// This Source Code Form is subject to the terms of the Mozilla Public

// License, v. 2.0. If a copy of the MPL was not distributed with this

// file, You can obtain one at https://mozilla.org/MPL/2.0/.

use crate::calendar::gregorian::Gregorian;

use crate::calendar::prelude::CommonDate;

use crate::calendar::prelude::CommonWeekOfYear;

use crate::calendar::prelude::Quarter;

use crate::calendar::prelude::ToFromCommonDate;

use crate::calendar::prelude::ToFromOrdinalDate;

use crate::calendar::AllowYearZero;

use crate::calendar::CalendarMoment;

use crate::calendar::HasLeapYears;

use crate::clock::TimeOfDay;

use crate::common::math::TermNum;

use crate::day_count::BoundedDayCount;

use crate::day_count::CalculatedBounds;

use crate::day_count::Epoch;

use crate::day_count::Fixed;

use crate::day_count::FromFixed;

use crate::day_count::ToFixed;

use crate::day_cycle::Weekday;

use crate::CalendarError;

use num_traits::FromPrimitive;

use std::cmp::Ordering;

use std::num::NonZero;

/// Represents a date in the ISO-8601 week-date calendar

///

/// ## Introduction

///

/// The ISO-8601 week-date is essentially an alternative naming system for Gregorian dates.

/// Instead of dividing a year into months, the ISO-8601 week-date divides the year into weeks.

///

/// Despite being derived from the Gregorian calendar, **the ISO-8601 has a different year

/// start and year end than the Gregorian.** If the Gregorian year X ends in the middle of

/// the ISO week, the next days may be in Gregorian year X+1 and ISO year X.

///

/// ## Basic Structure

///

/// Each year is divided into 52 weeks, except for "long years" which have 53 weeks. These

/// are common weeks with 7 days each, and start on Monday.

///

/// A long year occurs if the corresponding Gregorian year starts or ends on a Thursday.

///

/// ## Representation and Examples

///

/// The most obvious ways to create an ISO struct is to convert from the Gregorian, or

/// to aggregate the year, week number and weekday.

///

/// ```

/// use radnelac::calendar::*;

/// use radnelac::day_count::*;

/// use radnelac::day_cycle::*;

///

/// let g = Gregorian::try_new(2025, GregorianMonth::May, 15).unwrap();

/// let i = g.convert::<ISO>();

/// assert_eq!(i, ISO::try_new(2025, 20, Weekday::Thursday).unwrap());

/// ```

///

/// ## Further reading

/// + [Wikipedia](https://en.wikipedia.org/wiki/ISO_week_date)

/// + [Rachel by the Bay](https://rachelbythebay.com/w/2018/04/20/iso/)

///   + describes the confusion of intermingling documentation for ISO and Gregorian dates

#[derive(Debug, PartialEq, Clone, Copy)]

pub struct ISO {

    year: i32,

    week: NonZero<u8>,

    day: Weekday,

}

impl ISO {

    /// Attempt to create a new ISO week date

    pub fn try_new(year: i32, week: u8, day: Weekday) -> Result<Self, CalendarError> {

        if week < 1 || week > 53 || (week == 53 && 
!Self::is_leap(year)72
) {

            //This if statement is structured specifically to minimize calls to Self::is_leap.

            //Self::is_leap calls Gregorian calendar functions which may exceed the effective

            //bounds.

            return Err(CalendarError::InvalidWeek);

        }

        Ok(ISO {

            year: year,

            week: NonZero::<u8>::new(week).expect("Checked in if"),

            day: day,

        })

    }

    pub fn year(self) -> i32 {

        self.year

    }

    pub fn week(self) -> NonZero<u8> {

        self.week

    }

    /// Note that the numeric values of the Weekday enum are not consistent with ISO-8601.

