brotli/enc/hash.h

// Copyright 2010 Google Inc. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// A (forgetful) hash table to the data seen by the compressor, to
// help create backward references to previous data.

#ifndef BROTLI_ENC_HASH_H_
#define BROTLI_ENC_HASH_H_

#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <sys/types.h>
#include <algorithm>
#include <cstdlib>
#include <memory>
#include <string>

#include "./dictionary_hash.h"
#include "./fast_log.h"
#include "./find_match_length.h"
#include "./port.h"
#include "./prefix.h"
#include "./static_dict.h"
#include "./transform.h"

namespace brotli {

static const int kDistanceCacheIndex[] = {
  0, 1, 2, 3, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1,
};
static const int kDistanceCacheOffset[] = {
  0, 0, 0, 0, -1, 1, -2, 2, -3, 3, -1, 1, -2, 2, -3, 3
};

// kHashMul32 multiplier has these properties:
// * The multiplier must be odd. Otherwise we may lose the highest bit.
// * No long streaks of 1s or 0s.
// * There is no effort to ensure that it is a prime, the oddity is enough
//   for this use.
// * The number has been tuned heuristically against compression benchmarks.
static const uint32_t kHashMul32 = 0x1e35a7bd;

template<int kShiftBits, int kMinLength>
inline uint32_t Hash(const uint8_t *data) {
  if (kMinLength <= 3) {
    // If kMinLength is 2 or 3, we hash the first 3 bytes of data.
    uint32_t h = (BROTLI_UNALIGNED_LOAD32(data) & 0xffffff) * kHashMul32;
    // The higher bits contain more mixture from the multiplication,
    // so we take our results from there.
    return h >> (32 - kShiftBits);
  } else {
    // If kMinLength is at least 4, we hash the first 4 bytes of data.
    uint32_t h = BROTLI_UNALIGNED_LOAD32(data) * kHashMul32;
    // The higher bits contain more mixture from the multiplication,
    // so we take our results from there.
    return h >> (32 - kShiftBits);
  }
}

// Usually, we always choose the longest backward reference. This function
// allows for the exception of that rule.
//
// If we choose a backward reference that is further away, it will
// usually be coded with more bits. We approximate this by assuming
// log2(distance). If the distance can be expressed in terms of the
// last four distances, we use some heuristic constants to estimate
// the bits cost. For the first up to four literals we use the bit
// cost of the literals from the literal cost model, after that we
// use the average bit cost of the cost model.
//
// This function is used to sometimes discard a longer backward reference
// when it is not much longer and the bit cost for encoding it is more
// than the saved literals.
inline double BackwardReferenceScore(double average_cost,
                                     int copy_length,
                                     int backward_reference_offset) {
  return (copy_length * average_cost -
          1.20 * Log2Floor(backward_reference_offset));
}

inline double BackwardReferenceScoreUsingLastDistance(double average_cost,
                                                      int copy_length,
                                                      int distance_short_code) {
  static const double kDistanceShortCodeBitCost[16] = {
    -0.6, 0.95, 1.17, 1.27,
    0.93, 0.93, 0.96, 0.96, 0.99, 0.99,
    1.05, 1.05, 1.15, 1.15, 1.25, 1.25
  };
  return (average_cost * copy_length
          - kDistanceShortCodeBitCost[distance_short_code]);
}

