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C++

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/**
* consteval_huffman.hpp - Provides compile-time text compression.
* Written by Clyne Sullivan.
* https://github.com/tcsullivan/consteval-huffman
*/
#ifndef TCSULLIVAN_CONSTEVAL_HUFFMAN_HPP_
#define TCSULLIVAN_CONSTEVAL_HUFFMAN_HPP_
#include <algorithm>
#include <concepts>
#include <span>
#include <type_traits>
namespace detail
{
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// Provides a string container for the huffman compressor.
// Using this allows for automatic string data length measurement, as
// well as implementation of the _huffman suffix.
template<unsigned long int N>
struct huffman_string_container {
char data[N];
consteval huffman_string_container(const char (&s)[N]) noexcept {
std::copy(s, s + N, data);
}
consteval operator const char *() const noexcept {
return data;
}
consteval auto size() const noexcept {
return N;
}
};
}
/**
* Compresses the given data string using Huffman coding, providing a
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* minimal run-time interface for decompressing the data.
* @tparam data The string of data to be compressed.
*/
template<auto raw_data>
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requires(
std::same_as<std::remove_cvref_t<decltype(raw_data)>,
detail::huffman_string_container<raw_data.size()>> &&
raw_data.size() > 0)
class huffman_compressor
{
using size_t = long int;
using usize_t = unsigned long int;
// Note: class internals need to be defined before the public interface.
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// See the bottom of the class definition for usage.
private:
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// Node structure used to build a tree for calculating Huffman codes.
struct node {
int value = 0;
size_t freq = 0;
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// Below values are indices into the node list
int parent = -1;
int left = -1;
int right = -1;
};
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/**
* Builds a list of nodes for every character that appears in the given data.
* This list is sorted by increasing frequency.
* @return Compile-time allocated array of nodes
*/
consteval static auto build_node_list() noexcept {
// Build a list for counting every occuring value
auto list = std::span(new node[256] {}, 256);
for (int i = 0; i < 256; i++)
list[i].value = i;
for (usize_t i = 0; i < raw_data.size(); i++)
list[raw_data[i]].freq++;
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std::sort(list.begin(), list.end(),
[](const auto& a, const auto& b) { return a.freq < b.freq; });
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// Filter out the non-occuring values, and build a compact list to return
auto first_valid_node = std::find_if(list.begin(), list.end(),
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[](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();
return fit_list;
}
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/**
* Returns the count of how many nodes are in the node tree.
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*/
consteval static auto tree_count() noexcept {
auto list = build_node_list();
auto count = list.size() * 2 - 1;
delete[] list.data();
return count;
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}
/**
* Builds a tree out of the node list, allowing for the calculation of
* Huffman codes.
* @return Compile-time allocated tree of nodes, root node at index zero.
*/
consteval static auto build_node_tree() noexcept {
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auto list = build_node_list();
auto tree = std::span(new node[tree_count()] {}, tree_count());
auto list_end = list.end(); // Track end of list as it shrinks
auto tree_begin = tree.end(); // Build tree from bottom
int next_parent_node_value = 0x100; // Give parent nodes unique ids
while (1) {
// Create parent node for two least-occuring values
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node new_node {
next_parent_node_value++,
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list[0].freq + list[1].freq,
-1,
list[0].value,
list[1].value
};
// Move the two nodes into the tree and remove them from the list
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*--tree_begin = list[0];
*--tree_begin = list[1];
std::copy(list.begin() + 2, list_end--, list.begin());
if (std::distance(list.begin(), list_end) == 1) {
list.front() = new_node;
break;
}
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// Insert the parent node back into the list
auto insertion_point = std::find_if(list.begin(), list_end - 1,
[&new_node](const auto& n) { return n.freq >= new_node.freq; });
if (insertion_point != list_end - 1) {
*(list_end - 1) = node();
std::copy_backward(insertion_point, list_end - 1, list_end);
}
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*insertion_point = new_node;
}
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// Connect child nodes to their parents
tree[0] = list[0];
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;
});
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if (parent != iter)
iter->parent = std::distance(tree.begin(), parent);
}
}
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delete[] list.data();
return tree;
}
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/**
* Determines the size of the compressed data.
* @return A pair of total bytes used, and bits used in last byte.
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*/
consteval static auto compressed_size_info() noexcept {
auto tree = build_node_tree();
size_t bytes = 1, bits = 0;
for (usize_t i = 0; i < raw_data.size(); i++) {
auto leaf = std::find_if(tree.begin(), tree.end(),
[c = raw_data[i]](const auto& n) { return n.value == c; });
while (leaf->parent != -1) {
if (++bits == 8)
bits = 0, bytes++;
leaf = tree.begin() + leaf->parent;
}
}
delete[] tree.data();
return std::make_pair(bytes, bits);
}
/**
* Compresses the input data, storing the result in the object instance.
