AdventOfCode2020/03/main.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strings"
	"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)
}

type slope struct {
	rise     int
	run      int
	trees    int
	position int
}

type terrain struct {
	row []string
}

func (t terrain) isTree(poss int) bool {
	return t.row[poss%len(t.row)] == "#"
}

// You start on the open square (.) in the top-left corner and need to reach the bottom (below the bottom-most row on your map).
// The toboggan can only follow a few specific slopes (you opted for a cheaper model that prefers rational numbers);
// start by counting all the trees you would encounter for the slope right 3, down 1:
// From your starting position at the top-left, check the position that is right 3 and down 1.
// Then, check the position that is right 3 and down 1 from there, and so on until you go past the bottom of the map.
// The locations you'd check in the above example are marked here with O where there was an open square and X where there was a tree:
// ..##.........##.........##.........##.........##.........##.......  --->
// #..O#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
// .#....X..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
// ..#.#...#O#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
// .#...##..#..X...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
// ..#.##.......#.X#.......#.##.......#.##.......#.##.......#.##.....  --->
// .#.#.#....#.#.#.#.O..#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
// .#........#.#........X.#........#.#........#.#........#.#........#
// #.##...#...#.##...#...#.X#...#...#.##...#...#.##...#...#.##...#...
// #...##....##...##....##...#X....##...##....##...##....##...##....#
// .#..#...#.#.#..#...#.#.#..#...X.#.#..#...#.#.#..#...#.#.#..#...#.#  --->
// In this example, traversing the map using this slope would cause you to encounter 7 trees.
// Starting at the top-left corner of your map and following a slope of right 3 and down 1, how many trees would you encounter?

func partOne() {
	f, _ := os.Open("input")
	reader := bufio.NewReader(f)
	scanner := bufio.NewScanner(reader)

	depth := 0

	s := slope{
		rise:     1,
		run:      3,
		trees:    0,
		position: 0,
	}

	for scanner.Scan() {
		e := terrain{
			row: strings.Split(scanner.Text(), ""),
		}
		if depth%s.rise == 0 && e.isTree(s.position) {
			s.trees = s.trees + 1
		}
		s.position = s.position + s.run
		depth = depth + s.rise
	}

	fmt.Println(s.trees)
}

// Determine the number of trees you would encounter if, for each of the following slopes, you start at the top-left corner and traverse the map all the way to the bottom:
// Right 1, down 1.
// Right 3, down 1. (This is the slope you already checked.)
// Right 5, down 1.
// Right 7, down 1.
// Right 1, down 2.
// In the above example, these slopes would find 2, 7, 3, 4, and 2 tree(s) respectively; multiplied together, these produce the answer 336.

func partTwo() {
	f, _ := os.Open("input")
	reader := bufio.NewReader(f)
	scanner := bufio.NewScanner(reader)

	slopes := []*slope{
		{
			rise:     1,
			run:      1,
			trees:    0,
			position: 0,
		},
		{
			rise:     1,
			run:      3,
			trees:    0,
			position: 0,
		},
		{
			rise:     1,
			run:      5,
			trees:    0,
			position: 0,
		},
		{
			rise:     1,
			run:      7,
			trees:    0,
			position: 0,
		},
		{
			rise:     2,
			run:      1,
			trees:    0,
			position: 0,
		},
	}

	depth := 0

	for scanner.Scan() {
		e := terrain{
			row: strings.Split(scanner.Text(), ""),
		}
		for _, s := range slopes {
			if depth%s.rise == 0 {
				// check tree
				if e.isTree(s.position) {
					s.trees = s.trees + 1
				}
				// only increment position if we're "on" for this row; handles the rise > run case
				s.position = s.position + s.run
			}
			// always move on a row
			depth = depth + 1
		}
	}

	// can't be bothered to write out the loop
	fmt.Println(slopes[0].trees * slopes[1].trees * slopes[2].trees * slopes[3].trees * slopes[4].trees)
}