
+          case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
-            _ -> searchUpToD ds (Map.fromList ((\ (Endpoint x y script) -> (x - y, (x, script))) <$> extents))
+            _ -> searchUpToD ds (Map.fromList ((\ (Endpoint x y script) -> (x - y, (x, script))) <$> endpoints))
           where isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.

From 635c3fc00de6e65884cdd7742e83f1b76679b9f6 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:14:28 -0400
Subject: [PATCH 05/46] No need to allocate a list of envelopes.

---
 src/SES/Myers.hs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index abe10f01b..94c73e757 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -51,15 +51,15 @@ runSES :: (a -> b -> Bool) -> EditGraph a b -> EditScript a b
 runSES eq (EditGraph as bs)
   | null bs = This <$> toList as
   | null as = That <$> toList bs
-  | otherwise = reverse (searchUpToD [0..(n + m)] (Map.singleton 0 (0, [])))
+  | otherwise = reverse (searchUpToD 0 (Map.singleton 0 (0, [])))
   where (n, m) = (length as, length bs)
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
-        searchUpToD (d:ds) v =
+        searchUpToD d v =
           let endpoints = searchAlongK v . Diagonal <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
-            _ -> searchUpToD ds (Map.fromList ((\ (Endpoint x y script) -> (x - y, (x, script))) <$> endpoints))
+            _ -> searchUpToD (succ d) (Map.fromList ((\ (Endpoint x y script) -> (x - y, (x, script))) <$> endpoints))
           where isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.

From 7eb9c6c834375343870c8ad630019f21985e576e Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:15:48 -0400
Subject: [PATCH 06/46] Exhaustiveness has been restored.

---
 src/SES/Myers.hs | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 94c73e757..1180c4e57 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE GADTs, ImplicitParams, MultiParamTypeClasses, ScopedTypeVariables #-}
-{-# OPTIONS_GHC -Wno-incomplete-patterns #-}
 module SES.Myers
 ( EditScript
 , EditGraph(..)

From 7aaff0b6348878d24c9aaf300c9d7ee65fd35a6e Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:17:06 -0400
Subject: [PATCH 07/46] :fire: Diagonal.

---
 src/SES/Myers.hs | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 1180c4e57..2a1b39ee6 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -2,7 +2,6 @@
 module SES.Myers
 ( EditScript
 , EditGraph(..)
-, Diagonal(..)
 , Endpoint(..)
 , ses
 , MyersState
@@ -28,10 +27,6 @@ data EditGraph a b = EditGraph { as :: !(Array.Array Int a), bs :: !(Array.Array
 makeEditGraph :: (Foldable t, Foldable u) => t a -> u b -> EditGraph a b
 makeEditGraph as bs = EditGraph (Array.listArray (0, pred (length as)) (toList as)) (Array.listArray (0, pred (length bs)) (toList bs))
 
--- | A diagonal in the edit graph of lists of lengths n and m, numbered from -m to n.
-newtype Diagonal = Diagonal { unDiagonal :: Int }
-  deriving (Eq, Ix, Ord, Show)
-
 data Endpoint a b = Endpoint { x :: {-# UNPACK #-} !Int, y :: {-# UNPACK #-} !Int, script :: !(EditScript a b) }
   deriving (Eq, Show)
 
@@ -55,14 +50,14 @@ runSES eq (EditGraph as bs)
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD d v =
-          let endpoints = searchAlongK v . Diagonal <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
+          let endpoints = searchAlongK v <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
             _ -> searchUpToD (succ d) (Map.fromList ((\ (Endpoint x y script) -> (x - y, (x, script))) <$> endpoints))
           where isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
-                searchAlongK v (Diagonal k) = slideFrom $!
+                searchAlongK v k = slideFrom $!
                   if d == 0 || k < negate m || k > n then
                     -- The top-left corner, or otherwise out-of-bounds.
                     Endpoint 0 0 []

From 3cbc2b3505bb3ea2720934f93e7d13125a673725 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:18:32 -0400
Subject: [PATCH 08/46] Close over the state.

---
 src/SES/Myers.hs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 2a1b39ee6..853e42856 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -50,14 +50,14 @@ runSES eq (EditGraph as bs)
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD d v =
-          let endpoints = searchAlongK v <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
+          let endpoints = searchAlongK <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
             _ -> searchUpToD (succ d) (Map.fromList ((\ (Endpoint x y script) -> (x - y, (x, script))) <$> endpoints))
           where isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
-                searchAlongK v k = slideFrom $!
+                searchAlongK k = slideFrom $!
                   if d == 0 || k < negate m || k > n then
                     -- The top-left corner, or otherwise out-of-bounds.
                     Endpoint 0 0 []

From c4992ff950674be0dadbd6525ccdcf263af6309b Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:20:44 -0400
Subject: [PATCH 09/46] :fire: runSES.

---
 src/SES/Myers.hs | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 853e42856..f38992621 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -35,18 +35,12 @@ data Endpoint a b = Endpoint { x :: {-# UNPACK #-} !Int, y :: {-# UNPACK #-} !In
 
 -- | Compute the shortest edit script using Myers’ algorithm.
 ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript a b
-ses eq as bs = let graph = makeEditGraph as bs in runSES eq graph
-
-
--- Evaluation
-
--- | Compute the shortest edit script (diff) of an edit graph.
-runSES :: (a -> b -> Bool) -> EditGraph a b -> EditScript a b
-runSES eq (EditGraph as bs)
+ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
   | otherwise = reverse (searchUpToD 0 (Map.singleton 0 (0, [])))
-  where (n, m) = (length as, length bs)
+  where EditGraph as bs = makeEditGraph as' bs'
+        (n, m) = (length as, length bs)
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD d v =

From 854b2b1c622201110121591f16a00ce190133b3e Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:24:12 -0400
Subject: [PATCH 10/46] :fire: EditGraph.

