day 3 in the books

03/input

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03/main.go

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+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	partOne()
+	partTwo()
+}
+
+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)
+}