brotli/dec/bit_reader.h

/* Copyright 2013 Google Inc. All Rights Reserved.

   Distributed under MIT license.
   See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
*/

/* Bit reading helpers */

#ifndef BROTLI_DEC_BIT_READER_H_
#define BROTLI_DEC_BIT_READER_H_

#include <string.h>  /* memcpy */

#include "./port.h"
#include "./types.h"

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

#if (BROTLI_64_BITS)
#define BROTLI_SHORT_FILL_BIT_WINDOW_READ 4
typedef uint64_t reg_t;
#else
#define BROTLI_SHORT_FILL_BIT_WINDOW_READ 2
typedef uint32_t reg_t;
#endif

static const uint32_t kBitMask[33] = { 0x0000,
    0x00000001, 0x00000003, 0x00000007, 0x0000000F,
    0x0000001F, 0x0000003F, 0x0000007F, 0x000000FF,
    0x000001FF, 0x000003FF, 0x000007FF, 0x00000FFF,
    0x00001FFF, 0x00003FFF, 0x00007FFF, 0x0000FFFF,
    0x0001FFFF, 0x0003FFFF, 0x0007FFFF, 0x000FFFFF,
    0x001FFFFF, 0x003FFFFF, 0x007FFFFF, 0x00FFFFFF,
    0x01FFFFFF, 0x03FFFFFF, 0x07FFFFFF, 0x0FFFFFFF,
    0x1FFFFFFF, 0x3FFFFFFF, 0x7FFFFFFF, 0xFFFFFFFF
};

static BROTLI_INLINE uint32_t BitMask(uint32_t n) {
  if (IS_CONSTANT(n) || BROTLI_HAS_UBFX) {
    /* Masking with this expression turns to a single
       "Unsigned Bit Field Extract" UBFX instruction on ARM. */
    return ~((0xffffffffU) << n);
  } else {
    return kBitMask[n];
  }
}

typedef struct {
  reg_t val_;              /* pre-fetched bits */
  uint32_t bit_pos_;       /* current bit-reading position in val_ */
  const uint8_t* next_in;  /* the byte we're reading from */
  size_t avail_in;
} BrotliBitReader;

typedef struct {
  reg_t val_;
  uint32_t bit_pos_;
  const uint8_t* next_in;
  size_t avail_in;
} BrotliBitReaderState;

/* Initializes the bitreader fields. */
BROTLI_INTERNAL void BrotliInitBitReader(BrotliBitReader* const br);

/* Ensures that accumulator is not empty. May consume one byte of input.
   Returns 0 if data is required but there is no input available.
   For BROTLI_ALIGNED_READ this function also prepares bit reader for aligned
   reading. */
BROTLI_INTERNAL int BrotliWarmupBitReader(BrotliBitReader* const br);

static BROTLI_INLINE void BrotliBitReaderSaveState(
    BrotliBitReader* const from, BrotliBitReaderState* to) {
  to->val_ = from->val_;
  to->bit_pos_ = from->bit_pos_;
  to->next_in = from->next_in;
  to->avail_in = from->avail_in;
}

static BROTLI_INLINE void BrotliBitReaderRestoreState(
    BrotliBitReader* const to, BrotliBitReaderState* from) {
  to->val_ = from->val_;
  to->bit_pos_ = from->bit_pos_;
  to->next_in = from->next_in;
  to->avail_in = from->avail_in;
}

static BROTLI_INLINE uint32_t BrotliGetAvailableBits(
    const BrotliBitReader* br) {
  return (BROTLI_64_BITS ? 64 : 32) - br->bit_pos_;
}

/* Returns amount of unread bytes the bit reader still has buffered from the
   BrotliInput, including whole bytes in br->val_. */
static BROTLI_INLINE size_t BrotliGetRemainingBytes(BrotliBitReader* br) {
  return br->avail_in + (BrotliGetAvailableBits(br) >> 3);
}

