/*
 * eight_queens.c  --
 *
 *   Place eight queens on a chess board so that not two queens threaten
 *   each other. A chess board has 8 x 8 squares, and a queen threatens
 *   the chess pieces on the same row, the same column, or the same diagonal.
 */


#include <stdio.h>

struct position {
    int row;
    int col;
};


int is_valid(struct position queens[], int row, int col);
void print_chess_board(struct position queens[]);


int
main()
{
    struct position queens[8];
    int row, col, nchecks;

    row = col = nchecks = 0;
    while ( row < 8 ) {
        // Have partial solution

        if ( col >= 8 ) {
            // Cannot try any more positions on the current row
            // Backtrack
            row--;

            if ( row < 0 ) {
                // Could not find a solution; end search
                break;

            } else {
                // Retrieve the column stored for the current row and
                // try the next column
                col = queens[row].col + 1;
            }

        } else {
            // Check if current position is valid
            if ( is_valid(queens, row, col) ) {
                // Valid position; store it
                queens[row].row = row;
                queens[row].col = col;

                // Advance to next row
                row++;
                col = 0;

            } else {
                // Invalid position; try next column
                col++;
            }
        }

        nchecks++;
    }


    if (row == 8) {
        // Found solution
        printf("Found solution after checking %d positions\n", nchecks);
        print_chess_board(queens);
    } else {
        // Could not find solution
        printf("Could not find a solution after checkgin %d positions\n", nchecks);
    }

    return 0;
}


int
is_valid(struct position queens[], int row, int col)
{
    int i;

    if (row < 0 || row >=8 || col < 0 || col >=8)
        return 0;

    for (i=0; i<row; i++) {
        // Check not same column
        if (queens[i].col == col)
            return 0;

        // Check not same diagonal
        if (queens[i].row -  queens[i].col == row - col)
            return 0;

        if (queens[i].row + queens[i].col == row + col)
            return 0;
    }

    return 1;
}


void
print_chess_board(struct position queens[])
{
    int i,j;

    for (i=0; i<8; i++) {
        for (j=0; j<8; j++) {
            if (queens[i].col == j)
                printf("X ");
            else
                printf("o ");
        }

        printf("\n");
    }
}