diff --git a/2021/day12/main.go b/2021/day12/main.go
new file mode 100644
index 0000000..e679a70
--- /dev/null
+++ b/2021/day12/main.go
@@ -0,0 +1,153 @@
+package main
+
+import (
+	_ "embed"
+	"flag"
+	"fmt"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+//go:embed input.txt
+var input string
+
+func init() {
+	// do this in init (not main) so test file has same input
+	input = strings.TrimRight(input, "\n")
+	if len(input) == 0 {
+		panic("empty input.txt file")
+	}
+}
+
+// 610/1663
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	if part == 1 {
+		ans := part1(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	} else {
+		ans := part2(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	}
+}
+
+func part1(input string) int {
+	parsed := parseInput(input)
+
+	// part 1
+	// cannot visit SMALL caves more than once (lowercase ones)
+	// how many paths are there from start -> end
+
+	graph := map[string]map[string]bool{}
+	for _, pair := range parsed {
+		if graph[pair[0]] == nil {
+			graph[pair[0]] = map[string]bool{}
+		}
+		if graph[pair[1]] == nil {
+			graph[pair[1]] = map[string]bool{}
+		}
+		graph[pair[0]][pair[1]] = true
+		graph[pair[1]][pair[0]] = true
+	}
+
+	return walk(graph, "start", map[string]bool{"start": true}, []string{"start"})
+}
+
+func walk(graph map[string]map[string]bool, current string, visited map[string]bool, path []string) int {
+	if current == "end" {
+		return 1
+	}
+
+	var pathsToEnd int
+
+	for visitable := range graph[current] {
+		if visited[visitable] && strings.ToUpper(visitable) != visitable {
+			continue
+		}
+		visited[current] = true
+		// path is basically unused (and wasting memory), but useful to debug sometimes
+		path = append(path, visitable)
+
+		pathsToEnd += walk(graph, visitable, visited, path)
+
+		// backtrack
+		visited[visitable] = false
+		path = path[:len(path)-1]
+	}
+
+	return pathsToEnd
+}
+
+func part2(input string) int {
+	parsed := parseInput(input)
+
+	// cannot visit SMALL caves more than once (lowercase ones)
+	// how many paths are there from start -> end
+
+	graph := map[string]map[string]bool{}
+	for _, pair := range parsed {
+		if graph[pair[0]] == nil {
+			graph[pair[0]] = map[string]bool{}
+		}
+		if graph[pair[1]] == nil {
+			graph[pair[1]] = map[string]bool{}
+		}
+		graph[pair[0]][pair[1]] = true
+		graph[pair[1]][pair[0]] = true
+	}
+
+	return walk2(graph, "start", map[string]int{"start": 5}, []string{"start"}, false)
+}
+
+func walk2(graph map[string]map[string]bool, current string, visited map[string]int, path []string, doubleUsed bool) int {
+	if current == "end" {
+		fmt.Println("path", path)
+		return 1
+	}
+
+	visited[current]++
+
+	var pathsToEnd int
+
+	for visitable := range graph[current] {
+		if visitable == "start" {
+			continue
+		}
+
+		if strings.ToUpper(visitable) != visitable && visited[visitable] > 0 {
+			if doubleUsed {
+				continue
+			} else {
+				doubleUsed = true
+			}
+		}
+
+		path = append(path, visitable)
+		pathsToEnd += walk2(graph, visitable, visited, path, doubleUsed)
+
+		// backtrack
+		visited[visitable]--
+		path = path[:len(path)-1]
+		// backtrack doubleUsed IF this is a smallcave and reducing its visited count still has the
+		// cave marked as visited (aka count == 1)
+		if strings.ToUpper(visitable) != visitable && visited[visitable] == 1 {
+			doubleUsed = false
+		}
+	}
+
+	return pathsToEnd
+}
+
+func parseInput(input string) (ans [][]string) {
+	for _, line := range strings.Split(input, "\n") {
+		ans = append(ans, strings.Split(line, "-"))
+	}
+	return ans
+}
diff --git a/2021/day12/main_test.go b/2021/day12/main_test.go
new file mode 100644
index 0000000..0f911e1
--- /dev/null
+++ b/2021/day12/main_test.go
@@ -0,0 +1,115 @@
+package main
+
+import (
+	"testing"
+)
+
+var example = `start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end`
+
+var example2 = `dc-end
+HN-start
+start-kj
+dc-start
+dc-HN
+LN-dc
+HN-end
+kj-sa
+kj-HN
+kj-dc`
+
+var example3 = `fs-end
+he-DX
+fs-he
+start-DX
+pj-DX
+end-zg
+zg-sl
+zg-pj
+pj-he
+RW-he
+fs-DX
+pj-RW
+zg-RW
+start-pj
+he-WI
+zg-he
+pj-fs
+start-RW`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  10,
+		},
+		{
+			name:  "example2",
+			input: example2,
+			want:  19,
+		},
+		{
+			name:  "example3",
+			input: example3,
+			want:  226,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  3421,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part1(tt.input); got != tt.want {
+				t.Errorf("part1() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_part2(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  36,
+		},
+		{
+			name:  "example2",
+			input: example2,
+			want:  103,
+		},
+		{
+			name:  "example3",
+			input: example3,
+			want:  3509,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  84870,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part2(tt.input); got != tt.want {
+				t.Errorf("part2() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}