add compress function

suffixes.hs

@@ -10,10 +10,18 @@ data Trie = TBranch {
             }
   deriving (Show)
 
+
+compress t@(TBranch root []) = t
+compress (TBranch root children)
+  | length children == 1 =
+      let compressed = (compress $ head children) in TBranch (root++(getRoot compressed)) (getChildren compressed)
+  | otherwise = TBranch root (map compress children)
+
 notEmpty [] = False
 notEmpty _ = True
 
 buildTrie [] = TBranch "" []
+buildTrie ("":_) = TBranch "" []
 buildTrie words =
   let root   = head $ head words
       groups = groupTails $ map tail words
@@ -22,7 +30,13 @@ buildTrie words =
 trie words = TBranch "" (map buildTrie $ groupTails $ tails words)
 
 groupTails [] = []
-groupTails xs = L.groupBy ((==) `F.on` head) $ L.sort $ filter notEmpty xs
+groupTails xs = L.groupBy grouper $ L.sort xs
+  where grouper [] [] = True
+        grouper [] _ = False
+        grouper _ [] = False
+        grouper a b = (head a) == (head b)
 
 tails "" = []
 tails (w@(c:cs)) =  w : tails cs
+
+suffixTree word = compress $ trie word