main.go

package main

import (
	"fmt"
	"math"
	"strconv"

	"github.com/mbark/advent-of-code-2020/util"
)

var input = `
F75
L90
N5
W2
N5
L90
S1
R90
S4
E2
W1
F100
L180
W3
N1
R180
W1
S4
R270
S4
F23
E2
F17
N5
R90
F83
E2
F34
E3
S4
F19
W4
L90
W2
L90
E2
F79
L90
F80
N1
R90
F35
L90
N5
E3
S5
R90
F71
N1
F19
F49
R90
E4
L90
W1
R90
S2
E1
L180
F79
W4
L180
F74
W5
R90
F30
R90
W2
R90
F35
R90
F12
W2
F7
R90
E5
F68
E3
F21
N4
F22
W4
S2
F89
L90
N5
R90
F8
R90
W2
S3
R180
N2
R90
F49
N4
E5
F31
S3
E4
F61
E1
L90
N1
W5
F15
F13
E4
N2
R180
S2
L90
F92
S4
R180
L90
S1
L90
F98
N4
E3
F69
S3
R180
F59
S5
R90
F32
E5
S4
W4
F9
R180
F44
L90
W1
L180
F74
W1
F55
R270
S3
F62
W3
F61
N2
L90
F43
S3
F86
W1
S3
E3
F34
L90
F77
N1
F27
N5
W5
N3
F80
E2
L180
F59
W1
L90
N4
R90
W5
L180
W5
L90
F17
E5
L180
W4
R180
W4
F70
S5
R90
W4
W4
N3
W1
N2
E5
F53
N5
R90
R90
E2
N5
R270
S5
R90
N5
E1
R90
F76
R90
W3
S2
R90
S5
E1
L90
E2
F58
R90
F15
N2
L90
N5
F65
E5
N2
R90
W1
F40
S4
R90
F23
W4
R90
W4
F17
S2
R90
E4
F58
R90
L90
F64
N4
F63
E5
N1
F38
N3
F19
E5
S4
F14
R90
S3
E2
S4
E1
F12
W3
L90
F33
S4
W3
F29
R90
N3
F44
E5
N1
F49
W5
F52
N5
L90
S4
F31
R90
N3
L270
L270
E4
N4
W1
L90
S5
W4
R180
N3
L180
F2
S1
L180
N3
L90
N1
W4
R180
W5
R270
N4
L180
F51
R180
S5
F35
E3
R180
F1
S3
E2
F47
E4
S5
E1
L90
F16
S4
F17
N1
W2
L90
F99
E1
R180
N1
F78
L90
F18
S2
W3
N2
F56
R270
W1
R90
F53
L180
F6
W5
R90
W4
F27
R90
F73
E5
R90
W4
S4
W3
S2
E5
N5
R270
F43
W1
F77
E4
F85
W2
R90
N1
E1
F39
R90
W2
N4
E2
W4
F81
W4
F3
W4
R180
W2
N5
R90
N5
E2
S5
S3
E1
N4
W1
S3
F48
S2
F23
E2
R180
F89
L180
N3
E1
N5
W2
L90
E1
F80
R90
S1
E3
N5
L90
F11
W5
F70
N3
W1
L90
N5
F99
N5
F24
R180
E4
F57
S1
F9
S5
R90
W4
F16
E1
F18
F17
L180
S1
L90
S1
E2
N1
E4
F84
N1
F6
E1
L180
W3
L180
N3
F60
F43
S3
F48
W2
S1
R180
N1
S2
F78
L180
W4
F51
W5
N2
E2
N1
W4
F65
N3
L90
W3
N2
W2
F46
N4
W4
N2
W2
F74
R180
E4
R90
N1
F33
S4
L180
S1
F15
R180
S5
L180
F21
N4
F87
W4
L90
S4
F40
S5
W3
L180
F78
W2
F40
N3
E3
S4
W1
S4
E1
S4
W5
S2
F82
E5
R180
F7
W3
R90
N2
R90
W5
N5
W5
S3
E2
L90
E1
F77
E4
F70
W5
S2
W2
F7
W4
F8
R180
W5
L90
F49
W2
L90
S5
W1
S3
F74
W3
F76
W3
N1
E4
S2
F65
E1
W1
F85
E5
F67
R90
W1
F8
R180
N2
F94
W4
R90
S4
F81
N5
R90
E4
F9
N3
F49
R180
N3
F94
N4
R90
S2
F34
W2
E4
W5
F84
S4
W1
R180
W5
F13
W3
L90
N1
E1
R90
S2
R180
F81
L90
F36
W3
S4
W1
F3
R90
W3
R90
E5
R90
F46
L90
F63
N1
L90
W4
N1
S2
N3
L90
F64
W5
N4
F45
W4
S4
R90
W3
N5
R90
L180
E4
R270
S1
L180
E1
F22
E4
F11
W3
F12
W3
R90
S5
E3
F4
R90
S3
F3
L180
S2
F16
E5
R90
F75
W4
N1
S4
F81
L90
F69
N5
L90
W5
F63
N3
W2
L90
F39
E5
F81
W3
F1
L90
F31
W4
S4
F32
W3
N4
L180
N5
W2
F20
L90
F42
S3
E5
S4
L90
S2
L90
S3
W5
F54
R90
F85
S4
F22
W3
F21
R90
F66
S1
F22
S4
W5
N5
E3
F52
L180
W3
S1
W5
R180
F43
E5
F16
W4
S5
W5
N4
F97
E5
F4
E2
R180
F99
S5
F44
R90
W5
N2
L90
F46
E4
L180
E4
R90
F45
R90
S4
F30
R90
F23
N5
W2
F45
S3
F70
L90
W2
F90
W5
F81
E2
F15
L90
F90
N2
F78
S4
F3
F37
`

