Optimize color_parse_mem

Commit 5ef8d77 implemented color_parse_mem, a function for
parsing colors from a non-NUL-terminated string, by simply
allocating a new NUL-terminated string and calling
color_parse. This had a small but measurable speed impact on
a user format that used the advanced color parsing. E.g.,

  # uses quick parsing
  $ time ./git log --pretty=tformat:'%Credfoo%Creset' >/dev/null
  real    0m0.673s
  user    0m0.652s
  sys     0m0.016s

  # uses color_parse_mem
  $ time ./git log --pretty=tformat:'%C(red)foo%C(reset)' >/dev/null
  real    0m0.692s
  user    0m0.660s
  sys     0m0.032s

This patch implements color_parse_mem as the primary
function, with color_parse as a wrapper for strings. This
gives comparable timings to the first case above.

Original patch by René. Commit message and debugging by Jeff
King.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

color.c

@@ -40,30 +40,41 @@ static int parse_attr(const char *name, int len)
 }
 
 void color_parse(const char *value, const char *var, char *dst)
+{
+	color_parse_mem(value, strlen(value), var, dst);
+}
+
+void color_parse_mem(const char *value, int value_len, const char *var,
+		char *dst)
 {
 	const char *ptr = value;
+	int len = value_len;
 	int attr = -1;
 	int fg = -2;
 	int bg = -2;
 
-	if (!strcasecmp(value, "reset")) {
+	if (!strncasecmp(value, "reset", len)) {
 		strcpy(dst, "\033[m");
 		return;
 	}
 
 	/* [fg [bg]] [attr] */
-	while (*ptr) {
+	while (len > 0) {
 		const char *word = ptr;
-		int val, len = 0;
+		int val, wordlen = 0;
 
-		while (word[len] && !isspace(word[len]))
-			len++;
+		while (len > 0 && !isspace(word[wordlen])) {
+			wordlen++;
+			len--;
+		}
 
-		ptr = word + len;
-		while (*ptr && isspace(*ptr))
+		ptr = word + wordlen;
+		while (len > 0 && isspace(*ptr)) {
 			ptr++;
+			len--;
+		}
 
-		val = parse_color(word, len);
+		val = parse_color(word, wordlen);
 		if (val >= -1) {
 			if (fg == -2) {
 				fg = val;
@@ -75,7 +86,7 @@ void color_parse(const char *value, const char *var, char *dst)
 			}
 			goto bad;
 		}
-		val = parse_attr(word, len);
+		val = parse_attr(word, wordlen);
 		if (val < 0 || attr != -1)
 			goto bad;
 		attr = val;
@@ -115,7 +126,7 @@ void color_parse(const char *value, const char *var, char *dst)
 	*dst = 0;
 	return;
 bad:
-	die("bad color value '%s' for variable '%s'", value, var);
+	die("bad color value '%.*s' for variable '%s'", value_len, value, var);
 }
 
 int git_config_colorbool(const char *var, const char *value, int stdout_is_tty)
@@ -191,10 +202,3 @@ int color_fprintf_ln(FILE *fp, const char *color, const char *fmt, ...)
 	va_end(args);
 	return r;
 }
-
-void color_parse_mem(const char *value, int len, const char *var, char *dst)
-{
-	char *tmp = xmemdupz(value, len);
-	color_parse(tmp, var, dst);
-	free(tmp);
-}