//! Implementation of (bank) holidays.
//! Calendars are required to verify whether an exchange is open or if a certain
//! cash flow could be settled on a specific day. They are also needed to calculate
//! the amount of business days between to given dates.
//! Because of the settlement rules, bank holidays have an impact on how to
//! rollout cash flows from fixed income products.
//! The approach taken here is to define a set of rules to determine bank holidays.
//! From this set of rules, a calendar is generated by calculating all bank holidays
//! within a given range of years for fast access.

use time::{Date, Duration, Weekday};
use serde::{Deserialize, Serialize};
use thiserror::Error;

use std::collections::BTreeSet;

mod calendar_definitions;

pub use calendar_definitions::*;


#[derive(Error, Debug)]
/// Error class for calendar calculation
pub enum CalendarError {
    #[error("calendar could not been found")]
    CalendarNotFound,
    #[error("failed to create invalid date")]
    OutOfBound(#[from]time::error::ComponentRange),
    #[error("try to proceed beyond max date")]
    MaxDay,
    #[error("try to proceed before min date")]
    MinDay,
}

type Result<T> = std::result::Result<T, CalendarError>;

/// Specifies the nth week of a month
#[derive(Deserialize, Serialize, Debug, PartialEq)]
pub enum NthWeek {
    First,
    Second,
    Third,
    Fourth,
    Last,
    SecondLast,
    ThirdLast,
    FourthLast,
}

#[derive(Deserialize, Serialize, Debug, PartialEq)]
pub enum Holiday {
    /// Though weekends are no holidays, they need to be specified in the calendar. Weekends are assumed to be non-business days.
    /// In most countries, weekends include Saturday (`Sat`) and Sunday (`Sun`). Unfortunately, there are a few exceptions.
    WeekDay(Weekday),
    /// A holiday that occurs every year on the same day.
    /// `first` and `last` are the first and last year this day is a holiday (inclusively).
    YearlyDay {
        month: u8,
        day: u8,
        first: Option<i32>,
        last: Option<i32>,
    },
    /// Occurs every year, but is moved to next non-weekend day if it falls on a weekend.
    /// Note that Saturday and Sunday here assumed to be weekend days, even if these days
    /// are not defined as weekends in this calendar. If the next Monday is already a holiday,
    /// the date will be moved to the next available business day.
    /// `first` and `last` are the first and last year this day is a holiday (inclusively).
    MovableYearlyDay {
        month: u8,
        day: u8,
        first: Option<i32>,
        last: Option<i32>,
    },
    /// Occurs every year, but is moved to previous Friday if it falls on Saturday
    /// and to the next Monday if it falls on a Sunday.
    /// `first` and `last` are the first and last year this day is a holiday (inclusively).
    ModifiedMovableYearlyDay {
        month: u8,
        day: u8,
        first: Option<i32>,
        last: Option<i32>,
    },
    /// A single holiday which is valid only once in time.
    SingularDay(Date),
    /// A holiday that is defined in relative days (e.g. -2 for Good Friday) to Easter (Sunday).
    EasterOffset {
        offset: i32,
        first: Option<i32>,
        last: Option<i32>,
    },
    /// A holiday that falls on the nth (or last) weekday of a specific month, e.g. the first Monday in May.
    /// `first` and `last` are the first and last year this day is a holiday (inclusively).
    MonthWeekday {
        month: u8,
        weekday: Weekday,
        nth: NthWeek,
        first: Option<i32>,
        last: Option<i32>,
    },
}

/// Calendar for arbitrary complex holiday rules
#[derive(Debug, Clone)]
pub struct Calendar {
    holidays: BTreeSet<Date>,
    weekdays: Vec<Weekday>,
}

fn new_date(year: i32, month: u8, day: u8) -> Result<Date> {
    Ok(Date::from_calendar_date(year, month.try_into()?, day)?)
}

impl Calendar {
    /// Calculate all holidays and recognize weekend days for a given range of years
    /// from `start` to `end` (inclusively). The calculation is performed on the basis
    /// of a vector of holiday rules.
    pub fn calc_calendar(holiday_rules: &[Holiday], start: i32, end: i32) -> Result<Calendar> {
        let mut holidays = BTreeSet::new();
        let mut weekdays = Vec::new();

