aoc-2020/source/day3.d

import std.algorithm;
import std.format;
import std.range;
import std.stdio;
import std.uni;

void run(string[] args) {
	ulong count = stdin.byLine.countTrees;
	writeln(count);
}

ulong countTrees(Range)(Range lines) if (isInputRange!Range) {
	return lines.enumerate.filter!(x => (x.value[x.index * 3 % x.value.length] == '#')).count;
}

unittest {
	string[] field = [
		"..##.........##.........##.........##.........##.........##.......",
		"#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..#...#...#..",
		".#....#..#..#....#..#..#....#..#..#....#..#..#....#..#..#....#..#.",
		"..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#..#.#...#.#",
		".#...##..#..#...##..#..#...##..#..#...##..#..#...##..#..#...##..#.",
		"..#.##.......#.##.......#.##.......#.##.......#.##.......#.##.....",
		".#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#.#.#.#....#",
		".#........#.#........#.#........#.#........#.#........#.#........#",
		"#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...#.##...#...",
		"#...##....##...##....##...##....##...##....##...##....##...##....#",
		".#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#.#..#...#.#"
	];
	assert(field.countTrees == 7);
}