/src/libjxl/lib/jxl/ac_context.h

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// Copyright (c) the JPEG XL Project Authors. All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

#ifndef LIB_JXL_AC_CONTEXT_H_
#define LIB_JXL_AC_CONTEXT_H_

#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <vector>

#include "lib/jxl/base/compiler_specific.h"
#include "lib/jxl/base/status.h"
#include "lib/jxl/coeff_order_fwd.h"

namespace jxl {

// Block context used for scanning order, number of non-zeros, AC coefficients.
// Equal to the channel.
constexpr uint32_t kDCTOrderContextStart = 0;

// The number of predicted nonzeros goes from 0 to 1008. We use
// ceil(log2(predicted+1)) as a context for the number of nonzeros, so from 0 to
// 10, inclusive.
constexpr uint32_t kNonZeroBuckets = 37;

static const uint16_t kCoeffFreqContext[64] = {
    0xBAD, 0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  10, 11, 12, 13, 14,
    15,    15, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22,
    23,    23, 23, 23, 24, 24, 24, 24, 25, 25, 25, 25, 26, 26, 26, 26,
    27,    27, 27, 27, 28, 28, 28, 28, 29, 29, 29, 29, 30, 30, 30, 30,
};

static const uint16_t kCoeffNumNonzeroContext[64] = {
    0xBAD, 0,   31,  62,  62,  93,  93,  93,  93,  123, 123, 123, 123,
    152,   152, 152, 152, 152, 152, 152, 152, 180, 180, 180, 180, 180,
    180,   180, 180, 180, 180, 180, 180, 206, 206, 206, 206, 206, 206,
    206,   206, 206, 206, 206, 206, 206, 206, 206, 206, 206, 206, 206,
    206,   206, 206, 206, 206, 206, 206, 206, 206, 206, 206, 206,
};

// Supremum of ZeroDensityContext(x, y) + 1, when x + y < 64.
constexpr int kZeroDensityContextCount = 458;
// Supremum of ZeroDensityContext(x, y) + 1.
constexpr int kZeroDensityContextLimit = 474;

/* This function is used for entropy-sources pre-clustering.
 *
 * Ideally, each combination of |nonzeros_left| and |k| should go to its own
 * bucket; but it implies (64 * 63 / 2) == 2016 buckets. If there is other
 * dimension (e.g. block context), then number of primary clusters becomes too
 * big.
 *
 * To solve this problem, |nonzeros_left| and |k| values are clustered. It is
 * known that their sum is at most 64, consequently, the total number buckets
 * is at most A(64) * B(64).
 */
// TODO(user): investigate, why disabling pre-clustering makes entropy code
// less dense. Perhaps we would need to add HQ clustering algorithm that would
// be able to squeeze better by spending more CPU cycles.
static JXL_INLINE size_t ZeroDensityContext(size_t nonzeros_left, size_t k,
                                            size_t covered_blocks,
                                            size_t log2_covered_blocks,
                                            size_t prev) {
  JXL_DASSERT((static_cast<size_t>(1) << log2_covered_blocks) ==
              covered_blocks);
  nonzeros_left = (nonzeros_left + covered_blocks - 1) >> log2_covered_blocks;
  k >>= log2_covered_blocks;
  JXL_DASSERT(k > 0);
  JXL_DASSERT(k < 64);
  JXL_DASSERT(nonzeros_left > 0);
  // Asserting nonzeros_left + k < 65 here causes crashes in debug mode with
  // invalid input, since the (hot) decoding loop does not check this condition.
  // As no out-of-bound memory reads are issued even if that condition is
  // broken, we check this simpler condition which holds anyway. The decoder
  // will still mark a file in which that condition happens as not valid at the
  // end of the decoding loop, as `nzeros` will not be `0`.
  JXL_DASSERT(nonzeros_left < 64);
  return (kCoeffNumNonzeroContext[nonzeros_left] + kCoeffFreqContext[k]) * 2 +
         prev;
}

struct BlockCtxMap {
  std::vector<int> dc_thresholds[3];
  std::vector<uint32_t> qf_thresholds;
  std::vector<uint8_t> ctx_map;
  size_t num_ctxs, num_dc_ctxs;

  static constexpr uint8_t kDefaultCtxMap[] = {
      // Default ctx map clusters all the large transforms together.
      0, 1, 2, 2, 3,  3,  4,  5,  6,  6,  6,  6,  6,   //
      7, 8, 9, 9, 10, 11, 12, 13, 14, 14, 14, 14, 14,  //
      7, 8, 9, 9, 10, 11, 12, 13, 14, 14, 14, 14, 14,  //
  };
  static_assert(3 * kNumOrders ==
                    sizeof(kDefaultCtxMap) / sizeof *kDefaultCtxMap,
                "Update default context map");

  size_t Context(int dc_idx, uint32_t qf, size_t ord, size_t c) const {
    size_t qf_idx = 0;
    for (uint32_t t : qf_thresholds) {
      if (qf > t) qf_idx++;
    }
    size_t idx = c < 2 ? c ^ 1 : 2;
    idx = idx * kNumOrders + ord;
    idx = idx * (qf_thresholds.size() + 1) + qf_idx;
    idx = idx * num_dc_ctxs + dc_idx;
    return ctx_map[idx];
  }
  // Non-zero context is based on number of non-zeros and block context.
  // For better clustering, contexts with same number of non-zeros are grouped.
  constexpr uint32_t ZeroDensityContextsOffset(uint32_t block_ctx) const {
    return static_cast<uint32_t>(num_ctxs * kNonZeroBuckets +
                                 kZeroDensityContextCount * block_ctx);
  }

  // Context map for AC coefficients consists of 2 blocks:
  //  |num_ctxs x                : context for number of non-zeros in the block
  //   kNonZeroBuckets|            computed from block context and predicted
  //                               value (based top and left values)
  //  |num_ctxs x                : context for AC coefficient symbols,
  //   kZeroDensityContextCount|   computed from block context,
  //                               number of non-zeros left and
  //                               index in scan order
  constexpr uint32_t NumACContexts() const {
    return static_cast<uint32_t>(num_ctxs *
                                 (kNonZeroBuckets + kZeroDensityContextCount));
  }

  // Non-zero context is based on number of non-zeros and block context.
  // For better clustering, contexts with same number of non-zeros are grouped.
  inline uint32_t NonZeroContext(uint32_t non_zeros, uint32_t block_ctx) const {
    uint32_t ctx;
    if (non_zeros >= 64) non_zeros = 64;
    if (non_zeros < 8) {
      ctx = non_zeros;
    } else {
      ctx = 4 + non_zeros / 2;
    }
    return static_cast<uint32_t>(ctx * num_ctxs + block_ctx);
  }

  BlockCtxMap() {
    ctx_map.assign(std::begin(kDefaultCtxMap), std::end(kDefaultCtxMap));
    num_ctxs = *std::max_element(ctx_map.begin(), ctx_map.end()) + 1;
    num_dc_ctxs = 1;
  }
};

}  // namespace jxl

#endif  // LIB_JXL_AC_CONTEXT_H_

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Created: 2025-06-16 07:00