decode motion & modulo optimize patch by (BERO <bero at geocities dot co dot jp>)

Originally committed as revision 1872 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h263.c

@@ -3437,10 +3437,12 @@ static int h263_decode_motion(MpegEncContext * s, int pred, int f_code)
 
     sign = get_bits1(&s->gb);
     shift = f_code - 1;
-    val = (code - 1) << shift;
-    if (shift > 0)
+    val = code;
+    if (shift) {
+        val = (val - 1) << shift;
         val |= get_bits(&s->gb, shift);
         val++;
+    }
     if (sign)
         val = -val;
     val += pred;
@@ -3448,11 +3450,7 @@ static int h263_decode_motion(MpegEncContext * s, int pred, int f_code)
     /* modulo decoding */
     if (!s->h263_long_vectors) {
         l = 1 << (f_code + 4);
-        if (val < -l) {
-            val += l<<1;
-        } else if (val >= l) {
-            val -= l<<1;
-        }
+        val = ((val + l)&(l*2-1)) - l;
     } else {
         /* horrible h263 long vector mode */
         if (pred < -31 && val < -63)

libavcodec/mpeg12.c

@@ -1176,7 +1176,7 @@ static int mpeg_decode_mb(MpegEncContext *s,
 /* as h263, but only 17 codes */
 static int mpeg_decode_motion(MpegEncContext *s, int fcode, int pred)
 {
-    int code, sign, val, m, l, shift;
+    int code, sign, val, l, shift;
 
     code = get_vlc2(&s->gb, mv_vlc.table, MV_VLC_BITS, 2);
     if (code == 0) {
@@ -1188,22 +1188,19 @@ static int mpeg_decode_motion(MpegEncContext *s, int fcode, int pred)
 
     sign = get_bits1(&s->gb);
     shift = fcode - 1;
-    val = (code - 1) << shift;
-    if (shift > 0)
+    val = code;
+    if (shift) {
+        val = (val - 1) << shift;
         val |= get_bits(&s->gb, shift);
         val++;
+    }
     if (sign)
         val = -val;
     val += pred;
     
     /* modulo decoding */
     l = 1 << (shift+4);
-    m = 2 * l;
-    if (val < -l) {
-        val += m;
-    } else if (val >= l) {
-        val -= m;
-    }
+    val = ((val + l)&(l*2-1)) - l;
     return val;
 }
 

libavcodec/msmpeg4.c

@@ -1481,10 +1481,12 @@ static int msmpeg4v2_decode_motion(MpegEncContext * s, int pred, int f_code)
         return pred;
     sign = get_bits1(&s->gb);
     shift = f_code - 1;
-    val = (code - 1) << shift;
-    if (shift > 0)
+    val = code;
+    if (shift) {
+        val = (val - 1) << shift;
         val |= get_bits(&s->gb, shift);
         val++;
+    }
     if (sign)
         val = -val;