// Copyright 2012 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.

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

//

// Author: Jyrki Alakuijala (jyrki@google.com)

//

#include <assert.h>

#include <math.h>

#include "src/enc/backward_references_enc.h"

#include "src/enc/histogram_enc.h"

#include "src/dsp/lossless.h"

#include "src/dsp/lossless_common.h"

#include "src/dsp/dsp.h"

#include "src/utils/color_cache_utils.h"

#include "src/utils/utils.h"

#define MIN_BLOCK_SIZE 256  // minimum block size for backward references

#define MAX_ENTROPY    (1e30f)

// 1M window (4M bytes) minus 120 special codes for short distances.

#define WINDOW_SIZE ((1 << WINDOW_SIZE_BITS) - 120)

// Minimum number of pixels for which it is cheaper to encode a

// distance + length instead of each pixel as a literal.

#define MIN_LENGTH 4

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

static const uint8_t plane_to_code_lut[128] = {

 96,   73,  55,  39,  23,  13,   5,  1,  255, 255, 255, 255, 255, 255, 255, 255,

 101,  78,  58,  42,  26,  16,   8,  2,    0,   3,  9,   17,  27,  43,  59,  79,

 102,  86,  62,  46,  32,  20,  10,  6,    4,   7,  11,  21,  33,  47,  63,  87,

 105,  90,  70,  52,  37,  28,  18,  14,  12,  15,  19,  29,  38,  53,  71,  91,

 110,  99,  82,  66,  48,  35,  30,  24,  22,  25,  31,  36,  49,  67,  83, 100,

 115, 108,  94,  76,  64,  50,  44,  40,  34,  41,  45,  51,  65,  77,  95, 109,

 118, 113, 103,  92,  80,  68,  60,  56,  54,  57,  61,  69,  81,  93, 104, 114,

 119, 116, 111, 106,  97,  88,  84,  74,  72,  75,  85,  89,  98, 107, 112, 117

};

extern int VP8LDistanceToPlaneCode(int xsize, int dist);

int VP8LDistanceToPlaneCode(int xsize, int dist) {

  const int yoffset = dist / xsize;

  const int xoffset = dist - yoffset * xsize;

  if (xoffset <= 8 && yoffset < 8) {

    return plane_to_code_lut[yoffset * 16 + 8 - xoffset] + 1;

  } else if (xoffset > xsize - 8 && yoffset < 7) {

    return plane_to_code_lut[(yoffset + 1) * 16 + 8 + (xsize - xoffset)] + 1;

  }

  return dist + 120;

}

// Returns the exact index where array1 and array2 are different. For an index

// inferior or equal to best_len_match, the return value just has to be strictly

// inferior to best_len_match. The current behavior is to return 0 if this index

// is best_len_match, and the index itself otherwise.

// If no two elements are the same, it returns max_limit.

static WEBP_INLINE int FindMatchLength(const uint32_t* const array1,

                                       const uint32_t* const array2,

                                       int best_len_match, int max_limit) {

  // Before 'expensive' linear match, check if the two arrays match at the

  // current best length index.

  if (array1[best_len_match] != array2[best_len_match]) return 0;

  return VP8LVectorMismatch(array1, array2, max_limit);

}

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

//  VP8LBackwardRefs

struct PixOrCopyBlock {

  PixOrCopyBlock* next_;   // next block (or NULL)

  PixOrCopy* start_;       // data start

  int size_;               // currently used size

};

extern void VP8LClearBackwardRefs(VP8LBackwardRefs* const refs);

void VP8LClearBackwardRefs(VP8LBackwardRefs* const refs) {

  assert(refs != NULL);

  if (refs->tail_ != NULL) {

    *refs->tail_ = refs->free_blocks_;  // recycle all blocks at once

  }

  refs->free_blocks_ = refs->refs_;

  refs->tail_ = &refs->refs_;

  refs->last_block_ = NULL;

  refs->refs_ = NULL;

}

void VP8LBackwardRefsClear(VP8LBackwardRefs* const refs) {

  assert(refs != NULL);

  VP8LClearBackwardRefs(refs);

  while (refs->free_blocks_ != NULL) {

    PixOrCopyBlock* const next = refs->free_blocks_->next_;

    WebPSafeFree(refs->free_blocks_);

    refs->free_blocks_ = next;

