libwebp/src/enc/vp8l.c

// Copyright 2012 Google Inc. All Rights Reserved.
//
// This code is licensed under the same terms as WebM:
//  Software License Agreement:  http://www.webmproject.org/license/software/
//  Additional IP Rights Grant:  http://www.webmproject.org/license/additional/
// -----------------------------------------------------------------------------
//
// main entry for the lossless encoder.
//
// Author: Vikas Arora (vikaas.arora@gmail.com)
//

#ifdef USE_LOSSLESS_ENCODER

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

#include "./backward_references.h"
#include "./vp8enci.h"
#include "./vp8li.h"
#include "../dsp/lossless.h"
#include "../utils/bit_writer.h"
#include "../utils/huffman_encode.h"

#if defined(__cplusplus) || defined(c_plusplus)
extern "C" {
#endif

static const uint32_t kImageSizeBits = 14;

static int CompareColors(const void* p1, const void* p2) {
  const uint32_t a = *(const uint32_t*)p1;
  const uint32_t b = *(const uint32_t*)p2;
  if (a < b) {
    return -1;
  }
  if (a == b) {
    return 0;
  }
  return 1;
}

// If number of colors in the image is less than or equal to MAX_PALETTE_SIZE,
// creates a palette and returns true, else returns false.
static int AnalyzeAndCreatePalette(const uint32_t* const argb, int num_pix,
                                   uint32_t palette[MAX_PALETTE_SIZE],
                                   int* const palette_size) {
  int i, key;
  int num_colors = 0;
  uint8_t in_use[MAX_PALETTE_SIZE * 4] = { 0 };
  uint32_t colors[MAX_PALETTE_SIZE * 4];
  static const uint32_t kHashMul = 0x1e35a7bd;

  key = (kHashMul * argb[0]) >> PALETTE_KEY_RIGHT_SHIFT;
  colors[key] = argb[0];
  in_use[key] = 1;
  ++num_colors;

  for (i = 1; i < num_pix; ++i) {
    if (argb[i] == argb[i - 1]) {
      continue;
    }
    key = (kHashMul * argb[i]) >> PALETTE_KEY_RIGHT_SHIFT;
    while (1) {
      if (!in_use[key]) {
        colors[key] = argb[i];
        in_use[key] = 1;
        ++num_colors;
        if (num_colors > MAX_PALETTE_SIZE) {
          return 0;
        }
        break;
      } else if (colors[key] == argb[i]) {
        // The color is already there.
        break;
      } else {
        // Some other color sits there.
        // Do linear conflict resolution.
        ++key;
        key &= (MAX_PALETTE_SIZE * 4 - 1);  // key mask for 1K buffer.
      }
    }
  }

  num_colors = 0;
  for (i = 0; i < (int)(sizeof(in_use) / sizeof(in_use[0])); ++i) {
    if (in_use[i]) {
      palette[num_colors] = colors[i];
      ++num_colors;
    }
  }

  qsort(palette, num_colors, sizeof(*palette), CompareColors);
  *palette_size = num_colors;
  return 1;
}

static int AnalyzeEntropy(const uint32_t const *argb, int xsize, int ysize,
                          int* nonpredicted_bits, int* predicted_bits) {
  int i;
  VP8LHistogram* nonpredicted = NULL;
  VP8LHistogram* predicted = (VP8LHistogram*)malloc(2 * sizeof(*predicted));
  if (predicted == NULL) return 0;
  nonpredicted = predicted + 1;

  VP8LHistogramInit(predicted, 0);
  VP8LHistogramInit(nonpredicted, 0);
  for (i = 1; i < xsize * ysize; ++i) {
    uint32_t pix_diff;
    if ((argb[i] == argb[i - 1]) ||
        (i >= xsize && argb[i] == argb[i - xsize])) {
      continue;
    }
    VP8LHistogramAddSinglePixOrCopy(nonpredicted,
                                    PixOrCopyCreateLiteral(argb[i]));
    pix_diff = VP8LSubPixels(argb[i], argb[i - 1]);
    VP8LHistogramAddSinglePixOrCopy(predicted,
                                    PixOrCopyCreateLiteral(pix_diff));
  }
  *nonpredicted_bits = (int)VP8LHistogramEstimateBitsBulk(nonpredicted);
  *predicted_bits = (int)VP8LHistogramEstimateBitsBulk(predicted);
  free(predicted);
  return 1;
}

static int VP8LEncAnalyze(VP8LEncoder* const enc) {
  const WebPPicture* const pic = enc->pic_;
  assert(pic && pic->argb);

  enc->use_palette_ =
        AnalyzeAndCreatePalette(pic->argb, pic->width * pic->height,
                                enc->palette_, &enc->palette_size_);
  if (!enc->use_palette_) {
    int non_pred_entropy, pred_entropy;
    if (!AnalyzeEntropy(pic->argb, pic->width, pic->height,
                        &non_pred_entropy, &pred_entropy)) {
      return 0;
    }

    if (8 * pred_entropy < 7 * non_pred_entropy) {
      enc->use_predict_ = 1;
      enc->use_cross_color_ = 1;
    }
  }
  return 1;
}

