17-2.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"time"
)

type point struct {
	x int
	y int
	z int
	w int
}

const (
	activeRune   = '#'
	activeByte   = byte(activeRune)
	inactiveByte = byte('.')
	maxIndex     = 1000
)

var deltaList []point // a list of neighbors around a cube

func init() {
	deltaList = make([]point, 0)

	for w := -1; w <= 1; w++ {
		for z := -1; z <= 1; z++ {
			for y := -1; y <= 1; y++ {
				for x := -1; x <= 1; x++ {
					if !(x == 0 && y == 0 && z == 0 && w == 0) {
						deltaList = append(deltaList, point{x, y, z, w})
					}
				}
			}
		}
	}
}

func main() {

	t1 := time.Now()
	var active = make(map[point]bool)

	if len(os.Args) < 2 {
		panic(fmt.Sprintf("usage: %s <input>", os.Args[0]))
	}
	fh, err := os.Open(os.Args[1])
	if err != nil {
		panic(err)
	}

	scanner := bufio.NewScanner(fh)
	y := 0
	for scanner.Scan() {
		for x, r := range scanner.Text() {
			if r == activeRune {
				active[point{x, y, 0, 0}] = true
			}
		}
		y++
	}
	_ = fh.Close()

	for k := 0; k < 6; k++ {
		active = step(active)
	}

	fmt.Printf("part 2: %d cubes are active (%s)\n", len(active),time.Now().Sub(t1))

}

func step(act map[point]bool) map[point]bool {
	count := extendedCount(act)
	next := make(map[point]bool)
	for p, c := range count {
		if act[p] {
			if c == 2 || c == 3 {
				next[p] = true
			}
		} else {
			if c == 3 {
				next[p] = true
			}
		}
	}
	return next
}

// find the +1 environment around the active cubes
func extendedCount(act map[point]bool) map[point]int {
	ext := make(map[point]int)
	for p := range act {
		for _, d := range deltaList {
			ext[point{p.x + d.x, p.y + d.y, p.z + d.z, p.w + d.w}]++
		}
	}
	return ext
}