a really simple sudoku solver in pure c.

sudoku.c

#include <stdio.h>

int main();
int input_grid();
int validate_grid();
int solve_cell(int row, int column);
int is_valid(int row, int column, int value);
void print_grid();

int original[9][9], grid[9][9];

int main()
{
	if (! input_grid()) {
		printf("error: unable to input grid\n");
		return 0;
	}

	printf("\nvalidating puzzle... ");
	if (! validate_grid()) {
		printf("invalid!\n");
		return 0;
	}

	printf("valid.\n\nsolving following puzzle:\n");
	print_grid();

	if (! solve_cell(0, 0)) {
		printf("\nunfortunately, your puzzle is unsolvable\n");
		return 0;
	}

	printf("\npuzzle solved:\n");
	print_grid();
	return 0;
}

int input_grid()
{
	int character, row, column;
	row = column = 0;

	printf("enter rows line by line. use numbers for known cells, ");
	printf("zero or dot for missing cells.\nrow 1: ");

	while (row < 9) {
		character = getchar();

		if ('0' <= character && character <= '9' || character == '.') {
			if (column > 8) {
				printf("error: each row has maximum 9 cells\n");
				return 0;
			}

			if (character != '.')
				original[row][column] =
					grid[row][column] = character - '0';

			column++;

		} else if (character == '\n') {
			column = 0;
			row++;

			printf("row %i: ", row + 1);
		}
	}

	return 1;
}

int validate_grid()
{
	int i, j;

	for (i = 0; i < 9; i++)
		for (j = 0; j < 9; j++)
			if (grid[i][j])
				if (! is_valid(i, j, grid[i][j]))
					return 0;

	return 1;
}

int solve_cell(int row, int column)
{
	int number = 1;

	if (column == 9) {
		column = 0;
		row++;
	}

	if (row == 9)
		return 1;

	while (number < 10) {
		if (is_valid(row, column, number)) {
			grid[row][column] = number;

			if (solve_cell(row, column + 1))
				return 1;
		}

		grid[row][column] = 0;

		number++;
	}

	return 0;
}

int is_valid(int row, int column, int value)
{
	int i, j, r, c;

	if (original[row][column] != 0)
		if (original[row][column] != value)
			return 0;

	for (i = 0; i < 9; i++) {
		if (i != column)
			if (grid[row][i] == value)
				return 0;

		if (i != row)
			if (grid[i][column] == value)
				return 0;
	}

	r = (row / 3) * 3;
	c = (column / 3) * 3;
	for (i = r; i < r + 3; i++)
		for (j = c; j < c + 3; j++)
			if (i != row || j != column)
				if (grid[i][j] == value)
					return 0;

	return 1;
}

void print_grid()
{
	int i, j;

	for (i = 0; i < 10; i++) {
		if (i % 3 == 0)
			printf("+-------+-------+-------+\n");

		if (i == 9)
			return;

		for (j = 0; j < 9; j++) {
			if (j % 3 == 0)
				printf("| ");

			grid[i][j] != 0 ? printf("%d ", grid[i][j]) : printf(". ");
		}

		printf("|\n");
	}
}