text: // Copyright 2010 Google Inc. All Rights Reserved.
  //
  // Use of this source code is governed by a BSD-style license
  // that can be found in the COPYING file in the root of the source
  // tree. An additional intellectual property rights grant can be found
  // in the file PATENTS. All contributing project authors may
  // be found in the AUTHORS file in the root of the source tree.
  // -----------------------------------------------------------------------------
  //
  // Frame-reconstruction function. Memory allocation.
  //
  // Author: Skal (pascal.massimino@gmail.com)
  
  #include <stdlib.h>
  #include "src/dec/vp8i_dec.h"
  #include "src/utils/utils.h"
  
  //------------------------------------------------------------------------------
  // Main reconstruction function.
  
  static const uint16_t kScan[16] = {
    0 +  0 * BPS,  4 +  0 * BPS, 8 +  0 * BPS, 12 +  0 * BPS,
    0 +  4 * BPS,  4 +  4 * BPS, 8 +  4 * BPS, 12 +  4 * BPS,
    0 +  8 * BPS,  4 +  8 * BPS, 8 +  8 * BPS, 12 +  8 * BPS,
    0 + 12 * BPS,  4 + 12 * BPS, 8 + 12 * BPS, 12 + 12 * BPS
  };
  
  static int CheckMode(int mb_x, int mb_y, int mode) {
    if (mode == B_DC_PRED) {
      if (mb_x == 0) {
        return (mb_y == 0) ? B_DC_PRED_NOTOPLEFT : B_DC_PRED_NOLEFT;
      } else {
        return (mb_y == 0) ? B_DC_PRED_NOTOP : B_DC_PRED;
      }
    }
    return mode;
  }
  
  static void Copy32b(uint8_t* const dst, const uint8_t* const src) {
    memcpy(dst, src, 4);
  }
  
  static WEBP_INLINE void DoTransform(uint32_t bits, const int16_t* const src,
                                      uint8_t* const dst) {
    switch (bits >> 30) {
      case 3:
        VP8Transform(src, dst, 0);
        break;
      case 2:
        VP8TransformAC3(src, dst);
        break;
      case 1:
        VP8TransformDC(src, dst);
        break;
      default:
        break;
    }
  }
  
  static void DoUVTransform(uint32_t bits, const int16_t* const src,
                            uint8_t* const dst) {
    if (bits & 0xff) {    // any non-zero coeff at all?
      if (bits & 0xaa) {  // any non-zero AC coefficient?
        VP8TransformUV(src, dst);   // note we don't use the AC3 variant for U/V
      } else {
        VP8TransformDCUV(src, dst);
      }
    }
  }
  
  static void ReconstructRow(const VP8Decoder* const dec,
                             const VP8ThreadContext* ctx) {
    int j;
    int mb_x;
    const int mb_y = ctx->mb_y_;
    const int cache_id = ctx->id_;
    uint8_t* const y_dst = dec->yuv_b_ + Y_OFF;
    uint8_t* const u_dst = dec->yuv_b_ + U_OFF;
    uint8_t* const v_dst = dec->yuv_b_ + V_OFF;
  
    // Initialize left-most block.
    for (j = 0; j < 16; ++j) {
      y_dst[j * BPS - 1] = 129;
    }
    for (j = 0; j < 8; ++j) {
      u_dst[j * BPS - 1] = 129;
      v_dst[j * BPS - 1] = 129;
    }
  
    // Init top-left sample on left column too.
    if (mb_y > 0) {
      y_dst[-1 - BPS] = u_dst[-1 - BPS] = v_dst[-1 - BPS] = 129;
    } else {
      // we only need to do this init once at block (0,0).
      // Afterward, it remains valid for the whole topmost row.
      memset(y_dst - BPS - 1, 127, 16 + 4 + 1);
      memset(u_dst - BPS - 1, 127, 8 + 1);
      memset(v_dst - BPS - 1, 127, 8 + 1);
    }
  
    // Reconstruct one row.
    for (mb_x = 0; mb_x < dec->mb_w_; ++mb_x) {
      const VP8MBData* const block = ctx->mb_data_ + mb_x;
  
