2021-advent-25.go

package main

import (
	"io/ioutil"
	"log"
	"strings"
)

type Seafloor [][]rune

func (sf Seafloor) at(x, y int) string {
	return string(sf[y][x])
}

func (sf Seafloor) String() string {
	sb := strings.Builder{}
	for _, v := range sf {
		sb.WriteString(string(v))
		sb.WriteString("\n")
	}
	return sb.String()
}

func readInput(input string) Seafloor {
	bytes, err := ioutil.ReadFile(input)
	if err != nil {
		log.Fatal(err)
	}

	seaFloor := make(Seafloor, 0)

	for _, v := range strings.Split(string(bytes), "\n") {
		seaFloor = append(seaFloor, []rune(v))
	}
	return seaFloor
}

type coords struct{ x, y int }

func main() {
	in := readInput("input.txt")
	// fmt.Printf("%v", in)
	N := len(in)
	M := len(in[0])
	it := 1

	for {
		moved := false
		to_delete := make([]coords, 0)
		to_add := make(map[coords]rune)
		for y := range in {
			for x := range in[y] {
				t := in.at(x, y)
				if t == ">" {
					yn := y
					xn := x + 1
					if xn >= M {
						xn = 0
					}
					if in.at(xn, yn) == "." {
						moved = true
						to_delete = append(to_delete, coords{x, y})
						to_add[coords{xn, yn}] = '>'
					}
				}
			}
		}
		for _, c := range to_delete {
			in[c.y][c.x] = '.'
		}

		for c, v := range to_add {
			in[c.y][c.x] = v
		}

		to_delete = make([]coords, 0)
		to_add = make(map[coords]rune)

		for y := range in {
			for x := range in[y] {
				t := in.at(x, y)
				if t == "v" {
					yn := y + 1
					xn := x
					if yn >= N {
						yn = 0
					}
					if in.at(xn, yn) == "." {
						moved = true
						to_delete = append(to_delete, coords{x, y})
						to_add[coords{xn, yn}] = 'v'
					}
				}
			}
		}

		for _, c := range to_delete {
			in[c.y][c.x] = '.'
		}

		for c, v := range to_add {
			in[c.y][c.x] = v
		}

		if !moved {
			break
		}
		it += 1
	}
	println(it)
}