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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
+{
+ // 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
+ * minimal run-time interface for decompressing the data.
+ * @tparam data The string of data to be compressed.
+ */
+template<auto raw_data>
+ 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.
+ // See the bottom of the class definition for usage.
+private:
+ // 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 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++;
+
+ std::sort(list.begin(), list.end(),
+ [](const auto& a, const auto& b) { return a.freq < b.freq; });
+
+ // Filter out the non-occuring values, and build a compact list to return
+ 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();
+ return fit_list;
+ }
+
+ /**
+ * Returns the count of how many nodes are in the node tree.
+ */
+ consteval static auto tree_count() noexcept {
+ auto list = build_node_list();
+ auto count = list.size() * 2 - 1;
+ delete[] list.data();
+ return count;
+ }
+
+ /**
+ * 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 {
+ 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
+ node new_node {
+ next_parent_node_value++,
+ 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
+ *--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;
+ }
+
+ // 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);
+ }
+
+ *insertion_point = new_node;
+ }
+
+ // 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);
+ }
+ }
+
+ delete[] list.data();
+ return tree;
+ }
+
+ /**
+ * Determines the size of the compressed data.
+ * @return A pair of total bytes used, and bits used in last byte.
+ */
+ 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--) {
+ 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
+ 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;
+ }
+ 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;
+ }
+ decode_tree[i * 3 + 2] = j < tree_count() ? j - i : 0;
+ }
+
+ delete[] tree.data();
+ }
+
+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.
+ class decoder {
+ public:
+ using difference_type = std::ptrdiff_t;
+ using value_type = int;
+
+ decoder(const unsigned char *comp_data) noexcept
+ : m_data(comp_data),
+ m_table(comp_data + compressed_size_info().first) { get_next(); }
+ decoder() = default;
+
+ 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;
+ }
+
+ 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;
+ }
+ decoder& operator++() noexcept {
+ get_next();
+ return *this;
+ }
+ 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;
+
+ friend class huffman_compressor;
+ };
+
+ // Stick the forward_iterator check here just so it's run
+ consteval huffman_compressor() noexcept
+ requires (std::forward_iterator<decoder>)
+ {
+ if constexpr (bytes_saved() > 0) {
+ build_decode_tree();
+ compress();
+ } else {
+ std::copy(raw_data.data, raw_data.data + raw_data.size(),
+ compressed_data);
+ }
+ }
+
+ auto begin() const noexcept {
+ return decoder(compressed_data);
+ }
+ auto end() const noexcept {
+ 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;
+ }
+
+ auto size() const noexcept {
+ if constexpr (bytes_saved() > 0)
+ return compressed_size();
+ else
+ return uncompressed_size();
+ }
+
+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_
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