// Bundles multiple (2, 4 or 8) pixels into a single pixel.
// Returns the new xsize.
static void BundleColorMap(const uint32_t* const argb,
                           int width, int height, int xbits,
                           uint32_t* bundled_argb, int xs) {
  int x, y;
  const int bit_depth = 1 << (3 - xbits);
  uint32_t code = 0;

  for (y = 0; y < height; ++y) {
    for (x = 0; x < width; ++x) {
      const int mask = (1 << xbits) - 1;
      const int xsub = x & mask;
      if (xsub == 0) {
        code = 0;
      }
      // TODO(vikasa): simplify the bundling logic.
      code |= (argb[y * width + x] & 0xff00) << (bit_depth * xsub);
      bundled_argb[y * xs + (x >> xbits)] = 0xff000000 | code;
    }
  }
}

static int GetBackwardReferences(int width, int height,
                                 const uint32_t* argb,
                                 int quality, int use_color_cache,
                                 int cache_bits, int use_2d_locality,
                                 PixOrCopy** backward_refs,
                                 int* backward_refs_size) {
  int ok = 0;
  // Backward Reference using LZ77.
  int lz77_is_useful;
  int backward_refs_rle_size;
  int backward_refs_lz77_size;
  const int num_pix = width * height;
  VP8LHistogram* histo_rle;
  PixOrCopy* backward_refs_lz77 = (PixOrCopy*)
      malloc(num_pix * sizeof(*backward_refs_lz77));
  PixOrCopy* backward_refs_rle = (PixOrCopy*)
      malloc(num_pix * sizeof(*backward_refs_lz77));
  VP8LHistogram* histo_lz77 = (VP8LHistogram*)malloc(2 * sizeof(*histo_lz77));
  if (backward_refs_lz77 == NULL || backward_refs_rle == NULL ||
      histo_lz77 == NULL) {
    free(backward_refs_lz77);
    free(backward_refs_rle);
    goto End;
  }
  *backward_refs = NULL;
  histo_rle = histo_lz77 + 1;

  if (!VP8LBackwardReferencesHashChain(width, height, use_color_cache,
                                       argb, cache_bits, quality,
                                       backward_refs_lz77,
                                       &backward_refs_lz77_size)) {
    goto End;
  }
  VP8LHistogramInit(histo_lz77, cache_bits);
  VP8LHistogramCreate(histo_lz77, backward_refs_lz77, backward_refs_lz77_size);

  // Backward Reference using RLE only.
  VP8LBackwardReferencesRle(width, height, argb, backward_refs_rle,
                            &backward_refs_rle_size);

  VP8LHistogramInit(histo_rle, cache_bits);
  VP8LHistogramCreate(histo_rle, backward_refs_rle, backward_refs_rle_size);

  // Check if LZ77 is useful.
  lz77_is_useful = (VP8LHistogramEstimateBits(histo_rle) >
                    VP8LHistogramEstimateBits(histo_lz77));

  // Choose appropriate backward reference.
  if (lz77_is_useful) {
    // TraceBackwards is costly. Run it for higher qualities.
    const int try_lz77_trace_backwards = (quality >= 75);
    free(backward_refs_rle);
    if (try_lz77_trace_backwards) {
      const int recursion_level = (num_pix < 320 * 200) ? 1 : 0;
      int backward_refs_trace_size;
      PixOrCopy* backward_refs_trace;
      backward_refs_trace =
          (PixOrCopy*)malloc(num_pix * sizeof(*backward_refs_trace));
      if (backward_refs_trace == NULL) {
        free(backward_refs_lz77);
        goto End;
      }
      if (VP8LBackwardReferencesTraceBackwards(width, height, recursion_level,
                                               use_color_cache, argb,
                                               cache_bits, backward_refs_trace,
                                               &backward_refs_trace_size)) {
        free(backward_refs_lz77);
        *backward_refs = backward_refs_trace;
        *backward_refs_size = backward_refs_trace_size;
      } else {
        free(backward_refs_trace);
        *backward_refs = backward_refs_lz77;
        *backward_refs_size = backward_refs_lz77_size;
      }
    } else {
      *backward_refs = backward_refs_lz77;
      *backward_refs_size = backward_refs_lz77_size;
    }
  } else {
    free(backward_refs_lz77);
    *backward_refs = backward_refs_rle;
    *backward_refs_size = backward_refs_rle_size;
  }

  if (use_2d_locality) {
    // Use backward reference with 2D locality.
    VP8LBackwardReferences2DLocality(width, *backward_refs_size,
                                     *backward_refs);
  }
  ok = 1;

End:
  free(histo_lz77);
  if (!ok) {
    free(*backward_refs);
    *backward_refs = NULL;
  }
  return ok;
}

static void DeleteHistograms(VP8LHistogram** histograms, int size) {
  if (histograms != NULL) {
    int i;
    for (i = 0; i < size; ++i) {
      free(histograms[i]);
    }
    free(histograms);
  }
}

static int GetHistImageSymbols(int xsize, int ysize,
                               PixOrCopy* backward_refs,
                               int backward_refs_size,
                               int quality, int histogram_bits,
                               int cache_bits,
                               VP8LHistogram*** histogram_image,
                               int* histogram_image_size,
                               uint32_t* histogram_symbols) {
  // Build histogram image.
  int ok = 0;
  int i;
  int histogram_image_raw_size;
  VP8LHistogram** histogram_image_raw = NULL;

  *histogram_image = NULL;
  if (!VP8LHistogramBuildImage(xsize, ysize, histogram_bits, cache_bits,
                               backward_refs, backward_refs_size,
                               &histogram_image_raw,
                               &histogram_image_raw_size)) {
    goto Error;
  }
  // Collapse similar histograms.
  if (!VP8LHistogramCombine(histogram_image_raw, histogram_image_raw_size,
                            quality, histogram_image, histogram_image_size)) {
    goto Error;
  }
  // Refine histogram image.
  for (i = 0; i < histogram_image_raw_size; ++i) {
    histogram_symbols[i] = -1;
  }
  VP8LHistogramRefine(histogram_image_raw, histogram_image_raw_size,
                      histogram_symbols, *histogram_image_size,
                      *histogram_image);
  ok = 1;

Error:
  if (!ok) {
    DeleteHistograms(*histogram_image, *histogram_image_size);
  }
  DeleteHistograms(histogram_image_raw, histogram_image_raw_size);
  return ok;
}

// Heuristics for selecting the stride ranges to collapse.
static int ValuesShouldBeCollapsedToStrideAverage(int a, int b) {
  return abs(a - b) < 4;
}

