[PATCH] libbb: code shrink and speed up find_applet_by_name()

[Ron Yorston]

find_applet_by_name() determines the appropriate range of applet
indices to check for the given name and performs a linear search in
applet_names[].

Revise the code so the index of the upper bound of the range, 'max',
isn't calculated.  Instead check the value of the first non-matching
character to see if we've reached the end of the range.

This new code speeds up the time to find a valid applet name by 6%
and halves the time to detect that a given name is invalid.  The
average time to detect an invalid name is now the same as for a valid
one.

function                                             old     new   delta
find_applet_by_name                                  155     133     -22
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-22)             Total: -22 bytes

Signed-off-by: Ron Yorston <rmy at pobox.com>
---
 libbb/appletlib.c | 40 +++++++++++++++++-----------------------
 1 file changed, 17 insertions(+), 23 deletions(-)

diff --git a/libbb/appletlib.c b/libbb/appletlib.c
index 5f59f1273..2e61659fb 100644
--- a/libbb/appletlib.c
+++ b/libbb/appletlib.c
@@ -176,7 +176,7 @@ void FAST_FUNC bb_show_usage(void)
 
 int FAST_FUNC find_applet_by_name(const char *name)
 {
-	unsigned i, max;
+	unsigned i;
 	int j;
 	const char *p;
 
@@ -200,24 +200,21 @@ int FAST_FUNC find_applet_by_name(const char *name)
 #endif
 
 	p = applet_names;
-	i = 0;
 #if KNOWN_APPNAME_OFFSETS <= 0
-	max = NUM_APPLETS;
+	i = 0;
 #else
-	max = NUM_APPLETS * KNOWN_APPNAME_OFFSETS;
+	i = NUM_APPLETS * (KNOWN_APPNAME_OFFSETS - 1);
 	for (j = ARRAY_SIZE(applet_nameofs)-1; j >= 0; j--) {
 		const char *pp = applet_names + applet_nameofs[j];
 		if (strcmp(name, pp) >= 0) {
 			//bb_error_msg("name:'%s' >= pp:'%s'", name, pp);
 			p = pp;
-			i = max - NUM_APPLETS;
 			break;
 		}
-		max -= NUM_APPLETS;
+		i -= NUM_APPLETS;
 	}
-	max /= (unsigned)KNOWN_APPNAME_OFFSETS;
 	i /= (unsigned)KNOWN_APPNAME_OFFSETS;
-	//bb_error_msg("name:'%s' starting from:'%s' i:%u max:%u", name, p, i, max);
+	//bb_error_msg("name:'%s' starting from:'%s' i:%u", name, p, i);
 #endif
 
 	/* Open-coded linear search without strcmp/strlen calls for speed */
@@ -276,25 +273,22 @@ int FAST_FUNC find_applet_by_name(const char *name)
 	}
 	return -1;
 #else
-	while (i < max) {
-		char ch;
-		j = 0;
-		/* Do we see "name\0" in applet_names[p] position? */
-		while ((ch = *p) == name[j]) {
-			if (ch == '\0') {
+	while (*p) {
+		/* Do we see "name\0" at current position in applet_names? */
+		for (j = 0; *p == name[j]; ++j) {
+			if (*p++ == '\0') {
 				//bb_error_msg("found:'%s' i:%u", name, i);
 				return i; /* yes */
 			}
-			p++;
-			j++;
 		}
-		/* No.
-		 * p => 1st non-matching char in applet_names[],
-		 * skip to and including NUL.
-		 */
-		while (ch != '\0')
-			ch = *++p;
-		p++;
+		/* No.  Have we gone too far, alphabetically? */
+		if (*p > name[j]) {
+			//bb_error_msg("break:'%s' i:%u", name, i);
+			break;
+		}
+		/* No.  Move to the start of the next applet name. */
+		while (*p++ != '\0')
+			;
 		i++;
 	}
 	return -1;
-- 
2.29.2