adventofcode/2023/day16/main.go

package main

import (
	"fmt"

	h "git.bullercodeworks.com/brian/adventofcode/helpers"
)

func main() {
	inp := h.StdinToStringSlice()
	part1(inp)
	fmt.Println()
	part2(inp)
}

var (
	N = h.Coordinate{X: 0, Y: -1}
	E = h.Coordinate{X: 1, Y: 0}
	S = h.Coordinate{X: 0, Y: 1}
	W = h.Coordinate{X: -1, Y: 0}
)

// movement describes how a beam moving in a direction should react
// to a byte
var movement = map[h.Coordinate]map[byte][]h.Coordinate{
	N: {
		'|':  {N},
		'-':  {W, E},
		'\\': {W},
		'/':  {E},
		'.':  {N},
	},
	E: {
		'|':  {N, S},
		'-':  {E},
		'\\': {S},
		'/':  {N},
		'.':  {E},
	},
	S: {
		'|':  {S},
		'-':  {W, E},
		'\\': {E},
		'/':  {W},
		'.':  {S},
	},
	W: {
		'|':  {N, S},
		'-':  {W},
		'\\': {N},
		'/':  {S},
		'.':  {W},
	},
}

var tileHistory map[h.Coordinate][]h.Coordinate

func part1(input []string) {
	mm := h.StringSliceToCoordByteMap(input)
	tileHistory = make(map[h.Coordinate][]h.Coordinate)
	proc(&mm, E, h.Coordinate{X: 0, Y: 0})

	fmt.Println("# Part 1")
	fmt.Println(len(tileHistory))
}

func part2(input []string) {
	mm := h.StringSliceToCoordByteMap(input)

	maxTiles := h.MIN_INT
	// starts are a map of positions -> directions to test as a starting point
	starts := make(map[*h.Coordinate]h.Coordinate)
	for y := mm.TLY; y <= mm.BRY; y++ {
		for x := mm.TLX; x <= mm.BRX; x++ {
			if y != mm.TLY && x != mm.TLX && y != mm.BRY && x != mm.BRX {
				continue
			}
			if y == mm.TLY {
				// South facing beams
				starts[&h.Coordinate{Y: y, X: x}] = S
			} else if y == mm.BRY {
				// North facing beams
				starts[&h.Coordinate{Y: y, X: x}] = N
			}
			if x == mm.TLX {
				// East facing beams
				starts[&h.Coordinate{Y: y, X: x}] = E
			}
			if x == mm.BRX {
				// West facing beams
				starts[&h.Coordinate{Y: y, X: x}] = W
			}
		}
	}
	for pos, dir := range starts {
		tileHistory = make(map[h.Coordinate][]h.Coordinate)
		proc(&mm, dir, *pos)
		if len(tileHistory) > maxTiles {
			maxTiles = len(tileHistory)
		}
	}

	fmt.Println("# Part 2")
	fmt.Println(maxTiles)
}

func proc(mm *h.CoordByteMap, dir, pos h.Coordinate) {
	if !mm.ContainsCoord(pos) {
		return
	}
	if visits, ok := tileHistory[pos]; ok {
		for _, v := range visits {
			if v == dir {
				return
			}
		}
	}
	curr := mm.Get(pos)
	tileHistory[pos] = append(tileHistory[pos], dir)
	next := movement[dir][curr]
	for _, n := range next {
		proc(mm, n, pos.Add(n))
	}
}