// Change the population counts in a way that the consequent
// Hufmann tree compression, especially its rle-part will be more
// likely to compress this data more efficiently.
//
// length contains the size of the histogram.
// data contains the population counts.
static int OptimizeHuffmanForRle(int length, int* counts) {
  int stride;
  int limit;
  int sum;
  uint8_t* good_for_rle;
  // 1) Let's make the Huffman code more compatible with rle encoding.
  int i;
  for (; length >= 0; --length) {
    if (length == 0) {
      return 1;  // All zeros.
    }
    if (counts[length - 1] != 0) {
      // Now counts[0..length - 1] does not have trailing zeros.
      break;
    }
  }
  // 2) Let's mark all population counts that already can be encoded
  // with an rle code.
  good_for_rle = (uint8_t*)calloc(length, 1);
  if (good_for_rle == NULL) {
    return 0;
  }
  {
    // Let's not spoil any of the existing good rle codes.
    // Mark any seq of 0's that is longer as 5 as a good_for_rle.
    // Mark any seq of non-0's that is longer as 7 as a good_for_rle.
    int symbol = counts[0];
    int stride = 0;
    for (i = 0; i < length + 1; ++i) {
      if (i == length || counts[i] != symbol) {
        if ((symbol == 0 && stride >= 5) ||
            (symbol != 0 && stride >= 7)) {
          int k;
          for (k = 0; k < stride; ++k) {
            good_for_rle[i - k - 1] = 1;
          }
        }
        stride = 1;
        if (i != length) {
          symbol = counts[i];
        }
      } else {
        ++stride;
      }
    }
  }
  // 3) Let's replace those population counts that lead to more rle codes.
  stride = 0;
  limit = counts[0];
  sum = 0;
  for (i = 0; i < length + 1; ++i) {
    if (i == length || good_for_rle[i] ||
        (i != 0 && good_for_rle[i - 1]) ||
        !ValuesShouldBeCollapsedToStrideAverage(counts[i], limit)) {
      if (stride >= 4 || (stride >= 3 && sum == 0)) {
        int k;
        // The stride must end, collapse what we have, if we have enough (4).
        int count = (sum + stride / 2) / stride;
        if (count < 1) {
          count = 1;
        }
        if (sum == 0) {
          // Don't make an all zeros stride to be upgraded to ones.
          count = 0;
        }
        for (k = 0; k < stride; ++k) {
          // We don't want to change value at counts[i],
          // that is already belonging to the next stride. Thus - 1.
          counts[i - k - 1] = count;
        }
      }
      stride = 0;
      sum = 0;
      if (i < length - 3) {
        // All interesting strides have a count of at least 4,
        // at least when non-zeros.
        limit = (counts[i] + counts[i + 1] +
                 counts[i + 2] + counts[i + 3] + 2) / 4;
      } else if (i < length) {
        limit = counts[i];
      } else {
        limit = 0;
      }
    }
    ++stride;
    if (i != length) {
      sum += counts[i];
      if (stride >= 4) {
        limit = (sum + stride / 2) / stride;
      }
    }
  }
  free(good_for_rle);
  return 1;
}

// TODO(vikasa): Wrap bit_codes and bit_lengths in a Struct.
static int GetHuffBitLengthsAndCodes(
    int histogram_image_size, VP8LHistogram** histogram_image,
    int use_color_cache, int** bit_length_sizes,
    uint16_t*** bit_codes, uint8_t*** bit_lengths) {
  int i, k;
  int ok = 1;
  for (i = 0; i < histogram_image_size; ++i) {
    const int num_literals = VP8LHistogramNumCodes(histogram_image[i]);
    k = 0;
    // TODO(vikasa): Alloc one big buffer instead of allocating in the loop.
    (*bit_length_sizes)[5 * i] = num_literals;
    (*bit_lengths)[5 * i] = (uint8_t*)calloc(num_literals, 1);
    (*bit_codes)[5 * i] = (uint16_t*)
        malloc(num_literals * sizeof(*(*bit_codes)[5 * i]));
    if ((*bit_lengths)[5 * i] == NULL || (*bit_codes)[5 * i] == NULL) {
      ok = 0;
      goto Error;
    }

    // For each component, optimize histogram for Huffman with RLE compression.
    ok = ok && OptimizeHuffmanForRle(num_literals,
                                     histogram_image[i]->literal_);
    if (!use_color_cache) {
      // Implies that palette_bits == 0,
      // and so number of palette entries = (1 << 0) = 1.
      // Optimization might have smeared population count in this single
      // palette entry, so zero it out.
      histogram_image[i]->literal_[256 + kLengthCodes] = 0;
    }
    ok = ok && OptimizeHuffmanForRle(256, histogram_image[i]->red_);
    ok = ok && OptimizeHuffmanForRle(256, histogram_image[i]->blue_);
    ok = ok && OptimizeHuffmanForRle(256, histogram_image[i]->alpha_);
    ok = ok && OptimizeHuffmanForRle(DISTANCE_CODES_MAX,
                                     histogram_image[i]->distance_);

    // Create a Huffman tree (in the form of bit lengths) for each component.
    ok = ok && VP8LCreateHuffmanTree(histogram_image[i]->literal_, num_literals,
                                     15, (*bit_lengths)[5 * i]);
    for (k = 1; k < 5; ++k) {
      int val = 256;
      if (k == 4) {
        val = DISTANCE_CODES_MAX;
      }
      (*bit_length_sizes)[5 * i + k] = val;
      (*bit_lengths)[5 * i + k] = (uint8_t*)calloc(val, 1);
      (*bit_codes)[5 * i + k] = (uint16_t*)calloc(val, sizeof(bit_codes[0]));
      if ((*bit_lengths)[5 * i + k] == NULL ||
          (*bit_codes)[5 * i + k] == NULL) {
        ok = 0;
        goto Error;
      }
    }
    ok = ok && VP8LCreateHuffmanTree(histogram_image[i]->red_, 256, 15,
                                     (*bit_lengths)[5 * i + 1]) &&
        VP8LCreateHuffmanTree(histogram_image[i]->blue_, 256, 15,
                              (*bit_lengths)[5 * i + 2]) &&
        VP8LCreateHuffmanTree(histogram_image[i]->alpha_, 256, 15,
                              (*bit_lengths)[5 * i + 3]) &&
        VP8LCreateHuffmanTree(histogram_image[i]->distance_,
                              DISTANCE_CODES_MAX, 15,
                              (*bit_lengths)[5 * i + 4]);
    // Create the actual bit codes for the bit lengths.
    for (k = 0; k < 5; ++k) {
      int ix = 5 * i + k;
      VP8LConvertBitDepthsToSymbols((*bit_lengths)[ix], (*bit_length_sizes)[ix],
                                    (*bit_codes)[ix]);
    }
  }
  return ok;

