fix: not working with full board

src/main.c

@@ -3,8 +3,30 @@
 #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_LEN] = {
+		// 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, 0x020, 0x000, 0x000, 0x000, 0x004,
+		0x008, 0x000, 0x000, 0x080, 0x000, 0x004, 0x000, 0x000, 0x001,
+		0x040, 0x000, 0x000, 0x000, 0x002, 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);
+
+	for (int i = 0; i < 2; i++) {
+		sudoku_solve_step(board);
+		sudoku_print(board);
+	}
+
 	return EXIT_SUCCESS;
 }

src/sudoku.c

@@ -8,17 +8,55 @@
 
 #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));
+}
+
+void sudoku_place(u16 *brd, u16 val, uint idx) {
+	uint x = idx % SUDOKU_DPT;
+	uint y = idx / SUDOKU_DPT;
+
+	uint irow = idx - x;
+	uint ibox = SUDOKU_DPT * (y % SUDOKU_BOXLEN) + SUDOKU_BOXLEN * (x % SUDOKU_BOXLEN);
+
+	for (uint i = 0; i < SUDOKU_DPT; i++) {
+		setbrdpos(&brd[irow + i], val);
+		setbrdpos(&brd[x + (i * SUDOKU_DPT)], val);
+		setbrdpos(&brd[ibox + (SUDOKU_DPT * (i % SUDOKU_BOXLEN))], val);
+	}
+}
+
 void sudoku_solve_step(u16 *board) {
-	u16 bmask[SUDOKU_DEPTH_2]; // board mask
-	memcpy(bmask, board, SUDOKU_DEPTH_2 * 2);
-	// TODO: first, we must somehow loop through the columns
-	// TODO: then, we must write to the bmask or do *magic* for the bits designating to 1-9 to be unset if ONLY one of them is set
+	for (uint i = 0; i < SUDOKU_LEN; i++) {
+		switch (board[i]) {
+		case SUDOKU_1:
+		case SUDOKU_2:
+		case SUDOKU_3:
+		case SUDOKU_4:
+		case SUDOKU_5:
+		case SUDOKU_6:
+		case SUDOKU_7:
+		case SUDOKU_8:
+		case SUDOKU_9:
+			sudoku_place(board, board[i], i);
+			// printf("%i,%i\n", i % SUDOKU_DPT, i / SUDOKU_DPT);
+		}
+	}
 }
 
 void sudoku_print(const u16 *board) {
-	for (uint i = 0; i < SUDOKU_DEPTH_2; i++) {
+	printf("+———-———-———-———-———-———-———-———-———+\n");
+	for (uint i = 0; i < SUDOKU_LEN; 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;
@@ -31,7 +69,9 @@ void sudoku_print(const u16 *board) {
 		default:       tile = ' '; break;
 		};
 
-		printf("|%c ", tile);
+		printf("| %c ", tile);
+		// printf("|%3X", board[i]);
 		if (i % 9 == 8) printf("|\n");
 	}
+	printf("+———-———-———-———-———-———-———-———-———+\n");
 }

src/sudoku.h

@@ -4,8 +4,9 @@
 
 #include "util/intdef.h"
 
-#define SUDOKU_DEPTH   9                             // defines the "depth" of the puzzle.
-#define SUDOKU_DEPTH_2 (SUDOKU_DEPTH * SUDOKU_DEPTH) // computes how many squares there are
+#define SUDOKU_BOXLEN     3
+#define SUDOKU_DPT        9
+#define SUDOKU_LEN        (SUDOKU_DPT * SUDOKU_DPT)
 
 /* bitmask  */
 enum sudoku_bitmask {
@@ -18,10 +19,17 @@ enum sudoku_bitmask {
 	SUDOKU_7 = 0x040,
 	SUDOKU_8 = 0x080,
 	SUDOKU_9 = 0x100,
+	SUDOKU_ALL = 0x01FF,
 };
 
-/* TODO: write documentation */
+/* 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);
+
+/* loops through the board, updating all places that only have a single possibility set. */
 void sudoku_solve_step(u16 *board);
 
-/* TODO: write documentation */
+/* prints the sudoku board to `stdout` */
 void sudoku_print(const u16 *board);