  }

}

void VP8LBackwardRefsInit(VP8LBackwardRefs* const refs, int block_size) {

  assert(refs != NULL);

  memset(refs, 0, sizeof(*refs));

  refs->tail_ = &refs->refs_;

  refs->block_size_ =

      (block_size < MIN_BLOCK_SIZE) ? MIN_BLOCK_SIZE : block_size;

}

VP8LRefsCursor VP8LRefsCursorInit(const VP8LBackwardRefs* const refs) {

  VP8LRefsCursor c;

  c.cur_block_ = refs->refs_;

  if (refs->refs_ != NULL) {

    c.cur_pos = c.cur_block_->start_;

    c.last_pos_ = c.cur_pos + c.cur_block_->size_;

  } else {

    c.cur_pos = NULL;

    c.last_pos_ = NULL;

  }

  return c;

}

void VP8LRefsCursorNextBlock(VP8LRefsCursor* const c) {

  PixOrCopyBlock* const b = c->cur_block_->next_;

  c->cur_pos = (b == NULL) ? NULL : b->start_;

  c->last_pos_ = (b == NULL) ? NULL : b->start_ + b->size_;

  c->cur_block_ = b;

}

// Create a new block, either from the free list or allocated

static PixOrCopyBlock* BackwardRefsNewBlock(VP8LBackwardRefs* const refs) {

  PixOrCopyBlock* b = refs->free_blocks_;

  if (b == NULL) {   // allocate new memory chunk

    const size_t total_size =

        sizeof(*b) + refs->block_size_ * sizeof(*b->start_);

    b = (PixOrCopyBlock*)WebPSafeMalloc(1ULL, total_size);

    if (b == NULL) {

      refs->error_ |= 1;

      return NULL;

    }

    b->start_ = (PixOrCopy*)((uint8_t*)b + sizeof(*b));  // not always aligned

  } else {  // recycle from free-list

    refs->free_blocks_ = b->next_;

  }

  *refs->tail_ = b;

  refs->tail_ = &b->next_;

  refs->last_block_ = b;

  b->next_ = NULL;

  b->size_ = 0;

  return b;

}

extern void VP8LBackwardRefsCursorAdd(VP8LBackwardRefs* const refs,

                                      const PixOrCopy v);

void VP8LBackwardRefsCursorAdd(VP8LBackwardRefs* const refs,

                               const PixOrCopy v) {

  PixOrCopyBlock* b = refs->last_block_;

  if (b == NULL || b->size_ == refs->block_size_) {

    b = BackwardRefsNewBlock(refs);

    if (b == NULL) return;   // refs->error_ is set

  }

  b->start_[b->size_++] = v;

}

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

// Hash chains

int VP8LHashChainInit(VP8LHashChain* const p, int size) {

  assert(p->size_ == 0);

  assert(p->offset_length_ == NULL);

  assert(size > 0);

  p->offset_length_ =

      (uint32_t*)WebPSafeMalloc(size, sizeof(*p->offset_length_));

  if (p->offset_length_ == NULL) return 0;

  p->size_ = size;

  return 1;

}

void VP8LHashChainClear(VP8LHashChain* const p) {

  assert(p != NULL);

  WebPSafeFree(p->offset_length_);

  p->size_ = 0;

  p->offset_length_ = NULL;

}

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

#define HASH_MULTIPLIER_HI (0xc6a4a793ULL)

#define HASH_MULTIPLIER_LO (0x5bd1e996ULL)

static WEBP_INLINE uint32_t GetPixPairHash64(const uint32_t* const argb) {

  uint32_t key;

  key  = (argb[1] * HASH_MULTIPLIER_HI) & 0xffffffffu;

  key += (argb[0] * HASH_MULTIPLIER_LO) & 0xffffffffu;

  key = key >> (32 - HASH_BITS);

  return key;

}

// Returns the maximum number of hash chain lookups to do for a

// given compression quality. Return value in range [8, 86].

static int GetMaxItersForQuality(int quality) {

  return 8 + (quality * quality) / 128;

}

static int GetWindowSizeForHashChain(int quality, int xsize) {

  const int max_window_size = (quality > 75) ? WINDOW_SIZE

                            : (quality > 50) ? (xsize << 8)

                            : (quality > 25) ? (xsize << 6)

                            : (xsize << 4);

  assert(xsize > 0);

  return (max_window_size > WINDOW_SIZE) ? WINDOW_SIZE : max_window_size;

}

static WEBP_INLINE int MaxFindCopyLength(int len) {

  return (len < MAX_LENGTH) ? len : MAX_LENGTH;