/* Checks if there is at least num bytes left in the input ringbuffer (excluding
   the bits remaining in br->val_). */
static BROTLI_INLINE int BrotliCheckInputAmount(
    BrotliBitReader* const br, size_t num) {
  return br->avail_in >= num;
}

static BROTLI_INLINE uint16_t BrotliLoad16LE(const uint8_t* in) {
  if (BROTLI_LITTLE_ENDIAN) {
    return *((const uint16_t*)in);
  } else if (BROTLI_BIG_ENDIAN) {
    uint16_t value = *((const uint16_t*)in);
    return (uint16_t)(((value & 0xFFU) << 8) | ((value & 0xFF00U) >> 8));
  } else {
    return (uint16_t)(in[0] | (in[1] << 8));
  }
}

static BROTLI_INLINE uint32_t BrotliLoad32LE(const uint8_t* in) {
  if (BROTLI_LITTLE_ENDIAN) {
    return *((const uint32_t*)in);
  } else if (BROTLI_BIG_ENDIAN) {
    uint32_t value = *((const uint32_t*)in);
    return ((value & 0xFFU) << 24) | ((value & 0xFF00U) << 8) |
        ((value & 0xFF0000U) >> 8) | ((value & 0xFF000000U) >> 24);
  } else {
    uint32_t value = (uint32_t)(*(in++));
    value |= (uint32_t)(*(in++)) << 8;
    value |= (uint32_t)(*(in++)) << 16;
    value |= (uint32_t)(*(in++)) << 24;
    return value;
  }
}

#if (BROTLI_64_BITS)
static BROTLI_INLINE uint64_t BrotliLoad64LE(const uint8_t* in) {
  if (BROTLI_LITTLE_ENDIAN) {
    return *((const uint64_t*)in);
  } else if (BROTLI_BIG_ENDIAN) {
    uint64_t value = *((const uint64_t*)in);
    return
        ((value & 0xFFU) << 56) |
        ((value & 0xFF00U) << 40) |
        ((value & 0xFF0000U) << 24) |
        ((value & 0xFF000000U) << 8) |
        ((value & 0xFF00000000U) >> 8) |
        ((value & 0xFF0000000000U) >> 24) |
        ((value & 0xFF000000000000U) >> 40) |
        ((value & 0xFF00000000000000U) >> 56);
  } else {
    uint64_t value = (uint64_t)(*(in++));
    value |= (uint64_t)(*(in++)) << 8;
    value |= (uint64_t)(*(in++)) << 16;
    value |= (uint64_t)(*(in++)) << 24;
    value |= (uint64_t)(*(in++)) << 32;
    value |= (uint64_t)(*(in++)) << 40;
    value |= (uint64_t)(*(in++)) << 48;
    value |= (uint64_t)(*(in++)) << 56;
    return value;
  }
}
#endif

/* Guarantees that there are at least n_bits + 1 bits in accumulator.
   Precondition: accumulator contains at least 1 bit.
   n_bits should be in the range [1..24] for regular build. For portable
   non-64-bit little endian build only 16 bits are safe to request. */
static BROTLI_INLINE void BrotliFillBitWindow(
    BrotliBitReader* const br, uint32_t n_bits) {
#if (BROTLI_64_BITS)
  if (!BROTLI_ALIGNED_READ && IS_CONSTANT(n_bits) && (n_bits <= 8)) {
    if (br->bit_pos_ >= 56) {
      br->val_ >>= 56;
      br->bit_pos_ ^= 56;  /* here same as -= 56 because of the if condition */
      br->val_ |= BrotliLoad64LE(br->next_in) << 8;
      br->avail_in -= 7;
      br->next_in += 7;
    }
  } else if (!BROTLI_ALIGNED_READ && IS_CONSTANT(n_bits) && (n_bits <= 16)) {
    if (br->bit_pos_ >= 48) {
      br->val_ >>= 48;
      br->bit_pos_ ^= 48;  /* here same as -= 48 because of the if condition */
      br->val_ |= BrotliLoad64LE(br->next_in) << 16;
      br->avail_in -= 6;
      br->next_in += 6;
    }
  } else {
    if (br->bit_pos_ >= 32) {
      br->val_ >>= 32;
      br->bit_pos_ ^= 32;  /* here same as -= 32 because of the if condition */
      br->val_ |= ((uint64_t)BrotliLoad32LE(br->next_in)) << 32;
      br->avail_in -= BROTLI_SHORT_FILL_BIT_WINDOW_READ;
      br->next_in += BROTLI_SHORT_FILL_BIT_WINDOW_READ;
    }
  }
#else
  if (!BROTLI_ALIGNED_READ && IS_CONSTANT(n_bits) && (n_bits <= 8)) {
    if (br->bit_pos_ >= 24) {
      br->val_ >>= 24;
      br->bit_pos_ ^= 24;  /* here same as -= 24 because of the if condition */
      br->val_ |= BrotliLoad32LE(br->next_in) << 8;
      br->avail_in -= 3;
      br->next_in += 3;
    }
  } else {
    if (br->bit_pos_ >= 16) {
      br->val_ >>= 16;
      br->bit_pos_ ^= 16;  /* here same as -= 16 because of the if condition */
      br->val_ |= ((uint32_t)BrotliLoad16LE(br->next_in)) << 16;
      br->avail_in -= BROTLI_SHORT_FILL_BIT_WINDOW_READ;
      br->next_in += BROTLI_SHORT_FILL_BIT_WINDOW_READ;
    }
  }
#endif
}