      // Rotate in the left samples from previously decoded block. We move four
      // pixels at a time for alignment reason, and because of in-loop filter.
      if (mb_x > 0) {
        for (j = -1; j < 16; ++j) {
          Copy32b(&y_dst[j * BPS - 4], &y_dst[j * BPS + 12]);
        }
        for (j = -1; j < 8; ++j) {
          Copy32b(&u_dst[j * BPS - 4], &u_dst[j * BPS + 4]);
          Copy32b(&v_dst[j * BPS - 4], &v_dst[j * BPS + 4]);
        }
      }
      {
        // bring top samples into the cache
        VP8TopSamples* const top_yuv = dec->yuv_t_ + mb_x;
        const int16_t* const coeffs = block->coeffs_;
        uint32_t bits = block->non_zero_y_;
        int n;
  
        if (mb_y > 0) {
          memcpy(y_dst - BPS, top_yuv[0].y, 16);
          memcpy(u_dst - BPS, top_yuv[0].u, 8);
          memcpy(v_dst - BPS, top_yuv[0].v, 8);
        }
  
        // predict and add residuals
        if (block->is_i4x4_) {   // 4x4
          uint32_t* const top_right = (uint32_t*)(y_dst - BPS + 16);
  
          if (mb_y > 0) {
            if (mb_x >= dec->mb_w_ - 1) {    // on rightmost border
              memset(top_right, top_yuv[0].y[15], sizeof(*top_right));
            } else {
              memcpy(top_right, top_yuv[1].y, sizeof(*top_right));
            }
          }
          // replicate the top-right pixels below
          top_right[BPS] = top_right[2 * BPS] = top_right[3 * BPS] = top_right[0];
  
          // predict and add residuals for all 4x4 blocks in turn.
          for (n = 0; n < 16; ++n, bits <<= 2) {
            uint8_t* const dst = y_dst + kScan[n];
            VP8PredLuma4[block->imodes_[n]](dst);
            DoTransform(bits, coeffs + n * 16, dst);
          }
        } else {    // 16x16
          const int pred_func = CheckMode(mb_x, mb_y, block->imodes_[0]);
          VP8PredLuma16[pred_func](y_dst);
          if (bits != 0) {
            for (n = 0; n < 16; ++n, bits <<= 2) {
              DoTransform(bits, coeffs + n * 16, y_dst + kScan[n]);
            }
          }
        }
        {
          // Chroma
          const uint32_t bits_uv = block->non_zero_uv_;
          const int pred_func = CheckMode(mb_x, mb_y, block->uvmode_);
          VP8PredChroma8[pred_func](u_dst);
          VP8PredChroma8[pred_func](v_dst);
          DoUVTransform(bits_uv >> 0, coeffs + 16 * 16, u_dst);
          DoUVTransform(bits_uv >> 8, coeffs + 20 * 16, v_dst);
        }
  
        // stash away top samples for next block
        if (mb_y < dec->mb_h_ - 1) {
          memcpy(top_yuv[0].y, y_dst + 15 * BPS, 16);
          memcpy(top_yuv[0].u, u_dst +  7 * BPS,  8);
          memcpy(top_yuv[0].v, v_dst +  7 * BPS,  8);
        }
      }
      // Transfer reconstructed samples from yuv_b_ cache to final destination.
      {
        const int y_offset = cache_id * 16 * dec->cache_y_stride_;
        const int uv_offset = cache_id * 8 * dec->cache_uv_stride_;
        uint8_t* const y_out = dec->cache_y_ + mb_x * 16 + y_offset;
        uint8_t* const u_out = dec->cache_u_ + mb_x * 8 + uv_offset;
        uint8_t* const v_out = dec->cache_v_ + mb_x * 8 + uv_offset;
        for (j = 0; j < 16; ++j) {
          memcpy(y_out + j * dec->cache_y_stride_, y_dst + j * BPS, 16);
        }
        for (j = 0; j < 8; ++j) {
          memcpy(u_out + j * dec->cache_uv_stride_, u_dst + j * BPS, 8);
          memcpy(v_out + j * dec->cache_uv_stride_, v_dst + j * BPS, 8);
        }
      }
    }
  }
  
  //------------------------------------------------------------------------------
  // Filtering
  
