Rework C++ implementation for Huffman encoding

- Make implementation shorter and easier to understand - Make namespace hierarchy flat - Combine Leaf and Branch in one class Node for the tree - Make codebook explicit - Use std::string for string_to_encode - Remove noise (delete function with_new_bit() for instance) - Use unordered map for weights instead of vector - Instead inserting to the orphan array, sort it on every iteration (debatable) - Remove structure for cacheing bits for character, calculate it every time - Remove error handling - Change the encoding interface: encode() and decode() instead huffman::encode_string() and encoded.decoded()

Author: mika314

contents/huffman_encoding/code/c++/huffman.cpp

@@ -1,268 +1,124 @@
 #include <algorithm>
-#include <array>
-#include <bitset>
 #include <cassert>
-#include <cctype>
-#include <cstddef>
-#include <limits>
+#include <iostream>
 #include <memory>
 #include <string>
-#include <utility>
+#include <unordered_map>
 #include <vector>
 
-#include <iostream>
-
-using std::begin;
-using std::end;
-
-namespace huffman {
-
-[[noreturn]] inline void unreachable() {
- std::cerr << "this should never happen\n";
- std::abort();
-}
-
-// --- interface ---
-class codebook {
- struct node {
- int frequency;
-
- node(int freq) : frequency(freq) {}
- virtual ~node() = 0;
- };
-
- // never null
- using node_ptr = std::unique_ptr<node const>;
- using bitstring = std::vector<bool>;
-
- // this is a flatmap between char and a bitstring
- // there should only ever be one character with a given
- // value at any time.
- using encoder_t = std::vector<std::pair<char, bitstring>>;
-
- struct leaf final : node {
- char key;
-
- leaf(int freq, char key) : node(freq), key(key) {}
- };
-
- struct branch final : node {
- node_ptr lhs;
- node_ptr rhs;
-
- branch(node_ptr lhs, node_ptr rhs)
- : node(lhs->frequency + rhs->frequency), lhs(std::move(lhs)),
- rhs(std::move(rhs)) {}
- };
-
- // this allows us to share [codebook]s among encoded strings
- struct data {
- node_ptr decoder;
- encoder_t encoder;
- };
- std::shared_ptr<data const> underlying_;
+struct node {
+ node(int a_weight, char a_ch) : ch(a_ch), weight(a_weight) {}
+ node(std::unique_ptr<node>&& a_left, std::unique_ptr<node>&& a_right)
+ : left(std::move(a_left)), right(std::move(a_right)),
+ weight(left->weight + right->weight) {
+ left->parent = this;
+ left->branch = branch::left;
+ right->parent = this;
+ right->branch = branch::right;
+ }
 
-public:
- template <typename Iter>
- codebook(Iter const first, Iter const last);
+ // leaf fields
+ char ch = '\0';
 
- template <typename Iter>
- std::vector<bool> encode(Iter first, Iter last) const;
+ // branch fields
+ std::unique_ptr<node> left;
+ std::unique_ptr<node> right;
 
- template <typename Iter>
- std::string decode(Iter first, Iter last) const;
+ // common fields
+ node* parent = nullptr;
+ enum class branch { left, right };
+ branch branch = branch::left;
+ int weight = 0;
 };
 
-struct encoded_string {
- codebook codes;
- std::vector<bool> string;
-
- explicit encoded_string(std::string const& s)
- : codes(begin(s), end(s)), string(codes.encode(begin(s), end(s))) {}
-
- encoded_string(codebook codes, std::string const& s)
- : codes(codes), string(codes.encode(begin(s), end(s))) {}
+struct codebook {
+ // maps characters to leafs in the tree
+ std::unordered_map<char, node*> char_map;
 
- std::string decoded() const {
- return codes.decode(begin(string), end(string));
- }
+ // root of the tree
+ std::unique_ptr<node> root;
 };
 
