From 4c8e8b2d724fdb59af1d4ef0a09e434dab9d0d75 Mon Sep 17 00:00:00 2001
From: Joshua Clayton <joshua.clayton@gmail.com>
Date: Tue, 17 May 2016 08:33:41 -0400
Subject: [PATCH] Switch to faster implementation of grouping a list

Why?
====

Immediately after searching the codebase after tokens, we group results
together based on match. This can be slow, especially within large
codebases. This improves the time taken, previously O(n^2).

Code reference: http://stackoverflow.com/a/15412231
Corresponding benchmark: https://gist.github.com/joshuaclayton/3dcde3b19e2c3006ee922053edebc417
---
 src/Unused/Util.hs | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/src/Unused/Util.hs b/src/Unused/Util.hs
index 50e171f..27f5ae8 100644
--- a/src/Unused/Util.hs
+++ b/src/Unused/Util.hs
@@ -2,11 +2,11 @@ module Unused.Util
     ( groupBy
     ) where
 
-import Data.List (nub)
+import Control.Arrow ((&&&))
+import qualified Data.List as L
+import Data.Function
 
-groupBy :: Eq b => (a -> b) -> [a] -> [(b, [a])]
-groupBy f l =
-    fmap (\t -> (t, byTerm t)) uniqueTerms
-  where
-    byTerm t = filter ((== t) . f) l
-    uniqueTerms = nub $ fmap f l
+groupBy :: (Ord b) => (a -> b) -> [a] -> [(b, [a])]
+groupBy f = map (f . head &&& id)
+                   . L.groupBy ((==) `on` f)
+                   . L.sortBy (compare `on` f)