/* Mosltly like BrotliFillBitWindow, but guarantees only 16 bits and reads no
   more than BROTLI_SHORT_FILL_BIT_WINDOW_READ bytes of input. */
static BROTLI_INLINE void BrotliFillBitWindow16(BrotliBitReader* const br) {
  BrotliFillBitWindow(br, 17);
}

/* Pulls one byte of input to accumulator. */
static BROTLI_INLINE int BrotliPullByte(BrotliBitReader* const br) {
  if (br->avail_in == 0) {
    return 0;
  }
  br->val_ >>= 8;
#if (BROTLI_64_BITS)
  br->val_ |= ((uint64_t)*br->next_in) << 56;
#else
  br->val_ |= ((uint32_t)*br->next_in) << 24;
#endif
  br->bit_pos_ -= 8;
  --br->avail_in;
  ++br->next_in;
  return 1;
}

/* Returns currently available bits.
   The number of valid bits could be calclulated by BrotliGetAvailableBits. */
static BROTLI_INLINE reg_t BrotliGetBitsUnmasked(BrotliBitReader* const br) {
  return br->val_ >> br->bit_pos_;
}

/* Like BrotliGetBits, but does not mask the result.
   The result contains at least 16 valid bits. */
static BROTLI_INLINE uint32_t BrotliGet16BitsUnmasked(
    BrotliBitReader* const br) {
  BrotliFillBitWindow(br, 16);
  return (uint32_t)BrotliGetBitsUnmasked(br);
}

/* Returns the specified number of bits from br without advancing bit pos. */
static BROTLI_INLINE uint32_t BrotliGetBits(
    BrotliBitReader* const br, uint32_t n_bits) {
  BrotliFillBitWindow(br, n_bits);
  return (uint32_t)BrotliGetBitsUnmasked(br) & BitMask(n_bits);
}

/* Tries to peek the specified amount of bits. Returns 0, if there is not
   enough input. */
static BROTLI_INLINE int BrotliSafeGetBits(
    BrotliBitReader* const br, uint32_t n_bits, uint32_t* val) {
  while (BrotliGetAvailableBits(br) < n_bits) {
    if (!BrotliPullByte(br)) {
      return 0;
    }
  }
  *val = (uint32_t)BrotliGetBitsUnmasked(br) & BitMask(n_bits);
  return 1;
}

/* Advances the bit pos by n_bits. */
static BROTLI_INLINE void BrotliDropBits(
    BrotliBitReader* const br, uint32_t n_bits) {
  br->bit_pos_ += n_bits;
}

static BROTLI_INLINE void BrotliBitReaderUnload(BrotliBitReader* br) {
  uint32_t unused_bytes = BrotliGetAvailableBits(br) >> 3;
  uint32_t unused_bits = unused_bytes << 3;
  br->avail_in += unused_bytes;
  br->next_in -= unused_bytes;
  if (unused_bits == sizeof(br->val_) << 3) {
    br->val_ = 0;
  } else {
    br->val_ <<= unused_bits;
  }
  br->bit_pos_ += unused_bits;
}

