Markov chain implementation, generic on type and n-gram length.

markov_chain.h

/*
(C) Copyright Thierry Seegers 2016. Distributed under the following license:

Boost Software License - Version 1.0 - August 17th, 2003
Permission is hereby granted, free of charge, to any person or organization
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#include <cassert>
#include <cstdlib>
#include <iterator>
#include <map>
#include <queue>
#include <random>
#include <vector>

template<typename T, std::size_t N>
class markov_chain;

namespace detail
{
	// Because dereferencing involves computation, this iterator caches its value.
	template<typename T, std::size_t N>
	class const_iterator : public std::iterator<std::forward_iterator_tag, T>
	{
		std::deque<T> prefixes;
		markov_chain<T, N> const& chain;

		mutable struct state_t
		{
			bool cached : 1, end : 1;
		} state;

		mutable T suffix;

		const_iterator(std::deque<T> prefixes, markov_chain<T, N> const& chain, bool end) : prefixes(prefixes), chain(chain), state({false, end})
		{
			assert(prefixes.size() == N);
		}

		friend class markov_chain<T, N>;

	public:
		const reference operator*() const
		{
			assert(!state.end);
			assert(prefixes.size() == N);

			if(state.cached)
			{
				return suffix;
			}
			else
			{
				state.cached = true;
				return suffix = chain.lookup(prefixes.cbegin(), prefixes.cend());
			}
		}

		const_iterator operator++(int)
		{
			assert(!state.end);
			assert(prefixes.size() == N);

			const_iterator ci = *this;

			++*this;

			return ci;
		}

		const_iterator& operator++()
		{
			assert(!state.end);
			assert(prefixes.size() == N);

			this->operator*();

			prefixes.pop_front();
			prefixes.push_back(suffix);
			suffix = chain.lookup(prefixes.cbegin(), prefixes.cend());

			if(suffix == markov_chain<T, 1>::sentinel)
			{
				state.end = true;
				prefixes.assign(N, markov_chain<T, 1>::sentinel);
			}

			assert(prefixes.size() == N);

			return *this;
		}

		bool operator==(const_iterator const& other)
		{
			return state.end == other.state.end && prefixes == other.prefixes;
		}

		bool operator!=(const_iterator const& other)
		{
			return !(*this == other);
		}
	};

}

template<typename T, std::size_t N>
class markov_chain
{
public:
	using const_iterator = detail::const_iterator<T, N>;

	markov_chain()
	{}

	template<typename InputIt>
	markov_chain(InputIt first, InputIt last)
	{
		initialize(first, last, *this);
	}

	void add(typename std::deque<T>::const_iterator begin, typename std::deque<T>::const_iterator end, T const& suffix)
	{
		assert(distance(begin, end) == N);

		prefix_to_suffix[*begin].add(begin + 1, end, suffix);
	}

	T lookup(typename std::deque<T>::const_iterator begin, typename std::deque<T>::const_iterator end) const
	{
		assert(distance(begin, end) == N);

		return prefix_to_suffix.at(*begin).lookup(begin + 1, end);
	}

	const_iterator cbegin() const
	{
		return const_iterator{ std::deque<T>(N, markov_chain<T, 1>::sentinel), *this, false };
	}

	const_iterator cend() const
	{
		return const_iterator{ std::deque<T>(N, markov_chain<T, 1>::sentinel), *this, true };
	}

private:
	std::map<T, markov_chain<T, N - 1>> prefix_to_suffix;
};

template<typename T>
class markov_chain<T, 1>
{
public:
	using const_iterator = detail::const_iterator<T, 1>;

	static T sentinel;

	markov_chain()
	{}

	template<typename InputIt>
	markov_chain(InputIt first, InputIt last)
	{
		initialize(first, last, *this);
	}

	void add(typename std::deque<T>::const_iterator begin, typename std::deque<T>::const_iterator end, T const& suffix)
	{
		assert(distance(begin, end) == 1);

		prefix_to_suffix[*begin].push_back(suffix);
	}

	T lookup(typename std::deque<T>::const_iterator begin, typename std::deque<T>::const_iterator end) const
	{
		assert(distance(begin, end) == 1);

		auto const& suffixes = prefix_to_suffix.at(*begin);

		return suffixes[std::uniform_int_distribution<std::size_t>{0, suffixes.size() - 1}(random_engine)];
	}

	const_iterator cbegin() const
	{
		return const_iterator{ std::deque<T>(1, markov_chain<T, 1>::sentinel), *this, false };
	}

	const_iterator cend() const
	{
		return const_iterator{ std::deque<T>(1, markov_chain<T, 1>::sentinel), *this, true };
	}

private:
	static std::mt19937 random_engine;

	std::map<T, std::vector<T>> prefix_to_suffix;
};

template<typename T>
T markov_chain<T, 1>::sentinel = T{};

template<typename T>
std::mt19937 markov_chain<T, 1>::random_engine = std::mt19937(std::random_device{}());

template<typename InputIt, typename T, std::size_t N>
void initialize(InputIt first, InputIt last, markov_chain<T, N>& chain)
{
	std::deque<T> prefixes(N, markov_chain<T, 1>::sentinel);
	for(; first != last; ++first)
	{
		chain.add(prefixes.cbegin(), prefixes.cend(), *first);

		prefixes.pop_front();
		prefixes.push_back(*first);
	}

	chain.add(prefixes.cbegin(), prefixes.cend(), markov_chain<T, 1>::sentinel);
}