sudoku-solver/main.cpp

#include <iostream>
#include <list>
#include <vector>
#include <chrono>
#include <algorithm>
#include <iterator>

#define BOARD_SIZE 81
#define BOARD_DIMENSION 9

typedef std::pair<std::vector<int>, std::list<int>> move;

// Wrapper for the game board with the field data and some utility functions.
class Board {
public:
    int values[BOARD_SIZE];

    inline int coordinates_to_id(int x, int y) {
        return y * BOARD_DIMENSION + x;
    }

    inline int get(int x, int y) {
        return values[coordinates_to_id(x, y)];
    }

    inline void set(int x, int y, int value) {
        values[coordinates_to_id(x, y)] = value;
    }
};

// A cell in the game board -- used for the heap
class Cell {
public:
    Board *board;
    int x, y;

    int value;
    std::list<int> possible_values;

    Cell(Board *board, int x, int y) : board(board), x(x), y(y), value(board->get(x, y)) {}

    inline void set_value(int new_value) {
        value = new_value;
        update_possible_values();
    }

    void update_possible_values() {
        // Start off with all possible numbers
        possible_values = {1, 2, 3, 4, 5, 6, 7, 8 ,9};

        // Remove all from this row
        for (int dx = 0; dx < BOARD_DIMENSION; dx++) {
            possible_values.remove(board->get(dx, y));
        }

        // Remove all from this column
        for (int dy = 0; dy < BOARD_DIMENSION; dy++) {
            possible_values.remove(board->get(x, dy));
        }

        // Remove all from this quadrant
        for (int dy = 0; dy < 3; dy++) {
            for (int dx = 0; dx < 3; dx++) {
                possible_values.remove(board->get((x - x % 3) + dx, (y - y % 3) + dy));
            }
        }
    }

    bool operator<(const Cell &other) {
        return possible_values.size() < other.possible_values.size(); 
    }
};

class Sudoku {
private:
    Board *board = new Board();
    std::vector<Cell> heap;
    int value_count[9] = {0, 0, 0, 0, 0, 0, 0, 0, 0};
 
public:
    Sudoku(std::string s) {
        int x = 0;
        int y = 0;

        // Parse the input
        for (unsigned int i = 0; i < s.length(); i++) {
            board->values[i] = (int) (s[i] - '0');

            if (board->values[i] == 0) {
                Cell cell(board, x, y);
                heap.emplace_back(cell);
            }

            x++;
            if (x == BOARD_DIMENSION) {
                x -= BOARD_DIMENSION;
                y++;
            }
        }

        // Update values of cells initially
        for (Cell &cell : heap) {
            cell.update_possible_values();
        }

        std::make_heap(heap.begin(), heap.end());
    }

    // Update the possible values which a cell can take for every cell in the heap
    void update_all_cells() {
        for (Cell &cell : heap) {
            cell.update_possible_values();
        }

        std::sort_heap(heap.begin(), heap.end());
    }

    // Update the possible values of all cells in the heap which are affected by
    //  the value of the cell at the given position
    void update_affected_cells(int x, int y) {
        int x_quadrant = x - x % 3;
        int y_quadrant = y - y % 3;
        
        for (Cell &cell : heap) {
            if (cell.x == x // Column
                || cell.y == y // Row
                || (cell.x - cell.x % 3 == x_quadrant // Quadrant
                 && cell.y - cell.y % 3 == y_quadrant)) {
                
                cell.update_possible_values();
            }
        }

        std::sort_heap(heap.begin(), heap.end());
    }

    // Returns the best cell in the heap (the one with the most obvious choice)
    Cell get_next_best_empty_cell() {
		std::pop_heap(heap.begin(), heap.end());
		Cell c = heap.back();
        heap.pop_back();
        
		return c;
	}
 
    // Wrapper for solve_recurse, to be called from outside
    void solve() {
        auto start = std::chrono::high_resolution_clock::now();
        solve_recurse();

        auto end = std::chrono::high_resolution_clock::now();

        std::cout << std::endl;
        std::cout << "Time: " << std::chrono::duration_cast<std::chrono::microseconds>(end - start).count() << " microseconds" << std::endl;

        print_board();
    }

    // Recurisvely solve the CSP
    bool solve_recurse() {
        // If there are no more possible moves in the heap...
        if (heap.empty()) {
            // Check whether there are empty fields left
            for (int y = 0; y < BOARD_DIMENSION; y++) {
                for (int x = 0; x < BOARD_DIMENSION; x++) {
                    if (board->get(x, y) == 0) return false;
                }
            }

            // If not, we're done!
            return true;
        }

        Cell cell = get_next_best_empty_cell();

        // Sort the possible values by how often that value has been used in the board.
        // This is a simple heuristic based on the thought that a value which hasn't been used often is likely the safer choice.
        cell.possible_values.sort([&](const int& a, const int& b) {return value_count[a - 1] > value_count[b - 1];});

        // Iterate over all possible values this cell can take
        for (int value : cell.possible_values) {
            // Apply this value
            board->set(cell.x, cell.y, value);
            update_affected_cells(cell.x, cell.y);

            // Recurse; if we find a solution, we're done and can return true
            if (solve_recurse()) {
                return true;
            }

            // Seems like this was a dead end -- reset this move
            board->set(cell.x, cell.y, 0);
            update_affected_cells(cell.x, cell.y);
        }

        // Put the cell back into the heap -- it couldn't be used for any valid solution, so we'll need it somewhere else later
        heap.push_back(cell);
        std::push_heap(heap.begin(), heap.end());

        return false;
    }

    // Print the game board in a nice format
    void print_board() {
        std::cout << board->values[0] << " ";
		for (unsigned int i = 1; i < BOARD_SIZE; i++) {
			if (i % 3 == 0 && i % 9 != 0) {
				std::cout << "| ";
			}
			if (i % 9 == 0) {
				std::cout << std::endl;
			}
			if (i % 27 == 0) {
				std::cout << "------+-------+------" << std::endl;
			}
			std::cout << board->values[i] << " ";
		}

        std::cout << std::endl;
    }
};

int main() {
	//char str[82];
	//cin >> str;
	//Sudoku solver(str);
	Sudoku solver("006001849030000000000020006000400320400003000600008000010060003000005004029074005");
	//Sudoku solver("850002400720000009004000000000107002305000900040000000000080070017000000000036040");
	solver.solve();
	//solver.print_board();
    
    return 0;
}