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}

//------------------------------------------------------------------------------
// Compute susceptibility based on DCT-coeff histograms:
// the higher, the "easier" the macroblock is to compress.

const int VP8DspScan[16 + 4 + 4] = {
  // Luma
  0 +  0 * BPS,  4 +  0 * BPS, 8 +  0 * BPS, 12 +  0 * BPS,
  0 +  4 * BPS,  4 +  4 * BPS, 8 +  4 * BPS, 12 +  4 * BPS,
  0 +  8 * BPS,  4 +  8 * BPS, 8 +  8 * BPS, 12 +  8 * BPS,
  0 + 12 * BPS,  4 + 12 * BPS, 8 + 12 * BPS, 12 + 12 * BPS,

  0 + 0 * BPS,   4 + 0 * BPS, 0 + 4 * BPS,  4 + 4 * BPS,    // U
  8 + 0 * BPS,  12 + 0 * BPS, 8 + 4 * BPS, 12 + 4 * BPS     // V
};

// general-purpose util function
void VP8LSetHistogramData(const int distribution[MAX_COEFF_THRESH + 1],
                          VP8Histogram* const histo) {
  int max_value = 0, last_non_zero = 1;
  int k;
  for (k = 0; k <= MAX_COEFF_THRESH; ++k) {
    const int value = distribution[k];
    if (value > 0) {
      if (value > max_value) max_value = value;
      last_non_zero = k;
    }
  }
  histo->max_value = max_value;
  histo->last_non_zero = last_non_zero;
}

static void CollectHistogram(const uint8_t* ref, const uint8_t* pred,
                             int start_block, int end_block,
                             VP8Histogram* const histo) {
  int j;
  int distribution[MAX_COEFF_THRESH + 1] = { 0 };
  for (j = start_block; j < end_block; ++j) {
    int k;
    int16_t out[16];

    VP8FTransform(ref + VP8DspScan[j], pred + VP8DspScan[j], out);

    // Convert coefficients to bin.
    for (k = 0; k < 16; ++k) {
      const int v = abs(out[k]) >> 3;  // TODO(skal): add rounding?
      const int clipped_value = clip_max(v, MAX_COEFF_THRESH);
      ++distribution[clipped_value];
    }
  }
  VP8LSetHistogramData(distribution, histo);
}

//------------------------------------------------------------------------------
// run-time tables (~4k)

static uint8_t clip1[255 + 510 + 1];    // clips [-255,510] to [0,255]

// We declare this variable 'volatile' to prevent instruction reordering
// and make sure it's set to true _last_ (so as to be thread-safe)
static volatile int tables_ok = 0;

static WEBP_TSAN_IGNORE_FUNCTION void InitTables(void) {
  if (!tables_ok) {
    int i;
    for (i = -255; i <= 255 + 255; ++i) {
      clip1[255 + i] = clip_8b(i);
    }
    tables_ok = 1;
  }
}


//------------------------------------------------------------------------------
// Transforms (Paragraph 14.4)

#define STORE(x, y, v) \
  dst[(x) + (y) * BPS] = clip_8b(ref[(x) + (y) * BPS] + ((v) >> 3))

static const int kC1 = 20091 + (1 << 16);
static const int kC2 = 35468;
#define MUL(a, b) (((a) * (b)) >> 16)

static WEBP_INLINE void ITransformOne(const uint8_t* ref, const int16_t* in,
                                      uint8_t* dst) {
  int C[4 * 4], *tmp;
  int i;
  tmp = C;
  for (i = 0; i < 4; ++i) {    // vertical pass
    const int a = in[0] + in[8];
    const int b = in[0] - in[8];
    const int c = MUL(in[4], kC2) - MUL(in[12], kC1);
    const int d = MUL(in[4], kC1) + MUL(in[12], kC2);
    tmp[0] = a + d;
    tmp[1] = b + c;
    tmp[2] = b - c;
    tmp[3] = a - d;
    tmp += 4;
    in++;
  }

  tmp = C;
  for (i = 0; i < 4; ++i) {    // horizontal pass
    const int dc = tmp[0] + 4;
    const int a =  dc +  tmp[8];
    const int b =  dc -  tmp[8];
    const int c = MUL(tmp[4], kC2) - MUL(tmp[12], kC1);
    const int d = MUL(tmp[4], kC1) + MUL(tmp[12], kC2);
    STORE(0, i, a + d);
    STORE(1, i, b + c);
    STORE(2, i, b - c);
    STORE(3, i, a - d);
    tmp++;
  }
}

static void ITransform(const uint8_t* ref, const int16_t* in, uint8_t* dst,
                       int do_two) {
  ITransformOne(ref, in, dst);
  if (do_two) {