 Error:
  {
    int idx;
    for (idx = 0; idx <= 5 * i + k; ++idx) {
      free((*bit_lengths)[idx]);
      free((*bit_codes)[idx]);
    }
  }
  return 0;
}

static void ShiftHistogramImage(uint32_t* image , int image_size) {
  int i;
  for (i = 0; i < image_size; ++i) {
    image[i] <<= 8;
    image[i] |= 0xff000000;
  }
}

static void ClearHuffmanTreeIfOnlyOneSymbol(const int num_symbols,
                                            uint8_t* lengths,
                                            uint16_t* symbols) {
  int k;
  int count = 0;
  for (k = 0; k < num_symbols; ++k) {
    if (lengths[k] != 0) ++count;
    if (count > 1) return;
  }
  for (k = 0; k < num_symbols; ++k) {
    lengths[k] = 0;
    symbols[k] = 0;
  }
}

static void StoreHuffmanTreeOfHuffmanTreeToBitMask(
    VP8LBitWriter* const bw, const uint8_t* code_length_bitdepth) {
  // RFC 1951 will calm you down if you are worried about this funny sequence.
  // This sequence is tuned from that, but more weighted for lower symbol count,
  // and more spiking histograms.
  int i;
  static const uint8_t kStorageOrder[CODE_LENGTH_CODES] = {
    17, 18, 0, 1, 2, 3, 4, 5, 16, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
  };
  // Throw away trailing zeros:
  int codes_to_store = sizeof(kStorageOrder);
  for (; codes_to_store > 4; --codes_to_store) {
    if (code_length_bitdepth[kStorageOrder[codes_to_store - 1]] != 0) {
      break;
    }
  }
  // How many code length codes we write above the first four (see RFC 1951).
  VP8LWriteBits(bw, 4, codes_to_store - 4);
  for (i = 0; i < codes_to_store; ++i) {
    VP8LWriteBits(bw, 3, code_length_bitdepth[kStorageOrder[i]]);
  }
}

static void StoreHuffmanTreeToBitMask(
    VP8LBitWriter* const bw,
    const uint8_t* huffman_tree,
    const uint8_t* huffman_tree_extra_bits,
    const int num_symbols,
    const uint8_t* code_length_bitdepth,
    const uint16_t* code_length_bitdepth_symbols) {
  int i;
  for (i = 0; i < num_symbols; ++i) {
    const int ix = huffman_tree[i];
    VP8LWriteBits(bw, code_length_bitdepth[ix],
                  code_length_bitdepth_symbols[ix]);
    switch (ix) {
      case 16:
        VP8LWriteBits(bw, 2, huffman_tree_extra_bits[i]);
        break;
      case 17:
        VP8LWriteBits(bw, 3, huffman_tree_extra_bits[i]);
        break;
      case 18:
        VP8LWriteBits(bw, 7, huffman_tree_extra_bits[i]);
        break;
    }
  }
}

static int StoreHuffmanCode(VP8LBitWriter* const bw,
                            const uint8_t* const bit_lengths,
                            int bit_lengths_size) {
  int i;
  int ok = 0;
  int count = 0;
  int symbols[2] = { 0, 0 };
  int huffman_tree_size = 0;
  uint8_t code_length_bitdepth[CODE_LENGTH_CODES];
  uint16_t code_length_bitdepth_symbols[CODE_LENGTH_CODES];
  int huffman_tree_histogram[CODE_LENGTH_CODES];
  uint8_t* huffman_tree_extra_bits;
  uint8_t* huffman_tree = (uint8_t*)malloc(bit_lengths_size *
                                           (sizeof(*huffman_tree) +
                                            sizeof(*huffman_tree_extra_bits)));

  if (huffman_tree == NULL) goto End;
  huffman_tree_extra_bits =
      huffman_tree + (bit_lengths_size * sizeof(*huffman_tree));

  for (i = 0; i < bit_lengths_size; ++i) {
    if (bit_lengths[i] != 0) {
      if (count < 2) symbols[count] = i;
      ++count;
    }
  }
  if (count <= 2) {
    int num_bits = 4;
    // 0, 1 or 2 symbols to encode.
    VP8LWriteBits(bw, 1, 1);
    if (count == 0) {
      VP8LWriteBits(bw, 3, 0);
      ok = 1;
      goto End;
    }
    while (symbols[count - 1] >= (1 << num_bits)) num_bits += 2;
    VP8LWriteBits(bw, 3, (num_bits - 4) / 2 + 1);
    VP8LWriteBits(bw, 1, count - 1);
    for (i = 0; i < count; ++i) {
      VP8LWriteBits(bw, num_bits, symbols[i]);
    }
    ok = 1;
    goto End;
  }

