static int DecodeAlphaData()

in Extended/libwebp/src/dec/vp8l_dec.c [1005:1088]
80 lines of code
22 McCabe index (conditional complexity)

static int DecodeAlphaData(VP8LDecoder* const dec, uint8_t* const data,
                           int width, int height, int last_row) {
  int ok = 1;
  int row = dec->last_pixel_ / width;
  int col = dec->last_pixel_ % width;
  VP8LBitReader* const br = &dec->br_;
  VP8LMetadata* const hdr = &dec->hdr_;
  int pos = dec->last_pixel_;         // current position
  const int end = width * height;     // End of data
  const int last = width * last_row;  // Last pixel to decode
  const int len_code_limit = NUM_LITERAL_CODES + NUM_LENGTH_CODES;
  const int mask = hdr->huffman_mask_;
  const HTreeGroup* htree_group =
      (pos < last) ? GetHtreeGroupForPos(hdr, col, row) : NULL;
  assert(pos <= end);
  assert(last_row <= height);
  assert(Is8bOptimizable(hdr));

  while (!br->eos_ && pos < last) {
    int code;
    // Only update when changing tile.
    if ((col & mask) == 0) {
      htree_group = GetHtreeGroupForPos(hdr, col, row);
    }
    assert(htree_group != NULL);
    VP8LFillBitWindow(br);
    code = ReadSymbol(htree_group->htrees[GREEN], br);
    if (code < NUM_LITERAL_CODES) {  // Literal
      data[pos] = code;
      ++pos;
      ++col;
      if (col >= width) {
        col = 0;
        ++row;
        if (row <= last_row && (row % NUM_ARGB_CACHE_ROWS == 0)) {
          ExtractPalettedAlphaRows(dec, row);
        }
      }
    } 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 (pos >= dist && end - pos >= length) {
        CopyBlock8b(data + pos, dist, length);
      } else {
        ok = 0;
        goto End;
      }
      pos += length;
      col += length;
      while (col >= width) {
        col -= width;
        ++row;
        if (row <= last_row && (row % NUM_ARGB_CACHE_ROWS == 0)) {
          ExtractPalettedAlphaRows(dec, row);
        }
      }
      if (pos < last && (col & mask)) {
        htree_group = GetHtreeGroupForPos(hdr, col, row);
      }
    } else {  // Not reached
      ok = 0;
      goto End;
    }
    br->eos_ = VP8LIsEndOfStream(br);
  }
  // Process the remaining rows corresponding to last row-block.
  ExtractPalettedAlphaRows(dec, row > last_row ? last_row : row);

 End:
  br->eos_ = VP8LIsEndOfStream(br);
  if (!ok || (br->eos_ && pos < end)) {
    ok = 0;
    dec->status_ = br->eos_ ? VP8_STATUS_SUSPENDED
                            : VP8_STATUS_BITSTREAM_ERROR;
  } else {
    dec->last_pixel_ = pos;
  }
  return ok;
}