aoc2020/day3/part2/main.d

import std;

void main()
{
    File("input", "r").byLineCopy.array.productOfTreesForDefaultSlopes.writeln;
}

alias product = partial!(reverseArgs!(fold!((a, b) => a * b)), 1UL);

auto productOfTreesForDefaultSlopes(const char[][] input)
{
    return [tuple(1, 1), tuple(3, 1), tuple(5, 1), tuple(7, 1), tuple(1, 2)].map!(
            slope => input.dup.countTrees(slope[0], slope[1])).product;
}

auto countTrees(const char[][] input, int angleX, int angleY)
in(input.length > 0)
{
    immutable width = input[0].length;
    size_t x = 0;
    return input.stride(angleY).map!((row) {
        immutable result = row[x];
        x = (x + angleX) % width;
        return result;
    })
        .filter!(pos => pos == '#')
        .count;
}

unittest
{
    auto input = `..##.......
#...#...#..
.#....#..#.
..#.#...#.#
.#...##..#.
..#.##.....
.#.#.#....#
.#........#
#.##...#...
#...##....#
.#..#...#.#`.byChar.splitter("\n").map!array.array;

    assert(input.countTrees(1, 1) == 2);
}

unittest
{
    auto input = `..##.......
#...#...#..
.#....#..#.
..#.#...#.#
.#...##..#.
..#.##.....
.#.#.#....#
.#........#
#.##...#...
#...##....#
.#..#...#.#`.byChar.splitter("\n").map!array.array;

    assert(input.countTrees(3, 1) == 7);
}

unittest
{
    auto input = `..##.......
#...#...#..
.#....#..#.
..#.#...#.#
.#...##..#.
..#.##.....
.#.#.#....#
.#........#
#.##...#...
#...##....#
.#..#...#.#`.byChar.splitter("\n").map!array.array;

    assert(input.countTrees(5, 1) == 3);
}

unittest
{
    auto input = `..##.......
#...#...#..
.#....#..#.
..#.#...#.#
.#...##..#.
..#.##.....
.#.#.#....#
.#........#
#.##...#...
#...##....#
.#..#...#.#`.byChar.splitter("\n").map!array.array;

    assert(input.countTrees(7, 1) == 4);
}

unittest
{
    auto input = `..##.......
#...#...#..
.#....#..#.
..#.#...#.#
.#...##..#.
..#.##.....
.#.#.#....#
.#........#
#.##...#...
#...##....#
.#..#...#.#`.byChar.splitter("\n").map!array.array;

    assert(input.countTrees(1, 2) == 2);
}