libjxl

ans_common.cc (6256B)

---

 // 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 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.
 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