Improved the trie data structure to be more (memory) efficient

lib/trie.hpp

@@ -1,81 +1,106 @@
 #pragma once
 
-#include <boost/optional.hpp>
-
-#include <stack>
-#include <stdexcept>
+#include <algorithm>
+#include <memory>
 #include <utility>
 #include <vector>
 
 ///
-/// \brief A Trie datastructure used to remove prefixes in a set of words
-///
-/// The datastructure only works for words over integral unsigned types. In principle the symbols
-/// can be unbounded, however having very large symbols degrades the performance a lot. Some random
-/// testing shows that for symbols <= 50 the performance is similar to std::set (which is solving a
-/// different problem).
+/// \brief A Trie datastructure used to remove prefixes in a set of words.
+/// Insert-only. Iteration over the structure only uses longest matches.
 ///
 /// Tests : 1M words, avg words length 4 (geometric dist.), alphabet 50 symbols
-/// trie reduction 58% in 1.15s
-/// set  reduction 49% in 0.92s
+/// trie reduction 58% in 0.4s
+/// set  reduction 49% in 1.1s
 ///
 /// I did not implement any iterators, as those are quite hard to get right.
 /// There are, however, "internal iterators" exposed as a for_each() member
 /// function (if only we had coroutines already...)
 ///
+/// TODO: implement `bool member(...)`
+///
 template <typename T> struct trie {
-	static_assert(std::is_integral<T>::value && std::is_unsigned<T>::value, "");
-
 	/// \brief Inserts a word (given by iterators \p begin and \p end)
 	/// \returns true if the element was inserted, false if already there
 	template <typename Iterator> bool insert(Iterator && begin, Iterator && end) {
-		if (begin == end) return false;
+		if (!node) {
+			node.reset(new trie_node());
 
-		size_t i = *begin++;
-		if (i >= branches.size()) branches.resize(i + 1);
-
-		auto & b = branches[i];
-		if (b) return b->insert(begin, end);
-
-		b = trie();
-		b->insert(begin, end);
+			if (begin == end) {
 				return true;
 			}
+		}
+
+		return node->insert(begin, end);
+	}
 
 	/// \brief Inserts a word given as range \p r
 	/// \returns true if the element was inserted, false if already there
 	template <typename Range> bool insert(Range const & r) { return insert(begin(r), end(r)); }
 
 	/// \brief Applies \p function to all word (not to the prefixes)
+	template <typename Fun> void for_each(Fun && function) const {
+		if (node) {
+			node->for_each(std::forward<Fun>(function));
+		} else {
+			// empty set, so we don't call the function
+		}
+	}
+
+	/// \brief Empties the complete set
+	void clear() { node.reset(nullptr); }
+
+  private:
+	struct trie_node;
+	std::unique_ptr<trie_node> node = nullptr;
+
+	// A node always contains the empty word
+	struct trie_node {
+		template <typename Iterator> bool insert(Iterator && begin, Iterator && end) {
+			if (begin == end) return false;
+
+			T i = *begin++;
+			auto it = find(i);
+
+			if (it != data.end() && it->first == i) {
+				return it->second.insert(begin, end);
+			}
+
+			// else, does not yet exist
+			it = data.emplace(it, i, trie_node());
+			it->second.insert(begin, end);
+			return true;
+		}
+
 		template <typename Fun> void for_each(Fun && function) const {
 			std::vector<T> word;
 			return for_each_impl(std::forward<Fun>(function), word);
 		}
 
-	/// \brief Empties the complete set
-	void clear() { branches.clear(); }
-
 	  private:
 		template <typename Fun> void for_each_impl(Fun && function, std::vector<T> & word) const {
-		size_t count = 0;
-		for (T i = 0; i < branches.size(); ++i) {
-			auto const & b = branches[i];
-			if (b) {
-				++count;
-				word.push_back(i);
-				b->for_each_impl(function, word);
+			if (data.empty()) {
+				// we don't want function to modify word
+				const auto & cword = word;
+				function(cword);
+			}
+
+			for (auto const & kv : data) {
+				// for each letter, we extend the word, recurse and remove extension.
+				word.push_back(kv.first);
+				kv.second.for_each_impl(function, word);
 				word.resize(word.size() - 1);
 			}
 		}
 
-		if (count == 0) {
-			const auto & cword = word;
-			function(cword); // we don't want function to modify word
-			return;
-		}
+		typename std::vector<std::pair<T, trie_node>>::iterator find(T const & key) {
+			return std::lower_bound(
+			    data.begin(), data.end(), key,
+			    [](std::pair<T, trie_node> const & kv, T const & k) { return kv.first < k; });
 		}
 
-	std::vector<boost::optional<trie>> branches;
+		std::vector<std::pair<T, trie_node>> data;
+	};
 };
 
 /// \brief Flattens a trie \p t