---
 src/SES/Myers.hs | 25 ++-----------------------
 1 file changed, 2 insertions(+), 23 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index f38992621..8fb0554bd 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -1,7 +1,6 @@
 {-# LANGUAGE GADTs, ImplicitParams, MultiParamTypeClasses, ScopedTypeVariables #-}
 module SES.Myers
 ( EditScript
-, EditGraph(..)
 , Endpoint(..)
 , ses
 , MyersState
@@ -11,7 +10,6 @@ import Data.Array ((!))
 import qualified Data.Array as Array
 import qualified Data.IntMap.Lazy as Map
 import Data.Ix
-import Data.Functor.Classes
 import Data.These
 import GHC.Show hiding (show)
 import Prologue hiding (error)
@@ -19,14 +17,6 @@ import Prologue hiding (error)
 -- | An edit script, i.e. a sequence of changes/copies of elements.
 type EditScript a b = [These a b]
 
--- | Notionally the cartesian product of two sequences, represented as a simple wrapper around those arrays holding those sequences’ elements for O(1) lookups.
-data EditGraph a b = EditGraph { as :: !(Array.Array Int a), bs :: !(Array.Array Int b) }
-  deriving (Eq, Show)
-
--- | Construct an edit graph from Foldable sequences.
-makeEditGraph :: (Foldable t, Foldable u) => t a -> u b -> EditGraph a b
-makeEditGraph as bs = EditGraph (Array.listArray (0, pred (length as)) (toList as)) (Array.listArray (0, pred (length bs)) (toList bs))
-
 data Endpoint a b = Endpoint { x :: {-# UNPACK #-} !Int, y :: {-# UNPACK #-} !Int, script :: !(EditScript a b) }
   deriving (Eq, Show)
 
@@ -39,8 +29,8 @@ ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
   | otherwise = reverse (searchUpToD 0 (Map.singleton 0 (0, [])))
-  where EditGraph as bs = makeEditGraph as' bs'
-        (n, m) = (length as, length bs)
+  where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
+        (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD d v =
@@ -91,14 +81,3 @@ ses eq as' bs'
 
 -- | The state stored by Myers’ algorithm; an array of m + n + 1 values indicating the maximum x-index reached and path taken along each diagonal.
 type MyersState a b = Map.IntMap (Int, EditScript a b)
-
-
--- | Lifted showing of arrays.
-liftShowsVector :: Show i => (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> Array.Array i a -> ShowS
-liftShowsVector sp sl d = liftShowsPrec sp sl d . toList
-
-
--- Instances
-
-instance Show2 EditGraph where
-  liftShowsPrec2 sp1 sl1 sp2 sl2 d (EditGraph as bs) = showsBinaryWith (liftShowsVector sp1 sl1) (liftShowsVector sp2 sl2) "EditGraph" d as bs

From 25d66ede5f1bc7102749df4bfc85ba8aa75f3819 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:25:57 -0400
Subject: [PATCH 11/46] :fire: MyersState.

---
 src/SES/Myers.hs | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 8fb0554bd..1172b5328 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -3,7 +3,6 @@ module SES.Myers
 ( EditScript
 , Endpoint(..)
 , ses
-, MyersState
 ) where
 
 import Data.Array ((!))
@@ -21,8 +20,6 @@ data Endpoint a b = Endpoint { x :: {-# UNPACK #-} !Int, y :: {-# UNPACK #-} !In
   deriving (Eq, Show)
 
 
--- API
-
 -- | Compute the shortest edit script using Myers’ algorithm.
 ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript a b
 ses eq as' bs'
@@ -75,9 +72,3 @@ ses eq as' bs'
                   , b <- bs ! y
                   , a `eq` b  = slideFrom (Endpoint (succ x) (succ y) (These a b : script))
                   | otherwise =           (Endpoint       x        y               script)
-
-
--- Implementation details
-
--- | The state stored by Myers’ algorithm; an array of m + n + 1 values indicating the maximum x-index reached and path taken along each diagonal.
-type MyersState a b = Map.IntMap (Int, EditScript a b)

From 7ff6e1252c58a962bb24ec840a3479bb7e03d8b6 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:26:25 -0400
Subject: [PATCH 12/46] =?UTF-8?q?Don=E2=80=99t=20export=20Endpoint.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 src/SES/Myers.hs | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 1172b5328..5774ed346 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -1,7 +1,6 @@
 {-# LANGUAGE GADTs, ImplicitParams, MultiParamTypeClasses, ScopedTypeVariables #-}
 module SES.Myers
 ( EditScript
-, Endpoint(..)
 , ses
 ) where
 
@@ -16,7 +15,7 @@ import Prologue hiding (error)
 -- | An edit script, i.e. a sequence of changes/copies of elements.
 type EditScript a b = [These a b]
 
-data Endpoint a b = Endpoint { x :: {-# UNPACK #-} !Int, y :: {-# UNPACK #-} !Int, script :: !(EditScript a b) }
+data Endpoint a b = Endpoint { x :: {-# UNPACK #-} !Int, _y :: {-# UNPACK #-} !Int, _script :: !(EditScript a b) }
   deriving (Eq, Show)
 
 

From 158b6387a38819086ef0063d8c5808ef7cf48aea Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:26:43 -0400
Subject: [PATCH 13/46] :fire: redundant parens.

---
 src/SES/Myers.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 5774ed346..2f9087690 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -70,4 +70,4 @@ ses eq as' bs'
                   , a <- as ! x
                   , b <- bs ! y
                   , a `eq` b  = slideFrom (Endpoint (succ x) (succ y) (These a b : script))
-                  | otherwise =           (Endpoint       x        y               script)
+                  | otherwise =            Endpoint       x        y               script

From e505b532a942fc294c41a67fd9b5c999a0b915d7 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:28:57 -0400
Subject: [PATCH 14/46] Use the inRange method to bounds check x/y.

---
 src/SES/Myers.hs | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 2f9087690..83517d16c 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -65,9 +65,7 @@ ses eq as' bs'
 
                 -- | Slide down any diagonal edges from a given vertex.
                 slideFrom (Endpoint x y script)
-                  | x >= 0, x < n
-                  , y >= 0, y < m
-                  , a <- as ! x
-                  , b <- bs ! y
+                  | inRange (Array.bounds as) x, a <- as ! x
+                  , inRange (Array.bounds bs) y, b <- bs ! y
                   , a `eq` b  = slideFrom (Endpoint (succ x) (succ y) (These a b : script))
                   | otherwise =            Endpoint       x        y               script

From 0d22eabf5bf9ddd0a904586062e8b9a0a34cf376 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:31:38 -0400
Subject: [PATCH 15/46] Store Endpoints in the state.

