diff --git a/2021/day21/main.go b/2021/day21/main.go
new file mode 100644
index 0000000..de4cab5
--- /dev/null
+++ b/2021/day21/main.go
@@ -0,0 +1,90 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	lines := aoc.Input().ReadLines("day21.txt")
+	p1, p2 := parseInput(lines)
+	fmt.Println(part1(p1, p2))
+}
+
+func parseInput(lines []string) (p1, p2 int) {
+	aoc.MustScan(lines[0], "Player 1 starting position: %d", &p1)
+	aoc.MustScan(lines[1], "Player 2 starting position: %d", &p2)
+	return
+}
+
+func part1(p1, p2 int) int {
+	g := NewGame(p1, p2, &DD100{})
+	for i := 1; true; i++ {
+		hash := g.Turn(i)
+		if hash != 0 {
+			return hash
+		}
+	}
+	return 0
+}
+
+type Game struct {
+	p1, p2 int
+	s1, s2 int
+	die    Roller
+}
+
+func NewGame(p1, p2 int, die Roller) *Game {
+	return &Game{p1, p2, 0, 0, die}
+}
+
+func (g *Game) String() string {
+	return fmt.Sprintf("P1: %2d, S1: %5d | P1: %2d, S1: %5d", g.p1, g.s1, g.p2, g.s2)
+}
+
+func (g *Game) Turn(turns int) (hash int) {
+	if g.P1() {
+		return g.s2 * (6*turns - 3)
+	}
+	if g.P2() {
+		return g.s1 * (6 * turns)
+	}
+	return 0
+}
+
+func (g *Game) P1() (won bool) {
+	roll := g.die.Roll() + g.die.Roll() + g.die.Roll()
+	g.p1 = move(g.p1, roll)
+	g.s1 += score(g.p1)
+	return g.s1 >= 1000
+}
+
+func (g *Game) P2() (won bool) {
+	roll := g.die.Roll() + g.die.Roll() + g.die.Roll()
+	g.p2 = move(g.p2, roll)
+	g.s2 += score(g.p2)
+	return g.s2 >= 1000
+}
+
+func move(pos, roll int) int {
+	// 0-index for math, 1-index for position.
+	return ((pos-1)+roll)%10 + 1
+}
+
+func score(pos int) int {
+	// 0-index for math, 1-index for position.
+	return (pos-1)%10 + 1
+}
+
+type Roller interface {
+	Roll() int
+}
+
+type DD100 struct{ val int }
+
+func (d *DD100) Roll() int {
+	val := d.val
+	d.val = (d.val + 1) % 100
+	return val + 1
+}