}

int VP8LHashChainFill(VP8LHashChain* const p, int quality,

                      const uint32_t* const argb, int xsize, int ysize,

                      int low_effort) {

  const int size = xsize * ysize;

  const int iter_max = GetMaxItersForQuality(quality);

  const uint32_t window_size = GetWindowSizeForHashChain(quality, xsize);

  int pos;

  int argb_comp;

  uint32_t base_position;

  int32_t* hash_to_first_index;

  // Temporarily use the p->offset_length_ as a hash chain.

  int32_t* chain = (int32_t*)p->offset_length_;

  assert(size > 0);

  assert(p->size_ != 0);

  assert(p->offset_length_ != NULL);

  if (size <= 2) {

    p->offset_length_[0] = p->offset_length_[size - 1] = 0;

    return 1;

  }

  hash_to_first_index =

      (int32_t*)WebPSafeMalloc(HASH_SIZE, sizeof(*hash_to_first_index));

  if (hash_to_first_index == NULL) return 0;

  // Set the int32_t array to -1.

  memset(hash_to_first_index, 0xff, HASH_SIZE * sizeof(*hash_to_first_index));

  // Fill the chain linking pixels with the same hash.

  argb_comp = (argb[0] == argb[1]);

  for (pos = 0; pos < size - 2;) {

    uint32_t hash_code;

    const int argb_comp_next = (argb[pos + 1] == argb[pos + 2]);

    if (argb_comp && argb_comp_next) {

      // Consecutive pixels with the same color will share the same hash.

      // We therefore use a different hash: the color and its repetition

      // length.

      uint32_t tmp[2];

      uint32_t len = 1;

      tmp[0] = argb[pos];

      // Figure out how far the pixels are the same.

      // The last pixel has a different 64 bit hash, as its next pixel does

      // not have the same color, so we just need to get to the last pixel equal

      // to its follower.

      while (pos + (int)len + 2 < size && argb[pos + len + 2] == argb[pos]) {

        ++len;

      }

      if (len > MAX_LENGTH) {

        // Skip the pixels that match for distance=1 and length>MAX_LENGTH

        // because they are linked to their predecessor and we automatically

        // check that in the main for loop below. Skipping means setting no

        // predecessor in the chain, hence -1.

        memset(chain + pos, 0xff, (len - MAX_LENGTH) * sizeof(*chain));

        pos += len - MAX_LENGTH;

        len = MAX_LENGTH;

      }

      // Process the rest of the hash chain.

      while (len) {

        tmp[1] = len--;

        hash_code = GetPixPairHash64(tmp);

        chain[pos] = hash_to_first_index[hash_code];

        hash_to_first_index[hash_code] = pos++;

      }

      argb_comp = 0;

    } else {

      // Just move one pixel forward.

      hash_code = GetPixPairHash64(argb + pos);

      chain[pos] = hash_to_first_index[hash_code];

      hash_to_first_index[hash_code] = pos++;

      argb_comp = argb_comp_next;

    }

  }

  // Process the penultimate pixel.

  chain[pos] = hash_to_first_index[GetPixPairHash64(argb + pos)];

  WebPSafeFree(hash_to_first_index);

  // Find the best match interval at each pixel, defined by an offset to the

  // pixel and a length. The right-most pixel cannot match anything to the right

  // (hence a best length of 0) and the left-most pixel nothing to the left

  // (hence an offset of 0).

  assert(size > 2);

  p->offset_length_[0] = p->offset_length_[size - 1] = 0;

  for (base_position = size - 2; base_position > 0;) {

    const int max_len = MaxFindCopyLength(size - 1 - base_position);

    const uint32_t* const argb_start = argb + base_position;

    int iter = iter_max;

    int best_length = 0;

    uint32_t best_distance = 0;

    uint32_t best_argb;

    const int min_pos =

        (base_position > window_size) ? base_position - window_size : 0;

    const int length_max = (max_len < 256) ? max_len : 256;

    uint32_t max_base_position;

    pos = chain[base_position];

    if (!low_effort) {

      int curr_length;

      // Heuristic: use the comparison with the above line as an initialization.

      if (base_position >= (uint32_t)xsize) {

        curr_length = FindMatchLength(argb_start - xsize, argb_start,

                                      best_length, max_len);

        if (curr_length > best_length) {

          best_length = curr_length;

          best_distance = xsize;

        }

        --iter;

