libbb: code shrink and speed up find_applet_by_name()

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@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 17 insertions, 79 deletions

diff --git a/libbb/appletlib.c b/libbb/appletlib.c
index 5f59f1273..542211255 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;
 
@@ -200,105 +200,43 @@ 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 */
-
-#if 0 /*BB_UNALIGNED_MEMACCESS_OK && BB_LITTLE_ENDIAN*/
-        /* skip "[\0" name, it's surely not it */
-        if (ENABLE_TEST && LONE_CHAR(p, '['))
-                i++, p += 2;
-        /* All remaining applet names in p[] are at least 2 chars long */
-        /* name[] is also at least 2 chars long */
-
-        n32 = (name[0] << 0) | (name[1] << 8) | (name[2] << 16);
-        while (i < max) {
-                uint32_t p32;
-                char ch;
-
-                /* Quickly check match of the first 3 bytes */
-                move_from_unaligned32(p32, p);
-                p += 3;
-                if ((p32 & 0x00ffffff) != n32) {
-                        /* Most likely case: 3 first bytes do not match */
-                        i++;
-                        if ((p32 & 0x00ff0000) == '\0')
-                                continue; // p[2] was NUL
-                        p++;
-                        if ((p32 & 0xff000000) == '\0')
-                                continue; // p[3] was NUL
-                        /* p[0..3] aren't matching and none is NUL, check the rest */
-                        while (*p++ != '\0')
-                                continue;
-                        continue;
-                }
-
-                /* Unlikely branch: first 3 bytes ([0..2]) match */
-                if ((p32 & 0x00ff0000) == '\0') {
-                        /* name is 2-byte long, it is full match */
-                        //bb_error_msg("found:'%s' i:%u", name, i);
-                        return i;
-                }
-                /* Check remaining bytes [3..NUL] */
-                ch = (p32 >> 24);
-                j = 3;
-                while (ch == name[j]) {
-                        if (ch == '\0') {
-                                //bb_error_msg("found:'%s' i:%u", name, i);
-                                return i;
-                        }
-                        ch = *++p;
-                        j++;
-                }
-                /* Not a match. Skip it, including NUL */
-                while (ch != '\0')
-                        ch = *++p;
-                p++;
-                i++;
-        }
-        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')
+                        continue;
                 i++;
         }
         return -1;
-#endif
 }