aoc-2024/cmd/day_15/main.go

package main

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

func trim(bs []byte) []byte {
	if len(bs) > 0 && bs[len(bs)-1] == '\n' {
		return bs[:len(bs)-1]
	}
	return bs
}

func coordOf(i, w int) (x, y int) {
	y = int(float64(i) / float64(w))
	x = i - y*w
	return
}

func indexOf(x, y, w int) int {
	return x + y*w
}

type World struct {
	wmap                 []byte
	width, height, robot int
}

func (w *World) String() string {
	var b strings.Builder
	var n int
	var err error
	for i := 0; i < w.height; i++ {
		idx := i * w.width
		n, err = b.Write(w.wmap[idx : idx+w.width])
		if err != nil {
			panic(fmt.Sprintf("While writing bytes to string: %v", err))
		}
		if n != w.width {
			panic("Wrote fewer bytes to string than expected")
		}
		n, err = b.Write([]byte{'\n'})
		if err != nil {
			panic(fmt.Sprintf("While writing bytes to string: %v", err))
		}
		if n != 1 {
			panic("Wrote fewer bytes to string than expected")
		}
	}
	return b.String()
}

func (w *World) Robot(move byte) bool {
	x, y := coordOf(w.robot, w.width)
	var x2, y2 int
	switch move {
	case '<':
		x2, y2 = x-1, y
	case '>':
		x2, y2 = x+1, y
	case '^':
		x2, y2 = x, y-1
	case 'v':
		x2, y2 = x, y+1
	default:
		log.Fatalf("Unknown robot instruction %q", move)
	}

	// Test for world escape.
	if x2 < 0 || w.width <= x2 ||
		y2 < 0 || w.height <= y2 {
		return false
	}
	next := indexOf(x2, y2, w.width)
	delta := next - w.robot
	idx := next
	for w.wmap[idx] != '.' && w.wmap[idx] != '#' {
		idx += delta
	}
	if w.wmap[idx] == '#' { // Walls are immovable.
		return false
	}
	for idx != w.robot { // Move everything over.
		w.wmap[idx] = w.wmap[idx-delta]
		idx -= delta
	}
	w.robot = next
	w.wmap[idx] = '.' // Replace robot with empty space.
	return true
}

func main() {
	world := &World{
		wmap: make([]byte, 0, 512),
	}
	r := bufio.NewReader(os.Stdin)

	// Read map
	lastLinefeed := 0
	i := 0
	for {
		i++
		col := i - lastLinefeed
		b, err := r.ReadByte()
		if errors.Is(err, io.EOF) {
			log.Fatal("Unexpected end of file")
		}
		if err != nil {
			line := 1
			if world.width > 0 {
				line = i / world.width
			}
			log.Fatalf("While reading input at line %d, column %d: %v", line, col, err)
		}
		if b == '\n' {
			if col == 1 {
				break
			}
			if world.height == 0 {
				world.width = len(world.wmap)
			}
			lastLinefeed = i
			world.height++
			continue
		}
		switch b {
		case '@':
			world.robot = len(world.wmap)
		case '.', 'O', '#':
		default:
			line := 1
			if world.width > 0 {
				line = i / world.width
			}
			log.Fatalf("Unknown or unexpected map glyph %q at line %d, column %d", b, line, col)
		}
		world.wmap = append(world.wmap, b)
	}

	// Read movements
	i = 0
	for {
		i++
		col := i - lastLinefeed
		move, err := r.ReadByte()
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			log.Fatalf("While reading movements at line %d, column %d: %v", i, col, err)
		}
		switch move {
		case '\n':
			continue
		case '<', '>', '^', 'v':
			world.Robot(move)
		}
	}
	//fmt.Printf("%s\n", world)

	// Calculate sum of "GPS" coordinates
	sum := 0
	for i, b := range world.wmap {
		if b == 'O' {
			x, y := coordOf(i, world.width)
			sum += 100*y + x
		}
	}
	fmt.Println(sum)
}