Make generator always produce possible puzzles

sud.c

@@ -22,12 +22,15 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#define FILLP_RCP 3
+#define REM_PROB_NUMER 2
+#define REM_PROB_DENOM 3
+
+#define MAX_FILL_ATTEMPTS 32
 
 static void initposs(struct sudoku *sud)
 {
-	for (unsigned r = 0; r < SIDELEN; ++r) {
+	for (unsigned r = 0; r < NDIGITS; ++r) {
-		for (unsigned c = 0; c < SIDELEN; ++c) {
+		for (unsigned c = 0; c < NDIGITS; ++c) {
 			sud->cells[r][c].det = false;
 			for (unsigned i = 0; i < NDIGITS; ++i)
 				sud->cells[r][c].pvals[i] = true;
@@ -35,22 +38,67 @@ static void initposs(struct sudoku *sud)
 	}
 }
 
+static void clear(struct sudoku *sud, unsigned r, unsigned c)
+{
+	unsigned tr, tc, val;
+
+	/* Set undetermined with all possible values */
+	sud->cells[r][c].det = false;
+	for (unsigned i = 0; i < NDIGITS; ++i)
+		sud->cells[r][c].pvals[i] = true;
+
+	/* Update cell's possible values based off column. */
+	for (tr = 0; tr < NDIGITS; ++tr) {
+		if (tr == r || !sud->cells[tr][c].det)
+			continue;
+		val = sud->cells[tr][c].val;
+		sud->cells[r][c].pvals[val] = false;
+	}
+
+	/* Update possible values of cells in same row. */
+	for (tc = 0; tc < NDIGITS; ++tc) {
+		if (tc == c || !sud->cells[r][tc].det)
+			continue;
+		val = sud->cells[r][tc].val;
+		sud->cells[r][c].pvals[val] = false;
+	}
+
+	/* Update possible values of cells in same segment. */
+	const unsigned segr0 = SEGLEN * (r / SEGLEN);
+	const unsigned segc0 = SEGLEN * (c / SEGLEN);
+	for (tr = segr0; tr < segr0 + SEGLEN; ++tr) {
+		for (tc = segc0; tc < segc0 + SEGLEN; ++tc) {
+			if ((tr == r && tc == c) || !sud->cells[tr][tc].det)
+				continue;
+			val = sud->cells[tr][tc].val;
+			sud->cells[r][c].pvals[val] = false;
+		}
+	}
+}
+
 void gen(struct sudoku *sud)
 {
+	/* Generate a completed sudoku. */
+retry:
 	initposs(sud);
-
+	for (unsigned r = 0; r < NDIGITS; ++r) {
-	for (unsigned r = 0; r < SIDELEN; ++r) {
+		for (unsigned c = 0; c < NDIGITS; ++c) {
-		for (unsigned c = 0; c < SIDELEN; ++c) {
-			if (rand() % FILLP_RCP == 0) {
-				assert(!sud->cells[r][c].det);
-
 			unsigned val, n = 0;
 			do {
 				val = (unsigned)(rand() % NDIGITS);
 				++n;
+				if (n >= MAX_FILL_ATTEMPTS)
+					goto retry;
 			} while (update(sud, r, c, val) == NOT_ALLOWED);
 		}
 	}
+
+	/* Remove cells with probability of `1 / REMOVE_PROB_RECIP`. */
+	for (unsigned r = 0; r < NDIGITS; ++r) {
+		for (unsigned c = 0; c < NDIGITS; ++c) {
+			if (rand() % REM_PROB_DENOM < REM_PROB_NUMER)
+				clear(sud, r, c);
+		}
 	}
 }
 
@@ -66,14 +114,14 @@ enum update_res update(struct sudoku *sud, unsigned r, unsigned c, unsigned val)
 		return NOT_ALLOWED;
 
 	/* Update possible values of cells in same column. */
-	for (tr = 0; tr < SIDELEN; ++tr) {
+	for (tr = 0; tr < NDIGITS; ++tr) {
 		if (tr == r || sud->cells[tr][c].det)
 			continue;
 		sud->cells[tr][c].pvals[val] = false;
 	}
 
 	/* Update possible values of cells in same row. */
-	for (tc = 0; tc < SIDELEN; ++tc) {
+	for (tc = 0; tc < NDIGITS; ++tc) {
 		if (tc == c || sud->cells[r][tc].det)
 			continue;
 		sud->cells[r][tc].pvals[val] = false;
@@ -99,7 +147,7 @@ enum update_res update(struct sudoku *sud, unsigned r, unsigned c, unsigned val)
 
 void print(const struct sudoku *sud)
 {
-	for (unsigned r = 0; r < SIDELEN; ++r) {
+	for (unsigned r = 0; r < NDIGITS; ++r) {
 		/*
 		 * Print horizontal divider if on a segment boundary (but not
 		 * at the start).
@@ -107,7 +155,7 @@ void print(const struct sudoku *sud)
 		if (r != 0 && r % SEGLEN == 0)
 			puts("------+-------+------");
 
-		for (unsigned c = 0; c < SIDELEN; ++c) {
+		for (unsigned c = 0; c < NDIGITS; ++c) {
 			/*
 			 * Print vertical divider if on a segment boundary (but
 			 * not at the start).

sud.h

@@ -21,7 +21,6 @@
 
 #include <stdbool.h>
 
-#define SIDELEN 9
 #define SEGLEN 3
 #define NDIGITS 9
 
@@ -34,7 +33,7 @@ struct cellstate {
 };
 
 struct sudoku {
-	struct cellstate cells[SIDELEN][SIDELEN];
+	struct cellstate cells[NDIGITS][NDIGITS];
 };
 
 enum update_res { NOT_ALLOWED, ALREADY_DET, OK };