// A (forgetful) hash table to the data seen by the compressor, to
// help create backward references to previous data.
//
// This is a hash map of fixed size (kBucketSize). Starting from the
// given index, kBucketSweep buckets are used to store values of a key.
template <int kBucketBits, int kBucketSweep>
class HashLongestMatchQuickly {
 public:
  HashLongestMatchQuickly() {
    Reset();
  }
  void Reset() {
    // It is not strictly necessary to fill this buffer here, but
    // not filling will make the results of the compression stochastic
    // (but correct). This is because random data would cause the
    // system to find accidentally good backward references here and there.
    std::fill(&buckets_[0],
              &buckets_[sizeof(buckets_) / sizeof(buckets_[0])],
              0);
    num_dict_lookups_ = 0;
    num_dict_matches_ = 0;
  }
  // Look at 4 bytes at data.
  // Compute a hash from these, and store the value somewhere within
  // [ix .. ix+3].
  inline void Store(const uint8_t *data, const int ix) {
    const uint32_t key = Hash<kBucketBits, 4>(data);
    // Wiggle the value with the bucket sweep range.
    const uint32_t off = (static_cast<uint32_t>(ix) >> 3) % kBucketSweep;
    buckets_[key + off] = ix;
  }

  // Store hashes for a range of data.
  void StoreHashes(const uint8_t *data, size_t len, int startix, int mask) {
    for (int p = 0; p < len; ++p) {
      Store(&data[p & mask], startix + p);
    }
  }

  bool HasStaticDictionary() const { return false; }

  // Find a longest backward match of &ring_buffer[cur_ix & ring_buffer_mask]
  // up to the length of max_length.
  //
  // Does not look for matches longer than max_length.
  // Does not look for matches further away than max_backward.
  // Writes the best found match length into best_len_out.
  // Writes the index (&data[index]) of the start of the best match into
  // best_distance_out.
  inline bool FindLongestMatch(const uint8_t * __restrict ring_buffer,
                               const size_t ring_buffer_mask,
                               const float* __restrict literal_cost,
                               const size_t literal_cost_mask,
                               const double average_cost,
                               const int* __restrict distance_cache,
                               const uint32_t cur_ix,
                               const uint32_t max_length,
                               const uint32_t max_backward,
                               int * __restrict best_len_out,
                               int * __restrict best_len_code_out,
                               int * __restrict best_distance_out,
                               double* __restrict best_score_out) {
    const int best_len_in = *best_len_out;
    const int cur_ix_masked = cur_ix & ring_buffer_mask;
    int compare_char = ring_buffer[cur_ix_masked + best_len_in];
    double best_score = *best_score_out;
    int best_len = best_len_in;
    int backward = distance_cache[0];
    size_t prev_ix = cur_ix - backward;
    bool match_found = false;
    if (prev_ix < cur_ix) {
      prev_ix &= ring_buffer_mask;
      if (compare_char == ring_buffer[prev_ix + best_len]) {
        int len = FindMatchLengthWithLimit(&ring_buffer[prev_ix],
                                           &ring_buffer[cur_ix_masked],
                                           max_length);
        if (len >= 4) {
          best_score = BackwardReferenceScoreUsingLastDistance(average_cost,
                                                               len, 0);
          best_len = len;
          *best_len_out = len;
          *best_len_code_out = len;
          *best_distance_out = backward;
          *best_score_out = best_score;
          compare_char = ring_buffer[cur_ix_masked + best_len];
          if (kBucketSweep == 1) {
            return true;
          } else {
            match_found = true;
          }
        }
      }
    }
    const uint32_t key = Hash<kBucketBits, 4>(&ring_buffer[cur_ix_masked]);
    if (kBucketSweep == 1) {
      // Only one to look for, don't bother to prepare for a loop.
      prev_ix = buckets_[key];
      backward = cur_ix - prev_ix;
      prev_ix &= ring_buffer_mask;
      if (compare_char != ring_buffer[prev_ix + best_len_in]) {
        return false;
      }
      if (PREDICT_FALSE(backward == 0 || backward > max_backward)) {
        return false;
      }
      const int len = FindMatchLengthWithLimit(&ring_buffer[prev_ix],
                                               &ring_buffer[cur_ix_masked],
                                               max_length);
      if (len >= 4) {
        *best_len_out = len;
        *best_len_code_out = len;
        *best_distance_out = backward;
        *best_score_out = BackwardReferenceScore(average_cost, len, backward);
        return true;
      } else {
        return false;
      }
    } else {
      uint32_t *bucket = buckets_ + key;
      prev_ix = *bucket++;
      for (int i = 0; i < kBucketSweep; ++i, prev_ix = *bucket++) {
        const int backward = cur_ix - prev_ix;
        prev_ix &= ring_buffer_mask;
        if (compare_char != ring_buffer[prev_ix + best_len]) {
          continue;
        }
        if (PREDICT_FALSE(backward == 0 || backward > max_backward)) {
          continue;
        }
        const int len =
            FindMatchLengthWithLimit(&ring_buffer[prev_ix],
                                     &ring_buffer[cur_ix_masked],
                                     max_length);
        if (len >= 4) {
          const double score = BackwardReferenceScore(average_cost,
                                                      len, backward);
          if (best_score < score) {
            best_score = score;
            best_len = len;
            *best_len_out = best_len;
            *best_len_code_out = best_len;
            *best_distance_out = backward;
            *best_score_out = score;
            compare_char = ring_buffer[cur_ix_masked + best_len];
            match_found = true;
          }
        }
      }
      if (!match_found && num_dict_matches_ >= (num_dict_lookups_ >> 7)) {
        ++num_dict_lookups_;
        const uint32_t key = Hash<14, 4>(&ring_buffer[cur_ix_masked]) << 1;
        const uint16_t v = kStaticDictionaryHash[key];
        if (v > 0) {
          const int len = v & 31;
          const int dist = v >> 5;
          const int offset = kBrotliDictionaryOffsetsByLength[len] + len * dist;
          if (len <= max_length) {
            const int matchlen =
                FindMatchLengthWithLimit(&ring_buffer[cur_ix_masked],
                                         &kBrotliDictionary[offset], len);
            if (matchlen == len) {
              const size_t backward = max_backward + dist + 1;
              const double score = BackwardReferenceScore(average_cost,
                                                          len, backward);
              if (best_score < score) {
                ++num_dict_matches_;
                best_score = score;
                best_len = len;
                *best_len_out = best_len;
                *best_len_code_out = best_len;
                *best_distance_out = backward;
                *best_score_out = best_score;
                return true;
              }
            }
          }
        }
      }
      return match_found;
    }
  }

