Sudoku Solver (brute force)

sudoku-solve.c

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BLOCK (3)

#define WIDTH (BLOCK * BLOCK)
#if WIDTH % BLOCK > 0
#   error "Invalid block size"
#endif

bool read(int grid[WIDTH][WIDTH], FILE *fp)
{
    for (size_t k = 0; k < WIDTH * WIDTH; k++) {
        int *cell = &grid[k / WIDTH][k % WIDTH];
        if (fscanf(fp, "%d", cell) != 1 || *cell < 0 || *cell > WIDTH) {
            return 1;
        }
    }
    return 0;
}

void display(int grid[WIDTH][WIDTH])
{
    for (size_t row = 0; row < WIDTH; row++) {
        for (int col = 0; col < WIDTH; col++, putchar(' ')) {
            printf("%d", grid[row][col]);
        }
        putchar('\n');
    }
}

bool is_legal(int grid[WIDTH][WIDTH], int row, int col, int val)
{
    bool res = true;
    for (size_t i = 0, k = 0; i < BLOCK; i++) {
        for (size_t j = 0; j < BLOCK; j++, k++) {
            res &= grid[row][k] != val;
            res &= grid[k][col] != val;
            res &= grid[row - row % BLOCK + i][col - col % BLOCK + j] != val;
        }
    }
    return res;
}

int level = 0;

bool solve(int grid[WIDTH][WIDTH], int pos)
{
    printf("%*d\n", level * 3, pos);
    level++;

    if (pos == WIDTH * WIDTH) {
        level--;
        return true;
    }

    size_t row = pos / WIDTH, col = pos % WIDTH;

    if (grid[row][col] != 0) {
        bool u =  solve(grid, pos + 1);
        level--;
        return u;
    }

    for (size_t val = 1; val <= WIDTH; val++) {
        if (is_legal(grid, row, col, val)) {
            grid[row][col] = val;
            if (solve(grid, pos + 1)) {
                level--;
                return true;
            }
        }
    }
    grid[row][col] = 0;
    level--;
    return false;
}

int main(int argc, char *argv[])
{
    int grid[WIDTH][WIDTH] = {0};
    if (!(argc > 1 && strcmp("-0", argv[1]) == 0)) {
        if (read(grid, stdin)) {
            fprintf(stderr, "Error while reading!\n");
            exit(EXIT_FAILURE);
        }
    }
    solve(grid, 0);
    display(grid);
}