Function write_huffman_tree 

pub fn write_huffman_tree(depth: &[u8]) -> CompressedTree

Compresses a depth array into RLE-encoded code length codes.

// libjxl: enc_huffman_tree.cc
void WriteHuffmanTree(const uint8_t* depth, size_t length, size_t* tree_size,
                      uint8_t* tree, uint8_t* extra_bits_data) {
  uint8_t previous_value = 8;
  // Throw away trailing zeros.
  size_t new_length = length;
  for (size_t i = 0; i < length; ++i) {
    if (depth[length - i - 1] == 0) {
      --new_length;
    } else {
      break;
    }
  }
  // First gather statistics on if it is a good idea to do rle.
  bool use_rle_for_non_zero = false;
  bool use_rle_for_zero = false;
  if (length > 50) {
    DecideOverRleUse(depth, new_length, &use_rle_for_non_zero, &use_rle_for_zero);
  }
  // Actual rle coding.
  for (size_t i = 0; i < new_length;) {
    const uint8_t value = depth[i];
    size_t reps = 1;
    if ((value != 0 && use_rle_for_non_zero) || (value == 0 && use_rle_for_zero)) {
      for (size_t k = i + 1; k < new_length && depth[k] == value; ++k) {
        ++reps;
      }
    }
    if (value == 0) {
      WriteHuffmanTreeRepetitionsZeros(reps, tree_size, tree, extra_bits_data);
    } else {
      WriteHuffmanTreeRepetitions(previous_value, value, reps, tree_size, tree, extra_bits_data);
      previous_value = value;
    }
    i += reps;
  }
}