return try getLast(allocator.?, INPUT, 2020);

    return getLast(allocator.?, INPUT, 2020, u11);

    return try getLast(allocator.?, INPUT, 30_000_000);

    return getLast(allocator.?, INPUT, 30_000_000, u25);

fn getLast(allocator: std.mem.Allocator, input: Str, limit: usize) !usize {
    var spoken = std.AutoHashMap(usize, usize).init(allocator);

fn getLast(allocator: std.mem.Allocator, input: Str, limit: usize, comptime ItemType: anytype) !usize {
    var spoken = std.ArrayList(ItemType).init(allocator);

    var starting = std.mem.tokenize(u8, input, ",");
    var last: usize = undefined;

    // Zero means the number was not spoken yet.
    try spoken.appendNTimes(0, limit);

    var round: usize = 1;

    var last: ItemType = undefined;
    // Fill in starting numbers.
    var starting = std.mem.tokenize(u8, input, ",");
    var round: ItemType = 1;

        const num = try std.fmt.parseUnsigned(usize, st, 0);
        try spoken.put(num, round);

        const num = try std.fmt.parseUnsigned(ItemType, st, 0);
        spoken.items[num] = round;

        const next = round - (spoken.get(last) orelse round);
        try spoken.put(last, round);

        const next = round - if (spoken.items[last] != 0) spoken.items[last] else round;
        spoken.items[last] = round;

    return last;

    return @intCast(usize, last);