refactor and add comments to tictactoe_game.c

tictactoe_game.c

@@ -4,6 +4,9 @@
 #include <linux/kernel.h>
 #include <linux/printk.h>
 
+/*
+ * Initializes a new game instance.
+ */
 int tictactoe_game_init(ttt_game_t *game)
 {
 	game->next_turn = 'X';
@@ -17,69 +20,74 @@ int tictactoe_game_init(ttt_game_t *game)
 	return 0;
 }
 
-// returns 0 if there is no winner yet, -1 if it's a tie, and the respective player if they win
-char tictactoe_game_check_winner(ttt_game_t *game)
+/*
+ * Steps over the board, starting from position (start_x, start_y) and ending
+ * at the board's boundaries, checking if all cells are occupied by the same
+ * player.
+ *
+ * start_x and start_y must lie within the board's boundaries.
+ *
+ * Returns the respective player, or '\x00' if a cell isn't occupied.
+ */
+static char tictactoe_game_check_streak(ttt_game_t *game, size_t start_x,
+					size_t start_y, ssize_t step_x,
+					ssize_t step_y)
+{
+	char winner;
+	size_t x;
+	size_t y;
+
+	winner = game->board[start_x][start_y];
+	x = start_x + step_x;
+	y = start_y + step_y;
+
+	if (winner == ' ')
+		return 0;
+
+	while (x >= 0 && x < 3 && y >= 0 && y < 3) {
+		if (game->board[x][y] != winner)
+			return 0;
+		x += step_x;
+		y += step_y;
+	}
+
+	return winner;
+}
+
+/*
+ * Checks for a winner with the current board configuration.
+ *
+ * Returns
+ *   '\x00' if the game has not ended,
+ *   '\xFF' if the game is tied, or
+ *   the winner of the game.
+ */
+static char tictactoe_game_check_winner(ttt_game_t *game)
 {
 	char winner;
 
 	// check columns
 	for (int x = 0; x < 3; x++) {
-		winner = game->board[x][0];
-		if (winner != ' ') {
-			for (int y = 1; y < 3; y++) {
-				if (game->board[x][y] != winner) {
-					winner = 0;
-					break;
-				}
-			}
-			if (winner != 0)
-				return winner;
-		}
+		if ((winner = tictactoe_game_check_streak(game, x, 0, 0, 1)))
+			return winner;
 	}
 
 	// check rows
 	for (int y = 0; y < 3; y++) {
-		winner = game->board[0][y];
-		if (winner != ' ') {
-			for (int x = 1; x < 3; x++) {
-				if (game->board[x][y] != winner) {
-					winner = 0;
-					break;
-				}
-			}
-			if (winner != 0)
-				return winner;
-		}
+		if ((winner = tictactoe_game_check_streak(game, 0, y, 1, 0)))
+			return winner;
 	}
 
 	// check diagonal top-left <-> bottom-right
-	winner = game->board[0][0];
-	if (winner != ' ') {
-		for (int i = 0; i < 3; i++) {
-			if (game->board[i][i] != winner) {
-				winner = 0;
-				break;
-			}
-		}
-		if (winner != 0)
-			return winner;
-	}
+	if ((winner = tictactoe_game_check_streak(game, 0, 0, 1, 1)))
+		return winner;
 
 	// check diagonal bottom-left <-> top-right
-	winner = game->board[2][0];
-	if (winner != ' ') {
-		for (int i = 0; i < 3; i++) {
-			if (game->board[2 - i][i] != winner) {
-				winner = 0;
-				break;
-			}
-		}
-		if (winner != 0)
-			return winner;
-	}
+	if ((winner = tictactoe_game_check_streak(game, 0, 2, 1, -1)))
+		return winner;
 
 	// check for tie
-	winner = -1;
+	winner = '\xFF';
 	for (int x = 0; x < 3; x++) {
 		for (int y = 0; y < 3; y++) {
 			if (game->board[x][y] == ' ') {
@@ -92,6 +100,16 @@ char tictactoe_game_check_winner(ttt_game_t *game)
 	return winner;
 }
 
+/*
+ * Occupies the given board position for the player whose turn it is.
+ *
+ * This function does *not* check whether there is a winner first.
+ * x and y must lie within the board's boundaries.
+ *
+ * Returns
+ *   -1 if the given position is already occupied, or
+ *   0 on success.
+ */
 int tictactoe_game_make_turn(ttt_game_t *game, size_t x, size_t y)
 {
 	if (game->board[x][y] != ' ') {
@@ -110,6 +128,9 @@ int tictactoe_game_make_turn(ttt_game_t *game, size_t x, size_t y)
 	return 0;
 }
 
+/*
+ * Prints a human-readable visualization of the game state to buf.
+ */
 int tictactoe_game_snprint(ttt_game_t *game, char *buf, size_t count)
 {
 	int pos;
@@ -124,10 +145,10 @@ int tictactoe_game_snprint(ttt_game_t *game, char *buf, size_t count)
 		goto fail;
 	pos += ret;
 
-	if (game->winner == 0) {
+	if (!game->winner) {
 		ret = snprintf(buf + pos, count - pos, "It's %c's turn!\n",
 			       game->next_turn);
-	} else if (game->winner == -1) {
+	} else if (game->winner == '\xFF') {
 		ret = snprintf(buf + pos, count - pos, "It's a tie!\n");
 	} else {
 		ret = snprintf(buf + pos, count - pos, "Player %c won!\n",