write some basic testing/demo code

src/main.c

@@ -3,8 +3,35 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+#include "sudoku.h"
+
 int main(int argc, char **argv) {
 	(void)argc, (void)argv;
-	printf("%s\n", "Hello, World!");
+	u16 board[SUDOKU_DEPTH_2] = {
+		// clang-format off
+		0x010, 0x004, 0x000, 0x000, 0x040, 0x000, 0x000, 0x000, 0x000,
+		0x020, 0x000, 0x000, 0x001, 0x100, 0x010, 0x000, 0x000, 0x000,
+		0x000, 0x100, 0x080, 0x000, 0x000, 0x000, 0x000, 0x020, 0x000,
+		0x080, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x004,
+		0x008, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x001,
+		0x040, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x000, 0x020,
+		0x000, 0x020, 0x000, 0x000, 0x000, 0x000, 0x002, 0x080, 0x000,
+		0x000, 0x000, 0x000, 0x008, 0x001, 0x100, 0x000, 0x000, 0x010,
+		0x000, 0x000, 0x000, 0x000, 0x080, 0x000, 0x000, 0x040, 0x100,
+		// clang-format on
+	};
+	sudoku_init(board);
+	sudoku_print(board);
+	sudoku_solve_step(board);
+	sudoku_print(board);
+	sudoku_solve_step(board);
+	sudoku_print(board);
+	sudoku_solve_step(board);
+	sudoku_print(board);
+	sudoku_solve_step(board);
+	sudoku_print(board);
+	sudoku_solve_step(board);
+	sudoku_print(board);
+
 	return EXIT_SUCCESS;
 }

src/sudoku.c

@@ -47,6 +47,7 @@ void sudoku_print(const u16 *board) {
 	for (uint i = 0; i < SUDOKU_DEPTH_2; i++) {
 		char tile;
 		switch (board[i]) {
+		case 0:        tile = 'x'; break;
 		case SUDOKU_1: tile = '1'; break;
 		case SUDOKU_2: tile = '2'; break;
 		case SUDOKU_3: tile = '3'; break;
@@ -60,6 +61,7 @@ void sudoku_print(const u16 *board) {
 		};
 
 		printf("| %c ", tile);
+		// printf("|%3i", board[i]);
 		if (i % 9 == 8) printf("|\n");
 	}
 	printf("+———-———-———-———-———-———-———-———-———+\n");