 private:
  static const uint32_t kBucketSize = 1 << kBucketBits;
  uint32_t buckets_[kBucketSize + kBucketSweep];
  size_t num_dict_lookups_;
  size_t num_dict_matches_;
};

// A (forgetful) hash table to the data seen by the compressor, to
// help create backward references to previous data.
//
// This is a hash map of fixed size (kBucketSize) to a ring buffer of
// fixed size (kBlockSize). The ring buffer contains the last kBlockSize
// index positions of the given hash key in the compressed data.
template <int kBucketBits,
          int kBlockBits,
          int kMinLength,
          int kNumLastDistancesToCheck,
          bool kUseCostModel,
          bool kUseDictionary>
class HashLongestMatch {
 public:
  HashLongestMatch() : static_dict_(NULL) {
    Reset();
  }
  void Reset() {
    std::fill(&num_[0], &num_[sizeof(num_) / sizeof(num_[0])], 0);
    num_dict_lookups_ = 0;
    num_dict_matches_ = 0;
  }
  void SetStaticDictionary(const StaticDictionary *dict) {
    static_dict_ = dict;
  }
  bool HasStaticDictionary() const {
    return static_dict_ != NULL;
  }

  // Look at 3 bytes at data.
  // Compute a hash from these, and store the value of ix at that position.
  inline void Store(const uint8_t *data, const int ix) {
    const uint32_t key = Hash<kBucketBits, kMinLength>(data);
    const int minor_ix = num_[key] & kBlockMask;
    buckets_[key][minor_ix] = ix;
    ++num_[key];
  }

