aoc2025/day4/main.go

package main

import (
	"fmt"
	"os"
	"strings"

	"github.com/kelindar/bitmap"
)

func readLines(filename string) []string {
	raw, err := os.ReadFile(filename)
	if err != nil {
		panic(err)
	}
	return strings.Split(string(raw), "\n")
}

type warehouse struct {
	boxes  *bitmap.Bitmap
	width  uint32
	height uint32
}

func buildWarehouse(lines []string) warehouse {
	wh := warehouse{
		boxes: &bitmap.Bitmap{},
	}
	wh.height = uint32(len(lines))
	if wh.height == 0 {
		return wh
	}
	wh.width = uint32(len(lines[0]))
	if wh.width == 0 {
		return wh
	}
	for y, line := range lines {
		for x, char := range line {
			if char == '@' {
				set(wh.boxes, wh.width, wh.height, uint32(x), uint32(y))
			}
		}
	}
	return wh
}

func set(area *bitmap.Bitmap, width uint32, height uint32, x uint32, y uint32) {
	if x >= width || y >= height {
		return
	}
	area.Set(y*width + x)
}

func debug(area *bitmap.Bitmap, width uint32) {
	area.Range(func(offset uint32) {
		fmt.Printf("(%d, %d)\n", offset%width, offset/width)
	})
}

func buildNeighbors(width uint32, height uint32, offset uint32) *bitmap.Bitmap {
	y := offset / width
	x := offset % width
	var neighbors bitmap.Bitmap
	set(&neighbors, width, height, x+1, y)
	set(&neighbors, width, height, x-1, y)
	set(&neighbors, width, height, x+1, y+1)
	set(&neighbors, width, height, x-1, y+1)
	set(&neighbors, width, height, x+1, y-1)
	set(&neighbors, width, height, x-1, y-1)
	set(&neighbors, width, height, x, y+1)
	set(&neighbors, width, height, x, y-1)
	return &neighbors
}

func (wh warehouse) countAccessibleBoxes() uint32 {
	boxes := wh.boxes.Clone(nil)
	boxes.Filter(func(offset uint32) bool {
		neighbors := buildNeighbors(wh.width, wh.height, offset)
		neighbors.And(*wh.boxes)
		accessible := neighbors.Count() < 4
		return accessible
	})
	return uint32(boxes.Count())
}

func (wh *warehouse) removeAccessibleBoxes() uint32 {
	boxes := wh.boxes.Clone(nil)
	removed := uint32(0)
	boxes.Filter(func(offset uint32) bool {
		neighbors := buildNeighbors(wh.width, wh.height, offset)
		neighbors.And(*wh.boxes)
		accessible := neighbors.Count() < 4
		if accessible {
			removed++
		}
		return !accessible
	})
	wh.boxes = &boxes
	return removed
}

func main() {
	filename := "input0.txt"
	if len(os.Args) > 1 {
		filename = os.Args[1]
	}
	lines := readLines(filename)
	wh := buildWarehouse(lines)
	fmt.Printf("%v\n", wh.countAccessibleBoxes())
	total := uint32(0)
	for {
		count := wh.removeAccessibleBoxes()
		if count == 0 {
			break
		}
		total += count
	}
	fmt.Printf("%v\n", total)
}