var testInput = `
F10
N3
F7
R90
F11
`

type Instruction struct {
	Action rune
	Value  int
}

func main() {
	lines := util.ReadInput(input, "\n")
	var ins []Instruction
	for _, l := range lines {
		r := rune(l[0])
		v, _ := strconv.Atoi(l[1:])
		ins = append(ins, Instruction{Action: r, Value: v})
	}

	n, e := first(ins)
	n, e = abs(n, e)
	fmt.Printf("first %d (%d + %d)\n", n+e, n, e)

	n, e = second(ins)
	n, e = abs(n, e)
	fmt.Printf("second %d (%d + %d)\n", n+e, n, e)
}

func abs(n, e int) (int, int) {
	return int(math.Abs(float64(n))), int(math.Abs(float64(e)))
}

type Position struct {
	North int
	East  int
	Angle int
}

func first(instructions []Instruction) (int, int) {
	p := Position{North: 0, East: 0, Angle: 90}

	for _, i := range instructions {
		switch i.Action {
		case 'L':
			p.Angle += i.Value
			if p.Angle >= 360 {
				p.Angle -= 360
			}
		case 'R':
			p.Angle -= i.Value
			if p.Angle < 0 {
				p.Angle += 360
			}
		case 'F':
			ins := directionToInstruction(p.Angle, i.Value)
			p = movePosition(ins, p)
		default:
			p = movePosition(i, p)
		}
	}

	return p.North, p.East
}

func second(instructions []Instruction) (int, int) {
	p := Position{North: 0, East: 0}
	wp := Position{North: 1, East: 10}

	for _, i := range instructions {
		switch i.Action {
		case 'L':
			// L90 -> R270
			value := 360 - i.Value
			wp = rotateWpRight(value, wp)
		case 'R':
			wp = rotateWpRight(i.Value, wp)
		case 'F':
			p.North += wp.North * i.Value
			p.East += wp.East * i.Value
		default:
			wp = movePosition(i, wp)
		}
	}

	return p.North, p.East
}

func rotateWpRight(value int, wp Position) Position {
	for value > 0 {
		isNE := wp.North >= 0 && wp.East >= 0
		isSE := wp.North < 0 && wp.East >= 0
		isSW := wp.North < 0 && wp.East < 0
		isNW := wp.North >= 0 && wp.East < 0

		p := Position{}
		if isNE {
			p.North = -wp.East
			p.East = wp.North
		} else if isSE {
			p.North = -wp.East
			p.East = wp.North
		} else if isSW {
			p.North = -wp.East
			p.East = wp.North
		} else if isNW {
			p.North = -wp.East
			p.East = wp.North
		}

		wp = p
		value -= 90
	}

	return wp
}

func movePosition(i Instruction, p Position) Position {
	switch i.Action {
	case 'N':
		p.North += i.Value
	case 'S':
		p.North -= i.Value
	case 'E':
		p.East += i.Value
	case 'W':
		p.East -= i.Value
	default:
		panic(fmt.Sprintf("unkonw action %+v", i))
	}

	return p
}

func directionToInstruction(angle, value int) Instruction {
	var action rune
	switch angle {
	case 0:
		action = 'S'
	case 90:
		action = 'E'
	case 180:
		action = 'N'
	case 270:
		action = 'W'
	default:
		panic(fmt.Sprintf("unkonw angle %+v", angle))
	}

	return Instruction{Action: action, Value: value}
}