adventofcode/2023/day23/main.go

package main

import (
	"fmt"

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

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

func part1(inp []string) {
	fmt.Println("# Part 1")
	start, end := h.Coordinate{}, h.Coordinate{}
	start.X, start.Y = findStart(inp)
	end.X, end.Y = findEnd(inp)
	visited := map[h.Coordinate]int{start: 0}
	currentDir := S
	walk(inp, start, currentDir, visited)
	fmt.Println(visited[end])
}

func part2(inp []string) {
	fmt.Println("# Part 2")
	start, end := h.Coordinate{}, h.Coordinate{}
	start.X, start.Y = findStart(inp)
	end.X, end.Y = findEnd(inp)
	conns := getConnections(inp)
	conns[start] = true
	conns[end] = true

	paths := getPaths(inp, conns)
	visited := make([]bool, len(conns))
	visited[paths[start][0].index] = true
	fmt.Println(part2Walk(inp, paths, start, end, 0, visited))
}

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}
	dirs     = []h.Coordinate{N, E, S, W}
	badSlope = map[h.Coordinate]byte{
		N: 'v', E: '<', S: '^', W: '>',
	}
)

func getLeft(from h.Coordinate) h.Coordinate {
	switch {
	case from.Equals(N):
		return W
	case from.Equals(E):
		return N
	case from.Equals(S):
		return E
	default:
		return S
	}
}
func getRight(from h.Coordinate) h.Coordinate {
	switch {
	case from.Equals(N):
		return E
	case from.Equals(E):
		return S
	case from.Equals(S):
		return W
	default:
		return N
	}
}

func findStart(inp []string) (int, int) {
	y := 0
	for x := 0; x < len(inp[0]); x++ {
		if inp[y][x] == '.' {
			return x, y
		}
	}
	return -1, -1
}

func findEnd(inp []string) (int, int) {
	y := len(inp) - 1
	for x := 0; x < len(inp[0]); x++ {
		if inp[y][x] == '.' {
			return x, y
		}
	}
	return -1, -1
}

func isValidCoord(inp []string, c h.Coordinate) bool {
	return c.Y >= 0 && c.Y < len(inp) && c.X >= 0 && c.X < len(inp[0])
}

func walk(inp []string, start, currDir h.Coordinate, visited map[h.Coordinate]int) {
	curr := start
	currStep := visited[curr]

	for _, dir := range []h.Coordinate{currDir, getLeft(currDir), getRight(currDir)} {
		next := h.Coordinate{X: (curr.X + dir.X), Y: (curr.Y + dir.Y)}
		if isValidCoord(inp, next) && inp[next.Y][next.X] != '#' {
			bt := inp[next.Y][next.X]
			if badSlope[dir] == bt {
				continue
			}
			if val, found := visited[next]; !found || val < currStep+1 {
				visited[next] = currStep + 1
				walk(inp, next, dir, visited)
			}
		}
	}
}

func getConnections(inp []string) map[h.Coordinate]bool {
	conns := map[h.Coordinate]bool{}
	for y, line := range inp {
		for x, bt := range line {
			if bt == '#' {
				continue
			}
			pos := h.Coordinate{X: x, Y: y}
			neighbors := 0
			for _, nbr := range pos.GetOrthNeighbors() {
				if isValidCoord(inp, nbr) && inp[nbr.Y][nbr.X] != '#' {
					neighbors++
				}
			}
			if neighbors > 2 {
				conns[pos] = true
			}
		}
	}
	return conns
}

type Path struct {
	end           h.Coordinate
	length, index int
}

func getPaths(inp []string, conns map[h.Coordinate]bool) map[h.Coordinate][]Path {
	paths := map[h.Coordinate][]Path{}
	connIdx := 0
	for c := range conns {
		for _, d := range dirs {
			curr := h.Coordinate{X: c.X + d.X, Y: c.Y + d.Y}
			if isValidCoord(inp, curr) && inp[curr.Y][curr.X] != '#' {
				path := getPath(inp, c, curr, d, 1, conns)
				path.index = connIdx
				paths[c] = append(paths[c], path)
			}
		}
		connIdx++
	}
	return paths
}

func getPath(inp []string, start, curr, dir h.Coordinate, length int, conns map[h.Coordinate]bool) Path {
	for _, dir := range []h.Coordinate{dir, getLeft(dir), getRight(dir)} {
		next := h.Coordinate{X: curr.X + dir.X, Y: curr.Y + dir.Y}
		if inp[next.Y][next.X] != '#' {
			if _, found := conns[next]; found {
				return Path{next, length + 1, 0}
			} else {
				return getPath(inp, start, next, dir, length+1, conns)
			}
		}
	}
	return Path{
		end:    h.Coordinate{X: -1, Y: -1},
		length: 0, index: 0,
	}
}

func part2Walk(inp []string, paths map[h.Coordinate][]Path, start, end h.Coordinate, step int, visited []bool) int {
	var max int
	for _, path := range paths[start] {
		if path.end == end {
			return step + path.length
		}
		index := paths[path.end][0].index
		if !visited[index] {
			visited[index] = true
			max = h.Max(max, part2Walk(inp, paths, path.end, end, step+path.length, visited))
			visited[index] = false
		}
	}
	return max
}