  VP8LWriteBits(bw, 1, 0);
  VP8LCreateCompressedHuffmanTree(bit_lengths, bit_lengths_size,
                                  &huffman_tree_size, huffman_tree,
                                  huffman_tree_extra_bits);
  memset(huffman_tree_histogram, 0, sizeof(huffman_tree_histogram));
  for (i = 0; i < huffman_tree_size; ++i) {
    ++huffman_tree_histogram[huffman_tree[i]];
  }
  memset(code_length_bitdepth, 0, sizeof(code_length_bitdepth));
  memset(code_length_bitdepth_symbols, 0, sizeof(code_length_bitdepth_symbols));

  if (!VP8LCreateHuffmanTree(huffman_tree_histogram, CODE_LENGTH_CODES,
                             7, code_length_bitdepth)) {
    goto End;
  }
  VP8LConvertBitDepthsToSymbols(code_length_bitdepth, CODE_LENGTH_CODES,
                                code_length_bitdepth_symbols);
  StoreHuffmanTreeOfHuffmanTreeToBitMask(bw, code_length_bitdepth);
  ClearHuffmanTreeIfOnlyOneSymbol(CODE_LENGTH_CODES,
                                  code_length_bitdepth,
                                  code_length_bitdepth_symbols);
  {
    int num_trailing_zeros = 0;
    int trailing_zero_bits = 0;
    int trimmed_length;
    int write_length;
    int length;
    for (i = huffman_tree_size; i > 0; --i) {
      int ix = huffman_tree[i - 1];
      if (ix == 0 || ix == 17 || ix == 18) {
        ++num_trailing_zeros;
        trailing_zero_bits += code_length_bitdepth[ix];
        if (ix == 17) trailing_zero_bits += 3;
        if (ix == 18) trailing_zero_bits += 7;
      } else {
        break;
      }
    }
    trimmed_length = huffman_tree_size - num_trailing_zeros;
    write_length = (trimmed_length > 1 && trailing_zero_bits > 12);
    length = write_length ? trimmed_length : huffman_tree_size;
    VP8LWriteBits(bw, 1, write_length);
    if (write_length) {
      const int nbits = VP8LBitsLog2Ceiling(trimmed_length - 1);
      const int nbitpairs = nbits == 0 ? 1 : (nbits + 1) / 2;
      VP8LWriteBits(bw, 3, nbitpairs - 1);
      VP8LWriteBits(bw, nbitpairs * 2, trimmed_length - 2);
    }
    StoreHuffmanTreeToBitMask(bw, huffman_tree, huffman_tree_extra_bits,
                              length, code_length_bitdepth,
                              code_length_bitdepth_symbols);
  }
  ok = 1;

 End:
  free(huffman_tree);
  return ok;
}

static void StoreImageToBitMask(
    VP8LBitWriter* const bw, int width, int histo_bits,
    const PixOrCopy* literals, int literals_size,
    const uint32_t* histogram_symbols,
    uint8_t** const bitdepths, uint16_t** const bit_symbols) {
  // x and y trace the position in the image.
  int x = 0;
  int y = 0;
  const int histo_xsize = histo_bits ? VP8LSubSampleSize(width, histo_bits) : 1;
  int i;
  for (i = 0; i < literals_size; ++i) {
    const PixOrCopy v = literals[i];
    const int histogram_ix = histogram_symbols[histo_bits ?
                                               (y >> histo_bits) * histo_xsize +
                                               (x >> histo_bits) : 0];
    if (PixOrCopyIsCacheIdx(&v)) {
      const int code = PixOrCopyCacheIdx(&v);
      int literal_ix = 256 + kLengthCodes + code;
      VP8LWriteBits(bw, bitdepths[5 * histogram_ix][literal_ix],
                    bit_symbols[5 * histogram_ix][literal_ix]);
    } else if (PixOrCopyIsLiteral(&v)) {
      static const int order[] = {1, 2, 0, 3};
      int k;
      for (k = 0; k < 4; ++k) {
        const int code = PixOrCopyLiteral(&v, order[k]);
        VP8LWriteBits(bw, bitdepths[5 * histogram_ix + k][code],
                      bit_symbols[5 * histogram_ix + k][code]);
      }
    } else {
      int bits, n_bits;
      int code, distance;
      int len_ix;
      PrefixEncode(v.len, &code, &n_bits, &bits);
      len_ix = 256 + code;
      VP8LWriteBits(bw, bitdepths[5 * histogram_ix][len_ix],
                    bit_symbols[5 * histogram_ix][len_ix]);
      VP8LWriteBits(bw, n_bits, bits);

      distance = PixOrCopyDistance(&v);
      PrefixEncode(distance, &code, &n_bits, &bits);
      VP8LWriteBits(bw, bitdepths[5 * histogram_ix + 4][code],
                    bit_symbols[5 * histogram_ix + 4][code]);
      VP8LWriteBits(bw, n_bits, bits);
    }
    x += PixOrCopyLength(&v);
    while (x >= width) {
      x -= width;
      ++y;
    }
  }
}

static int EncodeImageInternal(VP8LBitWriter* const bw,
                               const uint32_t* const argb,
                               int width, int height, int quality,
                               int cache_bits, int histogram_bits) {
  int i;
  int ok = 0;
  int histogram_image_size;
  int write_histogram_image;
  int* bit_lengths_sizes = NULL;
  uint8_t** bit_lengths = NULL;
  uint16_t** bit_codes = NULL;
  const int use_2d_locality = 1;
  int backward_refs_size;
  const int use_color_cache = (cache_bits > 0);
  const int color_cache_size = use_color_cache ? (1 << cache_bits) : 0;
  const int histogram_image_xysize = VP8LSubSampleSize(width, histogram_bits) *
      VP8LSubSampleSize(height, histogram_bits);
  VP8LHistogram** histogram_image;
  PixOrCopy* backward_refs;
  uint32_t* histogram_symbols = (uint32_t*)
      calloc(histogram_image_xysize, sizeof(*histogram_symbols));

  if (histogram_symbols == NULL) goto Error;