  // kFilterExtraRows[] = How many extra lines are needed on the MB boundary
  // for caching, given a filtering level.
  // Simple filter:  up to 2 luma samples are read and 1 is written.
  // Complex filter: up to 4 luma samples are read and 3 are written. Same for
  //                 U/V, so it's 8 samples total (because of the 2x upsampling).
  static const uint8_t kFilterExtraRows[3] = { 0, 2, 8 };
  
  static void DoFilter(const VP8Decoder* const dec, int mb_x, int mb_y) {
    const VP8ThreadContext* const ctx = &dec->thread_ctx_;
    const int cache_id = ctx->id_;
    const int y_bps = dec->cache_y_stride_;
    const VP8FInfo* const f_info = ctx->f_info_ + mb_x;
    uint8_t* const y_dst = dec->cache_y_ + cache_id * 16 * y_bps + mb_x * 16;
    const int ilevel = f_info->f_ilevel_;
    const int limit = f_info->f_limit_;
    if (limit == 0) {
      return;
    }
    assert(limit >= 3);
    if (dec->filter_type_ == 1) {   // simple
      if (mb_x > 0) {
        VP8SimpleHFilter16(y_dst, y_bps, limit + 4);
      }
      if (f_info->f_inner_) {
        VP8SimpleHFilter16i(y_dst, y_bps, limit);
      }
      if (mb_y > 0) {
        VP8SimpleVFilter16(y_dst, y_bps, limit + 4);
      }
      if (f_info->f_inner_) {
        VP8SimpleVFilter16i(y_dst, y_bps, limit);
      }
    } else {    // complex
      const int uv_bps = dec->cache_uv_stride_;
      uint8_t* const u_dst = dec->cache_u_ + cache_id * 8 * uv_bps + mb_x * 8;
      uint8_t* const v_dst = dec->cache_v_ + cache_id * 8 * uv_bps + mb_x * 8;
      const int hev_thresh = f_info->hev_thresh_;
      if (mb_x > 0) {
        VP8HFilter16(y_dst, y_bps, limit + 4, ilevel, hev_thresh);
        VP8HFilter8(u_dst, v_dst, uv_bps, limit + 4, ilevel, hev_thresh);
      }
      if (f_info->f_inner_) {
        VP8HFilter16i(y_dst, y_bps, limit, ilevel, hev_thresh);
        VP8HFilter8i(u_dst, v_dst, uv_bps, limit, ilevel, hev_thresh);
      }
      if (mb_y > 0) {
        VP8VFilter16(y_dst, y_bps, limit + 4, ilevel, hev_thresh);
        VP8VFilter8(u_dst, v_dst, uv_bps, limit + 4, ilevel, hev_thresh);
      }
      if (f_info->f_inner_) {
        VP8VFilter16i(y_dst, y_bps, limit, ilevel, hev_thresh);
        VP8VFilter8i(u_dst, v_dst, uv_bps, limit, ilevel, hev_thresh);
      }
    }
  }
  
  // Filter the decoded macroblock row (if needed)
  static void FilterRow(const VP8Decoder* const dec) {
    int mb_x;
    const int mb_y = dec->thread_ctx_.mb_y_;
    assert(dec->thread_ctx_.filter_row_);
    for (mb_x = dec->tl_mb_x_; mb_x < dec->br_mb_x_; ++mb_x) {
      DoFilter(dec, mb_x, mb_y);
    }
  }
  
  //------------------------------------------------------------------------------
  // Precompute the filtering strength for each segment and each i4x4/i16x16 mode.
  
  static void PrecomputeFilterStrengths(VP8Decoder* const dec) {
    if (dec->filter_type_ > 0) {
      int s;
      const VP8FilterHeader* const hdr = &dec->filter_hdr_;
      for (s = 0; s < NUM_MB_SEGMENTS; ++s) {
        int i4x4;
        // First, compute the initial level
        int base_level;
        if (dec->segment_hdr_.use_segment_) {
          base_level = dec->segment_hdr_.filter_strength_[s];
          if (!dec->segment_hdr_.absolute_delta_) {
            base_level += hdr->level_;
          }
        } else {
          base_level = hdr->level_;
        }
        for (i4x4 = 0; i4x4 <= 1; ++i4x4) {
          VP8FInfo* const info = &dec->fstrengths_[s][i4x4];
          int level = base_level;
          if (hdr->use_lf_delta_) {
            level += hdr->ref_lf_delta_[0];
            if (i4x4) {
              level += hdr->mode_lf_delta_[0];
            }
          }
          level = (level < 0) ? 0 : (level > 63) ? 63 : level;
          if (level > 0) {
            int ilevel = level;
            if (hdr->sharpness_ > 0) {
              if (hdr->sharpness_ > 4) {
                ilevel >>= 2;
              } else {
                ilevel >>= 1;
              }
              if (ilevel > 9 - hdr->sharpness_) {
                ilevel = 9 - hdr->sharpness_;
              }
            }
            if (ilevel < 1) ilevel = 1;
            info->f_ilevel_ = ilevel;
            info->f_limit_ = 2 * level + ilevel;
            info->hev_thresh_ = (level >= 40) ? 2 : (level >= 15) ? 1 : 0;
          } else {
            info->f_limit_ = 0;  // no filtering
          }
          info->f_inner_ = i4x4;
        }
      }
    }
  }
  
