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  }
  if (y < enc->mb_h_ - 1) {  // top
    memcpy(it->y_top_, ysrc + 15 * BPS, 16);
    memcpy(it->uv_top_, uvsrc + 7 * BPS, 8 + 8);
  }
}

int VP8IteratorNext(VP8EncIterator* const it) {
  it->preds_ += 4;
  it->mb_ += 1;
  it->nz_ += 1;
  it->y_top_ += 16;
  it->uv_top_ += 16;
  it->x_ += 1;
  if (it->x_ == it->enc_->mb_w_) {
    VP8IteratorSetRow(it, ++it->y_);
  }
  return (0 < --it->count_down_);
}

//------------------------------------------------------------------------------
// Helper function to set mode properties

void VP8SetIntra16Mode(const VP8EncIterator* const it, int mode) {
  uint8_t* preds = it->preds_;
  int y;
  for (y = 0; y < 4; ++y) {
    memset(preds, mode, 4);
    preds += it->enc_->preds_w_;
  }
  it->mb_->type_ = 1;
}

void VP8SetIntra4Mode(const VP8EncIterator* const it, const uint8_t* modes) {
  uint8_t* preds = it->preds_;
  int y;
  for (y = 4; y > 0; --y) {
    memcpy(preds, modes, 4 * sizeof(*modes));
    preds += it->enc_->preds_w_;
    modes += 4;
  }
  it->mb_->type_ = 0;
}

void VP8SetIntraUVMode(const VP8EncIterator* const it, int mode) {
  it->mb_->uv_mode_ = mode;
}

void VP8SetSkip(const VP8EncIterator* const it, int skip) {
  it->mb_->skip_ = skip;
}

void VP8SetSegment(const VP8EncIterator* const it, int segment) {
  it->mb_->segment_ = segment;
}

//------------------------------------------------------------------------------
// Intra4x4 sub-blocks iteration
//
//  We store and update the boundary samples into an array of 37 pixels. They
//  are updated as we iterate and reconstructs each intra4x4 blocks in turn.
//  The position of the samples has the following snake pattern:
//
// 16|17 18 19 20|21 22 23 24|25 26 27 28|29 30 31 32|33 34 35 36  <- Top-right
// --+-----------+-----------+-----------+-----------+
// 15|         19|         23|         27|         31|
// 14|         18|         22|         26|         30|
// 13|         17|         21|         25|         29|
// 12|13 14 15 16|17 18 19 20|21 22 23 24|25 26 27 28|
// --+-----------+-----------+-----------+-----------+
// 11|         15|         19|         23|         27|
// 10|         14|         18|         22|         26|
//  9|         13|         17|         21|         25|
//  8| 9 10 11 12|13 14 15 16|17 18 19 20|21 22 23 24|
// --+-----------+-----------+-----------+-----------+
//  7|         11|         15|         19|         23|
//  6|         10|         14|         18|         22|
//  5|          9|         13|         17|         21|
//  4| 5  6  7  8| 9 10 11 12|13 14 15 16|17 18 19 20|
// --+-----------+-----------+-----------+-----------+
//  3|          7|         11|         15|         19|
//  2|          6|         10|         14|         18|
//  1|          5|          9|         13|         17|
//  0| 1  2  3  4| 5  6  7  8| 9 10 11 12|13 14 15 16|
// --+-----------+-----------+-----------+-----------+

// Array to record the position of the top sample to pass to the prediction
// functions in dsp.c.
static const uint8_t VP8TopLeftI4[16] = {
  17, 21, 25, 29,
  13, 17, 21, 25,
  9,  13, 17, 21,
  5,   9, 13, 17
};

void VP8IteratorStartI4(VP8EncIterator* const it) {
  const VP8Encoder* const enc = it->enc_;
  int i;

  it->i4_ = 0;    // first 4x4 sub-block
  it->i4_top_ = it->i4_boundary_ + VP8TopLeftI4[0];

  // Import the boundary samples
  for (i = 0; i < 17; ++i) {    // left
    it->i4_boundary_[i] = it->y_left_[15 - i];
  }
  for (i = 0; i < 16; ++i) {    // top
    it->i4_boundary_[17 + i] = it->y_top_[i];
  }
  // top-right samples have a special case on the far right of the picture
  if (it->x_ < enc->mb_w_ - 1) {
    for (i = 16; i < 16 + 4; ++i) {
      it->i4_boundary_[17 + i] = it->y_top_[i];
    }
  } else {    // else, replicate the last valid pixel four times
    for (i = 16; i < 16 + 4; ++i) {
      it->i4_boundary_[17 + i] = it->i4_boundary_[17 + 15];
    }
  }
  VP8IteratorNzToBytes(it);  // import the non-zero context
}