brotli/dec/bit_reader.h

/* Copyright 2013 Google Inc. All Rights Reserved.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.

   Bit reading helpers
*/

#ifndef BROTLI_DEC_BIT_READER_H_
#define BROTLI_DEC_BIT_READER_H_

#include <string.h>
#include "./streams.h"
#include "./types.h"

#if defined(__cplusplus) || defined(c_plusplus)
extern "C" {
#endif

#define BROTLI_MAX_NUM_BIT_READ   25
#define BROTLI_READ_SIZE          4096
#define BROTLI_IBUF_SIZE          (2 * BROTLI_READ_SIZE + 32)
#define BROTLI_IBUF_MASK          (2 * BROTLI_READ_SIZE - 1)

#define UNALIGNED_COPY64(dst, src) *(uint64_t*)(dst) = *(const uint64_t*)(src)

static const uint32_t kBitMask[BROTLI_MAX_NUM_BIT_READ] = {
  0, 1, 3, 7, 15, 31, 63, 127, 255, 511, 1023, 2047, 4095, 8191, 16383, 32767,
  65535, 131071, 262143, 524287, 1048575, 2097151, 4194303, 8388607, 16777215
};

typedef struct {
  /* Input byte buffer, consist of a ringbuffer and a "slack" region where */
  /* bytes from the start of the ringbuffer are copied. */
  uint8_t buf_[BROTLI_IBUF_SIZE];
  BrotliInput input_;    /* input callback */
  uint64_t    val_;      /* pre-fetched bits */
  size_t      pos_;      /* byte position in stream */
  int         bit_pos_;  /* current bit-reading position in val_ */
  size_t      end_pos_;  /* current end position in stream */
  int         eos_;      /* input stream is finished */
} BrotliBitReader;

int BrotliInitBitReader(BrotliBitReader* const br, BrotliInput input);

/* Return the prefetched bits, so they can be looked up. */
static BROTLI_INLINE uint32_t BrotliPrefetchBits(BrotliBitReader* const br) {
  return (uint32_t)(br->val_ >> br->bit_pos_);
}

/* For jumping over a number of bits in the bit stream when accessed with */
/* BrotliPrefetchBits and BrotliFillBitWindow. */
static BROTLI_INLINE void BrotliSetBitPos(BrotliBitReader* const br, int val) {
#ifdef BROTLI_DECODE_DEBUG
  int n_bits = val - br->bit_pos_;
  const uint32_t bval = (uint32_t)(br->val_ >> br->bit_pos_) & kBitMask[n_bits];
  printf("[BrotliReadBits]  %010ld %2d  val: %6x\n",
         (br->pos_ << 3) + br->bit_pos_ - 64, n_bits, bval);
#endif
  br->bit_pos_ = val;
}

/* Reload up to 64 bits byte-by-byte */
static BROTLI_INLINE void ShiftBytes(BrotliBitReader* const br) {
  while (br->bit_pos_ >= 8) {
    br->val_ >>= 8;
    br->val_ |= ((uint64_t)br->buf_[br->pos_ & BROTLI_IBUF_MASK]) << 56;
    ++br->pos_;
    br->bit_pos_ -= 8;
  }
}

/* Fills up the input ringbuffer by calling the input callback.

   Does nothing if there are at least 32 bytes present after current position.

   Returns 0 if either:
    - the input callback returned an error, or
    - there is no more input and the position is past the end of the stream.

   After encountering the end of the input stream, 32 additional zero bytes are
   copied to the ringbuffer, therefore it is safe to call this function after
   every 32 bytes of input is read.
*/
static BROTLI_INLINE int BrotliReadMoreInput(BrotliBitReader* const br) {
  if (br->pos_ + 32 < br->end_pos_) {
    return 1;
  } else if (br->eos_) {
    return (br->pos_ << 3) + br->bit_pos_ <= (br->end_pos_ << 3) + 64;
  } else {
    uint8_t* dst = br->buf_ + (br->end_pos_ & BROTLI_IBUF_MASK);
    int bytes_read = BrotliRead(br->input_, dst, BROTLI_READ_SIZE);
    if (bytes_read < 0) {
      return 0;
    }
    if (bytes_read < BROTLI_READ_SIZE) {
      br->eos_ = 1;
      /* Store 32 bytes of zero after the stream end. */
#if (defined(__x86_64__) || defined(_M_X64))
      *(uint64_t*)(dst + bytes_read) = 0;
      *(uint64_t*)(dst + bytes_read + 8) = 0;
      *(uint64_t*)(dst + bytes_read + 16) = 0;
      *(uint64_t*)(dst + bytes_read + 24) = 0;
#else
      memset(dst + bytes_read, 0, 32);
#endif
    }
    if (dst == br->buf_) {
      /* Copy the head of the ringbuffer to the slack region. */
#if (defined(__x86_64__) || defined(_M_X64))
      UNALIGNED_COPY64(br->buf_ + BROTLI_IBUF_SIZE - 32, br->buf_);
      UNALIGNED_COPY64(br->buf_ + BROTLI_IBUF_SIZE - 24, br->buf_ + 8);
      UNALIGNED_COPY64(br->buf_ + BROTLI_IBUF_SIZE - 16, br->buf_ + 16);
      UNALIGNED_COPY64(br->buf_ + BROTLI_IBUF_SIZE - 8, br->buf_ + 24);
#else
      memcpy(br->buf_ + (BROTLI_READ_SIZE << 1), br->buf_, 32);
#endif
    }
    br->end_pos_ += bytes_read;
    return 1;
  }
}

/* Advances the Read buffer by 5 bytes to make room for reading next 24 bits. */
static BROTLI_INLINE void BrotliFillBitWindow(BrotliBitReader* const br) {
  if (br->bit_pos_ >= 40) {
#if (defined(__x86_64__) || defined(_M_X64))
    br->val_ >>= 40;
    br->bit_pos_ -= 40;
    /* The expression below needs a little-endian arch to work correctly. */
    /* This gives a large speedup for decoding speed. */
    br->val_ |= *(const uint64_t*)(
        br->buf_ + (br->pos_ & BROTLI_IBUF_MASK)) << 24;
    br->pos_ += 5;
#else
    ShiftBytes(br);
#endif
  }
}

/* Reads the specified number of bits from Read Buffer. */
/* Requires that n_bits is positive. */
static BROTLI_INLINE uint32_t BrotliReadBits(
    BrotliBitReader* const br, int n_bits) {
  uint32_t val;
  BrotliFillBitWindow(br);
  val = (uint32_t)(br->val_ >> br->bit_pos_) & kBitMask[n_bits];
#ifdef BROTLI_DECODE_DEBUG
  printf("[BrotliReadBits]  %010ld %2d  val: %6x\n",
         (br->pos_ << 3) + br->bit_pos_ - 64, n_bits, val);
#endif
  br->bit_pos_ += n_bits;
  return val;
}

#if defined(__cplusplus) || defined(c_plusplus)
}    /* extern "C" */
#endif

#endif  /* BROTLI_DEC_BIT_READER_H_ */