Use uint16_t instead of struct for cell state

- Bits 0-8 are the possible values
- Bits 9-12 is the concrete value
- Bit 15 is 1 if the cell is determined

sud.h

@@ -20,21 +20,21 @@
 #define SUD_H
 
 #include <stdbool.h>
+#include <stdint.h>
 
 #define SEGLEN 3
-#define NDIGITS (SEGLEN * SEGLEN)
-#define NCELLS (NDIGITS * NDIGITS)
+#define NDIGITS 9
+#define NCELLS 81
 
-struct cellstate {
-	bool det;
-	union {
-		unsigned val;
-		bool pvals[NDIGITS];
-	};
-};
+#define DETMASK 0x8000
+#define VALSHIFT 9
+#define VALMASK 0x1e00
+
+#define DET(x) (x & DETMASK)
+#define VAL(x) ((x & VALMASK) >> VALSHIFT)
 
 struct sudoku {
-	struct cellstate cells[NDIGITS][NDIGITS];
+	uint16_t cells[NDIGITS][NDIGITS];
 };
 
 enum update_res { NOT_ALLOWED, ALREADY_DET, OK };
@@ -47,12 +47,6 @@ enum check_res { INCOMPLETE, INCORRECT, SOLVED };
  */
 bool load(struct sudoku *sud, const char *ptr);
 
-/**
- * Populate the sudoku with some random values. The proportion of
- * cells that are filled will be approximately `fill_prop`.
- */
-void gen(struct sudoku *sud, double fill_prop);
-
 /**
  * Attempt to update the cell at row `r`, column `c` to have the value
  * `val`. Returns `OK` on success, `ALREADY_DET` if the cell is