This uses a word of extra space but saves some time moving things around.
---
 src/SES/Myers.hs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 83517d16c..a87ff96f2 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -24,7 +24,7 @@ ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript
 ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
-  | otherwise = reverse (searchUpToD 0 (Map.singleton 0 (0, [])))
+  | otherwise = reverse (searchUpToD 0 (Map.singleton 0 (Endpoint 0 0 [])))
   where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
         (n, m) = (length as', length bs')
 
@@ -33,7 +33,7 @@ ses eq as' bs'
           let endpoints = searchAlongK <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
-            _ -> searchUpToD (succ d) (Map.fromList ((\ (Endpoint x y script) -> (x - y, (x, script))) <$> endpoints))
+            _ -> searchUpToD (succ d) (Map.fromList ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))
           where isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
@@ -53,7 +53,7 @@ ses eq as' bs'
                   else
                     -- The upper/right extent of the search region or edit graph, whichever is smaller.
                     moveRightFrom prev
-                  where getK k = let (x, script) = v Map.! k in Endpoint x (x - k) script
+                  where getK k = v Map.! k
                         prev = getK (pred k)
                         next = getK (succ k)
 

From 30b9e85ed31fa7eb104ea78f8d968788d96693e0 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:36:26 -0400
Subject: [PATCH 16/46] Use minimal Arrays for the state.

---
 src/SES/Myers.hs | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index a87ff96f2..e9be5bb91 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -6,7 +6,6 @@ module SES.Myers
 
 import Data.Array ((!))
 import qualified Data.Array as Array
-import qualified Data.IntMap.Lazy as Map
 import Data.Ix
 import Data.These
 import GHC.Show hiding (show)
@@ -24,7 +23,7 @@ ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript
 ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
-  | otherwise = reverse (searchUpToD 0 (Map.singleton 0 (Endpoint 0 0 [])))
+  | otherwise = reverse (searchUpToD 0 (Array.array (0, 0) [(0, Endpoint 0 0 [])]))
   where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
         (n, m) = (length as', length bs')
 
@@ -33,7 +32,7 @@ ses eq as' bs'
           let endpoints = searchAlongK <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
-            _ -> searchUpToD (succ d) (Map.fromList ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))
+            _ -> searchUpToD (succ d) (Array.array (negate d, d) ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))
           where isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
@@ -53,7 +52,7 @@ ses eq as' bs'
                   else
                     -- The upper/right extent of the search region or edit graph, whichever is smaller.
                     moveRightFrom prev
-                  where getK k = v Map.! k
+                  where getK k = v ! k
                         prev = getK (pred k)
                         next = getK (succ k)
 

From 51e05c39d38d3b1f2ffa6ba66d410eead4fc8cc8 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:52:48 -0400
Subject: [PATCH 17/46] Align moveDownFrom/moveRightFrom.

---
 src/SES/Myers.hs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index e9be5bb91..cd2f9a042 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -57,10 +57,10 @@ ses eq as' bs'
                         next = getK (succ k)
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.
-                moveDownFrom (Endpoint x y script) = Endpoint x (succ y) (if y < m then That (bs ! y) : script else script)
+                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y < m then That (bs ! y) : script else script)
 
                 -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-                moveRightFrom (Endpoint x y script) = Endpoint (succ x) y (if x < n then This (as ! x) : script else script)
+                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if x < n then This (as ! x) : script else script)
 
                 -- | Slide down any diagonal edges from a given vertex.
                 slideFrom (Endpoint x y script)

From f395d5c144c24e32fb5b3a02d1477e9b4638667b Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:54:39 -0400
Subject: [PATCH 18/46] Rename prev/next to left/up.

---
 src/SES/Myers.hs | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index cd2f9a042..40ced8af6 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -42,19 +42,19 @@ ses eq as' bs'
                     Endpoint 0 0 []
                   else if k == negate d || k == negate m then
                     -- The lower/left extent of the search region or edit graph, whichever is smaller.
-                    moveDownFrom next
+                    moveDownFrom up
                   else if k /= d && k /= n then
                     -- Somewhere in the interior of the search region and edit graph.
-                    if x prev < x next then
-                      moveDownFrom next
+                    if x left < x up then
+                      moveDownFrom up
                     else
-                      moveRightFrom prev
+                      moveRightFrom left
                   else
                     -- The upper/right extent of the search region or edit graph, whichever is smaller.
-                    moveRightFrom prev
+                    moveRightFrom left
                   where getK k = v ! k
-                        prev = getK (pred k)
-                        next = getK (succ k)
+                        left = getK (pred k)
+                        up   = getK (succ k)
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.
                 moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y < m then That (bs ! y) : script else script)

From 6a4148fd6fe6a8c8ee072130df2de9163a8248bc Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 11:57:50 -0400
Subject: [PATCH 19/46] s/negate/-/

---
 src/SES/Myers.hs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 40ced8af6..33125dd0d 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -29,18 +29,18 @@ ses eq as' bs'
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD d v =
-          let endpoints = searchAlongK <$> [ k | k <- [negate d, negate d + 2 .. d], inRange (negate m, n) k ] in
+          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (negate m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
-            _ -> searchUpToD (succ d) (Array.array (negate d, d) ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))
+            _ -> searchUpToD (succ d) (Array.array (-d, d) ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))
           where isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
                 searchAlongK k = slideFrom $!
-                  if d == 0 || k < negate m || k > n then
+                  if d == 0 || k < -m || k > n then
                     -- The top-left corner, or otherwise out-of-bounds.
                     Endpoint 0 0 []
-                  else if k == negate d || k == negate m then
+                  else if k == -d || k == -m then
                     -- The lower/left extent of the search region or edit graph, whichever is smaller.
                     moveDownFrom up
                   else if k /= d && k /= n then

From 05ec4dda6389fad40cec72694fa8a9e04985d1dd Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 12:02:34 -0400
Subject: [PATCH 20/46] k is already bounded.

---
 src/SES/Myers.hs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 33125dd0d..b4a5507ca 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -37,8 +37,8 @@ ses eq as' bs'
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
                 searchAlongK k = slideFrom $!
-                  if d == 0 || k < -m || k > n then
-                    -- The top-left corner, or otherwise out-of-bounds.
+                  if d == 0 then
+                    -- The top-left corner.
                     Endpoint 0 0 []
                   else if k == -d || k == -m then
                     -- The lower/left extent of the search region or edit graph, whichever is smaller.

