package main

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

// With the toboggan login problems resolved, you set off toward the airport. While travel by toboggan might be easy, it's certainly not safe: there's very minimal steering and the area is covered in trees. You'll need to see which angles will take you near the fewest trees.

// Due to the local geology, trees in this area only grow on exact integer coordinates in a grid. You make a map (your puzzle input) of the open squares (.) and trees (#) you can see. For example:

// ..##.......
// #...#...#..
// .#....#..#.
// ..#.#...#.#
// .#...##..#.
// ..#.##.....
// .#.#.#....#
// .#........#
// #.##...#...
// #...##....#
// .#..#...#.#

// These aren't the only trees, though; due to something you read about once involving arboreal genetics and biome stability, the same pattern repeats to the right many times:

// ..##.........##.........##.........##.........##.........##.......  --->
// #...#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..
// .#....#..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.
// ..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#
// .#...##..#..#...##..#..#...##..#..#...##..#..#...##..#..#...##..#.
// ..#.##.......#.##.......#.##.......#.##.......#.##.......#.##.....  --->
// .#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#
// .#........#.#........#.#........#.#........#.#........#.#........#
// #.##...#...#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...
// #...##....##...##....##...##....##...##....##...##....##...##....#
// .#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#  --->

// 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 main() {
	file, _ := os.Open("input.txt")
	defer file.Close()

	treeMap := [][]int{}

	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		treeRow := []int{}
		for _, char := range scanner.Text() {
			if char == '#' {
				treeRow = append(treeRow, 1)
			} else if char == '.' {
				treeRow = append(treeRow, 0)
			}
		}
		treeMap = append(treeMap, treeRow)
	}

	rowLength := len(treeMap[0])

	row := 0
	pos := 0
	trees := 0

	// Part 1

	for row < len(treeMap) {
		trees += treeMap[row][pos]
		row++
		pos += 3
		pos %= rowLength
	}

	fmt.Println("Part 1:", trees)

	// Part 2

	// 	Time to check the rest of the slopes - you need to minimize the probability of a sudden arboreal stop, after all.

	// 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.

	// What do you get if you multiply together the number of trees encountered on each of the listed slopes?

	trees = 1

	type slope struct {
		right, down int
	}

	for _, s := range []slope{{1, 1}, {3, 1}, {5, 1}, {7, 1}, {1, 2}} {
		slopeTrees := 0
		row = 0
		pos = 0
		for row < len(treeMap) {
			slopeTrees += treeMap[row][pos]
			row += s.down
			pos += s.right
			pos %= rowLength
		}
		trees *= slopeTrees
	}

	fmt.Println("Part 2:", trees)
}