    /// Use day_num for the numeric day number.

    pub fn day(self) -> Weekday {

        self.day

    }

    /// Represents Sunday as 7 instead of 0, as required by ISO-8601.

    pub fn day_num(self) -> u8 {

        (self.day as u8).adjusted_remainder(7)

    }

    pub fn new_year(year: i32) -> Self {

        ISO::try_new(year, 1, Weekday::Monday).expect("Week 1 known to be valid")

    }

}

impl PartialOrd for ISO {

    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {

        if self == other {

            Some(Ordering::Equal)

        } else if self.year != other.year {

            self.year.partial_cmp(&other.year)

        } else if self.week != other.week {

            self.week.partial_cmp(&other.week)

        } else {

            let self_day = (self.day as i64).adjusted_remainder(7);

            let other_day = (other.day as i64).adjusted_remainder(7);

            self_day.partial_cmp(&other_day)

        }

    }

}

impl AllowYearZero for ISO {}

impl CalculatedBounds for ISO {}

impl Epoch for ISO {

    fn epoch() -> Fixed {

        Gregorian::epoch()

    }

}

impl HasLeapYears for ISO {

    fn is_leap(i_year: i32) -> bool {

        //LISTING 5.3 (*Calendrical Calculations: The Ultimate Edition* by Reingold & Dershowitz.)

        let jan1 = Gregorian::try_year_start(i_year)

            .expect("Year known to be valid")

            .convert::<Weekday>();

        let dec31 = Gregorian::try_year_end(i_year)

            .expect("Year known to be valid")

            .convert::<Weekday>();

        jan1 == Weekday::Thursday || 
dec31 == Weekday::Thursday24

    }

}

impl FromFixed for ISO {

    fn from_fixed(fixed_date: Fixed) -> ISO {

        //LISTING 5.2 (*Calendrical Calculations: The Ultimate Edition* by Reingold & Dershowitz.)

        let date = fixed_date.get_day_i();

        let approx = Gregorian::ordinal_from_fixed(Fixed::cast_new(date - 3)).year;

        let next = ISO::new_year(approx + 1).to_fixed().get_day_i();

        let year = if date >= next { 
approx + 1179
 } else { 
approx7.77k
 };

        let current = ISO::new_year(year).to_fixed().get_day_i();

        let week = (date - current).div_euclid(7) + 1;

        debug_assert!(week < 55 && week > 0);

        //Calendrical Calculations stores "day" as 7 for Sunday, as per ISO.

        //However since we have an unambiguous enum, we can save such details for

        //functions that need it. We also adjust "to_fixed_unchecked"

        let day = Weekday::from_u8(date.modulus(7) as u8).expect("In range due to modulus.");

        ISO::try_new(year, week as u8, day).expect("Week known to be valid")

    }

}

impl ToFixed for ISO {

    fn to_fixed(self) -> Fixed {

        //LISTING 5.1 (*Calendrical Calculations: The Ultimate Edition* by Reingold & Dershowitz.)

        let g = CommonDate::new(self.year - 1, 12, 28);

        let w = NonZero::<i16>::from(self.week);

        //Calendrical Calculations stores "day" as 7 for Sunday, as per ISO.

        //However since we have an unambiguous enum, we can save such details for

        //functions that need it. We also adjust "from_fixed_unchecked"

        let day_i = (self.day as i64).adjusted_remainder(7);

        let result = Gregorian::try_from_common_date(g)

            .expect("month 12, day 28 is always valid for Gregorian")

            .nth_kday(w, Weekday::Sunday)

            .get_day_i()

            + day_i;

        Fixed::cast_new(result)

    }

}

impl Quarter for ISO {

    fn quarter(self) -> NonZero<u8> {

        NonZero::new(((self.week().get() - 1) / 14) + 1).expect("(m - 1)/14 > -1")

    }

}

/// Represents a date *and time* in the ISO Calendar

pub type ISOMoment = CalendarMoment<ISO>;

impl ISOMoment {

    pub fn year(self) -> i32 {

        self.date().year()

    }

    pub fn week(self) -> NonZero<u8> {

        self.date().week()

    }

    /// Note that the numeric values of the Weekday enum are not consistent with ISO-8601.