        for rule in holiday_rules {
            match rule {
                Holiday::SingularDay(date) => {
                    let year = date.year();
                    if year >= start && year <= end {
                        holidays.insert(*date);
                    }
                }
                Holiday::WeekDay(weekday) => {
                    weekdays.push(*weekday);
                }
                Holiday::YearlyDay {
                    month,
                    day,
                    first,
                    last,
                } => {
                    let (first, last) = Self::calc_first_and_last(start, end, first, last);
                    for year in first..last + 1 {
                        holidays.insert( new_date(year, *month, *day)?);
                    }
                }
                Holiday::MovableYearlyDay {
                    month,
                    day,
                    first,
                    last,
                } => {
                    let (first, last) = Self::calc_first_and_last(start, end, first, last);
                    for year in first..last + 1 {
                        let date = new_date(year, *month, *day)?;
                        let mut date = match date.weekday() {
                            Weekday::Saturday => date.next_day().ok_or(CalendarError::MaxDay)?.next_day().ok_or(CalendarError::MaxDay)?,
                            Weekday::Sunday => date.next_day().ok_or(CalendarError::MaxDay)?,
                            _ => date,
                        };
                        while holidays.get(&date).is_some() {
                            date = date.next_day().ok_or(CalendarError::MaxDay)?;
                        }
                        holidays.insert(date);
                    }
                }
                Holiday::ModifiedMovableYearlyDay {
                    month,
                    day,
                    first,
                    last,
                } => {
                    let (first, last) = Self::calc_first_and_last(start, end, first, last);
                    for year in first..last + 1 {
                        let date = new_date(year, *month, *day)?;
                        let moved_date = match date.weekday() {
                            Weekday::Saturday => date.previous_day().ok_or(CalendarError::MinDay)?,
                            Weekday::Sunday => date.next_day().ok_or(CalendarError::MaxDay)?,
                            _ => date,
                        };
                        if moved_date.month() == date.month() {
                            holidays.insert(moved_date);
                        } else {
                            holidays.insert(date);
                        }
                    }
                }
                Holiday::EasterOffset {
                    offset,
                    first,
                    last,
                } => {
                    let (first, last) = Self::calc_first_and_last(start, end, first, last);
                    for year in first..last + 1 {
                        let easter = computus::gregorian(year).unwrap();
                        let easter = new_date(easter.year, easter.month as u8, easter.day as u8)?;
                        let date = easter.checked_add(Duration::days(*offset as i64)).unwrap();
                        holidays.insert(date);
                    }
                }
                Holiday::MonthWeekday {
                    month,
                    weekday,
                    nth,
                    first,
                    last,
                } => {
                    let (first, last) = Self::calc_first_and_last(start, end, first, last);
                    for year in first..last + 1 {
                        let day = match nth {
                            NthWeek::First => 1,
                            NthWeek::Second => 8,
                            NthWeek::Third => 15,
                            NthWeek::Fourth => 22,
                            NthWeek::Last => last_day_of_month(year, *month),
                            NthWeek::SecondLast => last_day_of_month(year, *month)-7,
                            NthWeek::ThirdLast => last_day_of_month(year, *month)-14,
                            NthWeek::FourthLast => last_day_of_month(year, *month)-21,
                        };
                        let mut date = new_date(year, *month, day)?;
                        while date.weekday() != *weekday {
                            date = match nth {
                                NthWeek::Last 
                                | NthWeek::SecondLast
                                | NthWeek::ThirdLast
                                | NthWeek::FourthLast => date.previous_day().ok_or(CalendarError::MinDay)?,
                                _ => date.next_day().ok_or(CalendarError::MaxDay)?,
                            }
                        }
                        holidays.insert(date);
                    }
                }
            }
        }
        Ok(Calendar { holidays, weekdays })
    }

