Add filled() function as a faster alternative to check()

sud.c

@@ -241,3 +241,14 @@ enum check_res check(const struct sudoku *sud)
 	/* If we've got this far, all is well. */
 	return SOLVED;
 }
+
+bool filled(const struct sudoku *sud)
+{
+	for (unsigned r = 0; r < NDIGITS; ++r) {
+		for (unsigned c = 0; c < NDIGITS; ++c) {
+			if (!sud->cells[r][c].det)
+				return false;
+		}
+	}
+	return true;
+}

sud.h

@@ -73,4 +73,9 @@ void print(const struct sudoku *sud);
  */
 enum check_res check(const struct sudoku *sud);
 
+/**
+ * Determine whether all the sudoku's cells have been determined.
+ */
+bool filled(const struct sudoku *sud);
+
 #endif