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https://git.mirrors.martin98.com/https://github.com/google/draco
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117 lines
3.3 KiB
C++
117 lines
3.3 KiB
C++
// Copyright 2016 The Draco Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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#include "core/adaptive_rans_coding.h"
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#include <iostream>
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namespace draco {
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uint8_t clamp_probability(double p) {
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DCHECK_LE(p, 1.0);
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DCHECK_LE(0.0, p);
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uint32_t p_int = static_cast<uint32_t>((p * 256) + 0.5);
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p_int -= (p_int == 256);
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p_int += (p_int == 0);
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return static_cast<uint8_t>(p_int);
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}
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double update_probability(double old_p, bool bit) {
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static constexpr double w = 128.0;
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static constexpr double w0 = (w - 1.0) / w;
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static constexpr double w1 = 1.0 / w;
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return old_p * w0 + (!bit) * w1;
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}
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AdaptiveRAnsBitEncoder::AdaptiveRAnsBitEncoder() {}
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AdaptiveRAnsBitEncoder::~AdaptiveRAnsBitEncoder() { Clear(); }
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void AdaptiveRAnsBitEncoder::StartEncoding() { Clear(); }
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void AdaptiveRAnsBitEncoder::EndEncoding(EncoderBuffer *target_buffer) {
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// Buffer for ans to write.
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std::vector<uint8_t> buffer(bits_.size() + 16);
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AnsCoder ans_coder;
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ans_write_init(&ans_coder, buffer.data());
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// Unfortunaetly we have to encode the bits in reversed order, while the
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// probabilities that should be given are those of the forward sequence.
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double p0_f = 0.5;
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std::vector<uint8_t> p0s;
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p0s.reserve(bits_.size());
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for (bool b : bits_) {
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p0s.push_back(clamp_probability(p0_f));
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p0_f = update_probability(p0_f, b);
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}
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auto bit = bits_.rbegin();
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auto pit = p0s.rbegin();
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while (bit != bits_.rend()) {
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rabs_write(&ans_coder, *bit, *pit);
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++bit;
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++pit;
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}
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const uint32_t size_in_bytes = ans_write_end(&ans_coder);
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target_buffer->Encode(size_in_bytes);
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target_buffer->Encode(buffer.data(), size_in_bytes);
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Clear();
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}
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void AdaptiveRAnsBitEncoder::Clear() { bits_.clear(); }
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AdaptiveRAnsBitDecoder::AdaptiveRAnsBitDecoder() : p0_f_(0.5) {}
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AdaptiveRAnsBitDecoder::~AdaptiveRAnsBitDecoder() { Clear(); }
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void AdaptiveRAnsBitDecoder::StartDecoding(DecoderBuffer *source_buffer) {
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Clear();
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uint32_t size_in_bytes;
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source_buffer->Decode(&size_in_bytes);
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ans_read_init(&ans_decoder_, reinterpret_cast<uint8_t *>(const_cast<char *>(
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source_buffer->data_head())),
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size_in_bytes);
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source_buffer->Advance(size_in_bytes);
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}
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// TODO(hemmer): Consider moving these to the .h file.
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bool AdaptiveRAnsBitDecoder::DecodeNextBit() {
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const uint8_t p0 = clamp_probability(p0_f_);
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const bool bit = static_cast<bool>(rabs_read(&ans_decoder_, p0));
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p0_f_ = update_probability(p0_f_, bit);
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return bit;
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}
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void AdaptiveRAnsBitDecoder::DecodeBits32(int nbits, uint32_t *value) {
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DCHECK_EQ(true, nbits <= 32);
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DCHECK_EQ(true, nbits > 0);
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uint32_t result = 0;
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while (nbits) {
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result = (result << 1) + DecodeNextBit();
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--nbits;
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}
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*value = result;
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}
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void AdaptiveRAnsBitDecoder::Clear() {
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ans_read_end(&ans_decoder_);
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p0_f_ = 0.5;
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}
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} // namespace draco
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