aoc-2020/d12/main.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"math"
	"os"
	"strconv"
)

func processLines(path string, f func(string) (stop bool, err error)) error {
	file, err := os.Open(path)
	if err != nil {
		return err
	}
	defer file.Close()

	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		stop, err := f(scanner.Text())
		if stop || err != nil {
			return err
		}
	}

	if err := scanner.Err(); err != nil {
		return err
	}

	return nil
}

type ship struct {
	// Ship position and heading
	e, n, heading int
	// Waypoint positions
	pe, pn int
}

func (s *ship) part2(inst string) error {
	fmt.Println("Read inst", inst)
	c := inst[0]

	amt, err := strconv.Atoi(inst[1:])
	if err != nil {
		return err
	}

	switch c {
	case 'N':
		s.pn += amt
	case 'S':
		s.pn -= amt
	case 'W':
		s.pe -= amt
	case 'E':
		s.pe += amt
	case 'R':
		err = s.rotateWaypoint(amt)
	case 'L':
		err = s.rotateWaypoint(-amt)
	case 'F':
		err = s.moveToWaypoint(amt)
	default:
		err = fmt.Errorf("unknown instruction %s", inst)
	}

	fmt.Printf("After move %+v\n", s)

	return err
}

func (s *ship) moveToWaypoint(amt int) error {
	s.n += amt * s.pn
	s.e += amt * s.pe

	return nil
}

func (s *ship) rotateWaypoint(amt int) error {
	if amt < 0 {
		amt = 360 + amt
	}

	for i := 0; i < amt/90; i++ {
		s.pe, s.pn = s.pn, -s.pe
	}

	return nil
}

func (s *ship) forward(amt int) error {
	switch s.heading {
	case 0:
		s.n += amt
	case 90:
		s.e += amt
	case 180:
		s.n -= amt
	case 270:
		s.e -= amt
	default:
		return fmt.Errorf("unable to move in heading %d", s.heading)
	}

	return nil
}

func (s *ship) changeHeading(amt int) error {
	// Add to heading
	for amt < 0 {
		amt = 360 + amt
	}

	s.heading = (s.heading + amt) % 360

	return nil
}

func (s *ship) move(inst string) error {
	c := inst[0]

	amt, err := strconv.Atoi(inst[1:])
	if err != nil {
		return err
	}

	switch c {
	case 'N':
		s.n += amt
	case 'S':
		s.n -= amt
	case 'W':
		s.e -= amt
	case 'E':
		s.e += amt
	case 'R':
		return s.changeHeading(amt)
	case 'L':
		return s.changeHeading(-amt)
	case 'F':
		return s.forward(amt)
	default:
		return fmt.Errorf("unknown instruction %s", inst)
	}

	return nil
}

func part1() {
	s := ship{0, 0, 90, 0, 0}

	if err := processLines("input.txt", func(line string) (bool, error) {
		return false, s.move(line)
	}); err != nil {
		log.Fatal(err)
	}

	dist := int(math.Abs(float64(s.e)) + math.Abs(float64(s.n)))
	fmt.Printf("Resulting position %d,%d Distance: %d\n", s.e, s.n, dist)
}

func part2() {
	s := ship{0, 0, 90, 10, 1}

	if err := processLines("input.txt", func(line string) (bool, error) {
		return false, s.part2(line)
	}); err != nil {
		log.Fatal(err)
	}

	dist := int(math.Abs(float64(s.e)) + math.Abs(float64(s.n)))
	fmt.Printf("Resulting position %d,%d Distance: %d\n", s.e, s.n, dist)
}

func main() {
	part1()
	part2()
}