advent_of_code_2024/day_06/solution.go

package day_06

import (
	"strings"
)

type Direction int

const (
	UP    Direction = iota
	RIGHT Direction = iota
	DOWN  Direction = iota
	LEFT  Direction = iota
)

const (
	VISITED  = 'X'
	OCCUPIED = '#'
	FREE     = '.'
)

var toDirections = map[rune]Direction{
	'^': UP,
	'>': RIGHT,
	'v': DOWN,
	'<': LEFT,
}

var fromDirections = map[Direction]rune{
	UP:    '^',
	RIGHT: '>',
	DOWN:  'v',
	LEFT:  '<',
}

// task:https://adventofcode.com/2024/day/5
// short summary - count visited steps
func SolveBasic(input string) int {
	grid := visitationGrid(input)

	sum := 0
	for _, row := range grid {
		for _, value := range row {
			if value == VISITED {
				sum++
			}
		}
	}
	return sum
}

// task:https://adventofcode.com/2024/day/5#part2
// short summary - find all position that will create an infinite loop
func SolveComplex(input string) int {
	grid := visitationGrid(input)

	dirGrid := loadGrid(input)
	startX, startY, startDir := findPlayer(dirGrid)
	dirGrid[startY][startX] = FREE

	positions := 0

	for y, row := range grid {
		for x, value := range row {
			if value == VISITED && dirGrid[y][x] == FREE {
				dirGrid[y][x] = OCCUPIED

				curX := startX
				curY := startY
				curDir := startDir

				for inGrid(dirGrid, curX, curY) {
					if dirGrid[curY][curX] == fromDirections[curDir] {
						// we visited a same point second time in the same direction
						// which means we are in an infinite list
						positions++
						break
					}
					dirGrid[curY][curX] = fromDirections[curDir]
					curX, curY, curDir = nextSpot(dirGrid, curX, curY, curDir)

				}
				dirGrid[y][x] = FREE
				clearDirGrid(dirGrid)
			}
		}
	}
	return positions
}

func clearDirGrid(grid [][]rune) {
	for y, row := range grid {
		for x, value := range row {
			if value != OCCUPIED {
				grid[y][x] = FREE
			}
		}
	}
}

func nextSpot(grid [][]rune, x int, y int, dir Direction) (int, int, Direction) {
	for {
		nx, ny := calculateNext(x, y, dir)
		if !inGrid(grid, nx, ny) {
			return nx, ny, dir
		} else if grid[ny][nx] != OCCUPIED {
			return nx, ny, dir
		} else {
			dir = (dir + 1) % 4
		}
	}
}

func calculateNext(x int, y int, dir Direction) (int, int) {
	switch dir {
	case UP:
		return x, y - 1
	case RIGHT:
		return x + 1, y
	case DOWN:
		return x, y + 1
	case LEFT:
		return x - 1, y
	}
	panic("unknown direction")
}

func inGrid(grid [][]rune, x int, y int) bool {
	return x >= 0 && y >= 0 && x < len(grid[0]) && y < len(grid)
}

func findPlayer(grid [][]rune) (int, int, Direction) {
	for y, row := range grid {
		for x, value := range row {
			if dir, present := toDirections[value]; present {
				return x, y, dir
			}
		}
	}
	panic("Couldn't find player")
}

func loadGrid(input string) [][]rune {
	substrings := strings.Split(input, "\n")
	var grid [][]rune
	for _, line := range substrings {
		grid = append(grid, []rune(line))
	}
	return grid
}

func visitationGrid(input string) [][]rune {
	grid := loadGrid(input)
	x, y, dir := findPlayer(grid)

	for inGrid(grid, x, y) {
		grid[y][x] = VISITED
		x, y, dir = nextSpot(grid, x, y, dir)
	}
	return grid
}