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huffman.cpp
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huffman.cpp
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#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cstddef>
#include <limits>
#include <memory>
#include <string>
#include <utility>
#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_;
public:
template <typename Iter>
codebook(Iter const first, Iter const last);
template <typename Iter>
std::vector<bool> encode(Iter first, Iter last) const;
template <typename Iter>
std::string decode(Iter first, Iter last) const;
};
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))) {}
std::string decoded() const {
return codes.decode(begin(string), end(string));
}
};
// --- 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;
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;
});
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;
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();
}
}
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");
}
};
for (auto it = first; it != last;) {
auto p = decode_single(it, last);
it = p.first;
ret.push_back(p.second);
}
return ret;
}
} // namespace huffman
int main() {
std::string to_be_encoded = R"(bibbity bobbity)";
auto encoded = huffman::encoded_string(to_be_encoded);
std::cout << "Encoded, the string looks like: ";
for (bool b : encoded.string) {
std::cout << b;
}
std::cout << "\nand decoded, the string looks like: " << encoded.decoded();
std::cout << "\n\nAs opposed to the original, which is "
<< to_be_encoded.size() * 8 << " bits, the encoded has size "
<< encoded.string.size() << ".\n";
}