      }

      // Heuristic: compare to the previous pixel.

      curr_length =

          FindMatchLength(argb_start - 1, argb_start, best_length, max_len);

      if (curr_length > best_length) {

        best_length = curr_length;

        best_distance = 1;

      }

      --iter;

      // Skip the for loop if we already have the maximum.

      if (best_length == MAX_LENGTH) pos = min_pos - 1;

    }

    best_argb = argb_start[best_length];

    for (; pos >= min_pos && --iter; pos = chain[pos]) {

      int curr_length;

      assert(base_position > (uint32_t)pos);

      if (argb[pos + best_length] != best_argb) continue;

      curr_length = VP8LVectorMismatch(argb + pos, argb_start, max_len);

      if (best_length < curr_length) {

        best_length = curr_length;

        best_distance = base_position - pos;

        best_argb = argb_start[best_length];

        // Stop if we have reached a good enough length.

        if (best_length >= length_max) break;

      }

    }

    // We have the best match but in case the two intervals continue matching

    // to the left, we have the best matches for the left-extended pixels.

    max_base_position = base_position;

    while (1) {

      assert(best_length <= MAX_LENGTH);

      assert(best_distance <= WINDOW_SIZE);

      p->offset_length_[base_position] =

          (best_distance << MAX_LENGTH_BITS) | (uint32_t)best_length;

      --base_position;

      // Stop if we don't have a match or if we are out of bounds.

      if (best_distance == 0 || base_position == 0) break;

      // Stop if we cannot extend the matching intervals to the left.

      if (base_position < best_distance ||

          argb[base_position - best_distance] != argb[base_position]) {

        break;

      }

      // Stop if we are matching at its limit because there could be a closer

      // matching interval with the same maximum length. Then again, if the

      // matching interval is as close as possible (best_distance == 1), we will

      // never find anything better so let's continue.

      if (best_length == MAX_LENGTH && best_distance != 1 &&

          base_position + MAX_LENGTH < max_base_position) {

        break;

      }

      if (best_length < MAX_LENGTH) {

        ++best_length;

        max_base_position = base_position;

      }

    }

  }

  return 1;

}

static WEBP_INLINE void AddSingleLiteral(uint32_t pixel, int use_color_cache,

                                         VP8LColorCache* const hashers,

                                         VP8LBackwardRefs* const refs) {

  PixOrCopy v;

  if (use_color_cache) {

    const uint32_t key = VP8LColorCacheGetIndex(hashers, pixel);

    if (VP8LColorCacheLookup(hashers, key) == pixel) {

      v = PixOrCopyCreateCacheIdx(key);

    } else {

      v = PixOrCopyCreateLiteral(pixel);

      VP8LColorCacheSet(hashers, key, pixel);

    }

  } else {

    v = PixOrCopyCreateLiteral(pixel);

  }

  VP8LBackwardRefsCursorAdd(refs, v);

}

static int BackwardReferencesRle(int xsize, int ysize,

                                 const uint32_t* const argb,

                                 int cache_bits, VP8LBackwardRefs* const refs) {

  const int pix_count = xsize * ysize;

  int i, k;

  const int use_color_cache = (cache_bits > 0);

  VP8LColorCache hashers;

  if (use_color_cache && !VP8LColorCacheInit(&hashers, cache_bits)) {

    return 0;

  }

  VP8LClearBackwardRefs(refs);

  // Add first pixel as literal.

  AddSingleLiteral(argb[0], use_color_cache, &hashers, refs);

  i = 1;

  while (i < pix_count) {

    const int max_len = MaxFindCopyLength(pix_count - i);

    const int rle_len = FindMatchLength(argb + i, argb + i - 1, 0, max_len);

    const int prev_row_len = (i < xsize) ? 0 :

        FindMatchLength(argb + i, argb + i - xsize, 0, max_len);

    if (rle_len >= prev_row_len && rle_len >= MIN_LENGTH) {

      VP8LBackwardRefsCursorAdd(refs, PixOrCopyCreateCopy(1, rle_len));

      // We don't need to update the color cache here since it is always the

      // same pixel being copied, and that does not change the color cache

      // state.

      i += rle_len;

    } else if (prev_row_len >= MIN_LENGTH) {

      VP8LBackwardRefsCursorAdd(refs, PixOrCopyCreateCopy(xsize, prev_row_len));

      if (use_color_cache) {

        for (k = 0; k < prev_row_len; ++k) {

          VP8LColorCacheInsert(&hashers, argb[i + k]);