  // Calculate backward references from ARGB image.
  if (!GetBackwardReferences(width, height, argb, quality,
                             use_color_cache, cache_bits, use_2d_locality,
                             &backward_refs, &backward_refs_size)) {
    goto Error;
  }
  // Build histogram image & symbols from backward references.
  if (!GetHistImageSymbols(width, height, backward_refs, backward_refs_size,
                           quality, histogram_bits, cache_bits,
                           &histogram_image, &histogram_image_size,
                           histogram_symbols)) {
    goto Error;
  }
  // Create Huffman bit lengths & codes for each histogram image.
  bit_lengths_sizes = (int*)calloc(5 * histogram_image_size,
                                   sizeof(*bit_lengths_sizes));
  bit_lengths = (uint8_t**)calloc(5 * histogram_image_size,
                                  sizeof(*bit_lengths));
  bit_codes = (uint16_t**)calloc(5 * histogram_image_size,
                                 sizeof(*bit_codes));
  if (bit_lengths_sizes == NULL || bit_lengths == NULL || bit_codes == NULL ||
      !GetHuffBitLengthsAndCodes(histogram_image_size, histogram_image,
                                 use_color_cache, &bit_lengths_sizes,
                                 &bit_codes, &bit_lengths)) {
    goto Error;
  }

  // Huffman image + meta huffman.
  write_histogram_image = (histogram_image_size > 1);
  VP8LWriteBits(bw, 1, write_histogram_image);
  if (write_histogram_image) {
    int image_size_bits;
    uint32_t* histogram_argb = (uint32_t*)
        malloc(histogram_image_xysize * sizeof(*histogram_argb));
    if (histogram_argb == NULL) goto Error;
    memcpy(histogram_argb, histogram_symbols,
           histogram_image_xysize * sizeof(*histogram_argb));

    ShiftHistogramImage(histogram_argb, histogram_image_xysize);
    VP8LWriteBits(bw, 4, histogram_bits);
    if (!EncodeImageInternal(bw, histogram_argb,
                             VP8LSubSampleSize(width, histogram_bits),
                             VP8LSubSampleSize(height, histogram_bits),
                             quality, 0, 0)) {
      free(histogram_argb);
      goto Error;
    }
    image_size_bits = VP8LBitsLog2Ceiling(histogram_image_size - 1);
    VP8LWriteBits(bw, 4, image_size_bits);
    VP8LWriteBits(bw, image_size_bits, histogram_image_size - 2);
    free(histogram_argb);
  }

  // Color Cache parameters.
  VP8LWriteBits(bw, 1, use_color_cache);
  if (use_color_cache) {
    VP8LWriteBits(bw, 4, cache_bits);
  }

  // Store Huffman codes.
  for (i = 0; i < histogram_image_size; ++i) {
    int k;
    for (k = 0; k < 5; ++k) {
      const uint8_t* const cur_bit_lengths =  bit_lengths[5 * i + k];
      const int cur_bit_lengths_size = (k == 0) ?
                   256 + kLengthCodes + color_cache_size :
                   bit_lengths_sizes[5 * i + k];
      if (!StoreHuffmanCode(bw, cur_bit_lengths, cur_bit_lengths_size)) {
        goto Error;
      }
    }
  }

  // Free combined histograms.
  DeleteHistograms(histogram_image, histogram_image_size);
  histogram_image = NULL;

  // Emit no bits if there is only one symbol in the histogram.
  // This gives better compression for some images.
  for (i = 0; i < 5 * histogram_image_size; ++i) {
    ClearHuffmanTreeIfOnlyOneSymbol(bit_lengths_sizes[i], bit_lengths[i],
                                    bit_codes[i]);
  }
  // Store actual literals.
  StoreImageToBitMask(bw, width, histogram_bits, backward_refs,
                      backward_refs_size, histogram_symbols,
                      bit_lengths, bit_codes);
  ok = 1;

 Error:
  if (!ok) {
    DeleteHistograms(histogram_image, histogram_image_size);
  }
  free(backward_refs);
  for (i = 0; i < 5 * histogram_image_size; ++i) {
    free(bit_lengths[i]);
    free(bit_codes[i]);
  }
  free(bit_lengths_sizes);
  free(bit_lengths);
  free(bit_codes);
  free(histogram_symbols);
  return ok;
}

static int EvalAndApplySubtractGreen(VP8LBitWriter* const bw,
                                     VP8LEncoder* const enc,
                                     int width, int height) {
  if (!enc->use_palette_) {
    int i;
    VP8LHistogram* before = NULL;
    // Check if it would be a good idea to subtract green from red and blue.
    VP8LHistogram* after = (VP8LHistogram*)malloc(2 * sizeof(*after));
    if (after == NULL) return 0;
    before = after + 1;

    VP8LHistogramInit(before, 1);
    VP8LHistogramInit(after, 1);
    for (i = 0; i < width * height; ++i) {
      // We only impact entropy in red and blue components, don't bother
      // to look at others.
      const uint32_t c = enc->argb_[i];
      const int green = (c >> 8) & 0xff;
      ++(before->red_[(c >> 16) & 0xff]);
      ++(before->blue_[c & 0xff]);
      ++(after->red_[((c >> 16) - green) & 0xff]);
      ++(after->blue_[(c - green) & 0xff]);
    }
    // Check if subtracting green yields low entropy.
    if (VP8LHistogramEstimateBits(after) < VP8LHistogramEstimateBits(before)) {
      VP8LWriteBits(bw, 1, 1);
      VP8LWriteBits(bw, 2, 2);
      VP8LSubtractGreenFromBlueAndRed(enc->argb_, width * height);
    }
    free(after);
  }
  return 1;
}

static int ApplyPredictFilter(VP8LBitWriter* const bw,
                              VP8LEncoder* const enc,
                              int width, int height, int quality) {
  const int pred_bits = enc->transform_bits_;
  const int transform_width = VP8LSubSampleSize(width, pred_bits);
  const int transform_height = VP8LSubSampleSize(height, pred_bits);

