Faster regular expression pattern matching in the grammar compiler.

The sequence operator (x+y) was implemented by splitting the string to be
matched at all positions and trying to match the parts against the two
subpatterns. To reduce the number of splits, we now estimate the minimum and
maximum length of the string that the subpatterns could match. For common
cases, where one of the subpatterns is a string of known length, like
in (x+"y") or (x + ("a"|"o"|"u"|"e")+"y"), only one split will be tried.

src/compiler/GF/Grammar/PatternMatch.hs

@@ -14,7 +14,8 @@
 
 module GF.Grammar.PatternMatch (matchPattern,
 		     testOvershadow, 
-		     findMatch
+		     findMatch,
+                     measurePatt
 		    ) where
 
 import GF.Data.Operations
@@ -117,6 +118,7 @@ tryMatch (p,t) = do
         _ -> Bad (render (text "no match with negative pattern" <+> ppPatt Unqualified 0 p))
 
       (PSeq p1 p2, ([],K s, [])) -> matchPSeq p1 p2 s
+      (PMSeq mp1 mp2, ([],K s, [])) -> matchPMSeq mp1 mp2 s
 
       (PRep p1, ([],K s, [])) -> checks [
          trym (foldr (const (PSeq p1)) (PString "") 
@@ -129,13 +131,18 @@ tryMatch (p,t) = do
 
       _ -> Bad (render (text "no match in case expr for" <+> ppTerm Unqualified 0 t))
 
-matchPSeq p1 p2 s =
-  do let min1 = 0 --minLength p1
-         min2 = length s -- -minLength p2
-         cuts = [splitAt n s | n <- [min1 .. min2]]
+matchPMSeq (m1,p1) (m2,p2) s = matchPSeq' m1 p1 m2 p2 s
+--matchPSeq p1 p2 s = matchPSeq' (0,maxBound::Int) p1 (0,maxBound::Int) p2 s
+matchPSeq p1 p2 s = matchPSeq' (lengthBounds p1) p1 (lengthBounds p2) p2 s
+
+matchPSeq' b1@(min1,max1) p1 b2@(min2,max2) p2 s =
+  do let n = length s
+         lo = min1 `max` (n-max2)
+         hi = (n-min2) `min` max1
+         cuts = [splitAt i s | i <- [lo..hi]]
      matches <- checks [mapM tryMatch [(p1,K s1),(p2,K s2)] | (s1,s2) <- cuts]
      return (concat matches)
-{-
+
 -- | Estimate the minimal length of the string that a pattern will match
 minLength p =
   case p of
@@ -147,7 +154,31 @@ minLength p =
     PAs x p' -> minLength p' 
     PT t p' -> minLength p' 
     _ -> 0 -- safe underestimate
--}
+
+-- | Estimate the maximal length of the string that a pattern will match
+maxLength = maybe maxBound id . maxl  -- safe overestimate
+  where
+    maxl p =
+      case p of
+        PString s  -> Just (length s)
+        PSeq p1 p2 -> liftM2 (+) (maxl p1) (maxl p2)
+        PAlt p1 p2 -> liftM2 max (maxl p1) (maxl p2)
+        PChar      -> Just 1
+        PChars _   -> Just 1
+        PAs x p'   -> maxl p'
+        PT t p'    -> maxl p'
+        _          -> Nothing -- unknown length
+
+lengthBounds p = (minLength p,maxLength p)
+
+mPatt p = (lengthBounds p,measurePatt p)
+
+measurePatt p =
+  case p of
+    PSeq p1 p2 -> PMSeq (mPatt p1) (mPatt p2)
+    _ -> composSafePattOp measurePatt p
+
+
 isInConstantForm :: Term -> Bool
 isInConstantForm trm = case trm of
     Cn _     -> True