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// Copyright 2010 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// Boolean decoder
//
// Author: Skal (pascal.massimino@gmail.com)
//         Vikas Arora (vikaas.arora@gmail.com)

#ifndef WEBP_UTILS_BIT_READER_H_
#define WEBP_UTILS_BIT_READER_H_

#include <assert.h>
#ifdef _MSC_VER
#include <stdlib.h>  // _byteswap_ulong
#endif
#include "../webp/types.h"

#ifdef __cplusplus
extern "C" {
#endif

// The Boolean decoder needs to maintain infinite precision on the value_ field.
// However, since range_ is only 8bit, we only need an active window of 8 bits
// for value_. Left bits (MSB) gets zeroed and shifted away when value_ falls
// below 128, range_ is updated, and fresh bits read from the bitstream are
// brought in as LSB. To avoid reading the fresh bits one by one (slow), we
// cache BITS of them ahead. The total of (BITS + 8) bits must fit into a
// natural register (with type bit_t). To fetch BITS bits from bitstream we
// use a type lbit_t.
//
// BITS can be any multiple of 8 from 8 to 56 (inclusive).
// Pick values that fit natural register size.

#if defined(__i386__) || defined(_M_IX86)      // x86 32bit
#define BITS 24
#elif defined(__x86_64__) || defined(_M_X64)   // x86 64bit
#define BITS 56
#elif defined(__arm__) || defined(_M_ARM)      // ARM
#define BITS 24
#elif defined(__mips__)                        // MIPS
#define BITS 24
#else                                          // reasonable default
#define BITS 24  // TODO(skal): test aarch64 and find the proper BITS value.
#endif

//------------------------------------------------------------------------------
// Derived types and constants:
//   bit_t = natural register type for storing 'value_' (which is BITS+8 bits)
//   range_t = register for 'range_' (which is 8bits only)

#if (BITS > 24)
typedef uint64_t bit_t;
#else
typedef uint32_t bit_t;
#endif

typedef uint32_t range_t;

//------------------------------------------------------------------------------
// Bitreader

typedef struct VP8BitReader VP8BitReader;
struct VP8BitReader {
  // boolean decoder  (keep the field ordering as is!)
  bit_t value_;               // current value
  range_t range_;             // current range minus 1. In [127, 254] interval.
  int bits_;                  // number of valid bits left
  // read buffer
  const uint8_t* buf_;        // next byte to be read
  const uint8_t* buf_end_;    // end of read buffer
  int eof_;                   // true if input is exhausted
};

// Initialize the bit reader and the boolean decoder.
void VP8InitBitReader(VP8BitReader* const br,
                      const uint8_t* const start, const uint8_t* const end);

// Update internal pointers to displace the byte buffer by the
// relative offset 'offset'.
void VP8RemapBitReader(VP8BitReader* const br, ptrdiff_t offset);

// return the next value made of 'num_bits' bits
uint32_t VP8GetValue(VP8BitReader* const br, int num_bits);
static WEBP_INLINE uint32_t VP8Get(VP8BitReader* const br) {
  return VP8GetValue(br, 1);