aoc-2024/cmd/day_6/main.go

package main

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
)

func main() {
	r := bufio.NewReader(os.Stdin)
	sitMap := make([]byte, 0, 128)
	width := 0
	height := 0
	var guard *Cursor

	// Read map
	for {
		b, err := r.ReadByte()
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			log.Fatalf("While reading input: %v", err)
		}
		if b == '\n' {
			if width == 0 {
				width = len(sitMap)
			}
			continue
		}
		switch b {
		case Left, Right, Up, Down:
			tmp := NewCursor(width, len(sitMap), b)
			guard = &tmp
			// Replace ^ with . so we don't run into our past
			// self.
			b = '.'
		}
		sitMap = append(sitMap, b)
	}
	height = len(sitMap) / width

	// Walk map
	positions := make(map[Coord]struct{})

	for 0 <= guard.Pos.X && guard.Pos.X <= width &&
		0 <= guard.Pos.Y && guard.Pos.Y <= height {

		positions[guard.Pos] = struct{}{}

		idx := IndexOf(guard.Front(), width)
		if idx < 0 || len(sitMap) < idx {
			break
		}
		e := sitMap[idx]
		switch e {
		case '#':
			guard.Turn()
		case '.':
			guard.Forward()
		default:
			log.Fatalf("Unexpected map element %#v at line %d, column %d", string(e), guard.Pos.X+1, guard.Pos.Y+1)
		}
	}
	fmt.Println(len(positions))
}

type Orientation = byte

const (
	Left  Orientation = '<'
	Up    Orientation = '^'
	Down  Orientation = 'v'
	Right Orientation = '>'
)

type Coord struct{ X, Y int }

func IndexOf(c Coord, width int) int {
	return c.X + c.Y*width
}

func OrientationOf(dir Coord) Orientation {
	var o Orientation
	switch dir {
	case Coord{-1, 0}:
		o = Left
	case Coord{1, 0}:
		o = Right
	case Coord{0, -1}:
		o = Up
	case Coord{0, 1}:
		o = Down
	default:
		panic(fmt.Sprintf("Unknown direction %#v", dir))
	}
	return o
}

func NewCursor(width, idx int, o Orientation) Cursor {
	x := idx % width
	y := idx / width
	d := Coord{}

	switch o {
	case Left:
		d.X = -1
	case Right:
		d.X = 1
	case Up:
		d.Y = -1
	case Down:
		d.Y = 1
	}
	return Cursor{Pos: Coord{x, y}, Dir: d}
}

type Cursor struct {
	Pos, Dir Coord
}

func (c *Cursor) Forward() {
	c.Pos = c.Front()
}

func (c *Cursor) Turn() {
	tmp := c.Dir.X
	c.Dir.X = -1 * c.Dir.Y
	c.Dir.Y = tmp
}

func (c *Cursor) Front() Coord {
	return Coord{c.Pos.X + c.Dir.X, c.Pos.Y + c.Dir.Y}
}