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Clyne 4 years ago
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consteval_huffman.hpp
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@ -1,3 +1,10 @@
/**
* 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_
@ -5,106 +12,130 @@
#include <span>
/**
* Compresses given data at compile-time, while also providing utilities for decoding.
* @tparam data Expected to be a null-terminated `char` of data to be compressed.
* Compresses the given character 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().
*/
template<const char *data>
template<const char *data, std::size_t data_length = std::char_traits<char>::length(data)>
class huffman_compress
{
using size_t = unsigned long int;
// The internals for this class needed to be defined before they're used in
// the public interface. Scroll to the next `public` section for usable variables/functions.
// 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.
private:
// Node structure used for tree-building.
// Node structure used to build a tree for calculating Huffman codes.
struct node {
int value = 0;
size_t freq = 0;
// Below values are indices into the node list
int parent = -1;
int left = -1;
int right = -1;
};
// Builds a list of nodes for every character that appears in the data.
// This list is sorted by increasing frequency.
/**
* 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() {
auto table = std::span(new node[256] {}, 256);
for (int i = 0; i < 256; i++)
table[i].value = i;
for (size_t i = 0; data[i]; i++)
for (size_t i = 0; i < data_length; i++)
table[data[i]].freq++;
std::sort(table.begin(), table.end(), [](auto& a, auto& b) { return a.freq < b.freq; });
int empty_count;
for (empty_count = 0; table[empty_count].freq == 0; empty_count++);
auto iter = std::copy(table.begin() + empty_count, table.end(), table.begin());
auto first_valid_node = std::find_if(table.begin(), table.end(),
[](const auto& n) { return n.freq != 0; });
auto iter = std::copy(first_valid_node, table.end(), table.begin());
std::fill(iter, table.end(), node());
return table;
}
// Returns the count of how many nodes in build_node_list() are valid nodes.
/**
* Counts how many nodes in build_node_list() are valid.
* @return Number of valid nodes, i.e. the "size" of the list.
*/
consteval static auto node_count() {
auto table = build_node_list();
size_t i;
for (i = 0; table[i].value != 0; i++);
for (i = 0; table[i].freq != 0; i++);
delete[] table.data();
return i;
}
// Builds a tree out of the node list, allowing for compression and decompression.
// Returns the count of how many nodes are in the node tree.
public:
consteval static auto tree_count() {
return node_count() * 2 - 1;
}
/**
* 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() {
auto table = build_node_list();
auto list = build_node_list();
auto tree = std::span(new node[tree_count()] {}, tree_count());
auto end = node_count();
size_t endend = 255;
unsigned char endv = 0xFE;
while (table[1].freq != 0) {
node n { endv--,
table[0].freq + table[1].freq,
-1,
table[0].value,
table[1].value };
table[endend--] = table[0];
table[endend--] = table[1];
size_t insert;
for (insert = 0;
table[insert].freq != 0 && table[insert].freq < n.freq;
insert++);
std::copy_backward(table.begin() + insert,
table.begin() + end,
table.begin() + end + 1);
table[insert] = n;
std::copy(table.begin() + 2, table.begin() + end + 1, table.begin());
table[end - 1] = node();
table[end--] = node();
auto list_end = node_count();
auto tree_begin = tree.end();
int next_merged_node_value = 0x100;
while (list[1].freq != 0) {
// Create the merged parent node
node new_node {
next_merged_node_value++,
list[0].freq + list[1].freq,
-1,
list[0].value,
list[1].value
};
*--tree_begin = list[0];
*--tree_begin = list[1];
auto insertion_point = list.begin();
while (insertion_point->freq != 0 && insertion_point->freq < new_node.freq)
insertion_point++;
std::copy_backward(insertion_point, list.begin() + list_end, list.begin() + list_end + 1);
*insertion_point = new_node;
std::copy(list.begin() + 2, list.begin() + list_end + 1, list.begin());
list[list_end - 1] = node();
list[list_end--] = node();
}
std::copy(table.begin() + endend + 1, table.end(), table.begin() + 1);
for (size_t i = 1; i < 256 - endend; i++) {
if (table[i].parent == -1) {
for (size_t j = 0; j < i; j++) {
if (table[j].left == table[i].value || table[j].right == table[i].value) {
table[i].parent = j;
break;
}
}
// 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; });
if (parent != iter)
iter->parent = std::distance(tree.begin(), parent);
}
}
return table;
delete[] list.data();
return tree;
}
// Returns the count of how many nodes are in the node tree.
consteval static auto tree_count() {
auto table = build_node_tree();
size_t i;
for (i = 0; i < 256 && table[i].value != 0; i++);
delete[] table.data();
return i;
}
// Determines the size of the compressed data.
