#include <iostream>
#include <vector>
#include <queue>
#include <string>
#include <unordered_map>

#define HT_TREE_MARK '@'

struct ht {
	char chr;
	size_t weight;

	struct ht *left{};
	struct ht *right{};

	ht(char _chr, size_t _w, struct ht *l = nullptr, struct ht *r = nullptr) :
		chr(_chr), weight(_w), left(l), right(r) {}
};

void ht_build_codes(struct ht *tree, std::unordered_map<char, std::string> &ht_codes, std::string path="")
{
	if (tree == nullptr)
		return;
	if (tree->chr != HT_TREE_MARK) {
		std::clog << tree->chr << ": " << path << std::endl;
		ht_codes[tree->chr] = path;
	}
	ht_build_codes(tree->left, ht_codes, path + '0');
	ht_build_codes(tree->right, ht_codes, path + '1');
}

bool ht_comp_fn(const struct ht *n1, const struct ht *n2)
{
	// enforce stable sorting
	if (n1->weight == n2->weight)
		return false;
	return n1->weight > n2->weight;
};

int main()
{
	std::string s = "abracadabraacacacabbaadadeea";

	// count occurences (frequency, weight) of each character in the message
	std::unordered_map<char, size_t> chr_occu;
	for (const auto& c : s)
		++chr_occu[c];

	std::priority_queue<struct ht *, std::vector<struct ht *>, decltype(&ht_comp_fn)> nodes(ht_comp_fn);
	// create initial nodes, from here we start building the tree
	for (const auto& v : chr_occu) {
		auto *node = new struct ht(v.first, v.second);
		nodes.push(node);
	}

	auto printpq = [](decltype(nodes) n) {
		while (!n.empty()) {
			auto *cn = n.top();
			std::clog << cn->chr << ":" << cn->weight << " ";
			n.pop();
		}
		std::clog << std::endl;
	};

	while (nodes.size() > 1) { // when the nodes queue element count is 1, we already have our tree
		// remove the two first (the one with the lowest weights)
		auto *n1 = nodes.top();
		nodes.pop();
		auto *n2 = nodes.top();
		nodes.pop();

		// create a new tree with a weight equal to the sum of the two popped nodes
		auto *node = new struct ht(HT_TREE_MARK, n1->weight + n2->weight, n1, n2);
		nodes.push(node);
	}

	std::unordered_map<char, std::string> ht_codes;
	std::string enc_msg;
	enc_msg.reserve(255);
	ht_build_codes(nodes.top(), ht_codes);

	for (const auto& c : s)
		enc_msg += ht_codes[c];
	std::clog << s << " -> " << enc_msg << std::endl;

	return 0;
}