    /// Use day_num for the numeric day number.

    pub fn day(self) -> Weekday {

        self.date().day()

    }

    /// Represents Sunday as 7 instead of 0, as required by ISO-8601.

    pub fn day_num(self) -> u8 {

        self.date().day_num()

    }

    pub fn new_year(year: i32) -> Self {

        ISOMoment::new(ISO::new_year(year), TimeOfDay::midnight())

    }

}

#[cfg(test)]

mod tests {

    use super::*;

    use crate::calendar::prelude::HasLeapYears;

    use crate::calendar::prelude::ToFromCommonDate;

    use crate::day_count::FIXED_MAX;

    use proptest::proptest;

    const MAX_YEARS: i32 = (FIXED_MAX / 365.25) as i32;

    #[test]

    fn week_of_impl() {

        let g = Gregorian::try_from_common_date(CommonDate::new(2025, 5, 15))

            .unwrap()

            .to_fixed();

        let i = ISO::from_fixed(g);

        assert_eq!(i.week().get(), 20);

    }

    #[test]

    fn epoch() {

        let i0 = ISO::from_fixed(Fixed::cast_new(0));

        let i1 = ISO::from_fixed(Fixed::cast_new(-1));

        assert!(i0 > i1, 
"i0: {:?}, i1: {:?}"0
, i0, i1);

    }

    proptest! {

        #[test]

        fn first_week(year in -MAX_YEARS..MAX_YEARS) {

            // https://en.wikipedia.org/wiki/ISO_week_date

            // > If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in W01.

            // > If it is on a Friday, it is part of W53 of the previous year. If it is on a

            // > Saturday, it is part of the last week of the previous year which is numbered

            // > W52 in a common year and W53 in a leap year. If it is on a Sunday, it is part

            // > of W52 of the previous year.

            let g = Gregorian::try_from_common_date(CommonDate {

                year,

                month: 1,

                day: 1,

            }).unwrap();

            let f = g.to_fixed();

            let w = Weekday::from_fixed(f);

            let i = ISO::from_fixed(f);

            let expected_week: u8 = match w {

                Weekday::Monday => 1,

                Weekday::Tuesday => 1,

                Weekday::Wednesday => 1,

                Weekday::Thursday => 1,

                Weekday::Friday => 53,

                Weekday::Saturday => if Gregorian::is_leap(year - 1) {53} else {52},

                Weekday::Sunday => 52,

            };

            let expected_year: i32 = if expected_week == 1 { year } else { year - 1 };

            assert_eq!(i.day(), w);

            assert_eq!(i.week().get(), expected_week);

            assert_eq!(i.year(), expected_year);

            if expected_week == 53 {

                assert!(ISO::is_leap(i.year()));

            }

        }

        #[test]

        fn fixed_week_numbers(y1 in -MAX_YEARS..MAX_YEARS, y2 in -MAX_YEARS..MAX_YEARS) {

            // https://en.wikipedia.org/wiki/ISO_week_date

            // > For all years, 8 days have a fixed ISO week number

            // > (between W01 and W08) in January and February

            // Month       Days                Weeks

            // January     04  11  18  25      W01 – W04

            // February    01  08  15  22  29  W05 – W09

            let targets = [

                (1, 4), (1, 11), (1, 18), (1, 25),

                (2, 1), (2, 8), (2, 15), (2, 22),

            ];

            for target in targets {

                let g1 = Gregorian::try_from_common_date(CommonDate {

                    year: y1,

                    month: target.0,

                    day: target.1,

                }).unwrap();

                let g2 = Gregorian::try_from_common_date(CommonDate {

                    year: y2,

                    month: target.0,

                    day: target.1,

                }).unwrap();

                assert_eq!(g1.convert::<ISO>().week(), g2.convert::<ISO>().week());

            }

        }

    }

}