main.go

package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"path"
	"strconv"
	"strings"
)

func input() *os.File {
	input, err := os.Open(path.Join("2021", "5", "input.txt"))
	if err != nil {
		panic(err)
	}

	return input
}

type point struct {
	x, y int
}

func parsePoint(rawPoint string) point {
	parts := strings.Split(rawPoint, ",")

	x, err := strconv.Atoi(parts[0])
	if err != nil {
		panic(err)
	}

	y, err := strconv.Atoi(parts[1])
	if err != nil {
		panic(err)
	}

	return point{
		x: x,
		y: y,
	}
}

func parseLine(rawLine string) (point, point) {
	parts := strings.Split(rawLine, " -> ")

	return parsePoint(parts[0]), parsePoint(parts[1])
}

func solve(r io.Reader) {
	scanner := bufio.NewScanner(r)

	field := make(map[point]int)

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

		start, end := parseLine(line)

		walk(start, end, func(p point) {
			field[p] += 1
		})
	}

	if scanner.Err() != nil {
		panic(scanner.Err())
	}

	overlap := 0
	for _, v := range field {
		if v > 1 {
			overlap += 1
		}
	}

	fmt.Println(overlap)
}

func walk(start, end point, action func(p point)) {
	if start.y == end.y {
		if start.x > end.x {
			start, end = end, start
		}

		for x := start.x; x <= end.x; x++ {
			action(point{
				x: x,
				y: start.y,
			})
		}
	} else if start.x == end.x {
		if start.y > end.y {
			start, end = end, start
		}

		for y := start.y; y <= end.y; y++ {
			action(point{
				x: start.x,
				y: y,
			})
		}
	} else {
		xIter := 1
		if end.x < start.x {
			xIter = -1
		}

		yIter := 1
		if end.y < start.y {
			yIter = -1
		}

		x := start.x
		y := start.y
		for x != end.x && y != end.y {
			action(point{
				x: x,
				y: y,
			})

			x += xIter
			y += yIter
		}

		action(point{
			x: x,
			y: y,
		})
	}
}

func main() {
	solve(strings.NewReader("0,9 -> 5,9\n8,0 -> 0,8\n9,4 -> 3,4\n2,2 -> 2,1\n7,0 -> 7,4\n6,4 -> 2,0\n0,9 -> 2,9\n3,4 -> 1,4\n0,0 -> 8,8\n5,5 -> 8,2\n"))
	solve(input())
}