From e532f7d2fb4deddf72961eee4495dcb2ff6a3d5f Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 13:13:40 -0400
Subject: [PATCH 21/46] Simplify the search logic.

---
 src/SES/Myers.hs | 26 +++++++-------------------
 1 file changed, 7 insertions(+), 19 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index b4a5507ca..3ad601db0 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -23,13 +23,13 @@ ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript
 ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
-  | otherwise = reverse (searchUpToD 0 (Array.array (0, 0) [(0, Endpoint 0 0 [])]))
+  | otherwise = reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
   where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
         (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD d v =
-          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (negate m, n) k ] in
+          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (-m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
             _ -> searchUpToD (succ d) (Array.array (-d, d) ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))
@@ -37,30 +37,18 @@ ses eq as' bs'
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
                 searchAlongK k = slideFrom $!
-                  if d == 0 then
-                    -- The top-left corner.
-                    Endpoint 0 0 []
-                  else if k == -d || k == -m then
-                    -- The lower/left extent of the search region or edit graph, whichever is smaller.
+                  if k == -d || k == -m || k /= d && k /= n && x left < x up then
                     moveDownFrom up
-                  else if k /= d && k /= n then
-                    -- Somewhere in the interior of the search region and edit graph.
-                    if x left < x up then
-                      moveDownFrom up
-                    else
-                      moveRightFrom left
                   else
-                    -- The upper/right extent of the search region or edit graph, whichever is smaller.
                     moveRightFrom left
-                  where getK k = v ! k
-                        left = getK (pred k)
-                        up   = getK (succ k)
+                  where left = v ! pred k
+                        up   = v ! succ k
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.
-                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y < m then That (bs ! y) : script else script)
+                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if inRange (Array.bounds bs) y then That (bs ! y) : script else script)
 
                 -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if x < n then This (as ! x) : script else script)
+                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if inRange (Array.bounds as) x then This (as ! x) : script else script)
 
                 -- | Slide down any diagonal edges from a given vertex.
                 slideFrom (Endpoint x y script)

From c2daf3af07d75e5ca67a19f8f76a1c4df4f4ddf8 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 13:17:44 -0400
Subject: [PATCH 22/46] Avoid refetching the bounds.

---
 src/SES/Myers.hs | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 3ad601db0..300d428a3 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -25,6 +25,7 @@ ses eq as' bs'
   | null as = That <$> toList bs
   | otherwise = reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
   where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
+        (aBounds, bBounds) = (Array.bounds as, Array.bounds bs)
         (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
@@ -45,14 +46,14 @@ ses eq as' bs'
                         up   = v ! succ k
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.
-                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if inRange (Array.bounds bs) y then That (bs ! y) : script else script)
+                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if inRange bBounds y then That (bs ! y) : script else script)
 
                 -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if inRange (Array.bounds as) x then This (as ! x) : script else script)
+                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if inRange aBounds x then This (as ! x) : script else script)
 
                 -- | Slide down any diagonal edges from a given vertex.
                 slideFrom (Endpoint x y script)
-                  | inRange (Array.bounds as) x, a <- as ! x
-                  , inRange (Array.bounds bs) y, b <- bs ! y
+                  | inRange aBounds x, a <- as ! x
+                  , inRange bBounds y, b <- bs ! y
                   , a `eq` b  = slideFrom (Endpoint (succ x) (succ y) (These a b : script))
                   | otherwise =            Endpoint       x        y               script

From d71f97d1b58e1b929d3135bdb45b428049660b82 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 13:20:25 -0400
Subject: [PATCH 23/46] Save the initial bounds computation.

---
 src/SES/Myers.hs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 300d428a3..ea2fda4f1 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -24,8 +24,8 @@ ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
   | otherwise = reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
-  where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
-        (aBounds, bBounds) = (Array.bounds as, Array.bounds bs)
+  where (as, bs) = (Array.listArray aBounds (toList as'), Array.listArray bBounds (toList bs'))
+        (aBounds, bBounds) = ((0, pred n), (0, pred m))
         (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.

From 8a2c0bf6c2c1abcecb227df5cb389f9d3330288f Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 13:45:16 -0400
Subject: [PATCH 24/46] Less strict evaluation of the edit script.

---
 src/SES/Myers.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index ea2fda4f1..e55f2245f 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -14,7 +14,7 @@ import Prologue hiding (error)
 -- | An edit script, i.e. a sequence of changes/copies of elements.
 type EditScript a b = [These a b]
 
-data Endpoint a b = Endpoint { x :: {-# UNPACK #-} !Int, _y :: {-# UNPACK #-} !Int, _script :: !(EditScript a b) }
+data Endpoint a b = Endpoint { x :: {-# UNPACK #-} !Int, _y :: {-# UNPACK #-} !Int, _script :: EditScript a b }
   deriving (Eq, Show)
 
 

From 6a627255c9dcbe052e08621845a7b980da0f9252 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 13:53:15 -0400
Subject: [PATCH 25/46] Revert "Save the initial bounds computation."

This reverts commit e062aa06676723be75c47d376fcf66457d9d39aa.
---
 src/SES/Myers.hs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index e55f2245f..dfa8c43f3 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -24,8 +24,8 @@ ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
   | otherwise = reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
-  where (as, bs) = (Array.listArray aBounds (toList as'), Array.listArray bBounds (toList bs'))
-        (aBounds, bBounds) = ((0, pred n), (0, pred m))
+  where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
+        (aBounds, bBounds) = (Array.bounds as, Array.bounds bs)
         (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.

From c45b5a32b07f2225fd3eaa9de454626448cbb7bd Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 13:53:27 -0400
Subject: [PATCH 26/46] Revert "Avoid refetching the bounds."

