use time::{Date, Month, OffsetDateTime};

/// The start of the COVID-19 lockdowns
///
/// This is used as an epoch in order to convert from a given date to an integer seed.  This is
/// specified as part of the algorithm for randomly selecting from a weighted list.
pub const COVID_EPOCH: Date = match Date::from_calendar_date(2020, Month::January, 26) {
	Ok(d) => d,
	Err(_) => Date::MIN, // This never runs, but we can't unwrap, so this is what we're stuck with
};

/// A state to use as an initial state before seeding
///
/// Think of this as a ::new() method for a pcg64 generator.  Subsiquent calls should be
/// made to the [`pcg64_seed()`] methods or the [`seed_with_date()`]/[`seed_with_today()`]
/// methods
pub const INITIAL_STATE: u128 = 1312_1312_1312;

pub fn pcg64(state: &mut u128) -> u64 {
	let mut x = *state;
	pcg64_iterstate(state);
	let count = (x >> (128 - 6)) as u32; // Highest 6 bits
    x ^= x >> 35;
    ((x >> 58) as u64).rotate_right(count)
}

fn pcg64_iterstate(state: &mut u128) {
	const MULTIPLIER: u128 = 0xde92a69f6e2f9f25fd0d90f576075fbd;
	const INCREMENT: u128 = 621;
	*state = state.wrapping_mul(MULTIPLIER).wrapping_add(INCREMENT);
}

pub fn pcg64_seed(state: &mut u128, bytes: &[u8]) {
    for byte in bytes {
        *state = (*state ^ *byte as u128).rotate_right(8);
    }
	pcg64_iterstate(state);
}

/// Seed the generator with a given [`Date`] object
pub fn seed_with_date(state: &mut u128, date: Date) {
	pcg64_seed(
		state,
		&(
			(date - COVID_EPOCH)
			.whole_days()
			as u32
		).to_le_bytes()
	)
}

/// Seed the generator with the [`Date`] object representing today's date
///
/// Uses the system's local time, or falls back to UTC
pub fn seed_with_today(state: &mut u128) {
	seed_with_date(
		state,
		OffsetDateTime::now_local()
			.unwrap_or_else(|_| OffsetDateTime::now_utc())
			.date()
	)
}