[m-dev.] for review: new string__length mode

[Michael Day]

For optimisation of string__length(LongString, SmallNumber). Anyone think
this is actually worth it?

Estimated hours taken: 0.25

library/string.m:
	Added a new mode to string__length for checking if the length of a
	string is equal to a particular number.

Index: string.m
===================================================================
RCS file: /home/mercury1/repository/mercury/library/string.m,v
retrieving revision 1.137
diff -u -r1.137 string.m
--- string.m	2000/11/16 05:29:31	1.137
+++ string.m	2000/11/16 09:03:57
@@ -26,6 +26,7 @@
 :- pred string__length(string, int).
 :- mode string__length(in, uo) is det.
 :- mode string__length(ui, uo) is det.
+:- mode string__length(ui, in) is semidet.
 	% Determine the length of a string.
 	% An empty string has length zero.
 
@@ -1616,6 +1617,20 @@
 :- pragma c_code(string__length(Str::ui, Length::uo),
 		[will_not_call_mercury, thread_safe], "
 	Length = strlen(Str);
+").
+
+/*
+:- pred string__length(string, int).
+:- mode string__length(ui, in) is semidet.
+*/
+:- pragma c_code(string__length(Str::ui, Length::in),
+		[will_not_call_mercury, thread_safe], "
+	size_t j;
+	char *Str2 = Str;
+	for (j = Length; j != 0; j--, Str2++) {
+	    if (*Str2 == 0) break;
+	}
+	SUCCESS_INDICATOR = (*Str2 == 0 && j == 0);
 ").
 
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