revert merge of sudoku_init and sudoku_solve_step; this was incorrect and causes problems

src/main.c

@@ -20,6 +20,8 @@ int main(int argc, char **argv) {
 		0x000, 0x000, 0x000, 0x000, 0x080, 0x000, 0x000, 0x040, 0x100,
 		// clang-format on
 	};
+	sudoku_init(board);
+	sudoku_print(board);
 
 	for (int i = 0; i < 2; i++) {
 		sudoku_solve_step(board);

src/sudoku.c

@@ -8,6 +8,12 @@
 
 #include "util/intdef.h"
 
+void sudoku_init(u16 *board) {
+	for (uint i = 0; i < SUDOKU_LEN; i++) {
+		board[i] |= SUDOKU_ALL & -!board[i];
+	}
+}
+
 /* removes `val` from `*brd`, if `*brd` is not equal to `val` */
 static inline void setbrdpos(u16 *brd, u16 val) {
 	*brd &= ~(val & -(*brd != val));
@@ -29,8 +35,6 @@ void sudoku_place(u16 *brd, u16 val, uint idx) {
 
 void sudoku_solve_step(u16 *board) {
 	for (uint i = 0; i < SUDOKU_LEN; i++) {
-		board[i] |= SUDOKU_ALL & -!board[i];
-
 		switch (board[i]) {
 		case SUDOKU_1:
 		case SUDOKU_2:

src/sudoku.h

@@ -22,6 +22,9 @@ enum sudoku_bitmask {
 	SUDOKU_ALL = 0x01FF,
 };
 
+/* initialises the sudoku board to `SUDOKU_ALL`. (if the tile is empty) */
+void sudoku_init(u16 *board);
+
 /* places `val` at `idx` in `board`, and removes `val` from the possibilities in neighbouring tiles */
 void sudoku_place(u16 *board, u16 val, uint idx);