BrotliEstimateBitCostsForLiterals(18.24)

BrotliEstimateBitCostsForLiterals.c

void BrotliEstimateBitCostsForLiterals(size_t pos, size_t len, size_t mask,
                                       const uint8_t* data, float* cost) {
  if (BrotliIsMostlyUTF8(data, pos, mask, len, kMinUTF8Ratio)) {
    EstimateBitCostsForLiteralsUTF8(pos, len, mask, data, cost);
    return;
  } else {
    size_t histogram[256] = { 0 };
    size_t window_half = 2000;
    size_t in_window = BROTLI_MIN(size_t, window_half, len);

    /* Bootstrap histogram. */
    size_t i;
    for (i = 0; i < in_window; ++i) {
      ++histogram[data[(pos + i) & mask]];
    }

    /* Compute bit costs with sliding window. */
    for (i = 0; i < len; ++i) {
      size_t histo;
      if (i >= window_half) {
        /* Remove a byte in the past. */
        --histogram[data[(pos + i - window_half) & mask]];
        --in_window;
      }
      if (i + window_half < len) {
        /* Add a byte in the future. */
        ++histogram[data[(pos + i + window_half) & mask]];
        ++in_window;
      }
      histo = histogram[data[(pos + i) & mask]];
      if (histo == 0) {
        histo = 1;
      }
      {
        double lit_cost = FastLog2(in_window) - FastLog2(histo);
        lit_cost += 0.029;
        if (lit_cost < 1.0) {
          lit_cost *= 0.5;
          lit_cost += 0.5;
        }
        cost[i] = (float)lit_cost;
      }
    }
  }
}