  //------------------------------------------------------------------------------
  // Dithering
  
  // minimal amp that will provide a non-zero dithering effect
  #define MIN_DITHER_AMP 4
  
  #define DITHER_AMP_TAB_SIZE 12
  static const uint8_t kQuantToDitherAmp[DITHER_AMP_TAB_SIZE] = {
    // roughly, it's dqm->uv_mat_[1]
    8, 7, 6, 4, 4, 2, 2, 2, 1, 1, 1, 1
  };
  
  void VP8InitDithering(const WebPDecoderOptions* const options,
                        VP8Decoder* const dec) {
    assert(dec != NULL);
    if (options != NULL) {
      const int d = options->dithering_strength;
      const int max_amp = (1 << VP8_RANDOM_DITHER_FIX) - 1;
      const int f = (d < 0) ? 0 : (d > 100) ? max_amp : (d * max_amp / 100);
      if (f > 0) {
        int s;
        int all_amp = 0;
        for (s = 0; s < NUM_MB_SEGMENTS; ++s) {
          VP8QuantMatrix* const dqm = &dec->dqm_[s];
          if (dqm->uv_quant_ < DITHER_AMP_TAB_SIZE) {
            const int idx = (dqm->uv_quant_ < 0) ? 0 : dqm->uv_quant_;
            dqm->dither_ = (f * kQuantToDitherAmp[idx]) >> 3;
          }
          all_amp |= dqm->dither_;
        }
        if (all_amp != 0) {
          VP8InitRandom(&dec->dithering_rg_, 1.0f);
          dec->dither_ = 1;
        }
      }
      // potentially allow alpha dithering
      dec->alpha_dithering_ = options->alpha_dithering_strength;
      if (dec->alpha_dithering_ > 100) {
        dec->alpha_dithering_ = 100;
      } else if (dec->alpha_dithering_ < 0) {
        dec->alpha_dithering_ = 0;
      }
    }
  }
  
  // Convert to range: [-2,2] for dither=50, [-4,4] for dither=100
  static void Dither8x8(VP8Random* const rg, uint8_t* dst, int bps, int amp) {
    uint8_t dither[64];
    int i;
    for (i = 0; i < 8 * 8; ++i) {
      dither[i] = VP8RandomBits2(rg, VP8_DITHER_AMP_BITS + 1, amp);
    }
    VP8DitherCombine8x8(dither, dst, bps);
  }
  
  static void DitherRow(VP8Decoder* const dec) {
    int mb_x;
    assert(dec->dither_);
    for (mb_x = dec->tl_mb_x_; mb_x < dec->br_mb_x_; ++mb_x) {
      const VP8ThreadContext* const ctx = &dec->thread_ctx_;
      const VP8MBData* const data = ctx->mb_data_ + mb_x;
      const int cache_id = ctx->id_;
      const int uv_bps = dec->cache_uv_stride_;
      if (data->dither_ >= MIN_DITHER_AMP) {
        uint8_t* const u_dst = dec->cache_u_ + cache_id * 8 * uv_bps + mb_x * 8;
        uint8_t* const v_dst = dec->cache_v_ + cache_id * 8 * uv_bps + mb_x * 8;
        Dither8x8(&dec->dithering_rg_, u_dst, uv_bps, data->dither_);
        Dither8x8(&dec->dithering_rg_, v_dst, uv_bps, data->dither_);
      }
    }
  }
  
  //------------------------------------------------------------------------------
  // This function is called after a row of macroblocks is finished decoding.
  // It also takes into account the following restrictions:
  //  * In case of in-loop filtering, we must hold off sending some of the bottom
  //    pixels as they are yet unfiltered. They will be when the next macroblock
  //    row is decoded. Meanwhile, we must preserve them by rotating them in the
  //    cache area. This doesn't hold for the very bottom row of the uncropped
  //    picture of course.
  //  * we must clip the remaining pixels against the cropping area. The VP8Io
  //    struct must have the following fields set correctly before calling put():
  