  // Store hashes for a range of data.
  void StoreHashes(const uint8_t *data, size_t len, int startix, int mask) {
    for (int p = 0; p < len; ++p) {
      Store(&data[p & mask], startix + p);
    }
  }

  // Find a longest backward match of &data[cur_ix] up to the length of
  // max_length.
  //
  // Does not look for matches longer than max_length.
  // Does not look for matches further away than max_backward.
  // Writes the best found match length into best_len_out.
  // Writes the index (&data[index]) offset from the start of the best match
  // into best_distance_out.
  // Write the score of the best match into best_score_out.
  bool FindLongestMatch(const uint8_t * __restrict data,
                        const size_t ring_buffer_mask,
                        const float * __restrict literal_cost,
                        const size_t literal_cost_mask,
                        const double average_cost,
                        const int* __restrict distance_cache,
                        const uint32_t cur_ix,
                        uint32_t max_length,
                        const uint32_t max_backward,
                        int * __restrict best_len_out,
                        int * __restrict best_len_code_out,
                        int * __restrict best_distance_out,
                        double * __restrict best_score_out) {
    *best_len_code_out = 0;
    const size_t cur_ix_masked = cur_ix & ring_buffer_mask;
    double start_cost_diff4 = 0.0;
    double start_cost_diff3 = 0.0;
    double start_cost_diff2 = 0.0;
    if (kUseCostModel) {
      start_cost_diff4 = literal_cost == NULL ? 0 :
          literal_cost[cur_ix & literal_cost_mask] +
          literal_cost[(cur_ix + 1) & literal_cost_mask] +
          literal_cost[(cur_ix + 2) & literal_cost_mask] +
          literal_cost[(cur_ix + 3) & literal_cost_mask] -
          4 * average_cost;
      start_cost_diff3 = literal_cost == NULL ? 0 :
          literal_cost[cur_ix & literal_cost_mask] +
          literal_cost[(cur_ix + 1) & literal_cost_mask] +
          literal_cost[(cur_ix + 2) & literal_cost_mask] -
          3 * average_cost + 0.3;
      start_cost_diff2 = literal_cost == NULL ? 0 :
          literal_cost[cur_ix & literal_cost_mask] +
          literal_cost[(cur_ix + 1) & literal_cost_mask] -
          2 * average_cost + 1.2;
    }
    bool match_found = false;
    // Don't accept a short copy from far away.
    double best_score = *best_score_out;
    int best_len = *best_len_out;
    *best_len_out = 0;
    // Try last distance first.
    for (int i = 0; i < kNumLastDistancesToCheck; ++i) {
      const int idx = kDistanceCacheIndex[i];
      const int backward = distance_cache[idx] + kDistanceCacheOffset[i];
      size_t prev_ix = cur_ix - backward;
      if (prev_ix >= cur_ix) {
        continue;
      }
      if (PREDICT_FALSE(backward > max_backward)) {
        continue;
      }
      prev_ix &= ring_buffer_mask;

      if (cur_ix_masked + best_len > ring_buffer_mask ||
          prev_ix + best_len > ring_buffer_mask ||
          data[cur_ix_masked + best_len] != data[prev_ix + best_len]) {
        continue;
      }
      const size_t len =
          FindMatchLengthWithLimit(&data[prev_ix], &data[cur_ix_masked],
                                   max_length);
      if (len >= std::max(kMinLength, 3) ||
          (kMinLength == 2 && len == 2 && i < 2)) {
        // Comparing for >= 2 does not change the semantics, but just saves for
        // a few unnecessary binary logarithms in backward reference score,
        // since we are not interested in such short matches.
        double score = BackwardReferenceScoreUsingLastDistance(
            average_cost, len, i);
        if (kUseCostModel) {
          switch (len) {
            case 2: score += start_cost_diff2; break;
            case 3: score += start_cost_diff3; break;
            default: score += start_cost_diff4;
          }
        }
        if (best_score < score) {
          best_score = score;
          best_len = len;
          *best_len_out = best_len;
          *best_len_code_out = best_len;
          *best_distance_out = backward;
          *best_score_out = best_score;
          match_found = true;
        }
      }
    }
    if (kMinLength == 2) {
      int stop = int(cur_ix) - 64;
      if (stop < 0) { stop = 0; }
      start_cost_diff2 -= 1.0;
      for (int i = cur_ix - 1; i > stop; --i) {
        size_t prev_ix = i;
        const size_t backward = cur_ix - prev_ix;
        if (PREDICT_FALSE(backward > max_backward)) {
          break;
        }
        prev_ix &= ring_buffer_mask;
        if (data[cur_ix_masked] != data[prev_ix] ||
            data[cur_ix_masked + 1] != data[prev_ix + 1]) {
          continue;
        }
        int len = 2;
        const double score =
            average_cost * 2 - 2.3 * Log2Floor(backward) + start_cost_diff2;