    /// Calculate the next business day
    pub fn next_bday(&self, date: Date) -> Result<Date> {
        let mut date = date.next_day().ok_or(CalendarError::MaxDay)?;
        while !self.is_business_day(date) {
            date = date.next_day().ok_or(CalendarError::MaxDay)?;
        }
        Ok(date)
    }

    /// Calculate the previous business day
    pub fn prev_bday(&self, date: Date) -> Result<Date> {
        let mut date = date.previous_day().ok_or(CalendarError::MinDay)?;
        while !self.is_business_day(date) {
            date = date.previous_day().ok_or(CalendarError::MinDay)?;
        }
        Ok(date)
    }

    fn calc_first_and_last(
        start: i32,
        end: i32,
        first: &Option<i32>,
        last: &Option<i32>,
    ) -> (i32, i32) {
        let first = match first {
            Some(year) => std::cmp::max(start, *year),
            _ => start,
        };
        let last = match last {
            Some(year) => std::cmp::min(end, *year),
            _ => end,
        };
        (first, last)
    }

    /// Returns true if the date falls on a weekend
    pub fn is_weekend(&self, day: Date) -> bool {
        let weekday = day.weekday();
        for w_day in &self.weekdays {
            if weekday == *w_day {
                return true;
            }
        }
        false
    }

    /// Returns true if the specified day is a bank holiday
    pub fn is_holiday(&self, date: Date) -> bool {
        self.holidays.get(&date).is_some()
    }

    /// Returns true if the specified day is a business day
    pub fn is_business_day(&self, date: Date) -> bool {
        !self.is_weekend(date) && !self.is_holiday(date)
    }
}

pub trait CalendarProvider {
    fn get_calendar(&self, calendar_name: &str) -> Result<&Calendar>;
}

/// Returns true if the specified year is a leap year (i.e. Feb 29th exists for this year)
pub fn is_leap_year(year: i32) -> bool {
    new_date(year, 2, 29).is_ok()
}

/// Calculate the last day of a given month in a given year
pub fn last_day_of_month(year: i32, month: u8) -> u8 {
    if let Ok(date) = new_date(year, month + 1, 1) {
        date.previous_day().unwrap().day()
    } else {
        // last day of December
        31
    }
}

pub struct SimpleCalendar {
    cal: Calendar,
}

impl SimpleCalendar {
    pub fn new(cal: &Calendar) -> SimpleCalendar {
        SimpleCalendar { cal: cal.clone() }
    }
}

impl CalendarProvider for SimpleCalendar {
    fn get_calendar(&self, _calendar_name: &str) -> Result<&Calendar> {
        Ok(&self.cal)
    }
}

impl Default for SimpleCalendar {
    fn default() -> SimpleCalendar {
        SimpleCalendar {
            cal: Calendar::calc_calendar(&[], 2020, 2021).unwrap(),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use time::macros::date;

    #[test]
    fn fixed_dates_calendar() {
        let holidays = vec![
            Holiday::SingularDay(date!(2019-11-20)),
            Holiday::SingularDay(date!(2019-11-24)),
            Holiday::SingularDay(date!(2019-11-25)),
            Holiday::WeekDay(Weekday::Saturday),
            Holiday::WeekDay(Weekday::Sunday),
        ];
        let cal = Calendar::calc_calendar(&holidays, 2019, 2019).unwrap();

        assert_eq!(
            false,
            cal.is_business_day(date!(2019-11-20))
        );
        assert_eq!(true, cal.is_business_day(date!(2019-11-21)));
        assert_eq!(true, cal.is_business_day(date!(2019-11-22)));
        // weekend
        assert_eq!(
            false,
            cal.is_business_day(date!(2019-11-23))
        );
        assert_eq!(true, cal.is_weekend(date!(2019-11-23)));
        assert_eq!(false, cal.is_holiday(date!(2019-11-23)));
        // weekend and holiday
        assert_eq!(
            false,
            cal.is_business_day(date!(2019-11-24))
        );
        assert_eq!(true, cal.is_weekend(date!(2019-11-24)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-24)));
        assert_eq!(
            false,
            cal.is_business_day(date!(2019-11-25))
        );
        assert_eq!(true, cal.is_business_day(date!(2019-11-26)));
    }

