static int DecodeImageData()

in Extended/libwebp/src/dec/vp8l_dec.c [1110:1255]

• 134 lines of code
• 40 McCabe index (conditional complexity)

static int DecodeImageData(VP8LDecoder* const dec, uint32_t* const data,
                           int width, int height, int last_row,
                           ProcessRowsFunc process_func) {
  int row = dec->last_pixel_ / width;
  int col = dec->last_pixel_ % width;
  VP8LBitReader* const br = &dec->br_;
  VP8LMetadata* const hdr = &dec->hdr_;
  uint32_t* src = data + dec->last_pixel_;
  uint32_t* last_cached = src;
  uint32_t* const src_end = data + width * height;     // End of data
  uint32_t* const src_last = data + width * last_row;  // Last pixel to decode
  const int len_code_limit = NUM_LITERAL_CODES + NUM_LENGTH_CODES;
  const int color_cache_limit = len_code_limit + hdr->color_cache_size_;
  int next_sync_row = dec->incremental_ ? row : 1 << 24;
  VP8LColorCache* const color_cache =
      (hdr->color_cache_size_ > 0) ? &hdr->color_cache_ : NULL;
  const int mask = hdr->huffman_mask_;
  const HTreeGroup* htree_group =
      (src < src_last) ? GetHtreeGroupForPos(hdr, col, row) : NULL;
  assert(dec->last_row_ < last_row);
  assert(src_last <= src_end);

  while (src < src_last) {
    int code;
    if (row >= next_sync_row) {
      SaveState(dec, (int)(src - data));
      next_sync_row = row + SYNC_EVERY_N_ROWS;
    }
    // Only update when changing tile. Note we could use this test:
    // if "((((prev_col ^ col) | prev_row ^ row)) > mask)" -> tile changed
    // but that's actually slower and needs storing the previous col/row.
    if ((col & mask) == 0) {
      htree_group = GetHtreeGroupForPos(hdr, col, row);
    }
    assert(htree_group != NULL);
    if (htree_group->is_trivial_code) {
      *src = htree_group->literal_arb;
      goto AdvanceByOne;
    }
    VP8LFillBitWindow(br);
    if (htree_group->use_packed_table) {
      code = ReadPackedSymbols(htree_group, br, src);
      if (VP8LIsEndOfStream(br)) break;
      if (code == PACKED_NON_LITERAL_CODE) goto AdvanceByOne;
    } else {
      code = ReadSymbol(htree_group->htrees[GREEN], br);
    }
    if (VP8LIsEndOfStream(br)) break;
    if (code < NUM_LITERAL_CODES) {  // Literal
      if (htree_group->is_trivial_literal) {
        *src = htree_group->literal_arb | (code << 8);
      } else {
        int red, blue, alpha;
        red = ReadSymbol(htree_group->htrees[RED], br);
        VP8LFillBitWindow(br);
        blue = ReadSymbol(htree_group->htrees[BLUE], br);
        alpha = ReadSymbol(htree_group->htrees[ALPHA], br);
        if (VP8LIsEndOfStream(br)) break;
        *src = ((uint32_t)alpha << 24) | (red << 16) | (code << 8) | blue;
      }
    AdvanceByOne:
      ++src;
      ++col;
      if (col >= width) {
        col = 0;
        ++row;
        if (process_func != NULL) {
          if (row <= last_row && (row % NUM_ARGB_CACHE_ROWS == 0)) {
            process_func(dec, row);
          }
        }
        if (color_cache != NULL) {
          while (last_cached < src) {
            VP8LColorCacheInsert(color_cache, *last_cached++);
          }
        }
      }
    } else if (code < len_code_limit) {  // Backward reference
      int dist_code, dist;
      const int length_sym = code - NUM_LITERAL_CODES;
      const int length = GetCopyLength(length_sym, br);
      const int dist_symbol = ReadSymbol(htree_group->htrees[DIST], br);
      VP8LFillBitWindow(br);
      dist_code = GetCopyDistance(dist_symbol, br);
      dist = PlaneCodeToDistance(width, dist_code);

      if (VP8LIsEndOfStream(br)) break;
      if (src - data < (ptrdiff_t)dist || src_end - src < (ptrdiff_t)length) {
        goto Error;
      } else {
        CopyBlock32b(src, dist, length);
      }
      src += length;
      col += length;
      while (col >= width) {
        col -= width;
        ++row;
        if (process_func != NULL) {
          if (row <= last_row && (row % NUM_ARGB_CACHE_ROWS == 0)) {
            process_func(dec, row);
          }
        }
      }
      // Because of the check done above (before 'src' was incremented by
      // 'length'), the following holds true.
      assert(src <= src_end);
      if (col & mask) htree_group = GetHtreeGroupForPos(hdr, col, row);
      if (color_cache != NULL) {
        while (last_cached < src) {
          VP8LColorCacheInsert(color_cache, *last_cached++);
        }
      }
    } else if (code < color_cache_limit) {  // Color cache
      const int key = code - len_code_limit;
      assert(color_cache != NULL);
      while (last_cached < src) {
        VP8LColorCacheInsert(color_cache, *last_cached++);
      }
      *src = VP8LColorCacheLookup(color_cache, key);
      goto AdvanceByOne;
    } else {  // Not reached
      goto Error;
    }
  }

  br->eos_ = VP8LIsEndOfStream(br);
  if (dec->incremental_ && br->eos_ && src < src_end) {
    RestoreState(dec);
  } else if (!br->eos_) {
    // Process the remaining rows corresponding to last row-block.
    if (process_func != NULL) {
      process_func(dec, row > last_row ? last_row : row);
    }
    dec->status_ = VP8_STATUS_OK;
    dec->last_pixel_ = (int)(src - data);  // end-of-scan marker
  } else {
    // if not incremental, and we are past the end of buffer (eos_=1), then this
    // is a real bitstream error.
    goto Error;
  }
  return 1;

 Error:
  dec->status_ = VP8_STATUS_BITSTREAM_ERROR;
  return 0;
}