Added function for bit-packing compression-side
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110
src/bitpack.cc
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110
src/bitpack.cc
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#include "bitpack.hh"
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#include <algorithm>
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#include <cmath>
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using std::uint16_t;
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using std::vector;
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using uchar = unsigned char;
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using vuint16 = vector<uint16_t>;
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using vuchar = vector<uchar>;
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constexpr int ipow(int base, int exp) {
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int result = 1;
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for (;;) {
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if (exp & 1) {
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result *= base;
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}
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exp >>= 1;
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if (exp == 0) {
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break;
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}
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base *= base;
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}
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return result;
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}
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/**
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* Packs \p t_input into unsigned char, assuming the max value of t_input only
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* takes \p t_n bits
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*
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* \param t_input_begin pointer to the beginning of the vector of values to be
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* packed \param t_input_end pointer to the end of the input vector \param t_n
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* maximum size of an input value in bits \return Returns a vector of unsigned
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* char containing the packed values from t_input
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*/
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vuchar pack_n(const vuint16::const_iterator t_input_begin,
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const vuint16::const_iterator t_input_end, int t_n) {
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if (t_n == 16) {
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return pack_16(t_input_begin, t_input_end);
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}
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vuchar ret{};
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// max value with current number of bits + 1
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const int max_value = ipow(2, t_n);
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uchar current_char = 0;
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int step = t_n / 8;
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int left_shift = 0;
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int middle_shift = 0;
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int right_shift = 0;
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for (auto it = t_input_begin; it != t_input_end; ++it) {
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if (*t_input_end >= max_value) {
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ret.push_back(current_char);
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const auto next_vec = pack_n(it, t_input_end, t_n + 1);
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ret.insert(ret.end(), next_vec.begin(), next_vec.end());
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return ret;
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}
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left_shift += step;
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if (left_shift >= t_n) {
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left_shift = (left_shift - t_n) + step;
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}
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ret.push_back((current_char | (*it >> left_shift)) & 0xFF);
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current_char = 0;
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bool zero_right_shift = (right_shift == 0);
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right_shift -= step;
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if (right_shift < 0) {
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middle_shift = std::abs(right_shift);
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right_shift = 8 + std::abs(right_shift);
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if (!zero_right_shift) {
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ret.push_back((*it >> middle_shift) & 0xFF);
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}
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}
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if (right_shift == 0) {
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ret.push_back(*it & 0xff);
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current_char = 0;
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} else {
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current_char = (*it << right_shift) & 0xFF;
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}
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}
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if (current_char != 0) {
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ret.push_back(current_char);
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}
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return ret;
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}
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vuchar pack_16(const vuint16::const_iterator t_input_begin,
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const vuint16::const_iterator t_input_end) {
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vuchar ret{};
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std::for_each(t_input_begin, t_input_end, [&](const auto value) {
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ret.push_back((value >> 8) & 0xFF);
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ret.push_back(value & 0xFF);
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});
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return ret;
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}
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vuchar pack(const vuint16 t_input) {
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vuchar ret{};
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constexpr int max_value = ipow(2, 8);
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for(auto it = t_input.begin(); it != t_input.end(); ++it) {
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if(*it >= max_value) {
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const auto next_vec =
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pack_n(static_cast<vuint16::const_iterator>(it), t_input.end(), 9);
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ret.insert(ret.end(), next_vec.begin(), next_vec.end());
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return ret;
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}
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ret.push_back(static_cast<unsigned char>(*it));
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}
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return ret;
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}
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21
src/bitpack.hh
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21
src/bitpack.hh
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@ -0,0 +1,21 @@
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/**
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* \file bitpack.hh
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* \brief Header for bit-packing functions
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*/
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#ifndef LZW_SRC_BITPACK_H_
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#define LZW_SRC_BITPACK_H_
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#include <cstdint>
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#include <vector>
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/// \brief Packs std::uint16_t of n bits into unsigned char
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std::vector<unsigned char> pack_n(const std::vector<std::uint16_t>::const_iterator,
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const std::vector<std::uint16_t>::const_iterator,
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int);
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/// \brief Specialization of \ref pack_n for 16bits
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std::vector<unsigned char> pack_16(const std::vector<std::uint16_t>::const_iterator,
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const std::vector<std::uint16_t>::const_iterator);
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#endif /* LZW_SRC_BITPACK_H_ */
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