advent-of-code-2024/day6/loop_guard.go

package main

import (
    "os"
    "fmt"
    "strings"
)


// type definitions

type Direction int
const (
	Up Direction = iota + 1
	Down
	Left
	Right
)

type state struct {
    field *[][]rune
    guard_x int
    guard_y int
    direction Direction
}

func new_state(text string) *state {
    lines := strings.Split(text, "\n")
    field := make([][]rune, len(lines))
    guard := [2]int{-1,-1}

    for y, line := range lines {
        field[y] = []rune(line)

        for x, r := range field[y] {
            if r == '^' {
                guard = [2]int{x, y}
                field[y][x] = '.'
            }
        }
    }

    s := state{field: &field, guard_x: guard[0], guard_y: guard[1], direction: Up}
    return &s
}


// general util

func check(e error) {
    if e != nil { panic(e) }
}


// simulation functions

func iter(s *state) *state {
    new_guard_x := s.guard_x
    new_guard_y := s.guard_y
    
    // try to move
    switch {
        case s.direction == Up:
            new_guard_y--
        case s.direction == Down:
            new_guard_y++
        case s.direction == Left:
            new_guard_x--
        case s.direction == Right:
            new_guard_x++
    }
    
    // if guard is outside of the field
    if new_guard_x < 0 || new_guard_y < 0 || new_guard_x >= len((*s.field)[0]) || new_guard_y >= len(*s.field) {
        new_guard_x = -1
        new_guard_y = -1
    } else if (*s.field)[new_guard_y][new_guard_x] == '#' {
    // correct if path is blocked
        switch {
            case s.direction == Up:
                new_guard_y++
                new_guard_x++
                s.direction = Right
            case s.direction == Down:
                new_guard_y--
                new_guard_x--
                s.direction = Left
            case s.direction == Left:
                new_guard_x++
                new_guard_y--
                s.direction = Up
            case s.direction == Right:
                new_guard_x--
                new_guard_y++
                s.direction = Down
        }
    }

    // set new guard coordinates
    s.guard_x = new_guard_x
    s.guard_y = new_guard_y

    return s
}

func will_loop(s *state) bool {
    counter := 0
    // while guard is within the field
    for s.guard_x >= 0 && counter < 10000 {
        // iterate
        s = iter(s)
        counter++
    }
    if s.guard_x < 0 {
        // the guard is no longer in the field, not a loop
        return false
    } else {
        return true
    }
}

func print(s *state) {
    out := make([]string, 0, len(*s.field))
    for line := range *s.field {
        out = append(out, string(line))
    }
    fmt.Println(strings.Join(out, "\n"))
    fmt.Println("guard", s.guard_x, s.guard_y)
}

// main loop
func main() {
    // read in file
    dat, err := os.ReadFile("data.txt")
    check(err)
    dat_str := strings.TrimSpace(string(dat))
    
    stateobj := new_state(dat_str)
    start_of_guard := [2]int{stateobj.guard_x, stateobj.guard_y}

    // check loops
    counter := 0
    for y := range *stateobj.field {
        for x, elem := range (*stateobj.field)[y] {
            // skip already existing borders
            if elem == '#' { continue }
            // because the placement of the start position of the guard is not allowed
            if x == start_of_guard[0] && y == start_of_guard[1] { continue }

            // set border
            (*stateobj.field)[y][x] = '#'
            
            // check for loop
            if will_loop(stateobj) { counter++ }

            // restore state
            stateobj = new_state(dat_str)
        }
    }

    fmt.Println(counter)
}