From 9077b5189bc570724f3c9a10cf740bb070008c41 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Sun, 12 Dec 2021 21:33:04 -0800
Subject: [PATCH] 2021 Day 13

---
 2021/day13/main.go | 124 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 124 insertions(+)
 create mode 100644 2021/day13/main.go

diff --git a/2021/day13/main.go b/2021/day13/main.go
new file mode 100644
index 0000000..491511e
--- /dev/null
+++ b/2021/day13/main.go
@@ -0,0 +1,124 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	text := aoc.Input().ReadFile("day13.txt")
+	fmt.Println(part1(text))
+	fmt.Println(part2(text))
+}
+
+func part1(text string) int {
+	g, folds := parse(text)
+	g = g.Fold(folds[0])
+	return len(g)
+}
+
+func part2(text string) string {
+	g, folds := parse(text)
+	for _, f := range folds {
+		g = g.Fold(f)
+	}
+	return g.String()
+}
+
+func parse(text string) (Grid, []Fold) {
+	dots, folds := aoc.Cut(text, "\n\n")
+	var points []Point
+	for _, line := range strings.Split(dots, "\n") {
+		x, y := aoc.Cut(line, ",")
+		points = append(points, Point{
+			x: aoc.MustInt(x),
+			y: aoc.MustInt(y),
+		})
+	}
+
+	var fs []Fold
+	for _, line := range strings.Split(folds, "\n") {
+		axis, val := aoc.Cut(line[len("fold along "):], "=")
+		fs = append(fs, Fold{
+			axis: axis,
+			val:  aoc.MustInt(val),
+		})
+	}
+	return NewGrid(points), fs
+}
+
+type Point struct{ x, y int }
+
+type Grid map[Point]int
+
+type Fold struct {
+	axis string
+	val  int
+}
+
+func NewGrid(points []Point) Grid {
+	g := make(Grid)
+	for _, p := range points {
+		g[p] = 1
+	}
+	return g
+}
+
+func (g Grid) Fold(f Fold) Grid {
+	g2 := make(Grid)
+	for p, count := range g {
+		var p2 Point
+		switch axis := f.axis; axis {
+		case "x":
+			p2 = p.foldX(f.val)
+		case "y":
+			p2 = p.foldY(f.val)
+		default:
+			panic(axis)
+		}
+		g2[p2] += count
+	}
+	return g2
+}
+
+func (p Point) foldX(x int) Point {
+	if p.x > x {
+		return Point{x - (p.x - x), p.y}
+	}
+	return p
+}
+
+func (p Point) foldY(y int) Point {
+	if p.y > y {
+		return Point{p.x, y - (p.y - y)}
+	}
+	return p
+}
+
+func (g Grid) String() string {
+	xmax, ymax := 0, 0
+	for p := range g {
+		if p.x > xmax {
+			xmax = p.x
+		}
+		if p.y > ymax {
+			ymax = p.y
+		}
+	}
+
+	var sb strings.Builder
+	for y := 0; y <= ymax; y++ {
+		for x := 0; x <= xmax; x++ {
+			_, ok := g[Point{x, y}]
+			if ok {
+				sb.WriteString("█")
+			} else {
+				sb.WriteString(" ")
+			}
+		}
+		sb.WriteString("\n")
+	}
+	return sb.String()
+}