  #define MACROBLOCK_VPOS(mb_y)  ((mb_y) * 16)    // vertical position of a MB
  
  // Finalize and transmit a complete row. Return false in case of user-abort.
  static int FinishRow(void* arg1, void* arg2) {
    VP8Decoder* const dec = (VP8Decoder*)arg1;
    VP8Io* const io = (VP8Io*)arg2;
    int ok = 1;
    const VP8ThreadContext* const ctx = &dec->thread_ctx_;
    const int cache_id = ctx->id_;
    const int extra_y_rows = kFilterExtraRows[dec->filter_type_];
    const int ysize = extra_y_rows * dec->cache_y_stride_;
    const int uvsize = (extra_y_rows / 2) * dec->cache_uv_stride_;
    const int y_offset = cache_id * 16 * dec->cache_y_stride_;
    const int uv_offset = cache_id * 8 * dec->cache_uv_stride_;
    uint8_t* const ydst = dec->cache_y_ - ysize + y_offset;
    uint8_t* const udst = dec->cache_u_ - uvsize + uv_offset;
    uint8_t* const vdst = dec->cache_v_ - uvsize + uv_offset;
    const int mb_y = ctx->mb_y_;
    const int is_first_row = (mb_y == 0);
    const int is_last_row = (mb_y >= dec->br_mb_y_ - 1);
  
    if (dec->mt_method_ == 2) {
      ReconstructRow(dec, ctx);
    }
  
    if (ctx->filter_row_) {
      FilterRow(dec);
    }
  
    if (dec->dither_) {
      DitherRow(dec);
    }
  
    if (io->put != NULL) {
      int y_start = MACROBLOCK_VPOS(mb_y);
      int y_end = MACROBLOCK_VPOS(mb_y + 1);
      if (!is_first_row) {
        y_start -= extra_y_rows;
        io->y = ydst;
        io->u = udst;
        io->v = vdst;
      } else {
        io->y = dec->cache_y_ + y_offset;
        io->u = dec->cache_u_ + uv_offset;
        io->v = dec->cache_v_ + uv_offset;
      }
  
      if (!is_last_row) {
        y_end -= extra_y_rows;
      }
      if (y_end > io->crop_bottom) {
        y_end = io->crop_bottom;    // make sure we don't overflow on last row.
      }
      // If dec->alpha_data_ is not NULL, we have some alpha plane present.
      io->a = NULL;
      if (dec->alpha_data_ != NULL && y_start < y_end) {
        io->a = VP8DecompressAlphaRows(dec, io, y_start, y_end - y_start);
        if (io->a == NULL) {
          return VP8SetError(dec, VP8_STATUS_BITSTREAM_ERROR,
                             "Could not decode alpha data.");
        }
      }
      if (y_start < io->crop_top) {
        const int delta_y = io->crop_top - y_start;
        y_start = io->crop_top;
        assert(!(delta_y & 1));
        io->y += dec->cache_y_stride_ * delta_y;
        io->u += dec->cache_uv_stride_ * (delta_y >> 1);
        io->v += dec->cache_uv_stride_ * (delta_y >> 1);
        if (io->a != NULL) {
          io->a += io->width * delta_y;
        }
      }
      if (y_start < y_end) {
        io->y += io->crop_left;
        io->u += io->crop_left >> 1;
        io->v += io->crop_left >> 1;
        if (io->a != NULL) {
          io->a += io->crop_left;
        }
        io->mb_y = y_start - io->crop_top;
        io->mb_w = io->crop_right - io->crop_left;
        io->mb_h = y_end - y_start;
        ok = io->put(io);
      }
    }
    // rotate top samples if needed
    if (cache_id + 1 == dec->num_caches_) {
      if (!is_last_row) {
        memcpy(dec->cache_y_ - ysize, ydst + 16 * dec->cache_y_stride_, ysize);
        memcpy(dec->cache_u_ - uvsize, udst + 8 * dec->cache_uv_stride_, uvsize);
        memcpy(dec->cache_v_ - uvsize, vdst + 8 * dec->cache_uv_stride_, uvsize);
      }
    }
  
