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consteval-huffman
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decoder conforms to std::forward_iterator

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Clyne 4 years ago
parent 2fbe5393df
commit cfbe244e90
2 changed files with 55 additions and 31 deletions
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4
README.md
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@ -2,7 +2,7 @@
Allows for long string or data constants to be compressed at compile-time, with a small decoder routine for decompression at run-time.
Compression is achieved using Huffman coding, which works by creating codes for frequently-occuring characters.
Compression is achieved using [Huffman coding](https://en.wikipedia.org/wiki/Huffman_coding), which works by creating short (measured in bits) codes for frequently-occuring characters.
## Use cases
@ -14,7 +14,7 @@ A ~3.5kB string of JSON can be compressed down ~2.5kB ([see it on Godbolt](https
A ~40 line commented sample of Lisp can be reduced from 1,662 bytes to 1,244 (418 bytes saved) ([on Godbolt](https://godbolt.org/z/c64Pzz)).
Compression will work best on not-small blocks of text or data. This is because a decoding tree must be stored with the compressed data, requiring three bytes per value.
Compression will work best on larger blocks of text or data, as a decoding tree must be stored with the compressed data that requires three bytes per unique data byte.
## How to Use

82
consteval_huffman.hpp
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@ -11,19 +11,18 @@
#include <span>
/**
* Compresses the given character string using Huffman coding, providing a
* Compresses the given data string using Huffman coding, providing a
* minimal run-time interface for decompressing the data.
* @tparam data The string of data to be compressed.
* @tparam data_length The size in bytes of the data, defaults to using strlen().
* @tparam data_length The size in bytes of the data, defaulting to strlen() behavior.
*/
template<const char *data, auto data_length = std::char_traits<char>::length(data)>
requires(data_length > 0)
class huffman_compress
{
using size_t = long int;
// Jump to the bottom of this header for the public-facing features of this
// class.
// The internals needed to be defined before they were used.
// Note: class internals need to be defined before the public interface.
private:
// Node structure used to build a tree for calculating Huffman codes.
struct node {
@ -56,6 +55,8 @@ private:
auto first_valid_node = std::find_if(list.begin(), list.end(),
[](const auto& n) { return n.freq != 0; });
auto fit_size = std::distance(first_valid_node, list.end());
if (fit_size < 2)
fit_size = 2;
auto fit_list = std::span(new node[fit_size] {}, fit_size);
std::copy(first_valid_node, list.end(), fit_list.begin());
delete[] list.data();
@ -119,7 +120,9 @@ private:
for (auto iter = tree.begin(); ++iter != tree.end();) {
if (iter->parent == -1) {
auto parent = std::find_if(tree.begin(), iter,
[&iter](const auto& n) { return n.left == iter->value || n.right == iter->value; });
[&iter](const auto& n) {
return n.left == iter->value || n.right == iter->value;
});
if (parent != iter)
iter->parent = std::distance(tree.begin(), parent);
}
@ -230,34 +233,47 @@ public:
// Utility for decoding compressed data.
class decode_info {
public:
decode_info(const huffman_compress<data, data_length>& comp_data) :
using difference_type = std::ptrdiff_t;
using value_type = int;
decode_info(const huffman_compress<data, data_length>* comp_data) :
m_data(comp_data) { get_next(); }
decode_info() = default;
// Checks if another byte is available
operator bool() const {
decode_info& end() {
if constexpr (bytes_saved() > 0) {
const auto [size_bytes, last_bits] = m_data.compressed_size_info();
return m_pos < (size_bytes - 1) || m_bit > (8 - last_bits);
const auto [size_bytes, last_bits] = m_data->compressed_size_info();
m_pos = size_bytes - 1;
m_bit = 8 - last_bits;
} else {
return m_pos < data_length;
m_pos = data_length + 1;
}
return *this;
}
// Gets the current byte
int operator*() const { return m_current; }
// Moves to the next byte
int operator++() {
get_next();
bool operator==(const decode_info& other) const {
return m_data == other.m_data && m_bit == other.m_bit && m_pos == other.m_pos;
}
auto operator*() const {
return m_current;
}
decode_info& operator++() {
get_next();
return *this;
}
decode_info operator++(int) {
auto old = *this;
get_next();
return old;
}
private:
// Internal: moves to next byte
void get_next() {
if constexpr (bytes_saved() > 0) {
auto *node = m_data.decode_tree;
auto *node = m_data->decode_tree;
do {
bool bit = m_data.compressed_data[m_pos] & (1 << (m_bit - 1));
bool bit = m_data->compressed_data[m_pos] & (1 << (m_bit - 1));
if (--m_bit == 0)
m_bit = 8, m_pos++;
node += 3 * node[bit ? 2 : 1];
@ -268,7 +284,7 @@ public:
}
}
const huffman_compress<data>& m_data;
const huffman_compress<data> *m_data = nullptr;
size_t m_pos = 0;
unsigned char m_bit = 8;
int m_current = -1;
@ -276,7 +292,19 @@ public:
friend class huffman_compress;
};
consteval huffman_compress() {
auto begin() const {
return decode_info(this);
}
auto end() const {
return decode_info(this).end();
}
auto cbegin() const { begin(); }
auto cend() const { end(); }
// Stick the requires clause here just so it's run
consteval huffman_compress()
requires (std::forward_iterator<decode_info>)
{
if constexpr (bytes_saved() > 0) {
build_decode_tree();
compress();
@ -285,16 +313,12 @@ public:
}
}
// Creates a decoder object for iteratively decompressing the data.
auto get_decoder() const {
return decode_info(*this);
}
private:
// Contains the compressed data.
unsigned char compressed_data[bytes_saved() > 0 ? compressed_size_info().first : data_length] = {};
unsigned char compressed_data[bytes_saved() > 0 ? compressed_size_info().first : data_length] = {0};
// Contains a 'tree' that can be used to decompress the data.
unsigned char decode_tree[3 * tree_count()] = {};
unsigned char decode_tree[3 * tree_count()] = {0};
};
#endif // TCSULLIVAN_CONSTEVAL_HUFFMAN_HPP_

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