    #[test]
    fn test_yearly_day() {
        let holidays = vec![
            Holiday::YearlyDay {
                month: 11,
                day: 1,
                first: None,
                last: None,
            },
            Holiday::YearlyDay {
                month: 11,
                day: 2,
                first: Some(2019),
                last: None,
            },
            Holiday::YearlyDay {
                month: 11,
                day: 3,
                first: None,
                last: Some(2019),
            },
            Holiday::YearlyDay {
                month: 11,
                day: 4,
                first: Some(2019),
                last: Some(2019),
            },
        ];
        let cal = Calendar::calc_calendar(&holidays, 2018, 2020).unwrap();

        assert_eq!(true, cal.is_holiday(date!(2018-11-1)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-1)));
        assert_eq!(true, cal.is_holiday(date!(2020-11-1)));

        assert_eq!(false, cal.is_holiday(date!(2018-11-2)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-2)));
        assert_eq!(true, cal.is_holiday(date!(2020-11-2)));

        assert_eq!(true, cal.is_holiday(date!(2018-11-3)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-3)));
        assert_eq!(false, cal.is_holiday(date!(2020-11-3)));

        assert_eq!(false, cal.is_holiday(date!(2018-11-4)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-4)));
        assert_eq!(false, cal.is_holiday(date!(2020-11-4)));
    }

    #[test]
    fn test_movable_yearly_day() {
        let holidays = vec![
            Holiday::MovableYearlyDay {
                month: 11,
                day: 1,
                first: None,
                last: None,
            },
            Holiday::MovableYearlyDay {
                month: 11,
                day: 2,
                first: None,
                last: None,
            },
            Holiday::MovableYearlyDay {
                month: 11,
                day: 10,
                first: None,
                last: Some(2019),
            },
            Holiday::MovableYearlyDay {
                month: 11,
                day: 17,
                first: Some(2019),
                last: None,
            },
            Holiday::MovableYearlyDay {
                month: 11,
                day: 24,
                first: Some(2019),
                last: Some(2019),
            },
        ];
        let cal = Calendar::calc_calendar(&holidays, 2018, 2020).unwrap();
        assert_eq!(true, cal.is_holiday(date!(2018-11-1)));
        assert_eq!(true, cal.is_holiday(date!(2018-11-2)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-1)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-4)));
        assert_eq!(true, cal.is_holiday(date!(2020-11-2)));
        assert_eq!(true, cal.is_holiday(date!(2020-11-3)));

        assert_eq!(true, cal.is_holiday(date!(2018-11-12)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-11)));
        assert_eq!(false, cal.is_holiday(date!(2020-11-10)));
        assert_eq!(false, cal.is_holiday(date!(2018-11-19)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-18)));
        assert_eq!(true, cal.is_holiday(date!(2020-11-17)));
        assert_eq!(false, cal.is_holiday(date!(2018-11-26)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-25)));
        assert_eq!(false, cal.is_holiday(date!(2020-11-24)));
    }

    #[test]
    /// Good Friday example
    fn test_easter_offset() {
        let holidays = vec![Holiday::EasterOffset {
            offset: -2,
            first: None,
            last: None,
        }];
        let cal = Calendar::calc_calendar(&holidays, 2019, 2020).unwrap();
        assert_eq!(false, cal.is_business_day(date!(2019-4-19)));
        assert_eq!(false, cal.is_business_day(date!(2020-4-10)));
    }

