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98 lines
3 KiB
C++
98 lines
3 KiB
C++
#pragma once
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#include <boost/optional.hpp>
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#include <stack>
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#include <stdexcept>
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#include <utility>
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#include <vector>
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///
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/// \brief A Trie datastructure used to remove prefixes in a set of words
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///
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/// The datastructure only works for words over integral unsigned types. In principle the symbols
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/// can be unbounded, however having very large symbols degrades the performance a lot. Some random
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/// testing shows that for symbols <= 50 the performance is similar to std::set (which is solving a
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/// different problem).
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///
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/// Tests : 1M words, avg words length 4 (geometric dist.), alphabet 50 symbols
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/// trie reduction 58% in 1.15s
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/// set reduction 49% in 0.92s
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///
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/// I did not implement any iterators, as those are quite hard to get right.
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/// There are, however, "internal iterators" exposed as a for_each() member
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/// function (if only we had coroutines already...)
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///
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template <typename T> struct trie {
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static_assert(std::is_integral<T>::value && std::is_unsigned<T>::value, "");
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/// \brief Inserts a word (given by iterators \p begin and \p end)
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/// \returns true if the element was inserted, false if already there
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template <typename Iterator> bool insert(Iterator && begin, Iterator && end) {
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if (begin == end) return false;
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size_t i = *begin++;
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if (i >= branches.size()) branches.resize(i + 1);
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auto & b = branches[i];
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if (b) return b->insert(begin, end);
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b = trie();
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b->insert(begin, end);
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return true;
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}
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/// \brief Inserts a word given as range \p r
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/// \returns true if the element was inserted, false if already there
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template <typename Range> bool insert(Range const & r) { return insert(begin(r), end(r)); }
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/// \brief Applies \p function to all word (not to the prefixes)
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template <typename Fun> void for_each(Fun && function) const {
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std::vector<T> word;
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return for_each_impl(std::forward<Fun>(function), word);
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}
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/// \brief Empties the complete set
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void clear() { branches.clear(); }
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private:
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template <typename Fun> void for_each_impl(Fun && function, std::vector<T> & word) const {
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size_t count = 0;
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for (T i = 0; i < branches.size(); ++i) {
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auto const & b = branches[i];
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if (b) {
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++count;
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word.push_back(i);
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b->for_each_impl(function, word);
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word.resize(word.size() - 1);
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}
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}
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if (count == 0) {
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const auto & cword = word;
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function(cword); // we don't want function to modify word
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return;
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}
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}
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std::vector<boost::optional<trie>> branches;
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};
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/// \brief Flattens a trie \p t
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/// \returns an array of words (without the prefixes)
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template <typename T> std::vector<std::vector<T>> flatten(trie<T> const & t) {
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std::vector<std::vector<T>> ret;
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t.for_each([&ret](std::vector<T> const & w) { ret.push_back(w); });
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return ret;
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}
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/// \brief Returns size and total sum of symbols
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template <typename T> std::pair<size_t, size_t> total_size(trie<T> const & t) {
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size_t count = 0;
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size_t total_count = 0;
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t.for_each([&count, &total_count](std::vector<T> const & w) {
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++count;
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total_count += w.size();
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});
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return {count, total_count};
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}
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