advent-of-code-2024/day16/maze.go

package main

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


// ----------------------------------------
type Direction uint8

const (
    Up    Direction = iota
    Down  Direction = iota
    Left  Direction = iota
    Right Direction = iota
)


// ----------------------------------------
type Path struct {
    position  [2]uint
    direction Direction
}

func (p Path) StepUnbound(d Direction) Path {
    switch {
        case d == Down: p.position[1]++
        case d == Up: p.position[1]--
        case d == Left: p.position[0]--
        case d == Right: p.position[0]++
    }
    p.direction = d

    return p
}


// ----------------------------------------
type Maze struct {
    start [2]uint
    end   [2]uint
    field [][]byte
}

func NewMaze(str string) *Maze {
    var maze Maze

    for y, line := range strings.Split(str, "\n") {
        line_arr := make([]byte, len(line))
        for x, char := range []byte(line) {
            switch {
                case char == 'S':
                    line_arr[x] = '.'
                    maze.start = [...]uint{uint(x), uint(y)}
                case char == 'E':
                    line_arr[x] = '.'
                    maze.end = [...]uint{uint(x), uint(y)}
                default: line_arr[x] = char
            }
        }
        maze.field = append(maze.field, line_arr)
    }

    return &maze
}

func (m *Maze) Step(path Path) []Path {
    out := make([]Path, 0, 4)

    if m.field[path.position[1] + 1][path.position[0]] == '.' && path.direction != Up {
        out = append(out, path.StepUnbound(Down))
    }
    if m.field[path.position[1] - 1][path.position[0]] == '.' && path.direction != Down {
        out = append(out, path.StepUnbound(Up))
    }
    if m.field[path.position[1]][path.position[0] + 1] == '.' && path.direction != Left {
        out = append(out, path.StepUnbound(Right))
    }
    if m.field[path.position[1]][path.position[0] - 1] == '.' && path.direction != Right {
        out = append(out, path.StepUnbound(Left))
    }

    return out
}

func (m *Maze) Start() Path {
    return Path{m.start, Right}
}

func (m *Maze) Finished(path Path) bool {
    return m.end == path.position
}


func (m *Maze) Print() {
    for _, line := range m.field {
        fmt.Println(string(line))
    }
}


// ----------------------------------------
type MazeGraph struct {
    cost     uint
    children []*MazeGraph
}

var new_maze_graph_visited map[Path]uint

func NewMazeGraph(m *Maze, start Path, cost uint) *MazeGraph {
    children := make([]*MazeGraph, 0, 3)

    // finish reached?
    if m.Finished(start) { return &MazeGraph{ cost, children } }

    // check for loop
    value, exists := new_maze_graph_visited[start]
    if exists {
        if value <= cost { return nil } 
    }
    new_maze_graph_visited[start] = cost

    // build subgraphs/ children
    for _, path := range m.Step(start) {
        var child *MazeGraph
        if start.direction != path.direction {
            child = NewMazeGraph(m, path, cost + 1001)
        } else {
            child = NewMazeGraph(m, path, cost + 1)
        }

        if child != nil { children = append(children, child) }
    }

    // found dead end
    if len(children) == 0 { return nil }

    // collapse if there is only one possible way
    if len(children) == 0 { return children[0] }
    
    return &MazeGraph{ cost: cost, children: children }
}

func (graph *MazeGraph) Leaves() []*MazeGraph {
    out := make([]*MazeGraph, 0)

    if len(graph.children) == 0 {
        out = append(out, graph)
        return out
    }

    for _, g := range graph.children {
        out = append(out, g.Leaves()...)
    }
    return out
}


// ----------------------------------------
func check(err error) { if err != nil { panic(err) } }


func main() {
    dat, err := os.ReadFile("data.txt")
    check(err)

    input := strings.TrimSpace(string(dat))
    maze := NewMaze(input)

    var best *MazeGraph
    new_maze_graph_visited = make(map[Path]uint)

    for _, leave := range NewMazeGraph(maze, maze.Start(), 0).Leaves() {
        if best == nil || best.cost > leave.cost { best = leave }
    }

    fmt.Println(best.cost)
}