package day9

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

type Point struct {
	x int
	y int
}

func makeSnake(length int) []*Point {
	snake := []*Point{}

	for i := 0; i < length; i++ {
		snake = append(snake, &Point{0, 0})
	}

	return snake
}

func move(direction rune, point *Point) {
	switch direction {
	case 'U':
		point.y -= 1
	case 'R':
		point.x += 1
	case 'D':
		point.y += 1
	case 'L':
		point.x -= 1
	default:
		fmt.Println("PANIC! Bad direction", string(direction))
	}
}

func abs(x int) int {
	if x >= 0 {
		return x
	} else {
		return -x
	}
}

func areTouching(head *Point, tail *Point) bool {
	return abs(head.x-tail.x) <= 1 && abs(head.y-tail.y) <= 1
}

func updateTail(head *Point, tail *Point) {
	if areTouching(head, tail) {
		return
	}

	if head.x == tail.x {
		if head.y <= tail.y-2 {
			tail.y -= 1
			return
		}

		if head.y >= tail.y+2 {
			tail.y += 1
			return
		}
	}

	if head.y == tail.y {
		if head.x <= tail.x-2 {
			tail.x -= 1
			return
		}
		if head.x >= tail.x+2 {
			tail.x += 1
			return
		}
	}

	proposedTails := []Point{
		{tail.x - 1, tail.y - 1},
		{tail.x - 1, tail.y + 1},
		{tail.x + 1, tail.y - 1},
		{tail.x + 1, tail.y + 1}}

	for _, proposed := range proposedTails {
		if areTouching(head, &proposed) {
			tail.x = proposed.x
			tail.y = proposed.y
			return
		}
	}

	fmt.Println("PANIC, couldn't figure out next step for", *head, "and", *tail)
}

func contains(array *[]Point, point Point) bool {
	for _, val := range *array {
		if val == point {
			return true
		}
	}
	return false
}

func runMove(direction rune, amount int, snake []*Point, visitedPoints *[]Point) {
	for i := 0; i < amount; i++ {
		move(direction, snake[0])
		for t := 1; t < len(snake); t++ {
			updateTail(snake[t-1], snake[t])
		}
		lastPoint := snake[len(snake)-1]
		if !contains(visitedPoints, *lastPoint) {
			*visitedPoints = append(*visitedPoints, *lastPoint)
		}
	}
}

func Run(filename string, lengthStr string) {
	file, err := os.Open(filename)
	if err != nil {
		fmt.Println("Error opening file:", err)
		return
	}

	snakeLength, err := strconv.Atoi(lengthStr)
	if err != nil {
		fmt.Println("Error parsing snake length:", err)
		return
	}

	scanner := bufio.NewScanner(file)
	scanner.Split(bufio.ScanLines)

	snake := makeSnake(snakeLength)
	visitedPoints := []Point{}

	for scanner.Scan() {
		line := scanner.Text()

		amount, err := strconv.Atoi(line[2:])
		if err != nil {
			fmt.Println("Could not parse argument", line)
			return
		}

		runMove(rune(line[0]), amount, snake, &visitedPoints)
	}

	fmt.Println("The tail visited", len(visitedPoints), "points")

	file.Close()
}