From d3f6fdc3ea71386c2b9a9cd5a686455dbee3e60e Mon Sep 17 00:00:00 2001
From: Gregor Kleen <gkleen@yggdrasil.li>
Date: Mon, 21 May 2018 16:14:26 +0200
Subject: Introduce FSTs & Generalize input/output

`toFST` is currently invalid
---
 edit-lens/src/Control/DFST.lhs | 78 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 edit-lens/src/Control/DFST.lhs

(limited to 'edit-lens/src/Control/DFST.lhs')

diff --git a/edit-lens/src/Control/DFST.lhs b/edit-lens/src/Control/DFST.lhs
new file mode 100644
index 0000000..aec7bbb
--- /dev/null
+++ b/edit-lens/src/Control/DFST.lhs
@@ -0,0 +1,78 @@
+\begin{code}
+{-# LANGUAGE ScopedTypeVariables
+#-}
+
+{-|
+Description: Deterministic finite state transducers
+-}
+module Control.DFST
+  ( DFST(..)
+  , runDFST, runDFST'
+  , toFST
+  ) where
+
+import Data.Map.Strict (Map, (!?))
+import qualified Data.Map.Strict as Map
+
+import Data.Set (Set)
+import qualified Data.Set as Set
+
+import Data.Sequence (Seq(..))
+import qualified Data.Sequence as Seq
+
+import Data.Monoid
+
+import Numeric.Natural
+
+import Control.Monad
+import Control.Monad.State
+
+import Control.FST (FST(FST))
+import qualified Control.FST as FST
+
+
+data DFST state input output = DFST
+  { stInitial :: state
+  , stTransition :: Map (state, input) (state, Seq output)
+    -- ^ All @(s, c)@-combinations not mapped are assumed to map to @(s, Nothing)@
+  , stAccept :: Set state
+  }
+
+
+toFST :: forall state input output. (Ord state, Ord input, Ord output) => DFST state input output -> FST (state, Natural) input output
+-- ^ Split apart non-singleton outputs into a series of epsilon-transitions
+--
+-- This function is currently invalid since multiple out-edges in the `DFST` visit the same intermediate states (we should label intermediate states not only with an ordinal but also with the input Symbol from the `DFST`)
+toFST DFST{..} = flip execState initialFST $ forM_ (Map.toList stTransition) handleTransition
+  where
+    initialFST = FST
+      { stInitial = (stInitial, 0)
+      , stTransition = Map.empty
+      , stAccept = Set.map (,0) stAccept
+      }
+    addTransition :: forall state input output. (Ord state, Ord input, Ord output) => (state, Maybe input) -> (state, Maybe output) -> State (FST state input output) ()
+    addTransition k v = modify $ \f@FST{ stTransition } -> f { FST.stTransition = Map.insertWith Set.union k (Set.singleton v) stTransition }
+    handleTransition :: ((state, input), (state, Seq output)) -> State (FST (state, Natural) input output) ()
+    handleTransition ((st, inS), (st', outs)) = handleTransition' (st, 0) (Just inS) outs (st', 0)
+    handleTransition' :: (state, Natural) -> Maybe input -> Seq output -> (state, Natural) -> State (FST (state, Natural) input output) ()
+    handleTransition' from inS Empty to = addTransition (from, inS) (to, Nothing)
+    handleTransition' from inS (outS :<| Empty) to = addTransition (from, inS) (to, Just outS)
+    handleTransition' from@(st, i) inS (outS :<| oo) to = addTransition (from, inS) ((st, succ i), Just outS) >> handleTransition' (st, succ i) Nothing oo to
+  
+runDFST :: forall state input output. (Ord state, Ord input) => DFST state input output -> Seq input -> Maybe (Seq output)
+runDFST dfst@DFST{..} str = let (finalState, str') = runDFST' dfst stInitial str Seq.empty
+                            in str' <$ guard (finalState `Set.member` stAccept)
+
+runDFST' :: forall state input output. (Ord state, Ord input)
+         => DFST state input output
+         -> state               -- ^ Current state
+         -> Seq input           -- ^ Remaining input
+         -> Seq output          -- ^ Accumulator containing previous output
+         -> (state, Seq output) -- ^ Next state, altered output
+runDFST' _             st  Empty     acc = (st, acc)
+runDFST' dfst@DFST{..} st (c :<| cs) acc
+  | Just (st', mc') <- stTransition !? (st, c)
+  = runDFST' dfst st' cs $ acc <> mc'
+  | otherwise
+  = runDFST' dfst st cs acc
+\end{code}
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