aoc2020/day3/part1/main.d

import std;

void main()
{
    File("input", "r").byLineCopy.array.countTrees(3, 1).writeln;
}

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

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

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