From b5538b2540c1a07fcfac1ec9ddc839ed23df9c68 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Sat, 11 Dec 2021 22:12:58 -0800
Subject: [PATCH] 2021 Day 12

---
 2021/aoc/aoc.go    |   4 +
 2021/day12/main.go | 179 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 183 insertions(+)
 create mode 100644 2021/day12/main.go

diff --git a/2021/aoc/aoc.go b/2021/aoc/aoc.go
index fec2efe..9ecfc27 100644
--- a/2021/aoc/aoc.go
+++ b/2021/aoc/aoc.go
@@ -91,3 +91,7 @@ func Ints(text string) (ns []int) {
 	}
 	return
 }
+
+func ExLines(text string) []string {
+	return strings.Split(strings.TrimSpace(text), "\n")
+}
diff --git a/2021/day12/main.go b/2021/day12/main.go
new file mode 100644
index 0000000..1c7797f
--- /dev/null
+++ b/2021/day12/main.go
@@ -0,0 +1,179 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	lines := aoc.Input().ReadLines("day12.txt")
+	fmt.Println(part1(lines))
+	fmt.Println(part2(lines))
+}
+
+func part1(lines []string) int {
+	g := NewGraph(lines)
+	return g.Paths()
+}
+func part2(lines []string) int {
+	g := NewGraph(lines)
+	return g.Paths2()
+}
+
+type Graph struct {
+	Edges map[string][]string
+	Big   map[string]struct{}
+	Small map[string]struct{}
+}
+
+func NewGraph(lines []string) *Graph {
+	g := &Graph{
+		Edges: make(map[string][]string),
+		Big:   make(map[string]struct{}),
+		Small: make(map[string]struct{}),
+	}
+	for _, l := range lines {
+		a, b := aoc.Cut(l, "-")
+		g.Edges[a] = append(g.Edges[a], b)
+		g.Edges[b] = append(g.Edges[b], a)
+		if isBig(a) {
+			g.Big[a] = struct{}{}
+		} else {
+			g.Small[a] = struct{}{}
+		}
+		if isBig(b) {
+			g.Big[b] = struct{}{}
+		} else {
+			g.Small[b] = struct{}{}
+		}
+	}
+	return g
+}
+
+func (g Graph) Neighbors(name string) []string {
+	return g.Edges[name]
+}
+
+type path []string
+
+func (p path) String() string {
+	return strings.Join(p, ",")
+}
+
+func (p path) Contains(name string) bool {
+	for _, n := range p {
+		if n == name {
+			return true
+		}
+	}
+	return false
+}
+
+func (p path) Tip() string {
+	return p[len(p)-1]
+}
+
+func (p path) Plus(name string) path {
+	// Copy memory to avoid overwriting the last element in the slice.
+	p2 := make(path, len(p))
+	copy(p2, p)
+	return append(p2, name)
+}
+
+func (g Graph) Paths() (count int) {
+	queue := []path{{"start"}}
+	for len(queue) > 0 {
+		p := queue[0]
+		queue = queue[1:]
+		tip := p.Tip()
+		for _, n := range g.Neighbors(tip) {
+			ext := p.Plus(n)
+			if n == "end" {
+				count++
+				continue
+			}
+			if !isBig(n) && p.Contains(n) {
+				// Don't include small caves twice
+				continue
+			}
+			queue = append(queue, ext)
+		}
+	}
+	return
+}
+
+func isBig(name string) bool {
+	return strings.ToUpper(name) == name
+}
+
+type path2 struct {
+	caves   []string
+	doubled bool
+}
+
+func (p *path2) Plus(name string) (path2, bool) {
+	var caves []string
+	doubled := p.doubled
+	canDupe := isBig(name)
+	for _, c := range p.caves {
+		caves = append(caves, c)
+		if c != name {
+			// No dupe.
+			continue
+		}
+		if canDupe {
+			continue
+		}
+		// c == name for a small cave
+		if doubled {
+			// Already found a doubled cave, can't include this one.
+			return path2{}, false
+		}
+		// Contains two of c now.
+		doubled = true
+	}
+	return path2{
+		caves:   append(caves, name),
+		doubled: doubled,
+	}, true
+}
+
+func (p path2) String() string {
+	return strings.Join(p.caves, ",")
+}
+
+func (p path2) Tip() string {
+	return p.caves[len(p.caves)-1]
+}
+
+func (g Graph) Paths2() (count int) {
+	queue := []path2{
+		{
+			caves:   []string{"start"},
+			doubled: false,
+		},
+	}
+	for len(queue) > 0 {
+		p := queue[0]
+		queue = queue[1:]
+		tip := p.Tip()
+		for _, n := range g.Neighbors(tip) {
+			if n == "start" {
+				// No loops!
+				continue
+			}
+			ext, ok := p.Plus(n)
+			if n == "end" {
+				count++
+				continue
+			}
+			if !ok {
+				continue
+			}
+			queue = append(queue, ext)
+		}
+	}
+	return
+}