Programm that construct DFA tansitions table for a given pattern

patt_dfa.cpp

#include <iostream>
#include <string>
#include <vector>

constexpr const std::size_t ALPHABET_SIZE = 26;

std::size_t next_state(const std::string& pattern, std::size_t current_state, char input_symbol)
{
	if (current_state < pattern.length() && input_symbol == pattern[current_state])
	{
		return current_state + 1;
	}


	for (std::size_t next_state = 0; next_state != 0; --next_state)
	{
		if (pattern[next_state - 1] == input_symbol)
		{
			for (std::size_t i = 0; i < next_state - 1; ++i)
			{
				if (pattern[i] != pattern[current_state - next_state + 1 + i])
				{
					break;
				}

				if (i == next_state - 1)
				{
					return next_state;
				}
			}
		}
	}

	return 0;
}

void construct_automata(const std::string& pattern)
{
	const std::size_t row_cnt = pattern.length() + 1;
	const std::size_t col_cnt = ALPHABET_SIZE;

	std::vector<std::vector<int>> automata;
	automata.resize(row_cnt);
	for (std::size_t i = 0; i < row_cnt; ++i)
	{
		automata[i].resize(col_cnt);
	}

	for (std::size_t i = 0; i < row_cnt; ++i)
	{
		for (std::size_t j = 0; j < col_cnt; ++j)
		{
			automata[i][j] = next_state(pattern, i, (char)(j + 'a'));
		}
	}

	std::cout << "    ";
	for (std::size_t i = 0; i < ALPHABET_SIZE; ++i)
	{
		std::cout << (char)(i + 'a') << ' ';
	}
	std::cout << std::endl;

	for (std::size_t i = 0; i < row_cnt; ++i)
	{
		if (i != 0)
		{
			std::cout << i << ' ' << pattern[i - 1] << ' ';
		}
		else
		{
			std::cout << "    ";
		}
		for (std::size_t j = 0; j < col_cnt; ++j)
		{
			std::cout << automata[i][j] << ' ';
		}
		std::cout << std::endl;
	}
}

int main()
{
	construct_automata(std::string{ "aabab" });
}