[m-rev.] for review: efficiency improvement for string__hash

[Simon Taylor]

Estimated hours taken: 1
Branches: main, release

library/string.m:
	Avoid memory allocation in string__hash.

Index: library/string.m
===================================================================
RCS file: /home/mercury1/repository/mercury/library/string.m,v
retrieving revision 1.184
diff -u -u -r1.184 string.m
--- library/string.m	15 Nov 2002 04:50:37 -0000	1.184
+++ library/string.m	21 Nov 2002 02:33:18 -0000
@@ -1208,25 +1208,28 @@
 
 string__hash(String, HashVal) :-
 	string__length(String, Length),
-	string__to_int_list(String, CodeList),
-	string__hash_2(CodeList, 0, HashVal0),
+	string__hash_2(String, 0, Length, 0, HashVal0),
 	HashVal = HashVal0 `xor` Length.
 
-:- pred string__hash_2(list(int), int, int).
-:- mode string__hash_2(in, in, out) is det.
+:- pred string__hash_2(string, int, int, int, int).
+:- mode string__hash_2(in, in, in, in, out) is det.
 
-string__hash_2([], HashVal, HashVal).
-string__hash_2([X | Xs], HashVal0, HashVal) :-
-	string__combine_hash(HashVal0, X, HashVal1),
-	string__hash_2(Xs, HashVal1, HashVal).
+string__hash_2(String, Index, Length, HashVal0, HashVal) :-
+	( Index < Length ->
+		string__combine_hash(HashVal0,
+			char__to_int(string__unsafe_index(String, Index)),
+			HashVal1),
+		string__hash_2(String, Index + 1, Length, HashVal1, HashVal)
+	;
+		HashVal = HashVal0
+	).
 
 :- pred string__combine_hash(int, int, int).
 :- mode string__combine_hash(in, in, out) is det.
 
 string__combine_hash(H0, X, H) :-
-	H1 = H0 << 5,
-	H2 = H1 `xor` H0,
-	H = H2 `xor` X.
+	H1 = H0 `xor` (H0 << 5),
+	H = H1 `xor` X.
 
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