This reverts commit de7739b5cf19f93a1ae697350e3332c9a1ea3034.
---
 src/SES/Myers.hs | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index dfa8c43f3..c4f6349b5 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -25,7 +25,6 @@ ses eq as' bs'
   | null as = That <$> toList bs
   | otherwise = reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
   where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
-        (aBounds, bBounds) = (Array.bounds as, Array.bounds bs)
         (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
@@ -46,14 +45,14 @@ ses eq as' bs'
                         up   = v ! succ k
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.
-                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if inRange bBounds y then That (bs ! y) : script else script)
+                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if inRange (Array.bounds bs) y then That (bs ! y) : script else script)
 
                 -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if inRange aBounds x then This (as ! x) : script else script)
+                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if inRange (Array.bounds as) x then This (as ! x) : script else script)
 
                 -- | Slide down any diagonal edges from a given vertex.
                 slideFrom (Endpoint x y script)
-                  | inRange aBounds x, a <- as ! x
-                  , inRange bBounds y, b <- bs ! y
+                  | inRange (Array.bounds as) x, a <- as ! x
+                  , inRange (Array.bounds bs) y, b <- bs ! y
                   , a `eq` b  = slideFrom (Endpoint (succ x) (succ y) (These a b : script))
                   | otherwise =            Endpoint       x        y               script

From c736d2527c50b1dea0cf8608a744f06e2fb3d5af Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 13:54:59 -0400
Subject: [PATCH 27/46] Revert "Simplify the search logic."

This reverts commit 008658c2bfb06da29d56278b4b48f5b76d0dd211.
---
 src/SES/Myers.hs | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index c4f6349b5..0e5ae2238 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -23,13 +23,13 @@ ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript
 ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
-  | otherwise = reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
+  | otherwise = reverse (searchUpToD 0 (Array.array (0, 0) [(0, Endpoint 0 0 [])]))
   where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
         (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD d v =
-          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (-m, n) k ] in
+          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (negate m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
             _ -> searchUpToD (succ d) (Array.array (-d, d) ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))
@@ -37,18 +37,30 @@ ses eq as' bs'
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
                 searchAlongK k = slideFrom $!
-                  if k == -d || k == -m || k /= d && k /= n && x left < x up then
+                  if d == 0 then
+                    -- The top-left corner.
+                    Endpoint 0 0 []
+                  else if k == -d || k == -m then
+                    -- The lower/left extent of the search region or edit graph, whichever is smaller.
                     moveDownFrom up
+                  else if k /= d && k /= n then
+                    -- Somewhere in the interior of the search region and edit graph.
+                    if x left < x up then
+                      moveDownFrom up
+                    else
+                      moveRightFrom left
                   else
+                    -- The upper/right extent of the search region or edit graph, whichever is smaller.
                     moveRightFrom left
-                  where left = v ! pred k
-                        up   = v ! succ k
+                  where getK k = v ! k
+                        left = getK (pred k)
+                        up   = getK (succ k)
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.
-                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if inRange (Array.bounds bs) y then That (bs ! y) : script else script)
+                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y < m then That (bs ! y) : script else script)
 
                 -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if inRange (Array.bounds as) x then This (as ! x) : script else script)
+                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if x < n then This (as ! x) : script else script)
 
                 -- | Slide down any diagonal edges from a given vertex.
                 slideFrom (Endpoint x y script)

From f0147ae5137699d70ffe82850b1a5b69606dfdca Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 13:58:19 -0400
Subject: [PATCH 28/46] Prioritize the upper/right extent.

---
 src/SES/Myers.hs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 0e5ae2238..1bfbe6808 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -43,15 +43,15 @@ ses eq as' bs'
                   else if k == -d || k == -m then
                     -- The lower/left extent of the search region or edit graph, whichever is smaller.
                     moveDownFrom up
-                  else if k /= d && k /= n then
+                  else if k == d || k == n then
+                    -- The upper/right extent of the search region or edit graph, whichever is smaller.
+                    moveRightFrom left
+                  else
                     -- Somewhere in the interior of the search region and edit graph.
                     if x left < x up then
                       moveDownFrom up
                     else
                       moveRightFrom left
-                  else
-                    -- The upper/right extent of the search region or edit graph, whichever is smaller.
-                    moveRightFrom left
                   where getK k = v ! k
                         left = getK (pred k)
                         up   = getK (succ k)

From 231a746d58db0c945ec94b2b1abae9f7b515ad1b Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 14:00:13 -0400
Subject: [PATCH 29/46] Use unary - instead of negate.

---
 src/SES/Myers.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 1bfbe6808..35f0b4b2f 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -29,7 +29,7 @@ ses eq as' bs'
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD d v =
-          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (negate m, n) k ] in
+          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (-m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
             _ -> searchUpToD (succ d) (Array.array (-d, d) ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))

From 0bf24a86dbe23603eb826107da8135236fd6cb81 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 14:01:33 -0400
Subject: [PATCH 30/46] Force the evaluation of the state array.

---
 src/SES/Myers.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 35f0b4b2f..4879fcb49 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -28,7 +28,7 @@ ses eq as' bs'
         (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
-        searchUpToD d v =
+        searchUpToD d v = v `seq`
           let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (-m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script

From c4f45522af7cbe7b760576c57c1ecf9674c7845b Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 14:02:06 -0400
Subject: [PATCH 31/46] :fire: getK.

---
 src/SES/Myers.hs | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 4879fcb49..fe6a2e832 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -52,9 +52,8 @@ ses eq as' bs'
                       moveDownFrom up
                     else
                       moveRightFrom left
-                  where getK k = v ! k
-                        left = getK (pred k)
-                        up   = getK (succ k)
+                  where left = v ! pred k
+                        up   = v ! succ k
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.
                 moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y < m then That (bs ! y) : script else script)

From 922569fbe6f3c0ed2050983d51b948912b0ec4b6 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 14:46:57 -0400
Subject: [PATCH 32/46] Simplify the search along k logic again.

