// 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 <vector>

#include "lib/jxl/ans_params.h"

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

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

namespace jxl {

// 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