        }

      }

      i += prev_row_len;

    } else {

      AddSingleLiteral(argb[i], use_color_cache, &hashers, refs);

      i++;

    }

  }

  if (use_color_cache) VP8LColorCacheClear(&hashers);

  return !refs->error_;

}

static int BackwardReferencesLz77(int xsize, int ysize,

                                  const uint32_t* const argb, int cache_bits,

                                  const VP8LHashChain* const hash_chain,

                                  VP8LBackwardRefs* const refs) {

  int i;

  int i_last_check = -1;

  int ok = 0;

  int cc_init = 0;

  const int use_color_cache = (cache_bits > 0);

  const int pix_count = xsize * ysize;

  VP8LColorCache hashers;

  if (use_color_cache) {

    cc_init = VP8LColorCacheInit(&hashers, cache_bits);

    if (!cc_init) goto Error;

  }

  VP8LClearBackwardRefs(refs);

  for (i = 0; i < pix_count;) {

    // Alternative#1: Code the pixels starting at 'i' using backward reference.

    int offset = 0;

    int len = 0;

    int j;

    VP8LHashChainFindCopy(hash_chain, i, &offset, &len;;

    if (len >= MIN_LENGTH) {

      const int len_ini = len;

      int max_reach = 0;

      const int j_max =

          (i + len_ini >= pix_count) ? pix_count - 1 : i + len_ini;

      // Only start from what we have not checked already.

      i_last_check = (i > i_last_check) ? i : i_last_check;

      // We know the best match for the current pixel but we try to find the

      // best matches for the current pixel AND the next one combined.

      // The naive method would use the intervals:

      // [i,i+len) + [i+len, length of best match at i+len)

      // while we check if we can use:

      // [i,j) (where j<=i+len) + [j, length of best match at j)

      for (j = i_last_check + 1; j <= j_max; ++j) {

        const int len_j = VP8LHashChainFindLength(hash_chain, j);

        const int reach =

            j + (len_j >= MIN_LENGTH ? len_j : 1);  // 1 for single literal.

        if (reach > max_reach) {

          len = j - i;

          max_reach = reach;

          if (max_reach >= pix_count) break;

        }

      }

    } else {

      len = 1;

    }

    // Go with literal or backward reference.

    assert(len > 0);

    if (len == 1) {

      AddSingleLiteral(argb[i], use_color_cache, &hashers, refs);

    } else {

      VP8LBackwardRefsCursorAdd(refs, PixOrCopyCreateCopy(offset, len));

      if (use_color_cache) {

        for (j = i; j < i + len; ++j) VP8LColorCacheInsert(&hashers, argb[j]);

      }

    }

    i += len;

  }

  ok = !refs->error_;

 Error:

  if (cc_init) VP8LColorCacheClear(&hashers);

  return ok;

}

// Compute an LZ77 by forcing matches to happen within a given distance cost.

// We therefore limit the algorithm to the lowest 32 values in the PlaneCode

// definition.

#define WINDOW_OFFSETS_SIZE_MAX 32

static int BackwardReferencesLz77Box(int xsize, int ysize,

                                     const uint32_t* const argb, int cache_bits,

                                     const VP8LHashChain* const hash_chain_best,

                                     VP8LHashChain* hash_chain,

                                     VP8LBackwardRefs* const refs) {

  int i;

  const int pix_count = xsize * ysize;

  uint16_t* counts;

  int window_offsets[WINDOW_OFFSETS_SIZE_MAX] = {0};

  int window_offsets_new[WINDOW_OFFSETS_SIZE_MAX] = {0};

  int window_offsets_size = 0;

  int window_offsets_new_size = 0;

  uint16_t* const counts_ini =

      (uint16_t*)WebPSafeMalloc(xsize * ysize, sizeof(*counts_ini));

  int best_offset_prev = -1, best_length_prev = -1;

  if (counts_ini == NULL) return 0;

  // counts[i] counts how many times a pixel is repeated starting at position i.

  i = pix_count - 2;

  counts = counts_ini + i;

  counts[1] = 1;

  for (; i >= 0; --i, --counts) {

    if (argb[i] == argb[i + 1]) {

      // Max out the counts to MAX_LENGTH.

      counts[0] = counts[1] + (counts[1] != MAX_LENGTH);

    } else {

      counts[0] = 1;

    }

  }

  // Figure out the window offsets around a pixel. They are stored in a

  // spiraling order around the pixel as defined by VP8LDistanceToPlaneCode.

