Implement check() function to verify solver results

sud.h

@@ -22,7 +22,7 @@
 #include <stdbool.h>
 
 #define SEGLEN 3
-#define NDIGITS 9
+#define NDIGITS (SEGLEN * SEGLEN)
 
 struct cellstate {
 	bool det;
@@ -38,6 +38,8 @@ struct sudoku {
 
 enum update_res { NOT_ALLOWED, ALREADY_DET, OK };
 
+enum check_res { INCOMPLETE, INCORRECT, SOLVED };
+
 /**
  * Populate the sudoku with some random values.
  */
@@ -57,4 +59,10 @@ update(struct sudoku *sud, unsigned r, unsigned c, unsigned val);
  */
 void print(const struct sudoku *sud);
 
+/**
+ * Determine whether the sudoku has been solved correctly, contains
+ * invalid choices or is incomplete.
+ */
+enum check_res check(const struct sudoku *sud);    
+
 #endif