write a funciton to not remove the value if the value has already been set.

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

@@ -14,14 +14,19 @@ void sudoku_init(u16 *board) {
 	}
 }
 
-void sudoku_place(u16 *board, u16 val, uint idx) {
+/* removes `val` from `*brd`, if `*brd` is not equal to `val` */
+static inline void setbrdpos(u16 *brd, u16 val) {
+	*brd &= ~(val & -(*brd != val));
+}
+
+void sudoku_place(u16 *brd, u16 val, uint idx) {
 	uint icol = idx % SUDOKU_DEPTH;
 	uint irow = idx - icol;
 	uint ibox = idx - (idx % SUDOKU_DEPTH_SQRT);
 	for (uint i = 0; i < SUDOKU_DEPTH; i++) {
-		board[irow + i] &= ~val;
-		board[icol + (i * SUDOKU_DEPTH)] &= ~val;
-		board[ibox + (i / SUDOKU_DEPTH_SQRT * SUDOKU_DEPTH) + (i % SUDOKU_DEPTH_SQRT)] &= ~val;
+		setbrdpos(&brd[irow + i], val);
+		setbrdpos(&brd[icol + (i * SUDOKU_DEPTH)], val);
+		setbrdpos(&brd[ibox + (i / SUDOKU_DEPTH_SQRT * SUDOKU_DEPTH) + (i % SUDOKU_DEPTH_SQRT)], val);
 	}
 }
 
@@ -43,9 +48,11 @@ void sudoku_solve_step(u16 *board) {
 }
 
 void sudoku_print(const u16 *board) {
+	printf("+———-———-———-———-———-———-———-———-———+\n");
 	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;
@@ -58,7 +65,9 @@ void sudoku_print(const u16 *board) {
 		default:       tile = ' '; break;
 		};
 
-		printf("|%c ", tile);
+		printf("| %c ", tile);
+		// printf("|%3i", board[i]);
 		if (i % 9 == 8) printf("|\n");
 	}
+	printf("+———-———-———-———-———-———-———-———-———+\n");
 }