ArithmeticEncoder.cpp

#include "ArithmeticEncoder.hpp"
#include <cassert>
#include <cstdint>
#include <cstdio>

ArithmeticEncoder::ArithmeticEncoder(File* f) : x1(0), x2(0xffffffff), x(0), archive(f) {};

void ArithmeticEncoder::prefetch() {
  for (int i = 0; i < 4; ++i) {
    x = (x << 8) + (archive->getchar() & 255);
  }
}

void ArithmeticEncoder::flush() {
  archive->putChar(x1 >> 24); // flush first unequal byte of range
}

void ArithmeticEncoder::encodeBit(int p, int bit) {
  if (p == 0) p++;
  assert(p > 0 && p < 65536);
  uint32_t xMid = x1 + ((x2 - x1) >> 16) * p + (((x2 - x1) & 0xffff) * p >> 16);

  assert(xMid >= x1 && xMid < x2);
  bit != 0 ? (x2 = xMid) : (x1 = xMid + 1);
  while (((x1 ^ x2) & 0xFF000000) == 0) { // pass equal leading bytes of range
    archive->putChar(x2 >> 24);
    x1 <<= 8;
    x2 = (x2 << 8) + 255;
  }
}

int ArithmeticEncoder::decodeBit(int p) {
  if (p == 0) p++;
  assert(p > 0 && p < 65536);
  uint32_t xMid = x1 + ((x2 - x1) >> 16) * p +(((x2 - x1) & 0xffff) * p >> 16);
  assert(xMid >= x1 && xMid < x2);
  int bit = static_cast<int>(x <= xMid);
  bit != 0 ? (x2 = xMid) : (x1 = xMid + 1);
  while (((x1 ^ x2) & 0xFF000000) == 0) { // pass equal leading bytes of range
    x1 <<= 8;
    x2 = (x2 << 8) + 255;
    x = (x << 8) + (archive->getchar() & 255); // EOF is OK
  }
  return bit;
}