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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 <vector>
#include "lib/jxl/base/bits.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_