  {

    int x, y;

    for (y = 0; y <= 6; ++y) {

      for (x = -6; x <= 6; ++x) {

        const int offset = y * xsize + x;

        int plane_code;

        // Ignore offsets that bring us after the pixel.

        if (offset <= 0) continue;

        plane_code = VP8LDistanceToPlaneCode(xsize, offset) - 1;

        if (plane_code >= WINDOW_OFFSETS_SIZE_MAX) continue;

        window_offsets[plane_code] = offset;

      }

    }

    // For narrow images, not all plane codes are reached, so remove those.

    for (i = 0; i < WINDOW_OFFSETS_SIZE_MAX; ++i) {

      if (window_offsets[i] == 0) continue;

      window_offsets[window_offsets_size++] = window_offsets[i];

    }

    // Given a pixel P, find the offsets that reach pixels unreachable from P-1

    // with any of the offsets in window_offsets[].

    for (i = 0; i < window_offsets_size; ++i) {

      int j;

      int is_reachable = 0;

      for (j = 0; j < window_offsets_size && !is_reachable; ++j) {

        is_reachable |= (window_offsets[i] == window_offsets[j] + 1);

      }

      if (!is_reachable) {

        window_offsets_new[window_offsets_new_size] = window_offsets[i];

        ++window_offsets_new_size;

      }

    }

  }

  hash_chain->offset_length_[0] = 0;

  for (i = 1; i < pix_count; ++i) {

    int ind;

    int best_length = VP8LHashChainFindLength(hash_chain_best, i);

    int best_offset;

    int do_compute = 1;

    if (best_length >= MAX_LENGTH) {

      // Do not recompute the best match if we already have a maximal one in the

      // window.

      best_offset = VP8LHashChainFindOffset(hash_chain_best, i);

      for (ind = 0; ind < window_offsets_size; ++ind) {

        if (best_offset == window_offsets[ind]) {

          do_compute = 0;

          break;

        }

      }

    }

    if (do_compute) {

      // Figure out if we should use the offset/length from the previous pixel

      // as an initial guess and therefore only inspect the offsets in

      // window_offsets_new[].

      const int use_prev =

          (best_length_prev > 1) && (best_length_prev < MAX_LENGTH);

      const int num_ind =

          use_prev ? window_offsets_new_size : window_offsets_size;

      best_length = use_prev ? best_length_prev - 1 : 0;

      best_offset = use_prev ? best_offset_prev : 0;

      // Find the longest match in a window around the pixel.

      for (ind = 0; ind < num_ind; ++ind) {

        int curr_length = 0;

        int j = i;

        int j_offset =

            use_prev ? i - window_offsets_new[ind] : i - window_offsets[ind];

        if (j_offset < 0 || argb[j_offset] != argb[i]) continue;

        // The longest match is the sum of how many times each pixel is

        // repeated.

        do {

          const int counts_j_offset = counts_ini[j_offset];

          const int counts_j = counts_ini[j];

          if (counts_j_offset != counts_j) {

            curr_length +=

                (counts_j_offset < counts_j) ? counts_j_offset : counts_j;

            break;

          }

          // The same color is repeated counts_pos times at j_offset and j.

          curr_length += counts_j_offset;

          j_offset += counts_j_offset;

          j += counts_j_offset;

        } while (curr_length <= MAX_LENGTH && j < pix_count &&

                 argb[j_offset] == argb[j]);

        if (best_length < curr_length) {

          best_offset =

              use_prev ? window_offsets_new[ind] : window_offsets[ind];

          if (curr_length >= MAX_LENGTH) {

            best_length = MAX_LENGTH;

            break;

          } else {

            best_length = curr_length;

          }

        }

      }

    }

    assert(i + best_length <= pix_count);

    assert(best_length <= MAX_LENGTH);

    if (best_length <= MIN_LENGTH) {

      hash_chain->offset_length_[i] = 0;

      best_offset_prev = 0;

      best_length_prev = 0;

    } else {

      hash_chain->offset_length_[i] =

          (best_offset << MAX_LENGTH_BITS) | (uint32_t)best_length;

      best_offset_prev = best_offset;

      best_length_prev = best_length;

    }

  }

  hash_chain->offset_length_[0] = 0;

  WebPSafeFree(counts_ini);

  return BackwardReferencesLz77(xsize, ysize, argb, cache_bits, hash_chain,

                                refs);

