aoc2020/day3/part1/main.d

import std;

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

ulong 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(3, 1) == 7);
}