implement ladder graph creation

ladder.cabal

@@ -27,6 +27,7 @@ library
   exposed-modules:
       Data.AssocMap
       Graph
+      Ladder
       Lib
       PermutationMap
   other-modules:

src/Ladder.hs

@@ -0,0 +1,34 @@
+module Ladder (readDictionary, mkLadderGraph) where
+
+import Data.Char (isLower)
+import qualified Data.List as L
+import qualified Graph as G
+import qualified PermutationMap as PM
+import Prelude hiding (lines, words)
+
+type Dictionary = [String]
+
+readDictionary :: String -> IO Dictionary
+readDictionary filename = do
+  text <- readFile filename
+  let lines = L.lines text
+  let lowercaseWords = map (filter isLower) lines
+  return (L.nub lowercaseWords)
+
+mkLadderGraph :: Dictionary -> G.DiGraph String
+mkLadderGraph dict = G.buildDiGraph [(word, computeCandidates permMap word) | word <- dict]
+  where
+    permMap = PM.createPermutationMap dict
+
+computeCandidates :: PM.PermutationMap -> String -> [String]
+computeCandidates permMap word =
+  -- Delete the original word from the permutations list
+  L.delete word permutations
+  where
+    removed = [L.delete c word | c <- word]
+    added = [c : word | c <- ['a' .. 'z']]
+    modified = [x : L.delete y word | x <- ['a' .. 'z'], y <- word, x /= y]
+    -- Sort and deduplicate all the candidates
+    candidates = L.nub $ map L.sort (added ++ removed ++ modified ++ [word])
+    -- For each candidate, lookup all its permutations
+    permutations = L.concatMap (\w -> PM.findWithDefault [] w permMap) candidates