---
 src/SES/Myers.hs | 22 +++++++---------------
 1 file changed, 7 insertions(+), 15 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index fe6a2e832..8f90a00a6 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -23,40 +23,32 @@ ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript
 ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
-  | otherwise = reverse (searchUpToD 0 (Array.array (0, 0) [(0, Endpoint 0 0 [])]))
+  | otherwise = reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
   where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
         (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD d v = v `seq`
-          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (-m, n) k ] in
+          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (lower, upper) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
             _ -> searchUpToD (succ d) (Array.array (-d, d) ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))
-          where isComplete (Endpoint x y _) = x >= n && y >= m
+          where (lower, upper) = (-(min m d), min n d)
+                isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
                 searchAlongK k = slideFrom $!
-                  if d == 0 then
-                    -- The top-left corner.
-                    Endpoint 0 0 []
-                  else if k == -d || k == -m then
+                  if k == lower || k /= upper && x left < x up then
                     -- The lower/left extent of the search region or edit graph, whichever is smaller.
                     moveDownFrom up
-                  else if k == d || k == n then
+                  else
                     -- The upper/right extent of the search region or edit graph, whichever is smaller.
                     moveRightFrom left
-                  else
-                    -- Somewhere in the interior of the search region and edit graph.
-                    if x left < x up then
-                      moveDownFrom up
-                    else
-                      moveRightFrom left
                   where left = v ! pred k
                         up   = v ! succ k
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.
-                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y < m then That (bs ! y) : script else script)
+                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y >= 0 && y < m then That (bs ! y) : script else script)
 
                 -- | Move rightward from a given vertex, deleting the element for the corresponding column.
                 moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if x < n then This (as ! x) : script else script)

From 74c301f7dad35efa16437e20a987087c695e3e59 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 15:01:49 -0400
Subject: [PATCH 33/46] Define searchAlongK with guard clauses.

---
 src/SES/Myers.hs | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 8f90a00a6..486325b44 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -23,27 +23,26 @@ ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript
 ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
-  | otherwise = reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
+  | otherwise = n `seq` m `seq` reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
   where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
         (n, m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
-        searchUpToD d v = v `seq`
-          let endpoints = searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (lower, upper) k ] in
+        searchUpToD d v = d `seq` v `seq`
+          let endpoints = slideFrom . searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (-m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
             _ -> searchUpToD (succ d) (Array.array (-d, d) ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))
-          where (lower, upper) = (-(min m d), min n d)
-                isComplete (Endpoint x y _) = x >= n && y >= m
+          where isComplete (Endpoint x y _) = x >= n && y >= m
 
-                -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any), and sliding down any diagonal edges eagerly.
-                searchAlongK k = slideFrom $!
-                  if k == lower || k /= upper && x left < x up then
-                    -- The lower/left extent of the search region or edit graph, whichever is smaller.
-                    moveDownFrom up
-                  else
-                    -- The upper/right extent of the search region or edit graph, whichever is smaller.
-                    moveRightFrom left
+                -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any).
+                searchAlongK k
+                  | k == -d       = moveDownFrom up
+                  | k ==  d       = moveRightFrom left
+                  | k == -m       = moveDownFrom up
+                  | k ==  n       = moveRightFrom left
+                  | x left < x up = moveDownFrom up
+                  | otherwise     = moveRightFrom left
                   where left = v ! pred k
                         up   = v ! succ k
 

From 9fa104fb23b8d10ddeedfb4a6cad0e42b7348a2c Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 15:04:11 -0400
Subject: [PATCH 34/46] Factor out the downward/rightward moves.

---
 src/SES/Myers.hs | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 486325b44..d57efb5cc 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -37,13 +37,15 @@ ses eq as' bs'
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any).
                 searchAlongK k
-                  | k == -d       = moveDownFrom up
-                  | k ==  d       = moveRightFrom left
-                  | k == -m       = moveDownFrom up
-                  | k ==  n       = moveRightFrom left
-                  | x left < x up = moveDownFrom up
-                  | otherwise     = moveRightFrom left
-                  where left = v ! pred k
+                  | k == -d       = moveDown
+                  | k ==  d       = moveRight
+                  | k == -m       = moveDown
+                  | k ==  n       = moveRight
+                  | x left < x up = moveDown
+                  | otherwise     = moveRight
+                  where moveRight = moveRightFrom left
+                        moveDown  = moveDownFrom  up
+                        left = v ! pred k
                         up   = v ! succ k
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.

From 6947a56501cda0b67ada89217dbaa69ebb798759 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 15:27:08 -0400
Subject: [PATCH 35/46] Use BangPatterns to denote strictness.

---
 src/SES/Myers.hs | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index d57efb5cc..8833051fe 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE GADTs, ImplicitParams, MultiParamTypeClasses, ScopedTypeVariables #-}
+{-# LANGUAGE BangPatterns, GADTs, ImplicitParams, MultiParamTypeClasses, ScopedTypeVariables #-}
 module SES.Myers
 ( EditScript
 , ses
@@ -23,12 +23,12 @@ ses :: (Foldable t, Foldable u) => (a -> b -> Bool) -> t a -> u b -> EditScript
 ses eq as' bs'
   | null bs = This <$> toList as
   | null as = That <$> toList bs
-  | otherwise = n `seq` m `seq` reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
+  | otherwise = reverse (searchUpToD 0 (Array.array (1, 1) [(1, Endpoint 0 (-1) [])]))
   where (as, bs) = (Array.listArray (0, pred n) (toList as'), Array.listArray (0, pred m) (toList bs'))
-        (n, m) = (length as', length bs')
+        (!n, !m) = (length as', length bs')
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
-        searchUpToD d v = d `seq` v `seq`
+        searchUpToD !d !v =
           let endpoints = slideFrom . searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (-m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
@@ -36,7 +36,7 @@ ses eq as' bs'
           where isComplete (Endpoint x y _) = x >= n && y >= m
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any).
-                searchAlongK k
+                searchAlongK !k
                   | k == -d       = moveDown
                   | k ==  d       = moveRight
                   | k == -m       = moveDown

From 2ed1926f94f54fef9233823c8d436556bcfe962d Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 15:32:07 -0400
Subject: [PATCH 36/46] :fire: moveDown/moveRight.

---
 src/SES/Myers.hs | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 8833051fe..f4dd044b5 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -37,16 +37,17 @@ ses eq as' bs'
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any).
                 searchAlongK !k
-                  | k == -d       = moveDown
-                  | k ==  d       = moveRight
-                  | k == -m       = moveDown
-                  | k ==  n       = moveRight
-                  | x left < x up = moveDown
-                  | otherwise     = moveRight
-                  where moveRight = moveRightFrom left
-                        moveDown  = moveDownFrom  up
-                        left = v ! pred k
-                        up   = v ! succ k
+                  | k == -d = moveDownFrom (v ! succ k)
+                  | k ==  d = moveRightFrom (v ! pred k)
+                  | k == -m = moveDownFrom (v ! succ k)
+                  | k ==  n = moveRightFrom (v ! pred k)
+                  | otherwise =
+                    let left = v ! pred k
+                        up   = v ! succ k in
+                    if x left < x up then
+                      moveDownFrom up
+                    else
+                      moveRightFrom left
 
                 -- | Move downward from a given vertex, inserting the element for the corresponding row.
                 moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y >= 0 && y < m then That (bs ! y) : script else script)

From 8131dfaf756c38f66d386ebaa3e9062fdaeb5e74 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 15:33:06 -0400
Subject: [PATCH 37/46] Move moveDownFrom/moveRightFrom/slideFrom out a scope.