  VP8LResidualImage(width, height, pred_bits, enc->argb_, enc->argb_scratch_,
                    enc->transform_data_);
  VP8LWriteBits(bw, 1, 1);
  VP8LWriteBits(bw, 2, 0);
  VP8LWriteBits(bw, 4, pred_bits);
  if (!EncodeImageInternal(bw, enc->transform_data_,
                           transform_width, transform_height, quality, 0, 0)) {
    return 0;
  }
  return 1;
}

static int ApplyCrossColorFilter(VP8LBitWriter* const bw,
                                 VP8LEncoder* const enc,
                                 int width, int height, int quality) {
  const int ccolor_transform_bits = enc->transform_bits_;
  const int transform_width = VP8LSubSampleSize(width, ccolor_transform_bits);
  const int transform_height = VP8LSubSampleSize(height, ccolor_transform_bits);
  const int step = (quality == 0) ? 32 : 8;

  VP8LColorSpaceTransform(width, height, ccolor_transform_bits, step,
                          enc->argb_, enc->transform_data_);
  VP8LWriteBits(bw, 1, 1);
  VP8LWriteBits(bw, 2, 1);
  VP8LWriteBits(bw, 4, ccolor_transform_bits);
  if (!EncodeImageInternal(bw, enc->transform_data_,
                           transform_width, transform_height, quality, 0, 0)) {
    return 0;
  }
  return 1;
}

static void PutLE32(uint8_t* const data, uint32_t val) {
  data[0] = (val >>  0) & 0xff;
  data[1] = (val >>  8) & 0xff;
  data[2] = (val >> 16) & 0xff;
  data[3] = (val >> 24) & 0xff;
}

static WebPEncodingError WriteRiffHeader(VP8LEncoder* const enc,
                                         size_t riff_size, size_t vp8l_size) {
  const WebPPicture* const pic = enc->pic_;
  uint8_t riff[HEADER_SIZE + SIGNATURE_SIZE] = {
    'R', 'I', 'F', 'F', 0, 0, 0, 0, 'W', 'E', 'B', 'P',
    'V', 'P', '8', 'L', 0, 0, 0, 0, LOSSLESS_MAGIC_BYTE,
  };
  if (riff_size < (vp8l_size + TAG_SIZE + CHUNK_HEADER_SIZE)) {
    return VP8_ENC_ERROR_INVALID_CONFIGURATION;
  }
  PutLE32(riff + TAG_SIZE, (uint32_t)riff_size);
  PutLE32(riff + RIFF_HEADER_SIZE + TAG_SIZE, (uint32_t)vp8l_size);
  if (!pic->writer(riff, sizeof(riff), pic)) {
    return VP8_ENC_ERROR_BAD_WRITE;
  }
  return VP8_ENC_OK;
}

static WebPEncodingError WriteImage(VP8LEncoder* const enc,
                                    VP8LBitWriter* const bw) {
  size_t riff_size, vp8l_size, webpll_size, pad;
  const WebPPicture* const pic = enc->pic_;
  WebPEncodingError err = VP8_ENC_OK;
  const uint8_t* const webpll_data = VP8LBitWriterFinish(bw);

  webpll_size = VP8LBitWriterNumBytes(bw);
  vp8l_size = SIGNATURE_SIZE + webpll_size;
  pad = vp8l_size & 1;
  vp8l_size += pad;

  riff_size = TAG_SIZE + CHUNK_HEADER_SIZE + vp8l_size;
  err = WriteRiffHeader(enc, riff_size, vp8l_size);
  if (err != VP8_ENC_OK) goto Error;

  if (!pic->writer(webpll_data, webpll_size, pic)) {
    err = VP8_ENC_ERROR_BAD_WRITE;
    goto Error;
  }

  if (pad) {
    const uint8_t pad_byte[1] = { 0 };
    if (!pic->writer(pad_byte, 1, pic)) {
      err = VP8_ENC_ERROR_BAD_WRITE;
      goto Error;
    }
  }
  return VP8_ENC_OK;

 Error:
  return err;
}

static VP8LEncoder* InitVP8LEncoder(const WebPConfig* const config,
                                    WebPPicture* const picture) {
  const int method = config->method;
  const int histo_bits = 9 - (int)(config->quality / 16.f + .5f);
  VP8LEncoder* enc = (VP8LEncoder*)malloc(sizeof(*enc));
  if (enc == NULL) {
    WebPEncodingSetError(picture, VP8_ENC_ERROR_OUT_OF_MEMORY);
    return NULL;
  }
  memset(enc, 0, sizeof(*enc));

  enc->config_ = config;
  enc->pic_ = picture;
  enc->use_lz77_ = 1;

  enc->histo_bits_ =
      (histo_bits < 3) ? 3 : (histo_bits > 8) ? 8 : histo_bits;
  enc->transform_bits_ = (method < 4) ? 5 : (method > 4) ? 3 : 4;

  return enc;
}

static void WriteImageSize(VP8LEncoder* const enc, VP8LBitWriter* const bw) {
  WebPPicture* const pic = enc->pic_;
  const int width = pic->width - 1;
  const int height = pic->height -1;
  assert(width < WEBP_MAX_DIMENSION && height < WEBP_MAX_DIMENSION);

  VP8LWriteBits(bw, kImageSizeBits, width);
  VP8LWriteBits(bw, kImageSizeBits, height);
}

static void DeleteVP8LEncoder(VP8LEncoder* enc) {
  free(enc->argb_);
  free(enc);
}

// Allocates the memory for argb (W x H) buffer, 2 rows of context for
// prediction and transform data.
static WebPEncodingError AllocateTransformBuffer(VP8LEncoder* const enc,
                                                 int height, int width) {
  WebPEncodingError err = VP8_ENC_OK;
  const size_t tile_size = 1 << enc->transform_bits_;
  const size_t image_size = height * width;
  const size_t argb_scratch_size = (tile_size + 1) * width;
  const size_t transform_data_size =
      VP8LSubSampleSize(height, enc->transform_bits_) *
      VP8LSubSampleSize(width, enc->transform_bits_);
  const size_t total_size =
      image_size + argb_scratch_size + transform_data_size;
  uint32_t* mem = (uint32_t*)malloc(total_size * sizeof(*mem));
  if (mem == NULL) {
    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
    goto Error;
  }
  enc->argb_ = mem;
  mem += image_size;
  enc->argb_scratch_ = mem;
  mem += argb_scratch_size;
  enc->transform_data_ = mem;
  enc->current_width_ = width;

