avcodec/xbmenc: Avoid snprintf() for data->hex conversion

Use a small LUT instead. Improves performance.

Reviewed-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/xbmenc.c

@@ -20,11 +20,9 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include "libavutil/reverse.h"
 #include "avcodec.h"
 #include "codec_internal.h"
 #include "encode.h"
-#include "mathops.h"
 
 #define ANSI_MIN_READLINE 509
 
@@ -57,14 +55,25 @@ static int xbm_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     buf += snprintf(buf, 39, "static unsigned char image_bits[] = {\n");
     for (i = 0, l = lineout; i < avctx->height; i++) {
         for (j = 0; j < linesize; j++) {
-            buf += snprintf(buf, 6, " 0x%02X", ff_reverse[*ptr++]);
+            // 0..15 bitreversed as chars
+            static const char lut[] = {
+                '0', '8', '4', 'C', '2', 'A', '6', 'E',
+                '1', '9', '5', 'D', '3', 'B', '7', 'F'
+            };
+            buf[0] = ' ';
+            buf[1] = '0';
+            buf[2] = 'x';
+            buf[3] = lut[*ptr & 0xF];
+            buf[4] = lut[*ptr >> 4];
+            buf += 5;
+            ptr++;
             if (--commas <= 0) {
-                buf += snprintf(buf, 2, "\n");
+                *buf++ = '\n';
                 break;
             }
-            buf += snprintf(buf, 2, ",");
+            *buf++ = ',';
             if (--l <= 0) {
-                buf += snprintf(buf, 2, "\n");
+                *buf++ = '\n';
                 l = lineout;
             }
         }