-// --- implementation ---
-inline codebook::node::~node() {}
-
-inline std::vector<bool> with_new_bit(std::vector<bool> bits, bool b) {
- bits.push_back(b);
- return bits;
-}
-
-template <typename Iter>
-codebook::codebook(Iter const first, Iter const last) {
- struct helper {
- static node_ptr make_decoder(Iter const first, Iter const last) {
- // in this part of the function, we build up a frequency list
- // sorted by frequency, descending
- auto freq = std::vector<leaf>();
-
- std::for_each(first, last, [&freq](char c) {
- auto const it = std::find_if(
- begin(freq), end(freq), [c](auto const& p) { return p.key == c; });
- if (it != end(freq)) {
- // it's already in the list
- it->frequency += 1;
- } else {
- // it's not already in the list
- freq.emplace_back(1, c);
- }
- });
-
- if (freq.empty()) {
- throw std::invalid_argument("attempted to codebook an empty range");
- }
-
- std::sort(begin(freq), end(freq), [](auto const& lhs, auto const& rhs) {
- return lhs.frequency > rhs.frequency;
- });
-
- auto ret = std::vector<std::unique_ptr<node>>();
- std::transform(
- begin(freq), end(freq), std::back_inserter(ret), [](auto const l) {
- return std::make_unique<leaf>(l);
- });
-
- while (ret.size() > 1) {
- auto rhs = std::move(ret.back());
- ret.pop_back();
- auto lhs = std::move(ret.back());
- ret.pop_back();
-
- auto new_node =
- std::make_unique<branch>(std::move(lhs), std::move(rhs));
- auto const new_freq = new_node->frequency;
-
- // look for the first element with a smaller frequency
- auto const it =
- std::find_if(begin(ret), end(ret), [new_freq](auto const& n) {
- return n->frequency < new_freq;
- });
- // and insert the new_node before that element
- ret.insert(it, std::move(new_node));
- // in this way, we keep the list sorted by frequency
- }
-
- return std::move(ret.back());
- }
-
- static void
- encoder_rec(node const* cur, std::vector<bool> bits, encoder_t& out) {
- if (auto l = dynamic_cast<leaf const*>(cur)) {
- out.push_back(std::make_pair(l->key, std::move(bits)));
- } else if (auto b = dynamic_cast<branch const*>(cur)) {
- encoder_rec(b->lhs.get(), with_new_bit(bits, 0), out);
- encoder_rec(b->rhs.get(), with_new_bit(std::move(bits), 1), out);
- } else {
- unreachable();
- }
- }
-
- static encoder_t make_encoder(node const& decoder) {
- auto ret = encoder_t();
-
- encoder_rec(&decoder, std::vector<bool>(), ret);
-
- return ret;
- }
- };
-
- auto decoder = helper::make_decoder(first, last);
- auto encoder = helper::make_encoder(*decoder);
- underlying_ = std::make_shared<data const>(
- data{std::move(decoder), std::move(encoder)});
-}
-
-template <typename Iter>
-std::vector<bool> codebook::encode(Iter const first, Iter const last) const {
- std::vector<bool> ret;
+std::vector<bool> encode(const std::string& string_to_encode, codebook& cb) {
+ // first pass
+ // calculate weights
+ std::unordered_map<char, int> weights;
+ std::vector<std::unique_ptr<node>> orphans;
+ for (char ch : string_to_encode) {
+ ++weights[ch];
+ }
 
