// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).

use std::{collections::HashSet, fmt::Debug};

use icu_calendar::{
    Calendar, Date,
    cal::*,
    error::DateFromFieldsError,
    options::{DateFromFieldsOptions, MissingFieldsStrategy, Overflow},
    types::{DateFields, Month},
};

/// Test that a given calendar produces valid monthdays
///
/// `valid_md_condition`, given `(month_number, is_leap, day_number)`, should return whether or not
/// that combination is ever possible in that calendar
fn test_reference_year_impl<C>(cal: C, valid_md_condition: impl Fn(u8, bool, u8) -> ValidityState)
where
    C: Calendar + Debug + Copy,
{
    // Test that all dates in a certain range behave according to Temporal
    let mut month_days_seen = HashSet::new();
    let mut rd = Date::try_new_iso(1972, 12, 31).unwrap().to_rata_die();
    for _ in 1..2000 {
        let date = Date::from_rata_die(rd, cal);
        let month_day = (date.month().to_input(), date.day_of_month().0);
        let mut fields = DateFields::default();
        fields.month = Some(date.month().to_input());
        fields.day = Some(month_day.1);
        let mut options = DateFromFieldsOptions::default();
        options.missing_fields_strategy = Some(MissingFieldsStrategy::Ecma);
        let reference_date = Date::try_from_fields(fields, options, cal).unwrap();
        if month_days_seen.contains(&month_day) {
            assert_ne!(date, reference_date, "{cal:?}");
        } else {
            assert_eq!(date, reference_date, "{cal:?}");
            month_days_seen.insert(month_day);
        }
        rd -= 1;
    }
    // Test that all MonthDay values round-trip
    for month_number in 1..=14 {
        for is_leap in [false, true] {
            let mut valid_day_number = 1;
            let month_validity = valid_md_condition(month_number, is_leap, valid_day_number);
            // The last valid monthday produced with Overflow::Reject in this month
            let mut last_valid_date = None;
            for day_number in 1..=32 {
                let md_validity = valid_md_condition(month_number, is_leap, day_number);
                if md_validity == ValidityState::Valid {
                    valid_day_number = day_number;
                }
                let mut fields = DateFields::default();
                fields.month = Some(match is_leap {
                    false => Month::new(month_number),
                    true => Month::leap(month_number),
                });
                fields.day = Some(day_number);
                let mut options = DateFromFieldsOptions::default();
                options.overflow = Some(Overflow::Constrain);
                options.missing_fields_strategy = Some(MissingFieldsStrategy::Ecma);
                let reference_date = match Date::try_from_fields(fields, options, cal) {
                    Ok(d) => {
                        assert!(
                            month_validity != ValidityState::Invalid,
                            "try_from_fields passed but should have failed: {fields:?} => {d:?}"
                        );
                        d
                    }
                    Err(DateFromFieldsError::MonthNotInCalendar) => {
                        assert!(
                            month_validity == ValidityState::Invalid,
                            "try_from_fields failed but should have passed: {fields:?}"
                        );
                        continue;
                    }
                    Err(e) => {
                        panic!("Unexpected error in month day from fields: {e}");
                    }
                };

                // Test round-trip (to valid day number)
                if md_validity == ValidityState::ChineseConstrain {
                    let input_month = fields.month.unwrap();
                    let output_month = reference_date.month().to_input();
                    // When constraining in the Chinese calendar the month
                    // stays the same but loses leapiness.
                    assert_eq!(
                        input_month.number(),
                        output_month.number(),
                        "Month number should stay the same: {fields:?} {cal:?}"
                    );
                    assert_ne!(
                        input_month.is_leap(),
                        output_month.is_leap(),
                        "Leap month should turn into regular month: {fields:?} {cal:?}"
                    );
                } else {
                    assert_eq!(
                        fields.month.unwrap(),
                        reference_date.month().to_input(),
                        "{fields:?} {cal:?}"
                    );
                    assert_eq!(
                        valid_day_number,
                        reference_date.day_of_month().0,
                        "{fields:?} {cal:?}"
                    );
                }

