Skin engine, optimize hot paths

callgrind identified check_viewport and scan_int as pretty hot code paths on startup
and playback

check_viewport uses strlen to check the string has at least 4 characters
we can just check if str[3] != '\0' without walking potentially much further
and no function call..

scan_int was building a buffer for atoi to parse when we can just do it in the loop
directly

Change-Id: Ie028980333cbed4c066d8ea547a89cf4fad76808

lib/skin_parser/skin_scan.c

@@ -166,53 +166,42 @@ char* scan_string(const char** document)
 int scan_int(const char** document)
 {
 
-    const char *cursor = *document, *end;
+    const char *cursor = *document;
-    int length = 0;
+    int retval = 0;
-    char buffer[16];
+    int sign = 1;
-    int retval;
-    int i;
 
-    while(isdigit(*cursor) || *cursor == COMMENTSYM || *cursor == '-')
+    while(true)
     {
         if(*cursor == COMMENTSYM)
         {
             skip_comment(&cursor);
+            if (retval > 0) /* || sign < 0 already read a number */
+            {
+                break;
+            }
             continue;
         }
-
+        else if (*cursor == '-')
-        length++;
+        {
+            if (retval != 0)  /* only allow negative prior to numbers */
+                break;
+            sign = -1;
+        }
+        else if (isdigit(*cursor)) /* is digit*/
+        {
+            retval = (retval * 10) + (*cursor - '0');
+        }
+        else
+            break;
         cursor++;
     }
-    if (length > 15)
+    *document = cursor;
-        length = 15;
-    end = cursor;
-    /* Copying to the buffer while avoiding comments */
-    cursor = *document;
-    buffer[length] = '\0';
-    for(i = 0; i < length; i++)
-    {
-        if(*cursor == COMMENTSYM)
-        {
-            skip_comment(&cursor);
-            i--;
-            continue;
-        }
 
-        buffer[i] = *cursor;
+    return sign * retval;
-        cursor++;
-
-    }
-    retval = atoi(buffer);
-
-    *document = end;
-    return retval;
 }
 
 int check_viewport(const char* document)
 {
-    if(strlen(document) < 3)
-        return 0;
-
     if(document[0] != TAGSYM)
         return 0;
 
@@ -224,5 +213,8 @@ int check_viewport(const char* document)
        && document[2] != 'i')
         return 0;
 
+    if (document[3] == '\0')
+        return 0;
+
     return 1;
 }