Day 3, part 1

source/app.d

@@ -7,13 +7,15 @@ import std.getopt;
 
 import day1;
 import day2;
+import day3;
 import dayutil;
 
 immutable string progName = "aoc-2020";
 
 void function(string[])[] programs = [
 	&day1.run,
-	&day2.run
+	&day2.run,
+	&day3.run,
 ];
 
 void printUsage(string name) {

source/day3.d

@@ -0,0 +1,31 @@
+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);
+}