    return ok;
  }
  
  #undef MACROBLOCK_VPOS
  
  //------------------------------------------------------------------------------
  
  int VP8ProcessRow(VP8Decoder* const dec, VP8Io* const io) {
    int ok = 1;
    VP8ThreadContext* const ctx = &dec->thread_ctx_;
    const int filter_row =
        (dec->filter_type_ > 0) &&
        (dec->mb_y_ >= dec->tl_mb_y_) && (dec->mb_y_ <= dec->br_mb_y_);
    if (dec->mt_method_ == 0) {
      // ctx->id_ and ctx->f_info_ are already set
      ctx->mb_y_ = dec->mb_y_;
      ctx->filter_row_ = filter_row;
      ReconstructRow(dec, ctx);
      ok = FinishRow(dec, io);
    } else {
      WebPWorker* const worker = &dec->worker_;
      // Finish previous job *before* updating context
      ok &= WebPGetWorkerInterface()->Sync(worker);
      assert(worker->status_ == OK);
      if (ok) {   // spawn a new deblocking/output job
        ctx->io_ = *io;
        ctx->id_ = dec->cache_id_;
        ctx->mb_y_ = dec->mb_y_;
        ctx->filter_row_ = filter_row;
        if (dec->mt_method_ == 2) {  // swap macroblock data
          VP8MBData* const tmp = ctx->mb_data_;
          ctx->mb_data_ = dec->mb_data_;
          dec->mb_data_ = tmp;
        } else {
          // perform reconstruction directly in main thread
          ReconstructRow(dec, ctx);
        }
        if (filter_row) {            // swap filter info
          VP8FInfo* const tmp = ctx->f_info_;
          ctx->f_info_ = dec->f_info_;
          dec->f_info_ = tmp;
        }
        // (reconstruct)+filter in parallel
        WebPGetWorkerInterface()->Launch(worker);
        if (++dec->cache_id_ == dec->num_caches_) {
          dec->cache_id_ = 0;
        }
      }
    }
    return ok;
  }
  
  //------------------------------------------------------------------------------
  // Finish setting up the decoding parameter once user's setup() is called.
  
  VP8StatusCode VP8EnterCritical(VP8Decoder* const dec, VP8Io* const io) {
    // Call setup() first. This may trigger additional decoding features on 'io'.
    // Note: Afterward, we must call teardown() no matter what.
    if (io->setup != NULL && !io->setup(io)) {
      VP8SetError(dec, VP8_STATUS_USER_ABORT, "Frame setup failed");
      return dec->status_;
    }
  
    // Disable filtering per user request
    if (io->bypass_filtering) {
      dec->filter_type_ = 0;
    }
  
    // Define the area where we can skip in-loop filtering, in case of cropping.
    //
    // 'Simple' filter reads two luma samples outside of the macroblock
    // and filters one. It doesn't filter the chroma samples. Hence, we can
    // avoid doing the in-loop filtering before crop_top/crop_left position.
    // For the 'Complex' filter, 3 samples are read and up to 3 are filtered.
    // Means: there's a dependency chain that goes all the way up to the
    // top-left corner of the picture (MB #0). We must filter all the previous
    // macroblocks.
    {
      const int extra_pixels = kFilterExtraRows[dec->filter_type_];
      if (dec->filter_type_ == 2) {
        // For complex filter, we need to preserve the dependency chain.
        dec->tl_mb_x_ = 0;
        dec->tl_mb_y_ = 0;
      } else {
        // For simple filter, we can filter only the cropped region.
        // We include 'extra_pixels' on the other side of the boundary, since
        // vertical or horizontal filtering of the previous macroblock can
        // modify some abutting pixels.
        dec->tl_mb_x_ = (io->crop_left - extra_pixels) >> 4;
        dec->tl_mb_y_ = (io->crop_top - extra_pixels) >> 4;
        if (dec->tl_mb_x_ < 0) dec->tl_mb_x_ = 0;
        if (dec->tl_mb_y_ < 0) dec->tl_mb_y_ = 0;
      }
      // We need some 'extra' pixels on the right/bottom.
      dec->br_mb_y_ = (io->crop_bottom + 15 + extra_pixels) >> 4;
      dec->br_mb_x_ = (io->crop_right + 15 + extra_pixels) >> 4;
      if (dec->br_mb_x_ > dec->mb_w_) {
        dec->br_mb_x_ = dec->mb_w_;
      }
      if (dec->br_mb_y_ > dec->mb_h_) {
        dec->br_mb_y_ = dec->mb_h_;
      }
    }
    PrecomputeFilterStrengths(dec);
    return VP8_STATUS_OK;
  }
  
  int VP8ExitCritical(VP8Decoder* const dec, VP8Io* const io) {
    int ok = 1;
    if (dec->mt_method_ > 0) {
      ok = WebPGetWorkerInterface()->Sync(&dec->worker_);
    }
  