 Error:
  return err;
}

// Note: Expects "enc->palette_" to be set properly.
// Also, "enc->palette_" will be modified after this call and should not be used
// later.
static WebPEncodingError ApplyPalette(VP8LBitWriter* const bw,
                                      VP8LEncoder* const enc,
                                      int width, int height, int quality) {
  WebPEncodingError err = VP8_ENC_OK;
  int i;
  uint32_t* argb = enc->pic_->argb;
  uint32_t* const palette = enc->palette_;
  const int palette_size = enc->palette_size_;

  // Replace each input pixel by corresponding palette index.
  for (i = 0; i < width * height; ++i) {
    int k;
    for (k = 0; k < palette_size; ++k) {
      const uint32_t pix = argb[i];
      if (pix == palette[k]) {
        argb[i] = 0xff000000u | (k << 8);
        break;
      }
    }
  }

  // Save palette to bitstream.
  VP8LWriteBits(bw, 1, 1);
  VP8LWriteBits(bw, 2, 3);
  VP8LWriteBits(bw, 8, palette_size - 1);
  for (i = palette_size - 1; i >= 1; --i) {
    palette[i] = VP8LSubPixels(palette[i], palette[i - 1]);
  }
  if (!EncodeImageInternal(bw, palette, palette_size, 1, quality, 0, 0)) {
    err = VP8_ENC_ERROR_INVALID_CONFIGURATION;
    goto Error;
  }

  if (palette_size <= 16) {
    // Image can be packed (multiple pixels per uint32_t).
    int xbits = 1;
    if (palette_size <= 2) {
      xbits = 3;
    } else if (palette_size <= 4) {
      xbits = 2;
    }
    err = AllocateTransformBuffer(enc, height, VP8LSubSampleSize(width, xbits));
    if (err != VP8_ENC_OK) goto Error;
    BundleColorMap(argb, width, height, xbits, enc->argb_, enc->current_width_);
  }

 Error:
  return err;
}

int VP8LEncodeImage(const WebPConfig* const config,
                    WebPPicture* const picture) {
  int ok = 0;
  int cache_bits = 7;  // If equal to 0, don't use color cache.
  int width, height, quality;
  VP8LEncoder* enc = NULL;
  WebPEncodingError err = VP8_ENC_OK;
  VP8LBitWriter bw;

  if (config == NULL || picture == NULL) return 0;

  if (picture->argb == NULL) {
    err = VP8_ENC_ERROR_NULL_PARAMETER;
    goto Error;
  }

  enc = InitVP8LEncoder(config, picture);
  if (enc == NULL) {
    err = VP8_ENC_ERROR_NULL_PARAMETER;
    goto Error;
  }
  width = picture->width;
  height = picture->height;
  quality = config->quality;

  VP8LBitWriterInit(&bw, (width * height) >> 1);

  // ---------------------------------------------------------------------------
  // Analyze image (entropy, num_palettes etc)

  if (!VP8LEncAnalyze(enc)) {
    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
    goto Error;
  }

  // Write image size.
  WriteImageSize(enc, &bw);

  if (enc->use_palette_) {
    err = ApplyPalette(&bw, enc, width, height, quality);
    if (err != VP8_ENC_OK) goto Error;
    cache_bits = 0;  // Don't use color cache.
  }

  // In case image is not packed.
  if (enc->argb_ == NULL) {
    const size_t image_size = height * width;
    err = AllocateTransformBuffer(enc, height, width);
    if (err != VP8_ENC_OK) goto Error;
    memcpy(enc->argb_, picture->argb, image_size * sizeof(*enc->argb_));
    enc->current_width_ = width;
  }

  // ---------------------------------------------------------------------------
  // Apply transforms and write transform data.

  if (!EvalAndApplySubtractGreen(&bw, enc, enc->current_width_, height)) {
    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
    goto Error;
  }

  if (enc->use_predict_) {
    if (!ApplyPredictFilter(&bw, enc, enc->current_width_, height, quality)) {
      err = VP8_ENC_ERROR_INVALID_CONFIGURATION;
      goto Error;
    }
  }

  if (enc->use_cross_color_) {
    if (!ApplyCrossColorFilter(&bw, enc, enc->current_width_, height,
                               quality)) {
      err = VP8_ENC_ERROR_INVALID_CONFIGURATION;
      goto Error;
    }
  }

  VP8LWriteBits(&bw, 1, 0);  // No more transforms.

  // ---------------------------------------------------------------------------
  // Estimate the color cache size.

  if (cache_bits > 0) {
    if (quality > 25) {
      if (!VP8LCalculateEstimateForCacheSize(enc->argb_, enc->current_width_,
                                             height, &cache_bits)) {
        err = VP8_ENC_ERROR_INVALID_CONFIGURATION;
        goto Error;
      }
    } else {
      cache_bits = 0;  // Don't use color cache.
    }
  }

  // ---------------------------------------------------------------------------
  // Encode and write the transformed image.

  ok = EncodeImageInternal(&bw, enc->argb_, enc->current_width_, height,
                           quality, cache_bits, enc->histo_bits_);
  if (!ok) goto Error;

  err = WriteImage(enc, &bw);
  if (err != VP8_ENC_OK) {
    ok = 0;
    goto Error;
  }

 Error:
  VP8LBitWriterDestroy(&bw);
  DeleteVP8LEncoder(enc);
  if (!ok) {
    assert(err != VP8_ENC_OK);
    WebPEncodingSetError(picture, err);
  }
  return ok;
}

//------------------------------------------------------------------------------

#if defined(__cplusplus) || defined(c_plusplus)
}    // extern "C"
#endif

#endif