Create consteval_huffman.hpp

diff --git a/consteval_huffman.hpp b/consteval_huffman.hpp
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+#ifndef TCSULLIVAN_CONSTEVAL_HUFFMAN_HPP_
+#define TCSULLIVAN_CONSTEVAL_HUFFMAN_HPP_
+
+#include <algorithm>
+#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.
+ */
+template<auto data>
+class huffman_compress
+{
+public:
+    using size_t = unsigned long int;
+
+    // Contains the compressed data.
+    unsigned char output[size()] = {};
+    // Contains a 'tree' that can be used to decompress the data.
+    unsigned char decode_tree[3 * tree_count()] = {};
+
+    // 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; }
+
+    consteval huffman_compress() {
+        build_decode_tree();
+        compress();
+    }
+
+private:
+    // Node structure used for tree-building.
+    struct node {
+        int value = 0;
+        size_t freq = 0;
+
+        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.
+    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++)
+            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());
+        std::fill(iter, table.end(), node());
+        return table;
+    }
+    // Returns the count of how many nodes in build_node_list() are valid nodes.
+    consteval static auto node_count() {
+        auto table = build_node_list();
+        size_t i;
+        for (i = 0; table[i].value != 0; i++);
+        delete[] table.data();
+        return i;
+    }
+    // Builds a tree out of the node list, allowing for compression and decompression.
+    consteval static auto build_node_tree() {
+        auto table = build_node_list();
+        
+        auto end = node_count();
+        size_t endend = 255;
+        unsigned char endv = 0xFF;
+        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();
+        }
+        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;
+                    }
+                }
+            }
+        }
+
+        return table;
+    }
+    // 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].
+    consteval static auto output_size() {
+        auto tree = build_node_tree();
+        size_t bytes = 0, bits = 0;
+        for (size_t i = 0; i < std::char_traits<char>::length(data); i++) {
+            auto leaf = std::find_if(tree.begin(), tree.end(), [c = data[i]](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 + 1, bits);
+    }
+    // Compresses the input data, placing the result in `output`.
+    consteval void compress()
+    {
+        auto tree = build_node_tree();
+        size_t bytes = size();
+        int bits = 5;
+        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; });
+            while (leaf->parent != -1) {
+                auto parent = tree.begin() + leaf->parent;
+                if (parent->right == leaf->value)
+                    output[bytes - 1] |= (1 << bits);
+                if (++bits == 8) {
+                    bits = 0;
+                    if (--bytes == 0)
+                        return;
+                }
+                leaf = parent;
+            }
+        }
+        delete[] tree.data();
+    }
+    // Builds the tree that can be used for decompression, stored in `decode_tree`.
+    consteval void build_decode_tree() {
+        auto tree = build_node_tree();
+        for (size_t i = 0; i < tree_count(); i++) {
+            decode_tree[i] = tree[i].value;
+            decode_tree[i + 1] = std::max(tree[i].left, 0);
+            decode_tree[i + 1] = std::max(tree[i].right, 0);
+        }
+        delete[] tree.data();
+    }
+};
+
+#endif // TCSULLIVAN_CONSTEVAL_HUFFMAN_HPP_