    if (io->teardown != NULL) {
      io->teardown(io);
    }
    return ok;
  }
  
  //------------------------------------------------------------------------------
  // For multi-threaded decoding we need to use 3 rows of 16 pixels as delay line.
  //
  // Reason is: the deblocking filter cannot deblock the bottom horizontal edges
  // immediately, and needs to wait for first few rows of the next macroblock to
  // be decoded. Hence, deblocking is lagging behind by 4 or 8 pixels (depending
  // on strength).
  // With two threads, the vertical positions of the rows being decoded are:
  // Decode:  [ 0..15][16..31][32..47][48..63][64..79][...
  // Deblock:         [ 0..11][12..27][28..43][44..59][...
  // If we use two threads and two caches of 16 pixels, the sequence would be:
  // Decode:  [ 0..15][16..31][ 0..15!!][16..31][ 0..15][...
  // Deblock:         [ 0..11][12..27!!][-4..11][12..27][...
  // The problem occurs during row [12..15!!] that both the decoding and
  // deblocking threads are writing simultaneously.
  // With 3 cache lines, one get a safe write pattern:
  // Decode:  [ 0..15][16..31][32..47][ 0..15][16..31][32..47][0..
  // Deblock:         [ 0..11][12..27][28..43][-4..11][12..27][28...
  // Note that multi-threaded output _without_ deblocking can make use of two
  // cache lines of 16 pixels only, since there's no lagging behind. The decoding
  // and output process have non-concurrent writing:
  // Decode:  [ 0..15][16..31][ 0..15][16..31][...
  // io->put:         [ 0..15][16..31][ 0..15][...
  
  #define MT_CACHE_LINES 3
  #define ST_CACHE_LINES 1   // 1 cache row only for single-threaded case
  
  // Initialize multi/single-thread worker
  static int InitThreadContext(VP8Decoder* const dec) {
    dec->cache_id_ = 0;
    if (dec->mt_method_ > 0) {
      WebPWorker* const worker = &dec->worker_;
      if (!WebPGetWorkerInterface()->Reset(worker)) {
        return VP8SetError(dec, VP8_STATUS_OUT_OF_MEMORY,
                           "thread initialization failed.");
      }
      worker->data1 = dec;
      worker->data2 = (void*)&dec->thread_ctx_.io_;
      worker->hook = FinishRow;
      dec->num_caches_ =
        (dec->filter_type_ > 0) ? MT_CACHE_LINES : MT_CACHE_LINES - 1;
    } else {
      dec->num_caches_ = ST_CACHE_LINES;
    }
    return 1;
  }
  
  int VP8GetThreadMethod(const WebPDecoderOptions* const options,
                         const WebPHeaderStructure* const headers,
                         int width, int height) {
    if (options == NULL || options->use_threads == 0) {
      return 0;
    }
    (void)headers;
    (void)width;
    (void)height;
    assert(headers == NULL || !headers->is_lossless);
  #if defined(WEBP_USE_THREAD)
    if (width >= MIN_WIDTH_FOR_THREADS) return 2;
  #endif
    return 0;
  }
  
  #undef MT_CACHE_LINES
  #undef ST_CACHE_LINES
  
  //------------------------------------------------------------------------------
  // Memory setup
  
  static int AllocateMemory(VP8Decoder* const dec) {
    const int num_caches = dec->num_caches_;
    const int mb_w = dec->mb_w_;
    // Note: we use 'size_t' when there's no overflow risk, uint64_t otherwise.
    const size_t intra_pred_mode_size = 4 * mb_w * sizeof(uint8_t);
    const size_t top_size = sizeof(VP8TopSamples) * mb_w;
    const size_t mb_info_size = (mb_w + 1) * sizeof(VP8MB);
    const size_t f_info_size =
        (dec->filter_type_ > 0) ?
            mb_w * (dec->mt_method_ > 0 ? 2 : 1) * sizeof(VP8FInfo)
          : 0;
    const size_t yuv_size = YUV_SIZE * sizeof(*dec->yuv_b_);
    const size_t mb_data_size =
        (dec->mt_method_ == 2 ? 2 : 1) * mb_w * sizeof(*dec->mb_data_);
    const size_t cache_height = (16 * num_caches
                              + kFilterExtraRows[dec->filter_type_]) * 3 / 2;
    const size_t cache_size = top_size * cache_height;
    // alpha_size is the only one that scales as width x height.
    const uint64_t alpha_size = (dec->alpha_data_ != NULL) ?
        (uint64_t)dec->pic_hdr_.width_ * dec->pic_hdr_.height_ : 0ULL;
    const uint64_t needed = (uint64_t)intra_pred_mode_size
                          + top_size + mb_info_size + f_info_size
                          + yuv_size + mb_data_size
                          + cache_size + alpha_size + WEBP_ALIGN_CST;
    uint8_t* mem;
  
