mashzip/huffman_table.cpp

#include "huffman_table.h"
#include "node.h"

#include <map>
#include <set>
#include <queue>
#include <iostream>

#define HEADER_SIZE 128

void initialize_table(const std::map<int, std::set<char> > &huffmanLengths,
                    std::unordered_map<char, std::pair<int, short> > &table)
{
    int nextbl = 0;
    short code = 0;

    for (auto lenCodePairIt = huffmanLengths.begin(); lenCodePairIt != huffmanLengths.end(); lenCodePairIt++)
    {
        auto lenCodePair = *lenCodePairIt;

        for (auto it = lenCodePair.second.begin(); it != lenCodePair.second.end(); it++)
        {
            table[*it].first = lenCodePair.first; // save current bit length for code
            table[*it].second = code;

            // code := (code + 1) << ((bit length of the next symbol) − (current bit length))
            // code++;

            // if last symbol of length and has symbols with greater length
            if (next(it) == lenCodePair.second.end() && next(lenCodePairIt) != huffmanLengths.end()) {
                nextbl = (*next(lenCodePairIt)).first;
            } else {
                nextbl = lenCodePair.first;
            }

            code = (code + 1) << (nextbl - lenCodePair.first);
        }

        // code <<= 1;
    }
}

HuffmanTable::HuffmanTable(uint8_t *header) {
    int cnt1, cnt2;
    std::map<int, std::set<char> > huffmanLengths;

    for (int i = 0; i < HEADER_SIZE; i++) {
        cnt1 = ((header[i] & 0b11110000) >> 4);
        cnt2 = (header[i] & 0b1111);
        if (cnt1 != 0) huffmanLengths[cnt1].insert((char)(i * 2));
        if (cnt2 != 0) huffmanLengths[cnt2].insert((char)(i * 2 + 1));
    }

    // build up codes
    initialize_table(huffmanLengths, this->huffmanCodes);
}

void get_lengths(Node* root, int len,
            std::map<int, std::set<char> > &huffmanLengths)
{
    if (!root)
        return;
    // std::cerr << "Get lengths node: " << root->getChar() << " " << root->getFreq() << root->getLeft() << " " << root->getRight() << std::endl;
    // found a leaf node
    if (root->isLeaf()) {
        // huffmanCode[root->ch] = str;
        // std::cerr << "Got leaf: " << root->getChar() << std::endl;
        huffmanLengths[len].insert(root->getChar());
    }

    get_lengths(root->getLeft(), len + 1, huffmanLengths);
    get_lengths(root->getRight(), len + 1, huffmanLengths);
}

HuffmanTable::HuffmanTable(std::basic_istream<char> &is) {
    // count frequency of appearance of each character
    // and store it in a map
    std::unordered_map<char, int> freq;
    char ch;
    while (is.get(ch)) {
        freq[ch]++;
    }

    std::cerr << "Calculated freqs" << std::endl;

    // Create a priority queue to store live nodes of
    // Huffman tree;
    std::priority_queue<Node*, std::vector<Node*>, NodeComp> pq;

    // Create a leaf node for each character and add it
    // to the priority queue.
    for (auto pair: freq) {
        Node *new_node = new Node(pair.first, pair.second, nullptr, nullptr);
        pq.push(new_node);
    }

    std::cerr << "Filled PQ: " << pq.size() << std::endl;

    // do till there is more than one node in the queue
    while (pq.size() != 1)
    {
        // Remove the two nodes of highest priority
        // (lowest frequency) from the queue
        Node *left = pq.top(); pq.pop();
        Node *right = pq.top(); pq.pop();

        // Create a new internal node with these two nodes
        // as children and with frequency equal to the sum
        // of the two nodes' frequencies. Add the new node
        // to the priority queue.
        int sum = left->getFreq() + right->getFreq();
        Node *new_node = new Node('\0', sum, left, right);
        pq.push(new_node);
    }

    // std::cerr << "Built tree: " << pq.size() << " " << pq.top()->getFreq() << std::endl;

    // root stores pointer to root of Huffman Tree
    Node* root = pq.top();

    std::map<int, std::set<char> > huffmanLengths;
    get_lengths(root, 0, huffmanLengths);

    // std::cerr << "Got lengths: " << huffmanLengths.size() << std::endl;

    initialize_table(huffmanLengths, this->huffmanCodes);
}

std::pair<int, short> HuffmanTable::operator[](const char &c) {
    return huffmanCodes[c];
}

void HuffmanTable::write_symbol(obitstream &os, const char &c) {
    if (huffmanCodes.find(c) == huffmanCodes.end()) throw std::runtime_error("No code in table for char!");

    os.writebits(huffmanCodes[c].second, huffmanCodes[c].first);
}

uint8_t *HuffmanTable::to_header() {
    uint8_t *header = new uint8_t[HEADER_SIZE];

    for (size_t i = 0; i < HEADER_SIZE; i++)
    {
        if (huffmanCodes.find(2 * i) != huffmanCodes.end()) {
            int len = huffmanCodes[2 * i].first;
            if (len > 0xf) throw std::runtime_error("Codes longer than 0xf are not allowed!");

            header[i] |= (len & 0xf) << 4;
        }

        if (huffmanCodes.find(2 * i + 1) != huffmanCodes.end()) {
            int len = huffmanCodes[2 * i + 1].first;
            if (len > 0xf) throw std::runtime_error("Codes longer than 0xf are not allowed!");

            header[i] |= (len & 0xf);
        }
    }
    
    return header;
}