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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.
#include "lib/jxl/ans_common.h"
#include <numeric>
#include "lib/jxl/ans_params.h"
#include "lib/jxl/base/status.h"
namespace jxl {
std::vector<int32_t> CreateFlatHistogram(int length, int total_count) {
JXL_ASSERT(length > 0);
JXL_ASSERT(length <= total_count);
const int count = total_count / length;
std::vector<int32_t> result(length, count);
const int rem_counts = total_count % length;
for (int i = 0; i < rem_counts; ++i) {
++result[i];
}
return result;
}
// First, all trailing non-occurring symbols are removed from the distribution;
// if this leaves the distribution empty, a dummy symbol with max weight is
// added. This ensures that the resulting distribution sums to total table size.
// Then, `entry_size` is chosen to be the largest power of two so that
// `table_size` = ANS_TAB_SIZE/`entry_size` is at least as big as the
// distribution size.
// Note that each entry will only ever contain two different symbols, and
// consecutive ranges of offsets, which allows us to use a compact
// representation.
// Each entry is initialized with only the (symbol=i, offset) pairs; then
// positions for which the entry overflows (i.e. distribution[i] > entry_size)
// or is not full are computed, and put into a stack in increasing order.
// Missing symbols in the distribution are padded with 0 (because `table_size`
// >= number of symbols). The `cutoff` value for each entry is initialized to
// the number of occupied slots in that entry (i.e. `distributions[i]`). While
// the overflowing-symbol stack is not empty (which implies that the
// underflowing-symbol stack also is not), the top overfull and underfull
// positions are popped from the stack; the empty slots in the underfull entry
// are then filled with as many slots as needed from the overfull entry; such
// slots are placed after the slots in the overfull entry, and `offsets[1]` is
// computed accordingly. The formerly underfull entry is thus now neither
// underfull nor overfull, and represents exactly two symbols. The overfull
// entry might be either overfull or underfull, and is pushed into the
// corresponding stack.
void InitAliasTable(std::vector<int32_t> distribution, uint32_t range,
size_t log_alpha_size, AliasTable::Entry* JXL_RESTRICT a) {
while (!distribution.empty() && distribution.back() == 0) {
distribution.pop_back();
}
// Ensure that a valid table is always returned, even for an empty
// alphabet. Otherwise, a specially-crafted stream might crash the
// decoder.
if (distribution.empty()) {
distribution.emplace_back(range);
}
const size_t table_size = 1 << log_alpha_size;
#if JXL_ENABLE_ASSERT
int sum = std::accumulate(distribution.begin(), distribution.end(), 0);
#endif // JXL_ENABLE_ASSERT
JXL_ASSERT(static_cast<uint32_t>(sum) == range);
// range must be a power of two
JXL_ASSERT((range & (range - 1)) == 0);
JXL_ASSERT(distribution.size() <= table_size);
JXL_ASSERT(table_size <= range);
const uint32_t entry_size = range >> log_alpha_size; // this is exact
// Special case for single-symbol distributions, that ensures that the state
// does not change when decoding from such a distribution. Note that, since we
// hardcode offset0 == 0, it is not straightforward (if at all possible) to
// fix the general case to produce this result.
for (size_t sym = 0; sym < distribution.size(); sym++) {
if (distribution[sym] == ANS_TAB_SIZE) {
for (size_t i = 0; i < table_size; i++) {
a[i].right_value = sym;
a[i].cutoff = 0;
a[i].offsets1 = entry_size * i;
a[i].freq0 = 0;
a[i].freq1_xor_freq0 = ANS_TAB_SIZE;
}
return;
}
}
std::vector<uint32_t> underfull_posn;
std::vector<uint32_t> overfull_posn;
std::vector<uint32_t> cutoffs(1 << log_alpha_size);
// Initialize entries.
for (size_t i = 0; i < distribution.size(); i++) {
cutoffs[i] = distribution[i];
if (cutoffs[i] > entry_size) {
overfull_posn.push_back(i);
} else if (cutoffs[i] < entry_size) {
underfull_posn.push_back(i);
}
}
for (uint32_t i = distribution.size(); i < table_size; i++) {
cutoffs[i] = 0;
underfull_posn.push_back(i);
}
// Reassign overflow/underflow values.
while (!overfull_posn.empty()) {
uint32_t overfull_i = overfull_posn.back();
overfull_posn.pop_back();
JXL_ASSERT(!underfull_posn.empty());
uint32_t underfull_i = underfull_posn.back();
underfull_posn.pop_back();
uint32_t underfull_by = entry_size - cutoffs[underfull_i];
cutoffs[overfull_i] -= underfull_by;
// overfull positions have their original symbols
a[underfull_i].right_value = overfull_i;
a[underfull_i].offsets1 = cutoffs[overfull_i];
// Slots in the right part of entry underfull_i were taken from the end
// of the symbols in entry overfull_i.
if (cutoffs[overfull_i] < entry_size) {
underfull_posn.push_back(overfull_i);
} else if (cutoffs[overfull_i] > entry_size) {
overfull_posn.push_back(overfull_i);
}
}
for (uint32_t i = 0; i < table_size; i++) {
// cutoffs[i] is properly initialized but the clang-analyzer doesn't infer
// it since it is partially initialized across two for-loops.
// NOLINTNEXTLINE(clang-analyzer-core.UndefinedBinaryOperatorResult)
if (cutoffs[i] == entry_size) {
a[i].right_value = i;
a[i].offsets1 = 0;
a[i].cutoff = 0;
} else {
// Note that, if cutoff is not equal to entry_size,
// a[i].offsets1 was initialized with (overfull cutoff) -
// (entry_size - a[i].cutoff). Thus, subtracting
// a[i].cutoff cannot make it negative.
a[i].offsets1 -= cutoffs[i];
a[i].cutoff = cutoffs[i];
}
const size_t freq0 = i < distribution.size() ? distribution[i] : 0;
const size_t i1 = a[i].right_value;
const size_t freq1 = i1 < distribution.size() ? distribution[i1] : 0;
a[i].freq0 = static_cast<uint16_t>(freq0);
a[i].freq1_xor_freq0 = static_cast<uint16_t>(freq1 ^ freq0);
}
}
} // namespace jxl
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