diff --git a/Makefile b/Makefile
index ed7d98a..ecb8727 100644
--- a/Makefile
+++ b/Makefile
@@ -17,7 +17,7 @@
 CFLAGS += -std=c11 -pedantic -Wall -Wextra
 CFLAGS += -O2 -flto
 
-SRC = main.c
+SRC = main.c sud.c
 OBJ = $(SRC:.c=.o)
 
 sudoku: $(OBJ)
@@ -26,4 +26,7 @@ sudoku: $(OBJ)
 clean:
 	rm -f sudoku $(OBJ)
 
+main.o: sud.h
+sud.o: sud.h
+
 .PHONY: clean
diff --git a/main.c b/main.c
index 7ec51fa..8987058 100644
--- a/main.c
+++ b/main.c
@@ -16,7 +16,21 @@
  * <https://www.gnu.org/licenses/>.
  */
 
+#include "sud.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
 int main(void)
 {
+	unsigned seed = time(NULL);
+	printf("Seed: %u\n\n", seed);
+	srand(seed);
+
+	struct sudoku sud;
+	gen(&sud);
+	print(&sud);
+
 	return 0;
 }
diff --git a/sud.c b/sud.c
new file mode 100644
index 0000000..2723a2d
--- /dev/null
+++ b/sud.c
@@ -0,0 +1,125 @@
+/*
+ * 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/>.
+ */
+
+#include "sud.h"
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#define FILLP_RCP 3
+
+static void initposs(struct sudoku *sud)
+{
+	for (unsigned r = 0; r < SIDELEN; ++r) {
+		for (unsigned c = 0; c < SIDELEN; ++c) {
+			sud->cells[r][c].det = false;
+			for (unsigned i = 0; i < NDIGITS; ++i)
+				sud->cells[r][c].pvals[i] = true;
+		}
+	}
+}
+
+void gen(struct sudoku *sud)
+{
+	initposs(sud);
+
+	for (unsigned r = 0; r < SIDELEN; ++r) {
+		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;
+				} while (update(sud, r, c, val) == NOT_ALLOWED);
+			}
+		}
+	}
+}
+
+enum update_res update(struct sudoku *sud, unsigned r, unsigned c, unsigned val)
+{
+	unsigned tr, tc;
+	assert(val < NDIGITS);
+
+	/* Check that the cell is undetermined and the value is allowed. */
+	if (sud->cells[r][c].det)
+		return ALREADY_DET;
+	if (!sud->cells[r][c].pvals[val])
+		return NOT_ALLOWED;
+
+	/* Update possible values of cells in same column. */
+	for (tr = 0; tr < SIDELEN; ++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) {
+		if (tc == c || sud->cells[r][tc].det)
+			continue;
+		sud->cells[r][tc].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;
+			sud->cells[tr][tc].pvals[val] = false;
+		}
+	}
+
+	/* Set the cell's value. */
+	sud->cells[r][c].det = true;
+	sud->cells[r][c].val = val;
+
+	return OK;
+}
+
+void print(const struct sudoku *sud)
+{
+	for (unsigned r = 0; r < SIDELEN; ++r) {
+		/*
+		 * Print horizontal divider if on a segment boundary (but not
+		 * at the start).
+		 */
+		if (r != 0 && r % SEGLEN == 0)
+			puts("------+-------+------");
+
+		for (unsigned c = 0; c < SIDELEN; ++c) {
+			/*
+			 * Print vertical divider if on a segment boundary (but
+			 * not at the start).
+			 */
+			if (c != 0 && c % SEGLEN == 0)
+				fputs("| ", stdout);
+
+			if (sud->cells[r][c].det)
+				printf("%u ", sud->cells[r][c].val + 1);
+			else
+				fputs("  ", stdout);
+		}
+		putchar('\n');
+	}
+}
diff --git a/sud.h b/sud.h
new file mode 100644
index 0000000..ec976e1
--- /dev/null
+++ b/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