Implement basic generation and printing

I expect this will probably generate some impossible puzzles, but I'll
deal with that later.

sud.h

@@ -0,0 +1,61 @@
+/*
+ * Copyright (C) 2022 Camden Dixie O'Brien
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public
+ * License along with this program.  If not, see
+ * <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef SUD_H
+#define SUD_H
+
+#include <stdbool.h>
+
+#define SIDELEN 9
+#define SEGLEN 3
+#define NDIGITS 9
+
+struct cellstate {
+	bool det;
+	union {
+		unsigned val;
+		bool pvals[NDIGITS];
+	};
+};
+
+struct sudoku {
+	struct cellstate cells[SIDELEN][SIDELEN];
+};
+
+enum update_res { NOT_ALLOWED, ALREADY_DET, OK };
+
+/**
+ * Populate the sudoku with some random values.
+ */
+void gen(struct sudoku *sud);
+
+/**
+ * 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
+ * already determined or `NOT_ALLOWED` if the cell being `val` would
+ * violate the sudoku rules.
+ */
+enum update_res
+update(struct sudoku *sud, unsigned r, unsigned c, unsigned val);
+
+/**
+ * Print a string representation of the sudoku to stdout.
+ */
+void print(const struct sudoku *sud);
+
+#endif