    #[test]
    fn test_month_weekday() {
        let holidays = vec![
            Holiday::MonthWeekday {
                month: 11,
                weekday: Weekday::Monday,
                nth: NthWeek::First,
                first: None,
                last: None,
            },
            Holiday::MonthWeekday {
                month: 11,
                weekday: Weekday::Tuesday,
                nth: NthWeek::Second,
                first: None,
                last: None,
            },
            Holiday::MonthWeekday {
                month: 11,
                weekday: Weekday::Wednesday,
                nth: NthWeek::Third,
                first: None,
                last: None,
            },
            Holiday::MonthWeekday {
                month: 11,
                weekday: Weekday::Thursday,
                nth: NthWeek::Fourth,
                first: None,
                last: None,
            },
            Holiday::MonthWeekday {
                month: 11,
                weekday: Weekday::Friday,
                nth: NthWeek::Last,
                first: None,
                last: None,
            },
            Holiday::MonthWeekday {
                month: 11,
                weekday: Weekday::Saturday,
                nth: NthWeek::First,
                first: None,
                last: Some(2018),
            },
            Holiday::MonthWeekday {
                month: 11,
                weekday: Weekday::Sunday,
                nth: NthWeek::Last,
                first: Some(2020),
                last: None,
            },
        ];
        let cal = Calendar::calc_calendar(&holidays, 2018, 2020).unwrap();
        assert_eq!(true, cal.is_holiday(date!(2019-11-4)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-12)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-20)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-28)));
        assert_eq!(true, cal.is_holiday(date!(2019-11-29)));

        assert_eq!(true, cal.is_holiday(date!(2018-11-3)));
        assert_eq!(false, cal.is_holiday(date!(2019-11-2)));
        assert_eq!(false, cal.is_holiday(date!(2020-11-7)));
        assert_eq!(false, cal.is_holiday(date!(2018-11-25)));
        assert_eq!(false, cal.is_holiday(date!(2019-11-24)));
        assert_eq!(true, cal.is_holiday(date!(2020-11-29)));
    }

    #[test]
    /// Testing serialization and deserialization of holidays definitions
    fn serialize_cal_definition() {
        let holidays = vec![
            Holiday::MonthWeekday {
                month: 11,
                weekday: Weekday::Monday,
                nth: NthWeek::First,
                first: None,
                last: None,
            },
            Holiday::MovableYearlyDay {
                month: 11,
                day: 1,
                first: Some(2016),
                last: None,
            },
            Holiday::YearlyDay {
                month: 11,
                day: 3,
                first: None,
                last: Some(2019),
            },
            Holiday::SingularDay(date!(2019-11-25)),
            Holiday::WeekDay(Weekday::Saturday),
            Holiday::EasterOffset {
                offset: -2,
                first: None,
                last: None,
            },
        ];
        let json = serde_json::to_string_pretty(&holidays).unwrap();
        assert_eq!(
            json,
            r#"[
  {
    "MonthWeekday": {
      "month": 11,
      "weekday": 1,
      "nth": "First",
      "first": null,
      "last": null
    }
  },
  {
    "MovableYearlyDay": {
      "month": 11,
      "day": 1,
      "first": 2016,
      "last": null
    }
  },
  {
    "YearlyDay": {
      "month": 11,
      "day": 3,
      "first": null,
      "last": 2019
    }
  },
  {
    "SingularDay": [
      2019,
      329
    ]
  },
  {
    "WeekDay": 6
  },
  {
    "EasterOffset": {
      "offset": -2,
      "first": null,
      "last": null
    }
  }
]"#
        );
        let holidays2: Vec<Holiday> = serde_json::from_str(&json).unwrap();
        assert_eq!(holidays.len(), holidays2.len());
        for i in 0..holidays.len() {
            assert_eq!(holidays[i], holidays2[i]);
        }
    }

    #[test]
    fn test_modified_movable() {
        let holidays = vec![
            Holiday::ModifiedMovableYearlyDay {
                month: 12,
                day: 25,
                first: None,
                last: None,
            },
            Holiday::ModifiedMovableYearlyDay {
                month: 1,
                day: 1,
                first: None,
                last: None,
            },
        ];
        let cal = Calendar::calc_calendar(&holidays, 2020, 2023).unwrap();
        assert!(cal.is_holiday(date!(2020-12-25)));
        assert!(cal.is_holiday(date!(2021-12-24)));
        assert!(cal.is_holiday(date!(2022-12-26)));
        assert!(cal.is_holiday(date!(2023-12-25)));
        assert!(cal.is_holiday(date!(2020-1-1)));
        assert!(cal.is_holiday(date!(2021-1-1)));
        assert!(cal.is_holiday(date!(2022-1-1)));
        assert!(cal.is_holiday(date!(2023-1-2)));
    }
}