Implement check() function to verify solver results

sud.c

@@ -171,3 +171,64 @@ void print(const struct sudoku *sud)
 		putchar('\n');
 	}
 }
+
+static void zerocounts(unsigned counts[NDIGITS])
+{
+	for (unsigned i = 0; i < NDIGITS; ++i)
+		counts[i] = 0;
+}
+
+static bool checkcounts(unsigned counts[NDIGITS])
+{
+	for (unsigned i = 0; i < NDIGITS; ++i) {
+		if (counts[i] != 1)
+			return false;
+	}
+	return true;
+}
+
+enum check_res check(const struct sudoku *sud)
+{
+	unsigned r, c, i, j, digitcounts[NDIGITS];
+
+	/* Check each row. */
+	for (r = 0; r < NDIGITS; ++r) {
+		zerocounts(digitcounts);
+		for (c = 0; c < NDIGITS; ++c) {
+			if (!sud->cells[r][c].det)
+				return INCOMPLETE;
+			++digitcounts[sud->cells[r][c].val];
+		}
+		if (!checkcounts(digitcounts))
+			return INCORRECT;
+	}
+
+	/* Check each column. */
+	for (c = 0; c < NDIGITS; ++c) {
+		zerocounts(digitcounts);
+		for (r = 0; r < NDIGITS; ++r) {
+			if (!sud->cells[r][c].det)
+				return INCOMPLETE;
+			++digitcounts[sud->cells[r][c].val];
+		}
+		if (!checkcounts(digitcounts))
+			return INCORRECT;
+	}
+
+	/* Check each segment. */
+	for (i = 0; i < NDIGITS; ++i) {
+		zerocounts(digitcounts);
+		for (j = 0; j < NDIGITS; ++j) {
+			r = 3 * (i / 3) + j / 3;
+			c = 3 * (i % 3) + j % 3;
+			if (!sud->cells[r][c].det)
+				return INCOMPLETE;
+			++digitcounts[sud->cells[r][c].val];
+		}
+		if (!checkcounts(digitcounts))
+			return INCORRECT;
+	}
+
+	/* If we've got this far, all is well. */
+	return SOLVED;
+}