}

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

static void BackwardReferences2DLocality(int xsize,

                                         const VP8LBackwardRefs* const refs) {

  VP8LRefsCursor c = VP8LRefsCursorInit(refs);

  while (VP8LRefsCursorOk(&c)) {

    if (PixOrCopyIsCopy(c.cur_pos)) {

      const int dist = c.cur_pos->argb_or_distance;

      const int transformed_dist = VP8LDistanceToPlaneCode(xsize, dist);

      c.cur_pos->argb_or_distance = transformed_dist;

    }

    VP8LRefsCursorNext(&c);

  }

}

// Evaluate optimal cache bits for the local color cache.

// The input *best_cache_bits sets the maximum cache bits to use (passing 0

// implies disabling the local color cache). The local color cache is also

// disabled for the lower (<= 25) quality.

// Returns 0 in case of memory error.

static int CalculateBestCacheSize(const uint32_t* argb, int quality,

                                  const VP8LBackwardRefs* const refs,

                                  int* const best_cache_bits) {

  int i;

  const int cache_bits_max = (quality <= 25) ? 0 : *best_cache_bits;

  double entropy_min = MAX_ENTROPY;

  int cc_init[MAX_COLOR_CACHE_BITS + 1] = { 0 };

  VP8LColorCache hashers[MAX_COLOR_CACHE_BITS + 1];

  VP8LRefsCursor c = VP8LRefsCursorInit(refs);

  VP8LHistogram* histos[MAX_COLOR_CACHE_BITS + 1] = { NULL };

  int ok = 0;

  assert(cache_bits_max >= 0 && cache_bits_max <= MAX_COLOR_CACHE_BITS);

  if (cache_bits_max == 0) {

    *best_cache_bits = 0;

    // Local color cache is disabled.

    return 1;

  }

  // Allocate data.

  for (i = 0; i <= cache_bits_max; ++i) {

    histos[i] = VP8LAllocateHistogram(i);

    if (histos[i] == NULL) goto Error;

    if (i == 0) continue;

    cc_init[i] = VP8LColorCacheInit(&hashers[i], i);

    if (!cc_init[i]) goto Error;

  }

  // Find the cache_bits giving the lowest entropy. The search is done in a

  // brute-force way as the function (entropy w.r.t cache_bits) can be

  // anything in practice.

  while (VP8LRefsCursorOk(&c)) {

    const PixOrCopy* const v = c.cur_pos;

    if (PixOrCopyIsLiteral(v)) {

      const uint32_t pix = *argb++;

      const uint32_t a = (pix >> 24) & 0xff;

      const uint32_t r = (pix >> 16) & 0xff;

      const uint32_t g = (pix >>  8) & 0xff;

      const uint32_t b = (pix >>  0) & 0xff;

      // The keys of the caches can be derived from the longest one.

      int key = VP8LHashPix(pix, 32 - cache_bits_max);

      // Do not use the color cache for cache_bits = 0.

      ++histos[0]->blue_[b];

      ++histos[0]->literal_[g];

      ++histos[0]->red_[r];

      ++histos[0]->alpha_[a];

      // Deal with cache_bits > 0.

      for (i = cache_bits_max; i >= 1; --i, key >>= 1) {

        if (VP8LColorCacheLookup(&hashers[i], key) == pix) {

          ++histos[i]->literal_[NUM_LITERAL_CODES + NUM_LENGTH_CODES + key];

        } else {

          VP8LColorCacheSet(&hashers[i], key, pix);

          ++histos[i]->blue_[b];

          ++histos[i]->literal_[g];

          ++histos[i]->red_[r];

          ++histos[i]->alpha_[a];

        }

      }

    } else {

      // We should compute the contribution of the (distance,length)

      // histograms but those are the same independently from the cache size.

