tor-browser

ans_common.cc (6433B)

---

// 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 <cstddef>
#include <cstdint>
#include <numeric>
#include <vector>

#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_DASSERT(length > 0);
 JXL_DASSERT(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 placeholder 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.
Status InitAliasTable(std::vector<int32_t> distribution, uint32_t log_range,
                     size_t log_alpha_size,
                     AliasTable::Entry* JXL_RESTRICT a) {
 const uint32_t range = 1 << log_range;
 const size_t table_size = 1 << log_alpha_size;
 JXL_ENSURE(table_size <= range);
 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);
 }
 JXL_ENSURE(distribution.size() <= table_size);
 const uint32_t entry_size = range >> log_alpha_size;  // this is exact
 int single_symbol = -1;
 int sum = 0;
 // 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++) {
   int32_t v = distribution[sym];
   sum += v;
   if (v == ANS_TAB_SIZE) {
     JXL_ENSURE(single_symbol == -1);
     single_symbol = sym;
   }
 }
 JXL_ENSURE(static_cast<uint32_t>(sum) == range);
 if (single_symbol != -1) {
   uint8_t sym = single_symbol;
   JXL_ENSURE(single_symbol == sym);
   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 true;
 }

 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_ENSURE(!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);
 }
 return true;
}

}  // namespace jxl