        if (best_score < score) {
          best_score = score;
          best_len = len;
          *best_len_out = best_len;
          *best_len_code_out = best_len;
          *best_distance_out = backward;
          match_found = true;
        }
      }
    }
    const uint32_t key = Hash<kBucketBits, kMinLength>(&data[cur_ix_masked]);
    const int * __restrict const bucket = &buckets_[key][0];
    const int down = (num_[key] > kBlockSize) ? (num_[key] - kBlockSize) : 0;
    for (int i = num_[key] - 1; i >= down; --i) {
      int prev_ix = bucket[i & kBlockMask];
      if (prev_ix >= 0) {
        const size_t backward = cur_ix - prev_ix;
        if (PREDICT_FALSE(backward > max_backward)) {
          break;
        }
        prev_ix &= ring_buffer_mask;
        if (cur_ix_masked + best_len > ring_buffer_mask ||
            prev_ix + best_len > ring_buffer_mask ||
            data[cur_ix_masked + best_len] != data[prev_ix + best_len]) {
          continue;
        }
        const size_t len =
            FindMatchLengthWithLimit(&data[prev_ix], &data[cur_ix_masked],
                                     max_length);
        if (len >= std::max(kMinLength, 3)) {
          // Comparing for >= 3 does not change the semantics, but just saves
          // for a few unnecessary binary logarithms in backward reference
          // score, since we are not interested in such short matches.
          double score = BackwardReferenceScore(average_cost,
                                                len, backward);
          if (kUseCostModel) {
            score += (len >= 4) ? start_cost_diff4 : start_cost_diff3;
          }
          if (best_score < score) {
            best_score = score;
            best_len = len;
            *best_len_out = best_len;
            *best_len_code_out = best_len;
            *best_distance_out = backward;
            *best_score_out = best_score;
            match_found = true;
          }
        }
      }
    }
    if (!match_found && num_dict_matches_ >= (num_dict_lookups_ >> 7)) {
      uint32_t key = Hash<14, 4>(&data[cur_ix_masked]) << 1;
      for (int k = 0; k < 2; ++k, ++key) {
        ++num_dict_lookups_;
        const uint16_t v = kStaticDictionaryHash[key];
        if (v > 0) {
          const int len = v & 31;
          const int dist = v >> 5;
          const int offset = kBrotliDictionaryOffsetsByLength[len] + len * dist;
          if (len <= max_length) {
            const int matchlen =
                FindMatchLengthWithLimit(&data[cur_ix_masked],
                                         &kBrotliDictionary[offset], len);
            if (matchlen == len) {
              const size_t backward = max_backward + dist + 1;
              double score = BackwardReferenceScore(average_cost,
                                                    len, backward);
              if (kUseCostModel) {
                score += start_cost_diff4;
              }
              if (best_score < score) {
                ++num_dict_matches_;
                best_score = score;
                best_len = len;
                *best_len_out = best_len;
                *best_len_code_out = best_len;
                *best_distance_out = backward;
                *best_score_out = best_score;
                match_found = true;
                break;
              }
            }
          }
        }
      }
    }
    if (kUseDictionary && static_dict_ != NULL) {
      // We decide based on first 4 bytes how many bytes to test for.
      uint32_t prefix = BROTLI_UNALIGNED_LOAD32(&data[cur_ix_masked]);
      int maxlen = static_dict_->GetLength(prefix);
      for (int len = std::min<size_t>(maxlen, max_length);
           len > best_len && len >= 4; --len) {
        std::string snippet((const char *)&data[cur_ix_masked], len);
        int copy_len_code;
        int word_id;
        if (static_dict_->Get(snippet, &copy_len_code, &word_id)) {
          const size_t backward = max_backward + word_id + 1;
          const double score = (BackwardReferenceScore(average_cost,
                                                       len, backward) +
                                start_cost_diff4);
          if (best_score < score) {
            best_score = score;
            best_len = len;
            *best_len_out = best_len;
            *best_len_code_out = copy_len_code;
            *best_distance_out = backward;
            *best_score_out = best_score;
            match_found = true;
          }
        }
      }
    }
    return match_found;
  }