---
 src/SES/Myers.hs | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index f4dd044b5..97d379941 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -49,15 +49,15 @@ ses eq as' bs'
                     else
                       moveRightFrom left
 
-                -- | Move downward from a given vertex, inserting the element for the corresponding row.
-                moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y >= 0 && y < m then That (bs ! y) : script else script)
+        -- | Move downward from a given vertex, inserting the element for the corresponding row.
+        moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y >= 0 && y < m then That (bs ! y) : script else script)
 
-                -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-                moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if x < n then This (as ! x) : script else script)
+        -- | Move rightward from a given vertex, deleting the element for the corresponding column.
+        moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if x < n then This (as ! x) : script else script)
 
-                -- | Slide down any diagonal edges from a given vertex.
-                slideFrom (Endpoint x y script)
-                  | inRange (Array.bounds as) x, a <- as ! x
-                  , inRange (Array.bounds bs) y, b <- bs ! y
-                  , a `eq` b  = slideFrom (Endpoint (succ x) (succ y) (These a b : script))
-                  | otherwise =            Endpoint       x        y               script
+        -- | Slide down any diagonal edges from a given vertex.
+        slideFrom (Endpoint x y script)
+          | inRange (Array.bounds as) x, a <- as ! x
+          , inRange (Array.bounds bs) y, b <- bs ! y
+          , a `eq` b  = slideFrom (Endpoint (succ x) (succ y) (These a b : script))
+          | otherwise =            Endpoint       x        y               script

From 139c9513d18060cee274e75036403e56be807839 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 15:33:26 -0400
Subject: [PATCH 38/46] Align moveDownFrom/moveRightFrom.

---
 src/SES/Myers.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 97d379941..32a720fb2 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -53,7 +53,7 @@ ses eq as' bs'
         moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y >= 0 && y < m then That (bs ! y) : script else script)
 
         -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-        moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if x < n then This (as ! x) : script else script)
+        moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if x < n           then This (as ! x) : script else script)
 
         -- | Slide down any diagonal edges from a given vertex.
         slideFrom (Endpoint x y script)

From 9337631149de65873ae9d2c8fd74c0a77d478ce4 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 15:46:36 -0400
Subject: [PATCH 39/46] Safe array subscripting.
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

This halves our heap consumption. I don’t know how.
---
 src/SES/Myers.hs | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 32a720fb2..491426a24 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -4,7 +4,6 @@ module SES.Myers
 , ses
 ) where
 
-import Data.Array ((!))
 import qualified Data.Array as Array
 import Data.Ix
 import Data.These
@@ -37,27 +36,36 @@ ses eq as' bs'
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any).
                 searchAlongK !k
-                  | k == -d = moveDownFrom (v ! succ k)
-                  | k ==  d = moveRightFrom (v ! pred k)
-                  | k == -m = moveDownFrom (v ! succ k)
-                  | k ==  n = moveRightFrom (v ! pred k)
+                  | k == -d = moveDownFrom (v Array.! succ k)
+                  | k ==  d = moveRightFrom (v Array.! pred k)
+                  | k == -m = moveDownFrom (v Array.! succ k)
+                  | k ==  n = moveRightFrom (v Array.! pred k)
                   | otherwise =
-                    let left = v ! pred k
-                        up   = v ! succ k in
+                    let left = v Array.! pred k
+                        up   = v Array.! succ k in
                     if x left < x up then
                       moveDownFrom up
                     else
                       moveRightFrom left
 
         -- | Move downward from a given vertex, inserting the element for the corresponding row.
-        moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) (if y >= 0 && y < m then That (bs ! y) : script else script)
+        moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) $ case bs ! y of
+          Just b -> That b : script
+          _ -> script
 
         -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-        moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  (if x < n           then This (as ! x) : script else script)
+        moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  $ case as ! x of
+          Just a -> This a : script
+          _ -> script
 
         -- | Slide down any diagonal edges from a given vertex.
         slideFrom (Endpoint x y script)
-          | inRange (Array.bounds as) x, a <- as ! x
-          , inRange (Array.bounds bs) y, b <- bs ! y
+          | Just a <- as ! x
+          , Just b <- bs ! y
           , a `eq` b  = slideFrom (Endpoint (succ x) (succ y) (These a b : script))
           | otherwise =            Endpoint       x        y               script
+
+
+(!) :: Ix i => Array.Array i a -> i -> Maybe a
+(!) v i | inRange (Array.bounds v) i = Just $! v Array.! i
+        | otherwise = Nothing

From 4cdd66b12b6f2bee4326dcdabaf6bfd3e4446fd8 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 15:50:56 -0400
Subject: [PATCH 40/46] Mark moveDownFrom and moveRightFrom for inlining.

---
 src/SES/Myers.hs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 491426a24..7ddb73560 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -52,11 +52,13 @@ ses eq as' bs'
         moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) $ case bs ! y of
           Just b -> That b : script
           _ -> script
+        {-# INLINE moveDownFrom #-}
 
         -- | Move rightward from a given vertex, deleting the element for the corresponding column.
         moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  $ case as ! x of
           Just a -> This a : script
           _ -> script
+        {-# INLINE moveRightFrom #-}
 
         -- | Slide down any diagonal edges from a given vertex.
         slideFrom (Endpoint x y script)

From 676cfaa68ec8797c64db2fbac932e1694d084920 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 16:02:56 -0400
Subject: [PATCH 41/46] Try to inline !.

---
 src/SES/Myers.hs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 7ddb73560..c5b09e083 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -71,3 +71,4 @@ ses eq as' bs'
 (!) :: Ix i => Array.Array i a -> i -> Maybe a
 (!) v i | inRange (Array.bounds v) i = Just $! v Array.! i
         | otherwise = Nothing
+{-# INLINE (!) #-}

From d822c138b3235fc0620caa2aad401eb86684aade Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 16:06:20 -0400
Subject: [PATCH 42/46] Use bang patterns for strictness.

