AdventOfCode2020/24/main.go

package main

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

func main() {
	start := time.Now()
	partOne()
	duration := time.Since(start)
	partTwo()
	duration2 := time.Since(start)
	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
}

// Our grid is a hex, so let's represent that as follows:
// x runs east-west, y runs north-south. Yes, these coordinates here are backward.
//       2,-2      2,0     2,2     2,4
//  1,-2      1,-1     1,1     1,2
//       0,-2      0,0     0,2     0,4
// -1,-2     -1,-1    -1,1    -1,2
//      -2,-2     -2,0    -2,2    -2,4
// 0,0 ->  e ->  0, 2
//     ->  w ->  0,-2
//     -> se ->	-1, 1
//     -> sw -> -1,-1
//     -> ne ->  1, 1
//     -> nw ->  1,-1

// 0 == white, 1 == black
type tiles map[coord]int

type coord struct {
	x int
	y int
}

func walkGrid(directions []string) (int, int) {
	var y, x int
	for _, d := range directions {
		switch d {
		case "e":
			x = x + 2
		case "se":
			y = y - 1
			x = x + 1
		case "sw":
			y = y - 1
			x = x - 1
		case "w":
			x = x - 2
		case "ne":
			y = y + 1
			x = x + 1
		case "nw":
			y = y + 1
			x = x - 1
		}
	}
	return y, x
}

func parseLine(s string) []string {
	directions := []string{}
	for i := 0; i < len(s); {
		switch s[i] {
		case 'e':
			directions = append(directions, "e")
			i = i + 1
		case 's':
			if s[i+1] == 'e' {
				directions = append(directions, "se")
				i = i + 2
			} else if s[i+1] == 'w' {
				directions = append(directions, "sw")
				i = i + 2
			}
		case 'w':
			directions = append(directions, "w")
			i = i + 1
		case 'n':
			if s[i+1] == 'e' {
				directions = append(directions, "ne")
				i = i + 2
			} else if s[i+1] == 'w' {
				directions = append(directions, "nw")
				i = i + 2
			}
		}
	}
	return directions
}

func generation(t tiles) tiles {
	newTiles := tiles{}
	seenWhiteTiles := tiles{}
	for coord, tile := range t {
		// walk black tile list
		if tile == 1 {
			neighbors := getNeighbors(coord)
			bn := 0
			for _, n := range neighbors {
				if t[n] == 0 {
					seenWhiteTiles[n] = seenWhiteTiles[n] + 1
				} else {
					bn = bn + 1
				}
			}
			// black tiles only survive if they have one or two black neighbors
			if bn == 1 || bn == 2 {
				newTiles[coord] = 1
			}
		}
	}
	// white tiles turn black if they have two black neighbors
	for c, n := range seenWhiteTiles {
		if n == 2 {
			newTiles[c] = 1
		}
	}
	return newTiles
}

func getNeighbors(c coord) []coord {
	return []coord{
		{x: c.x + 2, y: c.y},     // e
		{x: c.x - 2, y: c.y},     // w
		{x: c.x + 1, y: c.y + 1}, // ne
		{x: c.x + 1, y: c.y - 1}, // nw
		{x: c.x - 1, y: c.y + 1}, // se
		{x: c.x - 1, y: c.y - 1}, // sw
	}
}

func countBlackTiles(t tiles) int {
	sum := 0
	for _, tile := range t {
		sum = sum + tile
	}
	return sum
}

func partOne() {
	f, _ := os.Open("testinput")
	reader := bufio.NewReader(f)
	scanner := bufio.NewScanner(reader)

	t := tiles{}

	for scanner.Scan() {
		line := scanner.Text()
		x, y := walkGrid(parseLine(line))
		if t[coord{x: x, y: y}] == 0 {
			t[coord{x: x, y: y}] = 1
		} else {
			t[coord{x: x, y: y}] = 0
		}
	}
	fmt.Println(countBlackTiles(t))
}

func partTwo() {
	f, _ := os.Open("input")
	reader := bufio.NewReader(f)
	scanner := bufio.NewScanner(reader)

	t := tiles{}

	for scanner.Scan() {
		line := scanner.Text()
		y, x := walkGrid(parseLine(line))
		if t[coord{x: x, y: y}] == 0 {
			t[coord{x: x, y: y}] = 1
		} else {
			t[coord{x: x, y: y}] = 0
		}
	}
	for day := 1; day <= 100; day = day + 1 {
		t = generation(t)
	}
	fmt.Println(countBlackTiles(t))

}