                // Test Overflow::Reject
                options.overflow = Some(Overflow::Reject);
                let reject_result = Date::try_from_fields(fields, options, cal);
                if md_validity == ValidityState::ChineseConstrain {
                    assert!(matches!(
                        reject_result,
                        Err(DateFromFieldsError::MonthNotInYear)
                    ))
                } else if valid_day_number == day_number {
                    assert_eq!(reject_result, Ok(reference_date));
                } else {
                    assert!(matches!(
                        reject_result,
                        Err(DateFromFieldsError::InvalidDay { .. })
                    ))
                }

                // Constrain *must* constrain to the last valid day in
                // the month, except for when the leap month turns into a regular month
                // in the Chinese/Dangi "nonexistant in modern times" leap month edge case.
                if let Ok(reject_result) = reject_result {
                    // We didn't need to constrain here, update the last valid date
                    last_valid_date = Some(reject_result);
                } else if let Some(last_valid_date) = last_valid_date
                    && md_validity != ValidityState::ChineseConstrain
                {
                    assert_eq!(
                        last_valid_date, reference_date,
                        "Constrain should constrain to the last valid date in the month"
                    );
                }

                // Test that ordinal months cause it to fail (even if the month code is still set)
                fields.ordinal_month = Some(month_number);
                let ordinal_result = Date::try_from_fields(fields, options, cal);
                assert!(matches!(
                    ordinal_result,
                    Err(DateFromFieldsError::NotEnoughFields)
                ));
            }
        }
    }
}

#[derive(PartialEq, Debug)]
enum ValidityState {
    Valid,
    Invalid,
    // Chinese/Korean have special behavior where certain leap month-day combos are specced
    // as constraining to the regular month, because they are have not been found in the modern
    // time range where implementations are expected to agree.
    ChineseConstrain,
}

impl From<bool> for ValidityState {
    fn from(other: bool) -> ValidityState {
        if other {
            ValidityState::Valid
        } else {
            ValidityState::Invalid
        }
    }
}

fn gregorian_md_condition(month_number: u8, is_leap: bool, day_number: u8) -> ValidityState {
    // No leap months
    if is_leap {
        return ValidityState::Invalid;
    }

    ValidityState::from(match month_number {
        1 | 3 | 5 | 7 | 8 | 10 | 12 => day_number <= 31,
        2 => day_number <= 29,
        4 | 6 | 9 | 11 => day_number <= 30,
        _ => {
            assert!(month_number > 12);
            // No other months
            false
        }
    })
}

fn chinese_md_condition(month_number: u8, is_leap: bool, day_number: u8) -> ValidityState {
    // https://tc39.es/proposal-intl-era-monthcode/#chinese-dangi-iso-reference-years

    if (month_number == 1 && is_leap)
        || (month_number == 2 && is_leap && day_number >= 30)
        || (month_number == 8 && is_leap && day_number >= 30)
        || (month_number == 9 && is_leap && day_number >= 30)
        || (month_number == 10 && is_leap && day_number >= 30)
        || (month_number == 11 && is_leap && day_number >= 30)
        || (month_number == 12 && is_leap)
    {
        return ValidityState::ChineseConstrain;
    }
    ValidityState::from(month_number <= 12 && day_number <= 30)
}

fn coptic_md_condition(month_number: u8, is_leap: bool, day_number: u8) -> ValidityState {
    // No leap months
    if is_leap {
        return ValidityState::Invalid;
    }
    ValidityState::from(match month_number {
        1..=12 => day_number <= 30,
        13 => day_number <= 6,
        _ => false,
    })
}

fn hijri_md_condition(month_number: u8, is_leap: bool, day_number: u8) -> ValidityState {
    // No leap months
    if is_leap {
        return ValidityState::Invalid;
    }
    ValidityState::from(month_number <= 12 && day_number <= 30)
}

fn hijri_tabular_md_condition(month_number: u8, is_leap: bool, day_number: u8) -> ValidityState {
    // No leap months
    if is_leap {
        return ValidityState::Invalid;
    }