 private:
  // Number of hash buckets.
  static const uint32_t kBucketSize = 1 << kBucketBits;

  // Only kBlockSize newest backward references are kept,
  // and the older are forgotten.
  static const uint32_t kBlockSize = 1 << kBlockBits;

  // Mask for accessing entries in a block (in a ringbuffer manner).
  static const uint32_t kBlockMask = (1 << kBlockBits) - 1;

  // Number of entries in a particular bucket.
  uint16_t num_[kBucketSize];

  // Buckets containing kBlockSize of backward references.
  int buckets_[kBucketSize][kBlockSize];

  size_t num_dict_lookups_;
  size_t num_dict_matches_;

  const StaticDictionary *static_dict_;
};

struct Hashers {
  typedef HashLongestMatchQuickly<16, 1> H1;
  typedef HashLongestMatchQuickly<17, 4> H2;
  typedef HashLongestMatch<14, 4, 4, 4, false, false> H3;
  typedef HashLongestMatch<14, 5, 4, 4, false, false> H4;
  typedef HashLongestMatch<15, 6, 4, 10, false, false> H5;
  typedef HashLongestMatch<15, 7, 4, 10, false, false> H6;
  typedef HashLongestMatch<15, 8, 4, 16, false, false> H7;
  typedef HashLongestMatch<15, 8, 4, 16, true, true> H8;
  typedef HashLongestMatch<15, 8, 2, 16, true, false> H9;

  void Init(int type) {
    switch (type) {
      case 1: hash_h1.reset(new H1); break;
      case 2: hash_h2.reset(new H2); break;
      case 3: hash_h3.reset(new H3); break;
      case 4: hash_h4.reset(new H4); break;
      case 5: hash_h5.reset(new H5); break;
      case 6: hash_h6.reset(new H6); break;
      case 7: hash_h7.reset(new H7); break;
      case 8: hash_h8.reset(new H8); break;
      case 9: hash_h9.reset(new H9); break;
      default: break;
    }
  }

  void SetStaticDictionary(const StaticDictionary *dict) {
    if (hash_h8.get() != NULL) hash_h8->SetStaticDictionary(dict);
  }

  std::unique_ptr<H1> hash_h1;
  std::unique_ptr<H2> hash_h2;
  std::unique_ptr<H3> hash_h3;
  std::unique_ptr<H4> hash_h4;
  std::unique_ptr<H5> hash_h5;
  std::unique_ptr<H6> hash_h6;
  std::unique_ptr<H7> hash_h7;
  std::unique_ptr<H8> hash_h8;
  std::unique_ptr<H9> hash_h9;
};

}  // namespace brotli

#endif  // BROTLI_ENC_HASH_H_