- auto& encoder = underlying_->encoder;
- std::for_each(first, last, [&ret, &encoder](char c) {
- auto const it =
- std::find_if(begin(encoder), end(encoder), [c](auto const& p) {
- return p.first == c;
+ // build tree
+ for (auto&& w : weights) {
+ orphans.push_back(std::make_unique<node>(w.second, w.first));
+ cb.char_map[orphans.back()->ch] = orphans.back().get();
+ }
+ // make parents for orphans until only one root orphan is remained
+ while (orphans.size() > 1) {
+ std::sort(
+ std::begin(orphans),
+ std::end(orphans),
+ [](const std::unique_ptr<node>& a, const std::unique_ptr<node>& b) {
+ return a->weight > b->weight;
  });
- if (it != end(encoder)) {
- auto const& code = it->second;
- std::copy(begin(code), end(code), std::back_inserter(ret));
- } else {
- throw std::invalid_argument(
- "The range has a character which was not in the huffman set");
- }
- });
-
- return ret;
-}
-
-template <typename Iter>
-std::string codebook::decode(Iter const first, Iter const last) const {
- std::string ret;
-
- node const* const top = underlying_->decoder.get();
-
- // returns a pair:
- // the second member is the decoded character
- // the first member is the place we've gotten to in the range
- // i.e., if [0] is an 'a', and we have
- // [it, last) = { 0, 1, 1, 0 }
- // we return (it', 'a') such that
- // [it', last) = { 1, 1, 0 }
- auto decode_single =
- [top](Iter it, Iter const last) -> std::pair<Iter, char> {
- node const* current_node = top;
+ auto right = std::move(orphans.back());
+ orphans.pop_back();
+ auto left = std::move(orphans.back());
+ orphans.pop_back();
+ orphans.push_back(
+ std::make_unique<node>(std::move(left), std::move(right)));
+ }
 
- for (; it != last; ++it) {
- if (auto l = dynamic_cast<leaf const*>(current_node)) {
- return std::make_pair(it, l->key);
- } else if (auto b = dynamic_cast<branch const*>(current_node)) {
- if (*it) {
- current_node = b->rhs.get();
- } else {
- current_node = b->lhs.get();
- }
- } else {
- unreachable();
- }
+ // move last orphan to codebook
+ assert(orphans.size() == 1);
+ cb.root = std::move(orphans.front());
+
+ // second pass
+ // encoding
+ std::vector<bool> res;
+ for (auto ch : string_to_encode) {
+ auto node = cb.char_map[ch];
+ std::vector<bool> compressed_char;
+ while (node->parent != nullptr) {
+ compressed_char.push_back(node->branch != node::branch::left);
+ node = node->parent;
  }
+ res.insert(std::end(res), compressed_char.rbegin(), compressed_char.rend());
+ }
+ return res;
+}
 
- if (auto l = dynamic_cast<leaf const*>(current_node)) {
- return std::make_pair(last, l->key);
- } else {
- throw std::invalid_argument(
- "The range was not encoded with this huffman set");
+std::string decode(const codebook& cb, const std::vector<bool>& compressed_bit_stream) {
+ std::string res;
+ node* node = cb.root.get();
+ for (auto bit : compressed_bit_stream) {
+ if (!bit)
+ node = node->left.get();
+ else
+ node = node->right.get();
+ if (node->right == nullptr) {
+ res.push_back(node->ch);
+ node = cb.root.get();
  }
- };
-
- for (auto it = first; it != last;) {
- auto p = decode_single(it, last);
- it = p.first;
- ret.push_back(p.second);
  }
-
- return ret;
+ return res;
 }
 
-} // namespace huffman
-
 int main() {
- std::string to_be_encoded = R"(bibbity bobbity)";
+ std::string to_be_encoded = "bibbity bobbity";
+
+ codebook cb;
 
- auto encoded = huffman::encoded_string(to_be_encoded);
+ auto encoded = encode(to_be_encoded, cb);
 
  std::cout << "Encoded, the string looks like: ";
- for (bool b : encoded.string) {
+ for (bool b : encoded) {
  std::cout << b;
  }
- std::cout << "\nand decoded, the string looks like: " << encoded.decoded();
+
+ std::cout << "\nand decoded, the string looks like: "
+ << decode(cb, encoded);
  std::cout << "\n\nAs opposed to the original, which is "
  << to_be_encoded.size() * 8 << " bits, the encoded has size "
- << encoded.string.size() << ".\n";
+ << encoded.size() << ".\n";
 }