From dcb25bbbb8e9c12f9632feedb9ab17776d0ecec2 Mon Sep 17 00:00:00 2001
From: Benjamin Summers <benjamin@tlon.io>
Date: Wed, 18 Sep 2019 15:07:53 -0700
Subject: [PATCH] Track terminal state in pure code.

---
 pkg/king/lib/Vere/Term/Logic.hs | 88 +++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 pkg/king/lib/Vere/Term/Logic.hs

diff --git a/pkg/king/lib/Vere/Term/Logic.hs b/pkg/king/lib/Vere/Term/Logic.hs
new file mode 100644
index 000000000..89a0d00e1
--- /dev/null
+++ b/pkg/king/lib/Vere/Term/Logic.hs
@@ -0,0 +1,88 @@
+module Vere.Term.Logic
+    ( SpinnerCause(..), Ev(..), Ef(..)
+    , init
+    , step
+    , drawState
+    ) where
+
+import UrbitPrelude hiding (Empty, getCurrentTime, init)
+
+import Data.Sequence (Seq((:<|), Empty))
+import Urbit.Time    (Wen)
+
+
+--------------------------------------------------------------------------------
+
+data SpinnerCause = User | Event Text
+  deriving (Show)
+
+type SpinnerState = Maybe SpinnerCause
+
+{-
+   %line -- Output a line above the edit line.
+   %spin -- Set the spinner state.
+   %bell -- Ring a bell (no change to the state).
+   %draw -- Redraw the current line (no change to the state).
+   %move -- Move the cursor position.
+   %edit -- Set the edit line, moving the cursor to the end.
+   %more -- Write the edit line to history, and clear it.
+-}
+data Ev
+    = EvLine Text
+    | EvSpin SpinnerState
+    | EvMove Word
+    | EvBell
+    | EvDraw
+    | EvEdit Text
+    | EvMore
+  deriving (Show)
+
+data Ef
+    = EfClear
+    | EfWrite Text
+    | EfShift Int
+    | EfRing
+    | EfSpin SpinnerState
+  deriving (Show)
+
+data St = St
+    { sHistory :: Seq Text
+    , sLine    :: Text
+    , sCurPos  :: Word
+    , sSpinner :: SpinnerState
+    }
+  deriving (Show)
+
+--------------------------------------------------------------------------------
+
+init :: Wen -> St
+init now = St mempty "" 0 Nothing
+
+step :: St -> Ev -> St
+step st@St{..} = \case
+    EvLine t -> st & record t
+    EvSpin s -> st { sSpinner = s }
+    EvMove w -> st { sCurPos = min w (word $ length sLine) }
+    EvEdit t -> st { sLine = t, sCurPos = word (length t) }
+    EvMore   -> st { sLine = "", sCurPos = 0 } & record sLine
+    EvBell   -> st
+    EvDraw   -> st
+  where
+    word :: Integral i => i -> Word
+    word = fromIntegral
+
+    record :: Text -> St -> St
+    record t st@St{..} = st { sHistory = trim (sHistory |> t) }
+
+    trim :: Seq a -> Seq a
+    trim Empty             = Empty
+    trim s | length s < 20 = s
+    trim (_ :<| s)         = s
+
+drawState :: St -> [Ev]
+drawState St{..} = hist <> out <> cur <> spin
+  where
+    hist = EvLine <$> toList sHistory
+    out  = if null sLine   then [] else [EvEdit sLine]
+    cur  = if 0 == sCurPos then [] else [EvMove $ fromIntegral $ sCurPos]
+    spin = maybe [] (singleton . EvSpin . Just) sSpinner