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115 lines
3.6 KiB
C++
115 lines
3.6 KiB
C++
/* Copyright 2013 Google Inc. All Rights Reserved.
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Distributed under MIT license.
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See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
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*/
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// Sliding window over the input data.
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#ifndef BROTLI_ENC_RINGBUFFER_H_
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#define BROTLI_ENC_RINGBUFFER_H_
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#include "./port.h"
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#include "./types.h"
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namespace brotli {
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// A RingBuffer(window_bits, tail_bits) contains `1 << window_bits' bytes of
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// data in a circular manner: writing a byte writes it to:
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// `position() % (1 << window_bits)'.
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// For convenience, the RingBuffer array contains another copy of the
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// first `1 << tail_bits' bytes:
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// buffer_[i] == buffer_[i + (1 << window_bits)], if i < (1 << tail_bits),
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// and another copy of the last two bytes:
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// buffer_[-1] == buffer_[(1 << window_bits) - 1] and
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// buffer_[-2] == buffer_[(1 << window_bits) - 2].
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class RingBuffer {
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public:
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RingBuffer(int window_bits, int tail_bits)
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: size_(1u << window_bits),
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mask_((1u << window_bits) - 1),
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tail_size_(1u << tail_bits),
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pos_(0) {
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static const size_t kSlackForEightByteHashingEverywhere = 7;
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const size_t buflen = size_ + tail_size_;
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data_ = new uint8_t[2 + buflen + kSlackForEightByteHashingEverywhere];
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buffer_ = data_ + 2;
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for (size_t i = 0; i < kSlackForEightByteHashingEverywhere; ++i) {
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buffer_[buflen + i] = 0;
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}
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// Initialize the last two bytes and their copy to zero.
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buffer_[-2] = buffer_[size_ - 2] = 0;
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buffer_[-1] = buffer_[size_ - 1] = 0;
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}
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~RingBuffer() {
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delete [] data_;
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}
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// Push bytes into the ring buffer.
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void Write(const uint8_t *bytes, size_t n) {
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const size_t masked_pos = pos_ & mask_;
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// The length of the writes is limited so that we do not need to worry
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// about a write
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WriteTail(bytes, n);
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if (PREDICT_TRUE(masked_pos + n <= size_)) {
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// A single write fits.
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memcpy(&buffer_[masked_pos], bytes, n);
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} else {
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// Split into two writes.
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// Copy into the end of the buffer, including the tail buffer.
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memcpy(&buffer_[masked_pos], bytes,
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std::min(n, (size_ + tail_size_) - masked_pos));
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// Copy into the beginning of the buffer
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memcpy(&buffer_[0], bytes + (size_ - masked_pos),
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n - (size_ - masked_pos));
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}
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buffer_[-2] = buffer_[size_ - 2];
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buffer_[-1] = buffer_[size_ - 1];
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pos_ += static_cast<uint32_t>(n);
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if (pos_ > (1u << 30)) { /* Wrap, but preserve not-a-first-lap feature. */
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pos_ = (pos_ & ((1u << 30) - 1)) | (1u << 30);
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}
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}
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void Reset() {
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pos_ = 0;
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}
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// Logical cursor position in the ring buffer.
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uint32_t position() const { return pos_; }
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// Bit mask for getting the physical position for a logical position.
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uint32_t mask() const { return mask_; }
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uint8_t *start() { return &buffer_[0]; }
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const uint8_t *start() const { return &buffer_[0]; }
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private:
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void WriteTail(const uint8_t *bytes, size_t n) {
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const size_t masked_pos = pos_ & mask_;
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if (PREDICT_FALSE(masked_pos < tail_size_)) {
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// Just fill the tail buffer with the beginning data.
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const size_t p = size_ + masked_pos;
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memcpy(&buffer_[p], bytes, std::min(n, tail_size_ - masked_pos));
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}
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}
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// Size of the ringbuffer is (1 << window_bits) + tail_size_.
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const uint32_t size_;
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const uint32_t mask_;
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const uint32_t tail_size_;
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// Position to write in the ring buffer.
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uint32_t pos_;
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// The actual ring buffer containing the copy of the last two bytes, the data,
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// and the copy of the beginning as a tail.
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uint8_t *data_;
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// The start of the ringbuffer.
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uint8_t *buffer_;
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};
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} // namespace brotli
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#endif // BROTLI_ENC_RINGBUFFER_H_
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