*/
consteval void compress() noexcept {
auto tree = build_node_tree();
// Set up byte and bit count (note, we're compressing the data backwards)
auto [bytes, bits] = compressed_size_info();
if (bits > 0)
bits = 8 - bits;
else
bits = 0, bytes--;
// Compress data backwards, because we obtain the Huffman codes backwards
// as we traverse towards the parent node.
for (auto i = raw_data.size(); i > 0; i--) {
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auto leaf = std::find_if(tree.begin(), tree.end(),
[c = raw_data[i - 1]](auto& n) { return n.value == c; });
while (leaf->parent != -1) {
auto parent = tree.begin() + leaf->parent;
if (parent->right == leaf->value)
compressed_data[bytes - 1] |= (1 << bits);
if (++bits == 8)
bits = 0, --bytes;
leaf = parent;
}
}
delete[] tree.data();
}
/**
* Builds the decode tree, used to decompress the data.
* Format: three bytes per node.
* 1. Node value, 2. Distance to left child, 3. Distance to right child.
*/
consteval void build_decode_tree() noexcept {
auto tree = build_node_tree();
auto decode_tree = compressed_data + compressed_size_info().first;
for (usize_t i = 0; i < tree_count(); i++) {
// Only store node value if it represents a data value
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decode_tree[i * 3] = tree[i].value <= 0xFF ? tree[i].value : 0;
usize_t j;
// Find the left child of this node
for (j = i + 1; j < tree_count(); j++) {
if (tree[i].left == tree[j].value)
break;
}
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decode_tree[i * 3 + 1] = j < tree_count() ? j - i : 0;
// Find the right child of this node
for (j = i + 1; j < tree_count(); j++) {
if (tree[i].right == tree[j].value)
break;
}
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decode_tree[i * 3 + 2] = j < tree_count() ? j - i : 0;
}
delete[] tree.data();
}
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public:
consteval static auto compressed_size() noexcept {
return compressed_size_info().first + 3 * tree_count();
}
consteval static auto uncompressed_size() noexcept {
return raw_data.size();
}
consteval static size_t bytes_saved() noexcept {
size_t diff = uncompressed_size() - compressed_size();
return diff > 0 ? diff : 0;
}
// Utility for decoding compressed data.
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class decoder {
public:
using difference_type = std::ptrdiff_t;
using value_type = int;
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decoder(const unsigned char *comp_data) noexcept
: m_data(comp_data),
m_table(comp_data + compressed_size_info().first) { get_next(); }
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decoder() = default;
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constexpr static decoder end(const unsigned char *comp_data) noexcept {
decoder ender;
ender.m_data = comp_data;
if constexpr (bytes_saved() > 0) {
const auto [size_bytes, last_bits] = compressed_size_info();
ender.m_data += size_bytes - 1;
ender.m_bit = 1 << (7 - last_bits);
} else {
ender.m_data += raw_data.size() + 1;
}
return ender;
}
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bool operator==(const decoder& other) const noexcept {
return m_data == other.m_data && m_bit == other.m_bit;
}
auto operator*() const noexcept {
return m_current;
}
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decoder& operator++() noexcept {
get_next();
return *this;
}
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decoder operator++(int) noexcept {
auto old = *this;
get_next();
return old;
}
private:
void get_next() noexcept {
if (*this == end(m_data))
return;
if constexpr (bytes_saved() > 0) {
auto *node = m_table;
int data = *m_data;
auto bit = m_bit;
do {
node += (data & bit) ? node[2] * 3u : node[1] * 3u;
bit >>= 1;
if (!bit)
bit = 0x80, data = *++m_data;
} while (node[1] != 0);
m_bit = bit;
m_current = *node;
} else {
m_current = *m_data++;
}
}
const unsigned char *m_data = nullptr;
const unsigned char *m_table = nullptr;
unsigned char m_bit = 0x80;
int m_current = -1;
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friend class huffman_compressor;
};
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// Stick the forward_iterator check here just so it's run
consteval huffman_compressor() noexcept
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requires (std::forward_iterator<decoder>)
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{
if constexpr (bytes_saved() > 0) {
build_decode_tree();
compress();
} else {
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std::copy(raw_data.data, raw_data.data + raw_data.size(),
compressed_data);
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}
}
auto begin() const noexcept {
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return decoder(compressed_data);
}
auto end() const noexcept {
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return decoder::end(compressed_data);
}
auto cbegin() const noexcept { return begin(); }
auto cend() const noexcept { return end(); }
// For accessing the compressed data
auto data() const noexcept {
if constexpr (bytes_saved() > 0)
return compressed_data;
else
return raw_data;
}
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auto size() const noexcept {
if constexpr (bytes_saved() > 0)
return compressed_size();
else
return uncompressed_size();
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}
private:
// Contains the compressed data, followed by the decoding tree.
unsigned char compressed_data[
bytes_saved() > 0 ? compressed_size_info().first + 3 * tree_count()
: raw_data.size()] = {0};
};
template <detail::huffman_string_container hsc>
constexpr auto operator ""_huffman()
{
return huffman_compressor<hsc>();
}
#endif // TCSULLIVAN_CONSTEVAL_HUFFMAN_HPP_