AdventOfCode2021/05/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)
}

func makeScanner(test bool) *bufio.Scanner {
	var f *os.File
	if test {
		f, _ = os.Open("inputs/testinput")
	} else {
		f, _ = os.Open("inputs/input")
	}
	reader := bufio.NewReader(f)
	return bufio.NewScanner(reader)
}

func mustAtoi(line []byte) int {
	i, err := strconv.Atoi(string(line))
	if err != nil {
		panic(err)
	}
	return i
}

type point struct {
	x int
	y int
}

type grid struct {
	m map[int]int
}

func (g *grid) AddLine(start, end point, includeHorizontal bool) {
	for _, p := range LineFromPoints(start, end, includeHorizontal) {
		g.m[p.x+p.y*1000]++
	}
}

func LineFromPoints(start, end point, includeHorizontal bool) []point {
	if start.x != end.x && start.y != end.y && !includeHorizontal {
		return []point{}
	}
	var xstep, ystep int
	if start.x > end.x {
		xstep = -1
	}
	if start.x < end.x {
		xstep = 1
	}
	if start.y > end.y {
		ystep = -1
	}
	if start.y < end.y {
		ystep = 1
	}
	points := []point{}
	for i, j := start.x, start.y; i != end.x+xstep || j != end.y+ystep; i, j = i+xstep, j+ystep {
		points = append(points, point{x: i, y: j})
	}
	return points
}

func parsePoints(line string) (point, point) {
	var start, end point
	var num []byte // accumulator
	var state int  // parser state: 0: x1 1: y1 2: x2 3: y2
	for i := 0; i < len(line); i++ {
		switch line[i] {
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			num = append(num, line[i])
		case ' ':
			start.y = mustAtoi(num)
			num = []byte{}
			i += 3 // jump past '-> '
			state = 2
		case ',':
			if state == 0 {
				start.x = mustAtoi(num)
				num = []byte{}
				state = 1
			}
			if state == 2 {
				end.x = mustAtoi(num)
				num = []byte{}
				state = 3
			}
		}
	}
	end.y = mustAtoi(num)
	return start, end
}

func partOne() {
	scanner := makeScanner(false)

	grid := &grid{
		m: make(map[int]int, 1000),
	}
	for scanner.Scan() {
		line := scanner.Text()
		start, end := parsePoints(line)
		grid.AddLine(start, end, false)
	}
	var danger int
	for i := range grid.m {
		if grid.m[i] > 1 {
			danger++
		}
	}
	fmt.Println(danger)
}

func partTwo() {
	scanner := makeScanner(false)

	grid := &grid{
		m: make(map[int]int, 1000),
	}
	for scanner.Scan() {
		line := scanner.Text()
		start, end := parsePoints(line)
		grid.AddLine(start, end, true)
	}
	var danger int
	for i := range grid.m {
		if grid.m[i] > 1 {
			danger++
		}
	}
	fmt.Println(danger)
}