diff --git a/2021/day25/main.go b/2021/day25/main.go
new file mode 100644
index 0000000..7a2634a
--- /dev/null
+++ b/2021/day25/main.go
@@ -0,0 +1,123 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	lines := aoc.Input().ReadLines("day25.txt")
+	fmt.Println(part1(lines))
+}
+
+func part1(lines []string) int {
+	f := NewFloor(lines)
+	moved := true
+	i := 0
+	for moved {
+		f, moved = f.Tick()
+		i++
+	}
+	return i
+}
+
+type RC struct{ r, c int }
+
+type Floor struct {
+	east, south map[RC]bool
+	rows, cols  int
+}
+
+func NewFloor(lines []string) Floor {
+	f := Floor{
+		east:  make(map[RC]bool),
+		south: make(map[RC]bool),
+		rows:  len(lines),
+		cols:  len(lines[0]),
+	}
+	for r, row := range lines {
+		for c, char := range row {
+			rc := RC{r, c}
+			switch char {
+			case 'v':
+				f.south[rc] = true
+			case '>':
+				f.east[rc] = true
+			}
+		}
+	}
+	return f
+}
+
+func (f Floor) Occupied(rc RC) bool {
+	return f.east[rc] || f.south[rc]
+}
+
+func (f Floor) EastOf(rc RC) RC {
+	c := (rc.c + 1) % f.cols
+	return RC{rc.r, c}
+}
+
+func (f Floor) SouthOf(rc RC) RC {
+	r := (rc.r + 1) % f.rows
+	return RC{r, rc.c}
+}
+
+func (f Floor) Tick() (Floor, bool) {
+	next, eastward := f.TickEast()
+	next, southward := next.TickSouth()
+	return next, eastward || southward
+}
+
+func (f Floor) TickEast() (Floor, bool) {
+	next := f
+	next.east = make(map[RC]bool)
+	moved := false
+	for rc := range f.east {
+		t := f.EastOf(rc)
+		if f.Occupied(t) {
+			next.east[rc] = true
+		} else {
+			next.east[t] = true
+			moved = true
+		}
+	}
+	return next, moved
+}
+
+func (f Floor) TickSouth() (Floor, bool) {
+	next := f
+	next.south = make(map[RC]bool)
+	moved := false
+	for rc := range f.south {
+		t := f.SouthOf(rc)
+		if f.Occupied(t) {
+			next.south[rc] = true
+		} else {
+			next.south[t] = true
+			moved = true
+		}
+	}
+	return next, moved
+}
+
+func (f Floor) String() string {
+	var sb strings.Builder
+	for r := 0; r < f.rows; r++ {
+		for c := 0; c < f.cols; c++ {
+			rc := RC{r, c}
+			switch {
+			case f.east[rc]:
+				sb.WriteByte('>')
+			case f.south[rc]:
+				sb.WriteByte('v')
+			default:
+				sb.WriteByte('.')
+			}
+		}
+		sb.WriteString("\n")
+	}
+	return sb.String()
+}