g2meet: factor out chunk seeking

This version requires fewer code and is safer.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/g2meet.c

@@ -646,7 +646,7 @@ static int g2m_decode_frame(AVCodecContext *avctx, void *data,
     int magic;
     int got_header = 0;
     uint32_t chunk_size;
-    int chunk_type;
+    int chunk_type, chunk_start;
     int i;
     int ret;
 
@@ -672,8 +672,9 @@ static int g2m_decode_frame(AVCodecContext *avctx, void *data,
     }
 
     while (bytestream2_get_bytes_left(&bc) > 5) {
-        chunk_size = bytestream2_get_le32(&bc) - 1;
-        chunk_type = bytestream2_get_byte(&bc);
+        chunk_size  = bytestream2_get_le32(&bc) - 1;
+        chunk_type  = bytestream2_get_byte(&bc);
+        chunk_start = bytestream2_tell(&bc);
         if (chunk_size > bytestream2_get_bytes_left(&bc)) {
             av_log(avctx, AV_LOG_ERROR, "Invalid chunk size %d type %02X\n",
                    chunk_size, chunk_type);
@@ -722,8 +723,6 @@ static int g2m_decode_frame(AVCodecContext *avctx, void *data,
             c->tiles_x = (c->width  + c->tile_width  - 1) / c->tile_width;
             c->tiles_y = (c->height + c->tile_height - 1) / c->tile_height;
             c->bpp = bytestream2_get_byte(&bc);
-            chunk_size -= 21;
-            bytestream2_skip(&bc, chunk_size);
             if (g2m_init_buffers(c)) {
                 ret = AVERROR(ENOMEM);
                 goto header_fail;
@@ -734,7 +733,6 @@ static int g2m_decode_frame(AVCodecContext *avctx, void *data,
             if (!c->tiles_x || !c->tiles_y) {
                 av_log(avctx, AV_LOG_WARNING,
                        "No display info - skipping tile\n");
-                bytestream2_skip(&bc, bytestream2_get_bytes_left(&bc));
                 break;
             }
             if (chunk_size < 2) {
@@ -750,7 +748,6 @@ static int g2m_decode_frame(AVCodecContext *avctx, void *data,
                        c->tile_x, c->tile_y, c->tiles_x, c->tiles_y);
                 break;
             }
-            chunk_size -= 2;
             ret = 0;
             switch (c->compression) {
             case COMPR_EPIC_J_B:
@@ -760,13 +757,12 @@ static int g2m_decode_frame(AVCodecContext *avctx, void *data,
             case COMPR_KEMPF_J_B:
                 ret = kempf_decode_tile(c, c->tile_x, c->tile_y,
                                         buf + bytestream2_tell(&bc),
-                                        chunk_size);
+                                        chunk_size - 2);
                 break;
             }
             if (ret && c->framebuf)
                 av_log(avctx, AV_LOG_ERROR, "Error decoding tile %d,%d\n",
                        c->tile_x, c->tile_y);
-            bytestream2_skip(&bc, chunk_size);
             break;
         case CURSOR_POS:
             if (chunk_size < 5) {
@@ -776,7 +772,6 @@ static int g2m_decode_frame(AVCodecContext *avctx, void *data,
             }
             c->cursor_x = bytestream2_get_be16(&bc);
             c->cursor_y = bytestream2_get_be16(&bc);
-            bytestream2_skip(&bc, chunk_size - 4);
             break;
         case CURSOR_SHAPE:
             if (chunk_size < 8) {
@@ -787,17 +782,17 @@ static int g2m_decode_frame(AVCodecContext *avctx, void *data,
             bytestream2_init(&tbc, buf + bytestream2_tell(&bc),
                              chunk_size - 4);
             g2m_load_cursor(avctx, c, &tbc);
-            bytestream2_skip(&bc, chunk_size);
             break;
         case CHUNK_CC:
         case CHUNK_CD:
-            bytestream2_skip(&bc, chunk_size);
             break;
         default:
             av_log(avctx, AV_LOG_WARNING, "Skipping chunk type %02X\n",
                    chunk_type);
-            bytestream2_skip(&bc, chunk_size);
         }
+
+        /* navigate to next chunk */
+        bytestream2_skip(&bc, chunk_start + chunk_size - bytestream2_tell(&bc));
     }
     if (got_header)
         c->got_header = 1;