const std = @import("std");
const mem = std.mem;
const math = std.math;

pub fn main() !void {
    const input: []const u8 = @embedFile("input.txt");
    const part1, const part2 = try problem(input);
    std.debug.print("Part 1: {}\n", .{part1});
    std.debug.print("Part 2: {}\n", .{part2});
}

fn problem(input: []const u8) !struct { u64, u64 } {
    const lines: mem.TokenIterator(u8, .scalar) = mem.tokenizeScalar(u8, input, '\n');
    const part1: u64 = try p1(lines);
    const part2: u64 = try p2(lines);
    return .{ part1, part2 };
}

fn parseline(l: []const u8) !i64 {
    return switch (l[0]) {
        'L' => try std.fmt.parseInt(i64, l[1..], 10),
        'R' => - try std.fmt.parseInt(i64, l[1..], 10),
        else => error{InvalidLine}.InvalidLine,
    };
}

inline fn rotatepos(pos: u32, n: i64) u32 {
    return @truncate(@abs(@mod(pos + n, 100)));
}

fn countzeros(pos: u64, n: i64) u64 {
    var zeros: u64 = 0;
    var x: u32 = @intCast(pos);
    var clicks: u64 = @abs(n);
    const y: i64 = if (n < 0) -1 else 1;
    while (clicks > 0) : (clicks -= 1){
        x = rotatepos(x, y);
        if (x == 0) zeros += 1;
    }
    return zeros;
}

fn p1(constLines: mem.TokenIterator(u8, .scalar)) !u64 {
    var pos: u32 = 50;
    var zeros: u64 = 0;
    var lines = constLines;
    while (lines.next()) |line| {
        pos = rotatepos(pos, try parseline(line));
        if (pos == 0) zeros += 1;
    }
    return zeros;
}

fn p2(constLines: mem.TokenIterator(u8, .scalar)) !u64 {
    var pos: u32 = 50;
    var zeros: u64 = 0;
    var lines = constLines;
    while (lines.next()) |line| {
        const n = try parseline(line);
        zeros += countzeros(pos, n);
        pos = rotatepos(pos, n);
    }
    return zeros;
}

test "Sample Input" {
    const input: []const u8 = @embedFile("input_sample.txt");
    const part1, const part2 = try problem(input);
    std.debug.print("Part 1: {}\n", .{part1});
    std.debug.print("Part 2: {}\n", .{part2});
    try std.testing.expectEqual(3, part1);
    try std.testing.expectEqual(6, part2);
}