// This file is part of ICU4X.

//

// The contents of this file implement algorithms from Calendrical Calculations

// by Reingold & Dershowitz, Cambridge University Press, 4th edition (2018),

// which have been released as Lisp code at <https://github.com/EdReingold/calendar-code2/>

// under the Apache-2.0 license. Accordingly, this file is released under

// the Apache License, Version 2.0 which can be found at the calendrical_calculations

// package root or at http://www.apache.org/licenses/LICENSE-2.0.

use crate::helpers::{i64_to_i32, I32CastError};

use crate::rata_die::RataDie;

// The Gregorian epoch is equivalent to first day in fixed day measurement

const EPOCH: RataDie = RataDie::new(1);

/// Whether or not `year` is a leap year

pub fn is_leap_year(year: i32) -> bool {

    year % 4 == 0 && (year % 400 == 0 || year % 100 != 0)

}

// Fixed is day count representation of calendars starting from Jan 1st of year 1.

// The fixed calculations algorithms are from the Calendrical Calculations book.

//

/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1167-L1189>

pub fn fixed_from_iso(year: i32, month: u8, day: u8) -> RataDie {

    let prev_year = (year as i64) - 1;

    // Calculate days per year

    let mut fixed: i64 = (EPOCH.to_i64_date() - 1) + 365 * prev_year;

    // Calculate leap year offset

    let offset = prev_year.div_euclid(4) - prev_year.div_euclid(100) + prev_year.div_euclid(400);

    // Adjust for leap year logic

    fixed += offset;

    // Days of current year

    fixed += (367 * (month as i64) - 362).div_euclid(12);

    // Leap year adjustment for the current year

    fixed += if month <= 2 {

        0

    } else if is_leap_year(year) {

        -1

    } else {

        -2

    };

    // Days passed in current month

    fixed += day as i64;

    RataDie::new(fixed)

}

/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1191-L1217>

pub(crate) fn iso_year_from_fixed(date: RataDie) -> i64 {

    // Shouldn't overflow because it's not possbile to construct extreme values of RataDie

    let date = date - EPOCH;

    // 400 year cycles have 146097 days

    let (n_400, date) = (date.div_euclid(146097), date.rem_euclid(146097));

    // 100 year cycles have 36524 days

    let (n_100, date) = (date.div_euclid(36524), date.rem_euclid(36524));

    // 4 year cycles have 1461 days

    let (n_4, date) = (date.div_euclid(1461), date.rem_euclid(1461));

    let n_1 = date.div_euclid(365);

    let year = 400 * n_400 + 100 * n_100 + 4 * n_4 + n_1;

    if n_100 == 4 || n_1 == 4 {

        year

    } else {

        year + 1

    }

}

fn iso_new_year(year: i32) -> RataDie {

    fixed_from_iso(year, 1, 1)

}

/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1525-L1540>

pub fn iso_from_fixed(date: RataDie) -> Result<(i32, u8, u8), I32CastError> {

    let year = iso_year_from_fixed(date);

    let year = i64_to_i32(year)?;

    // Calculates the prior days of the adjusted year, then applies a correction based on leap year conditions for the correct ISO date conversion.

    let prior_days = date - iso_new_year(year);

    let correction = if date < fixed_from_iso(year, 3, 1) {

        0

    } else if is_leap_year(year) {

        1

    } else {

        2

    };

    let month = (12 * (prior_days + correction) + 373).div_euclid(367) as u8; // in 1..12 < u8::MAX

    let day = (date - fixed_from_iso(year, month, 1) + 1) as u8; // <= days_in_month < u8::MAX

    Ok((year, month, day))

}