      // As those constant contributions are in the end added to the other

      // histogram contributions, we can safely ignore them.

      int len = PixOrCopyLength(v);

      uint32_t argb_prev = *argb ^ 0xffffffffu;

      // Update the color caches.

      do {

        if (*argb != argb_prev) {

          // Efficiency: insert only if the color changes.

          int key = VP8LHashPix(*argb, 32 - cache_bits_max);

          for (i = cache_bits_max; i >= 1; --i, key >>= 1) {

            hashers[i].colors_[key] = *argb;

          }

          argb_prev = *argb;

        }

        argb++;

      } while (--len != 0);

    }

    VP8LRefsCursorNext(&c);

  }

  for (i = 0; i <= cache_bits_max; ++i) {

    const double entropy = VP8LHistogramEstimateBits(histos[i]);

    if (i == 0 || entropy < entropy_min) {

      entropy_min = entropy;

      *best_cache_bits = i;

    }

  }

  ok = 1;

Error:

  for (i = 0; i <= cache_bits_max; ++i) {

    if (cc_init[i]) VP8LColorCacheClear(&hashers[i]);

    VP8LFreeHistogram(histos[i]);

  }

  return ok;

}

// Update (in-place) backward references for specified cache_bits.

static int BackwardRefsWithLocalCache(const uint32_t* const argb,

                                      int cache_bits,

                                      VP8LBackwardRefs* const refs) {

  int pixel_index = 0;

  VP8LColorCache hashers;

  VP8LRefsCursor c = VP8LRefsCursorInit(refs);

  if (!VP8LColorCacheInit(&hashers, cache_bits)) return 0;

  while (VP8LRefsCursorOk(&c)) {

    PixOrCopy* const v = c.cur_pos;

    if (PixOrCopyIsLiteral(v)) {

      const uint32_t argb_literal = v->argb_or_distance;

      const int ix = VP8LColorCacheContains(&hashers, argb_literal);

      if (ix >= 0) {

        // hashers contains argb_literal

        *v = PixOrCopyCreateCacheIdx(ix);

      } else {

        VP8LColorCacheInsert(&hashers, argb_literal);

      }

      ++pixel_index;

    } else {

      // refs was created without local cache, so it can not have cache indexes.

      int k;

      assert(PixOrCopyIsCopy(v));

      for (k = 0; k < v->len; ++k) {

        VP8LColorCacheInsert(&hashers, argb[pixel_index++]);

      }

    }

    VP8LRefsCursorNext(&c);

  }

  VP8LColorCacheClear(&hashers);

  return 1;

}

static VP8LBackwardRefs* GetBackwardReferencesLowEffort(

    int width, int height, const uint32_t* const argb,

    int* const cache_bits, const VP8LHashChain* const hash_chain,

    VP8LBackwardRefs* const refs_lz77) {

  *cache_bits = 0;

  if (!BackwardReferencesLz77(width, height, argb, 0, hash_chain, refs_lz77)) {

    return NULL;

  }

  BackwardReferences2DLocality(width, refs_lz77);

  return refs_lz77;

}

extern int VP8LBackwardReferencesTraceBackwards(

    int xsize, int ysize, const uint32_t* const argb, int cache_bits,

    const VP8LHashChain* const hash_chain,

    const VP8LBackwardRefs* const refs_src, VP8LBackwardRefs* const refs_dst);

static VP8LBackwardRefs* GetBackwardReferences(

    int width, int height, const uint32_t* const argb, int quality,

    int lz77_types_to_try, int* const cache_bits,

    const VP8LHashChain* const hash_chain, VP8LBackwardRefs* best,

    VP8LBackwardRefs* worst) {

  const int cache_bits_initial = *cache_bits;

  double bit_cost_best = -1;

  VP8LHistogram* histo = NULL;

  int lz77_type, lz77_type_best = 0;

  VP8LHashChain hash_chain_box;

  memset(&hash_chain_box, 0, sizeof(hash_chain_box));

  histo = VP8LAllocateHistogram(MAX_COLOR_CACHE_BITS);

  if (histo == NULL) goto Error;

  for (lz77_type = 1; lz77_types_to_try;

       lz77_types_to_try &= ~lz77_type, lz77_type <<= 1) {

    int res = 0;

    double bit_cost;

    int cache_bits_tmp = cache_bits_initial;

    if ((lz77_types_to_try & lz77_type) == 0) continue;

    switch (lz77_type) {

      case kLZ77RLE:

        res = BackwardReferencesRle(width, height, argb, 0, worst);

        break;

      case kLZ77Standard:

        // Compute LZ77 with no cache (0 bits), as the ideal LZ77 with a color

        // cache is not that different in practice.

        res = BackwardReferencesLz77(width, height, argb, 0, hash_chain, worst);

        break;

      case kLZ77Box:

        if (!VP8LHashChainInit(&hash_chain_box, width * height)) goto Error;

        res = BackwardReferencesLz77Box(width, height, argb, 0, hash_chain,

                                        &hash_chain_box, worst);

        break;

      default:

        assert(0);

    }