/* Reads the specified number of bits from br and advances the bit pos.
   Precondition: accumulator MUST contain at least n_bits. */
static BROTLI_INLINE void BrotliTakeBits(
  BrotliBitReader* const br, uint32_t n_bits, uint32_t* val) {
  *val = (uint32_t)BrotliGetBitsUnmasked(br) & BitMask(n_bits);
  BROTLI_LOG(("[BrotliReadBits]  %d %d %d val: %6x\n",
      (int)br->avail_in, (int)br->bit_pos_, n_bits, (int)*val));
  BrotliDropBits(br, n_bits);
}

/* Reads the specified number of bits from br and advances the bit pos.
   Assumes that there is enough input to perform BrotliFillBitWindow. */
static BROTLI_INLINE uint32_t BrotliReadBits(
    BrotliBitReader* const br, uint32_t n_bits) {
  if (BROTLI_64_BITS || (n_bits <= 16)) {
    uint32_t val;
    BrotliFillBitWindow(br, n_bits);
    BrotliTakeBits(br, n_bits, &val);
    return val;
  } else {
    uint32_t low_val;
    uint32_t high_val;
    BrotliFillBitWindow(br, 16);
    BrotliTakeBits(br, 16, &low_val);
    BrotliFillBitWindow(br, 8);
    BrotliTakeBits(br, n_bits - 16, &high_val);
    return low_val | (high_val << 16);
  }
}

/* Tries to read the specified amount of bits. Returns 0, if there is not
   enough input. n_bits MUST be positive. */
static BROTLI_INLINE int BrotliSafeReadBits(
    BrotliBitReader* const br, uint32_t n_bits, uint32_t* val) {
  while (BrotliGetAvailableBits(br) < n_bits) {
    if (!BrotliPullByte(br)) {
      return 0;
    }
  }
  BrotliTakeBits(br, n_bits, val);
  return 1;
}

/* Advances the bit reader position to the next byte boundary and verifies
   that any skipped bits are set to zero. */
static BROTLI_INLINE int BrotliJumpToByteBoundary(BrotliBitReader* br) {
  uint32_t pad_bits_count = BrotliGetAvailableBits(br) & 0x7;
  uint32_t pad_bits = 0;
  if (pad_bits_count != 0) {
    BrotliTakeBits(br, pad_bits_count, &pad_bits);
  }
  return pad_bits == 0;
}

/* Peeks a byte at specified offset.
   Precondition: bit reader is parked to a byte boundary.
   Returns -1 if operation is not feasible. */
static BROTLI_INLINE int BrotliPeekByte(BrotliBitReader* br, size_t offset) {
  uint32_t available_bits = BrotliGetAvailableBits(br);
  size_t bytes_left = available_bits >> 3;
  BROTLI_DCHECK((available_bits & 7) == 0);
  if (offset < bytes_left) {
    return (BrotliGetBitsUnmasked(br) >> (unsigned)(offset << 3)) & 0xFF;
  }
  offset -= bytes_left;
  if (offset < br->avail_in) {
    return br->next_in[offset];
  }
  return -1;
}

/* Copies remaining input bytes stored in the bit reader to the output. Value
   num may not be larger than BrotliGetRemainingBytes. The bit reader must be
   warmed up again after this. */
static BROTLI_INLINE void BrotliCopyBytes(uint8_t* dest,
                                          BrotliBitReader* br, size_t num) {
  while (BrotliGetAvailableBits(br) >= 8 && num > 0) {
    *dest = (uint8_t)BrotliGetBitsUnmasked(br);
    BrotliDropBits(br, 8);
    ++dest;
    --num;
  }
  memcpy(dest, br->next_in, num);
  br->avail_in -= num;
  br->next_in += num;
}

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

#endif  /* BROTLI_DEC_BIT_READER_H_ */