    if month_number > 12 {
        return ValidityState::Invalid;
    }

    // Odd months have 30 days, even months have 29, except for M12 in a leap year
    ValidityState::from(if month_number.is_multiple_of(2) {
        if month_number == 12 {
            day_number <= 30
        } else {
            day_number <= 29
        }
    } else {
        day_number <= 30
    })
}

fn hebrew_md_condition(month_number: u8, is_leap: bool, day_number: u8) -> ValidityState {
    if is_leap {
        return ValidityState::from(month_number == 5 && day_number <= 30);
    }
    ValidityState::from(match month_number {
        1 | 2 | 3 | 5 | 7 | 9 | 11 => day_number <= 30,
        // Tevet, Adar, Iyar, Tammuz, Elul
        4 | 6 | 8 | 10 | 12 => day_number <= 29,
        _ => {
            assert!(month_number > 12);
            // No other months
            false
        }
    })
}

#[test]
fn test_reference_year_buddhist() {
    test_reference_year_impl(Buddhist, gregorian_md_condition)
}

#[test]
fn test_reference_year_chinese() {
    test_reference_year_impl(ChineseTraditional::new(), chinese_md_condition)
}

#[test]
fn test_reference_year_coptic() {
    test_reference_year_impl(Coptic, coptic_md_condition)
}

#[test]
fn test_reference_year_korean() {
    test_reference_year_impl(KoreanTraditional::new(), chinese_md_condition)
}

#[test]
fn test_reference_year_ethiopian() {
    test_reference_year_impl(Ethiopian::new(), coptic_md_condition)
}

#[test]
fn test_reference_year_ethiopian_amete_alem() {
    test_reference_year_impl(
        Ethiopian::new_with_era_style(EthiopianEraStyle::AmeteAlem),
        coptic_md_condition,
    )
}

#[test]
fn test_reference_year_gregorian() {
    test_reference_year_impl(Gregorian, gregorian_md_condition)
}

#[test]
fn test_reference_year_julian() {
    test_reference_year_impl(Julian, gregorian_md_condition)
}

#[test]
fn test_reference_year_hebrew() {
    test_reference_year_impl(Hebrew, hebrew_md_condition)
}

#[test]
fn test_reference_year_indian() {
    test_reference_year_impl(Indian, |month_number, is_leap, day_number| {
        if is_leap {
            // No leap months
            return ValidityState::Invalid;
        }
        // First half of the year has long months, second half short
        ValidityState::from(if month_number <= 6 {
            day_number <= 31
        } else if month_number <= 12 {
            day_number <= 30
        } else {
            // No larger months
            false
        })
    })
}

#[test]
fn test_reference_year_hijri_tabular_type_ii_friday() {
    test_reference_year_impl(
        Hijri::new_tabular(HijriTabularLeapYears::TypeII, HijriTabularEpoch::Friday),
        hijri_tabular_md_condition,
    )
}

#[test]
fn test_reference_year_hijri_tabular_type_ii_thursday() {
    test_reference_year_impl(
        Hijri::new_tabular(HijriTabularLeapYears::TypeII, HijriTabularEpoch::Thursday),
        hijri_tabular_md_condition,
    )
}

#[test]
fn test_reference_year_hijri_umm_al_qura() {
    test_reference_year_impl(Hijri::new_umm_al_qura(), hijri_md_condition)
}

#[test]
fn test_reference_year_iso() {
    test_reference_year_impl(Iso, gregorian_md_condition)
}

#[test]
fn test_reference_year_japanese() {
    test_reference_year_impl(Japanese::new(), gregorian_md_condition)
}

#[test]
fn test_reference_year_persian() {
    test_reference_year_impl(Persian, |month_number, is_leap, day_number| {
        if is_leap {
            // No leap months
            return ValidityState::Invalid;
        }
        // First half of the year has long months, second half short
        ValidityState::from(if month_number <= 6 {
            day_number <= 31
        } else if month_number <= 12 {
            day_number <= 30
        } else {
            // No larger months
            false
        })
    })
}

#[test]
fn test_reference_year_roc() {
    test_reference_year_impl(Roc, gregorian_md_condition)
}