// Returns a pair: [total byte size, bits used in last byte].
/**
* Determines the size of the compressed data.
* Returns a pair: [total byte size, bits used in last byte].
*/
consteval static auto output_size() {
auto tree = build_node_tree();
size_t bytes = 1, bits = 0;
for (size_t i = 0; i < std::char_traits<char>::length(data); i++) {
for (size_t i = 0; i < data_length; i++) {
auto leaf = std::find_if(tree.begin(), tree.end(),
[c = data[i]](auto& n) { return n.value == c; });
while (leaf->parent != -1) {
@ -121,15 +152,15 @@ private:
consteval void compress()
{
auto tree = build_node_tree();
size_t bytes = size();
size_t bytes = output_size().first;
int bits;
if (auto bitscount = output_size().second; bitscount > 0)
bits = 8 - bitscount;
else
bits = 0, bytes--;
for (size_t i = std::char_traits<char>::length(data); i > 0; i--) {
auto leaf = std::find_if(tree.begin(), tree.begin() + tree_count(),
[c = data[i - 1]](auto& n) { return n.value == c; });
for (size_t i = data_length; i > 0; i--) {
auto leaf = std::find_if(tree.begin(), tree.end(),
[c = data[i - 1]](auto& n) { return n.value == c; });
while (leaf->parent != -1) {
auto parent = tree.begin() + leaf->parent;
if (parent->right == leaf->value)
@ -146,48 +177,39 @@ private:
auto tree = build_node_tree();
for (size_t i = 0; i < tree_count(); i++) {
decode_tree[i * 3] = tree[i].value;
decode_tree[i * 3] = tree[i].value <= 0xFF ? tree[i].value : 0;
size_t j;
for (j = i + 1; j < tree_count(); j++) {
if (tree[i].left == tree[j].value)
break;
}
decode_tree[i * 3 + 1] = j < tree_count() ? j : 0;
decode_tree[i * 3 + 1] = j < tree_count() ? j - i : 0;
for (j = i + 1; j < tree_count(); j++) {
if (tree[i].right == tree[j].value)
break;
}
decode_tree[i * 3 + 2] = j < tree_count() ? j : 0;
decode_tree[i * 3 + 2] = j < tree_count() ? j - i : 0;
}
delete[] tree.data();
}
public:
// Returns the size of the compressed data, in bytes.
consteval static auto size() { return output_size().first; }
// Returns how many of the bits in the last byte of `output` are actually part of the data.
consteval static auto lastbitscount() { return output_size().second; }
// Contains the compressed data.
unsigned char output[size()] = {};
unsigned char output[output_size().first] = {};
// Contains a 'tree' that can be used to decompress the data.
unsigned char decode_tree[3 * tree_count()] = {};
consteval huffman_compress() {
build_decode_tree();
compress();
}
public:
unsigned char decode_tree[3 * tree_count()] = {};
// Utility for decoding compressed data.
class decode_info {
public:
decode_info(const huffman_compress<data>& data_) :
m_data(data_) { get_next(); }
decode_info(const huffman_compress<data, data_length>& comp_data) :
m_data(comp_data) { get_next(); }
// Checks if another byte is available
operator bool() const {
return m_pos < (m_data.size() - 1) || m_bit > (8 - m_data.lastbitscount());
const auto [size_bytes, last_bits_count] = m_data.output_size();
return m_pos < (size_bytes - 1) || m_bit > (8 - last_bits_count);
}
// Gets the current byte
int operator*() const { return m_current; }
@ -200,12 +222,12 @@ public:
private:
// Internal: moves to next byte
void get_next() {
auto node = m_data.decode_tree;
auto *node = m_data.decode_tree;
do {
bool bit = m_data.output[m_pos] & (1 << (m_bit - 1));
if (--m_bit == 0)
m_bit = 8, m_pos++;
node = m_data.decode_tree + 3 * node[bit ? 2 : 1];
node += 3 * node[bit ? 2 : 1];
} while (node[1] != 0);
m_current = *node;
}
@ -214,8 +236,25 @@ public:
size_t m_pos = 0;
unsigned char m_bit = 8;
int m_current = -1;
friend class huffman_compress;
};
consteval huffman_compress() {
build_decode_tree();
compress();
}
consteval static auto compressed_size() {
return output_size().first + output_size().second;
}
consteval static auto uncompressed_size() {
return data_length;
}
consteval static auto bytes_saved() {
return uncompressed_size() - compressed_size();
}
// Creates a decoder object for iteratively decompressing the data.
auto get_decoder() const {
return decode_info(*this);
@ -223,3 +262,4 @@ public:
};
#endif // TCSULLIVAN_CONSTEVAL_HUFFMAN_HPP_

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