AdventOfCode2020/12/main.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"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)
}

type ship struct {
	horizontal int
	vertical   int
	direction  rune // E W N S
}

// both horizontal/vertical are relative to ship
type waypoint struct {
	horizontal int
	vertical   int
}

func parseLine(s string) (rune, int) {
	i, _ := strconv.Atoi(s[1:])
	return []rune(s)[0], i
}

func movePartOne(s ship, action rune, vector int) ship {
	switch action {
	case 'R', 'L':
		s.direction = rotateShip(s.direction, vector, action)
	case 'N':
		s.vertical = s.vertical + vector
	case 'S':
		s.vertical = s.vertical - vector
	case 'E':
		s.horizontal = s.horizontal + vector
	case 'W':
		s.horizontal = s.horizontal - vector
	case 'F':
		s = movePartOne(s, s.direction, vector)
	}
	return s
}

func movePartTwo(s ship, w waypoint, action rune, vector int) (ship, waypoint) {
	switch action {
	case 'R', 'L':
		w = rotateWaypoint(w, action, vector)
	case 'N':
		w.vertical = w.vertical + vector
	case 'S':
		w.vertical = w.vertical - vector
	case 'E':
		w.horizontal = w.horizontal + vector
	case 'W':
		w.horizontal = w.horizontal - vector
	case 'F':
		s.horizontal = s.horizontal + (w.horizontal * vector)
		s.vertical = s.vertical + (w.vertical * vector)
	}
	return s, w
}

// for example E, 90, L => N; E, 180, R => W; E, 90, R => S
func rotateShip(current rune, degrees int, rl rune) rune {
	// easy
	if degrees == 180 {
		switch current {
		case 'N':
			return 'S'
		case 'S':
			return 'N'
		case 'E':
			return 'W'
		case 'W':
			return 'E'
		}
	}
	// translate L to R
	if rl == 'L' {
		if degrees == 90 {
			degrees = 270
		} else {
			degrees = 90
		}
	}
	// the R set
	if degrees == 90 {
		switch current {
		case 'N':
			return 'E'
		case 'S':
			return 'W'
		case 'E':
			return 'S'
		case 'W':
			return 'N'
		}
	} else if degrees == 270 {
		switch current {
		case 'N':
			return 'W'
		case 'S':
			return 'E'
		case 'E':
			return 'N'
		case 'W':
			return 'S'
		}
	}

	return current // we're made a terrible mistake somewhere
}

func rotateWaypoint(w waypoint, action rune, vector int) waypoint {
	if vector == 180 {
		w.horizontal = -w.horizontal
		w.vertical = -w.vertical
	} else if action == 'R' {
		if vector == 90 {
			w.horizontal, w.vertical = w.vertical, -w.horizontal
		}
		if vector == 270 {
			w.horizontal, w.vertical = -w.vertical, w.horizontal
		}
	} else if action == 'L' {
		if vector == 90 {
			w.horizontal, w.vertical = -w.vertical, w.horizontal
		}
		if vector == 270 {
			w.horizontal, w.vertical = w.vertical, -w.horizontal
		}
	}
	return w
}

func manhattan(i, j int) int {
	return abs(i) + abs(j)
}

func abs(x int) int {
	if x < 0 {
		return -x
	}
	return x
}

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

	s := ship{
		horizontal: 0,
		vertical:   0,
		direction:  'E',
	}

	for scanner.Scan() {
		dir, vec := parseLine(scanner.Text())
		s = movePartOne(s, dir, vec)
	}

	fmt.Println(manhattan(s.horizontal, s.vertical))
}

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

	s := ship{
		horizontal: 0,
		vertical:   0,
		// no longer care about ship direction
	}
	w := waypoint{
		horizontal: 10,
		vertical:   1,
	}

	for scanner.Scan() {
		dir, vec := parseLine(scanner.Text())
		s, w = movePartTwo(s, w, dir, vec)
	}

	fmt.Println(manhattan(s.horizontal, s.vertical))
}