---
 src/SES/Myers.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index c5b09e083..ce3c7f596 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -69,6 +69,6 @@ ses eq as' bs'
 
 
 (!) :: Ix i => Array.Array i a -> i -> Maybe a
-(!) v i | inRange (Array.bounds v) i = Just $! v Array.! i
+(!) v i | inRange (Array.bounds v) i, !a <- v Array.! i = Just a
         | otherwise = Nothing
 {-# INLINE (!) #-}

From a084544487c13b663ee5708dbb27a768b66e5205 Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 16:07:42 -0400
Subject: [PATCH 43/46] Evaluate endpoints eagerly.

---
 src/SES/Myers.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index ce3c7f596..0c68a3953 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -28,7 +28,7 @@ ses eq as' bs'
 
         -- Search an edit graph for the shortest edit script up to a given proposed edit distance, building on the results of previous searches.
         searchUpToD !d !v =
-          let endpoints = slideFrom . searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (-m, n) k ] in
+          let !endpoints = slideFrom . searchAlongK <$> [ k | k <- [-d, -d + 2 .. d], inRange (-m, n) k ] in
           case find isComplete endpoints of
             Just (Endpoint _ _ script) -> script
             _ -> searchUpToD (succ d) (Array.array (-d, d) ((\ e@(Endpoint x y _) -> (x - y, e)) <$> endpoints))

From 3bf95e05f428396eda5e3967b9edd6dc9bc0c55c Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 16:33:47 -0400
Subject: [PATCH 44/46] Tighten up moveDownFrom/moveRightFrom.

---
 src/SES/Myers.hs | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 0c68a3953..ae6538866 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -49,15 +49,11 @@ ses eq as' bs'
                       moveRightFrom left
 
         -- | Move downward from a given vertex, inserting the element for the corresponding row.
-        moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) $ case bs ! y of
-          Just b -> That b : script
-          _ -> script
+        moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) $ maybe script ((: script) . That) (bs ! y)
         {-# INLINE moveDownFrom #-}
 
         -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-        moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  $ case as ! x of
-          Just a -> This a : script
-          _ -> script
+        moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  $ maybe script ((: script) . This) (as ! x)
         {-# INLINE moveRightFrom #-}
 
         -- | Slide down any diagonal edges from a given vertex.

From 47a33a7916e3a17069a31fe69b698441e11a396e Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 17:02:29 -0400
Subject: [PATCH 45/46] Rename ! to !?.

---
 src/SES/Myers.hs | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index ae6538866..819c622ea 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -49,22 +49,22 @@ ses eq as' bs'
                       moveRightFrom left
 
         -- | Move downward from a given vertex, inserting the element for the corresponding row.
-        moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) $ maybe script ((: script) . That) (bs ! y)
+        moveDownFrom  (Endpoint x y script) = Endpoint       x (succ y) $ maybe script ((: script) . That) (bs !? y)
         {-# INLINE moveDownFrom #-}
 
         -- | Move rightward from a given vertex, deleting the element for the corresponding column.
-        moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  $ maybe script ((: script) . This) (as ! x)
+        moveRightFrom (Endpoint x y script) = Endpoint (succ x)      y  $ maybe script ((: script) . This) (as !? x)
         {-# INLINE moveRightFrom #-}
 
         -- | Slide down any diagonal edges from a given vertex.
         slideFrom (Endpoint x y script)
-          | Just a <- as ! x
-          , Just b <- bs ! y
+          | Just a <- as !? x
+          , Just b <- bs !? y
           , a `eq` b  = slideFrom (Endpoint (succ x) (succ y) (These a b : script))
           | otherwise =            Endpoint       x        y               script
 
 
-(!) :: Ix i => Array.Array i a -> i -> Maybe a
-(!) v i | inRange (Array.bounds v) i, !a <- v Array.! i = Just a
-        | otherwise = Nothing
-{-# INLINE (!) #-}
+(!?) :: Ix i => Array.Array i a -> i -> Maybe a
+(!?) v i | inRange (Array.bounds v) i, !a <- v Array.! i = Just a
+         | otherwise = Nothing
+{-# INLINE (!?) #-}

From 21508fb6c31c4d697938b1a9d37ceb841fb10fbe Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@me.com>
Date: Wed, 21 Jun 2017 17:05:13 -0400
Subject: [PATCH 46/46] Resume the unqualified import of !.

---
 src/SES/Myers.hs | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/src/SES/Myers.hs b/src/SES/Myers.hs
index 819c622ea..b8c32fd2e 100644
--- a/src/SES/Myers.hs
+++ b/src/SES/Myers.hs
@@ -4,6 +4,7 @@ module SES.Myers
 , ses
 ) where
 
+import Data.Array ((!))
 import qualified Data.Array as Array
 import Data.Ix
 import Data.These
@@ -36,13 +37,13 @@ ses eq as' bs'
 
                 -- Search an edit graph for the shortest edit script along a specific diagonal, moving onto a given diagonal from one of its in-bounds adjacent diagonals (if any).
                 searchAlongK !k
-                  | k == -d = moveDownFrom (v Array.! succ k)
-                  | k ==  d = moveRightFrom (v Array.! pred k)
-                  | k == -m = moveDownFrom (v Array.! succ k)
-                  | k ==  n = moveRightFrom (v Array.! pred k)
+                  | k == -d = moveDownFrom (v ! succ k)
+                  | k ==  d = moveRightFrom (v ! pred k)
+                  | k == -m = moveDownFrom (v ! succ k)
+                  | k ==  n = moveRightFrom (v ! pred k)
                   | otherwise =
-                    let left = v Array.! pred k
-                        up   = v Array.! succ k in
+                    let left = v ! pred k
+                        up   = v ! succ k in
                     if x left < x up then
                       moveDownFrom up
                     else
@@ -65,6 +66,6 @@ ses eq as' bs'
 
 
 (!?) :: Ix i => Array.Array i a -> i -> Maybe a
-(!?) v i | inRange (Array.bounds v) i, !a <- v Array.! i = Just a
+(!?) v i | inRange (Array.bounds v) i, !a <- v ! i = Just a
          | otherwise = Nothing
 {-# INLINE (!?) #-}