libbb: code shrink and speed up index_in_strings()

Rewrite index_in_strings() to replace calls to strcmp()/strlen().
With this change searching for valid names in the applet_names
array (for example) is 40% faster.

The code has to assume the strings aren't sorted, so will always scan
the entire array when presented with an invalid name.

function                                             old     new   delta
index_in_strings                                      63      56      -7
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-7)               Total: -7 bytes

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

1 files changed, 10 insertions, 4 deletions

diff --git a/libbb/compare_string_array.c b/libbb/compare_string_array.c
index a06e57d3d..d8cd033a3 100644
--- a/libbb/compare_string_array.c
+++ b/libbb/compare_string_array.c
@@ -63,13 +63,19 @@ int FAST_FUNC index_in_str_array(const char *const string_array[], const char *k
 
 int FAST_FUNC index_in_strings(const char *strings, const char *key)
 {
-        int idx = 0;
+        int j, idx = 0;
 
         while (*strings) {
-                if (strcmp(strings, key) == 0) {
-                        return idx;
+                /* Do we see "key\0" at current position in strings? */
+                for (j = 0; *strings == key[j]; ++j) {
+                        if (*strings++ == '\0') {
+                                //bb_error_msg("found:'%s' i:%u", key, idx);
+                                return idx; /* yes */
+                        }
                 }
-                strings += strlen(strings) + 1; /* skip NUL */
+                /* No.  Move to the start of the next string. */
+                while (*strings++ != '\0')
+                        continue;
                 idx++;
         }
         return -1;