    if (needed != (size_t)needed) return 0;  // check for overflow
    if (needed > dec->mem_size_) {
      WebPSafeFree(dec->mem_);
      dec->mem_size_ = 0;
      dec->mem_ = WebPSafeMalloc(needed, sizeof(uint8_t));
      if (dec->mem_ == NULL) {
        return VP8SetError(dec, VP8_STATUS_OUT_OF_MEMORY,
                           "no memory during frame initialization.");
      }
      // down-cast is ok, thanks to WebPSafeMalloc() above.
      dec->mem_size_ = (size_t)needed;
    }
  
    mem = (uint8_t*)dec->mem_;
    dec->intra_t_ = mem;
    mem += intra_pred_mode_size;
  
    dec->yuv_t_ = (VP8TopSamples*)mem;
    mem += top_size;
  
    dec->mb_info_ = ((VP8MB*)mem) + 1;
    mem += mb_info_size;
  
    dec->f_info_ = f_info_size ? (VP8FInfo*)mem : NULL;
    mem += f_info_size;
    dec->thread_ctx_.id_ = 0;
    dec->thread_ctx_.f_info_ = dec->f_info_;
    if (dec->filter_type_ > 0 && dec->mt_method_ > 0) {
      // secondary cache line. The deblocking process need to make use of the
      // filtering strength from previous macroblock row, while the new ones
      // are being decoded in parallel. We'll just swap the pointers.
      dec->thread_ctx_.f_info_ += mb_w;
    }
  
    mem = (uint8_t*)WEBP_ALIGN(mem);
    assert((yuv_size & WEBP_ALIGN_CST) == 0);
    dec->yuv_b_ = mem;
    mem += yuv_size;
  
    dec->mb_data_ = (VP8MBData*)mem;
    dec->thread_ctx_.mb_data_ = (VP8MBData*)mem;
    if (dec->mt_method_ == 2) {
      dec->thread_ctx_.mb_data_ += mb_w;
    }
    mem += mb_data_size;
  
    dec->cache_y_stride_ = 16 * mb_w;
    dec->cache_uv_stride_ = 8 * mb_w;
    {
      const int extra_rows = kFilterExtraRows[dec->filter_type_];
      const int extra_y = extra_rows * dec->cache_y_stride_;
      const int extra_uv = (extra_rows / 2) * dec->cache_uv_stride_;
      dec->cache_y_ = mem + extra_y;
      dec->cache_u_ = dec->cache_y_
                    + 16 * num_caches * dec->cache_y_stride_ + extra_uv;
      dec->cache_v_ = dec->cache_u_
                    + 8 * num_caches * dec->cache_uv_stride_ + extra_uv;
      dec->cache_id_ = 0;
    }
    mem += cache_size;
  
    // alpha plane
    dec->alpha_plane_ = alpha_size ? mem : NULL;
    mem += alpha_size;
    assert(mem <= (uint8_t*)dec->mem_ + dec->mem_size_);
  
    // note: left/top-info is initialized once for all.
    memset(dec->mb_info_ - 1, 0, mb_info_size);
    VP8InitScanline(dec);   // initialize left too.
  
    // initialize top
    memset(dec->intra_t_, B_DC_PRED, intra_pred_mode_size);
  
    return 1;
  }
  
  static void InitIo(VP8Decoder* const dec, VP8Io* io) {
    // prepare 'io'
    io->mb_y = 0;
    io->y = dec->cache_y_;
    io->u = dec->cache_u_;
    io->v = dec->cache_v_;
    io->y_stride = dec->cache_y_stride_;
    io->uv_stride = dec->cache_uv_stride_;
    io->a = NULL;
  }
  
  int VP8InitFrame(VP8Decoder* const dec, VP8Io* const io) {
    if (!InitThreadContext(dec)) return 0;  // call first. Sets dec->num_caches_.
    if (!AllocateMemory(dec)) return 0;
    InitIo(dec, io);
    VP8DspInit();  // Init critical function pointers and look-up tables.
    return 1;
  }
  
  //------------------------------------------------------------------------------
  