mp3: Make the extrasize explicit

Initialize the bit buffer with the correct size (amount of bits that will
be read) instead of relying on the bitstream reader overreading the
correct values.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Diego Biurrun <diego@biurrun.de>

libavcodec/mpegaudiodec_template.c

@@ -70,6 +70,7 @@ typedef struct MPADecodeContext {
     MPA_DECODE_HEADER
     uint8_t last_buf[LAST_BUF_SIZE];
     int last_buf_size;
+    int extrasize;
     /* next header (used in free format parsing) */
     uint32_t free_format_next_header;
     GetBitContext gb;
@@ -798,9 +799,10 @@ static void exponents_from_scale_factors(MPADecodeContext *s, GranuleDef *g,
 static void switch_buffer(MPADecodeContext *s, int *pos, int *end_pos,
                           int *end_pos2)
 {
-    if (s->in_gb.buffer && *pos >= s->gb.size_in_bits) {
+    if (s->in_gb.buffer && *pos >= s->gb.size_in_bits - s->extrasize * 8) {
         s->gb           = s->in_gb;
         s->in_gb.buffer = NULL;
+        s->extrasize    = 0;
         assert((get_bits_count(&s->gb) & 7) == 0);
         skip_bits_long(&s->gb, *pos - *end_pos);
         *end_pos2 =
@@ -832,7 +834,7 @@ static int huffman_decode(MPADecodeContext *s, GranuleDef *g,
     int i;
     int last_pos, bits_left;
     VLC *vlc;
-    int end_pos = FFMIN(end_pos2, s->gb.size_in_bits);
+    int end_pos = FFMIN(end_pos2, s->gb.size_in_bits - s->extrasize * 8);
 
     /* low frequencies (called big values) */
     s_index = 0;
@@ -1354,19 +1356,16 @@ static int mp_decode_layer3(MPADecodeContext *s)
     if (!s->adu_mode) {
         int skip;
         const uint8_t *ptr = s->gb.buffer + (get_bits_count(&s->gb)>>3);
-        int extrasize = av_clip(get_bits_left(&s->gb) >> 3, 0,
-                                FFMAX(0, LAST_BUF_SIZE - s->last_buf_size));
+        s->extrasize = av_clip((get_bits_left(&s->gb) >> 3) - s->extrasize, 0,
+                               FFMAX(0, LAST_BUF_SIZE - s->last_buf_size));
         assert((get_bits_count(&s->gb) & 7) == 0);
         /* now we get bits from the main_data_begin offset */
         ff_dlog(s->avctx, "seekback:%d, lastbuf:%d\n",
                 main_data_begin, s->last_buf_size);
 
-        memcpy(s->last_buf + s->last_buf_size, ptr, extrasize);
+        memcpy(s->last_buf + s->last_buf_size, ptr, s->extrasize);
         s->in_gb = s->gb;
-        init_get_bits(&s->gb, s->last_buf, s->last_buf_size*8);
-#if !UNCHECKED_BITSTREAM_READER
-        s->gb.size_in_bits_plus8 += extrasize * 8;
-#endif
+        init_get_bits(&s->gb, s->last_buf, (s->last_buf_size + s->extrasize) * 8);
         s->last_buf_size <<= 3;
         for (gr = 0; gr < nb_granules && (s->last_buf_size >> 3) < main_data_begin; gr++) {
             for (ch = 0; ch < s->nb_channels; ch++) {
@@ -1377,15 +1376,17 @@ static int mp_decode_layer3(MPADecodeContext *s)
             }
         }
         skip = s->last_buf_size - 8 * main_data_begin;
-        if (skip >= s->gb.size_in_bits && s->in_gb.buffer) {
-            skip_bits_long(&s->in_gb, skip - s->gb.size_in_bits);
+        if (skip >= s->gb.size_in_bits - s->extrasize * 8 && s->in_gb.buffer) {
+            skip_bits_long(&s->in_gb, skip - s->gb.size_in_bits + s->extrasize * 8);
             s->gb           = s->in_gb;
             s->in_gb.buffer = NULL;
+            s->extrasize    = 0;
         } else {
             skip_bits_long(&s->gb, skip);
         }
     } else {
         gr = 0;
+        s->extrasize = 0;
     }
 
     for (; gr < nb_granules; gr++) {
@@ -1553,7 +1554,7 @@ static int mp_decode_frame(MPADecodeContext *s, OUT_INT **samples,
         s->last_buf_size=0;
         if (s->in_gb.buffer) {
             align_get_bits(&s->gb);
-            i = get_bits_left(&s->gb)>>3;
+            i = (get_bits_left(&s->gb) >> 3) - s->extrasize;
             if (i >= 0 && i <= BACKSTEP_SIZE) {
                 memmove(s->last_buf, s->gb.buffer + (get_bits_count(&s->gb)>>3), i);
                 s->last_buf_size=i;
@@ -1561,12 +1562,12 @@ static int mp_decode_frame(MPADecodeContext *s, OUT_INT **samples,
                 av_log(s->avctx, AV_LOG_ERROR, "invalid old backstep %d\n", i);
             s->gb           = s->in_gb;
             s->in_gb.buffer = NULL;
+            s->extrasize    = 0;
         }
 
         align_get_bits(&s->gb);
         assert((get_bits_count(&s->gb) & 7) == 0);
-        i = get_bits_left(&s->gb) >> 3;
-
+        i = (get_bits_left(&s->gb) >> 3) - s->extrasize;
         if (i < 0 || i > BACKSTEP_SIZE || nb_frames < 0) {
             if (i < 0)
                 av_log(s->avctx, AV_LOG_ERROR, "invalid new backstep %d\n", i);