//! # DateSeries and BDateSeries Implementations
//!
//! This module provides two date-handling types using the [`chrono`](https://docs.rs/chrono) crate:
//!
//! - [`DateSeries`]: Stores any set of calendar dates and allows adding/subtracting *calendar days*.
//! - [`BDateSeries`]: Stores only Monday–Friday business days and interprets add/sub as *business day* shifts,
//!   skipping weekends (e.g., adding 1 to Friday goes to Monday).
//!
//! Both types also provide a [`from_iso8601_range`](#method.from_iso8601_range) constructor
//! that builds a date series (or business‑date series) from a start/end string (YYYY‑MM‑DD).

use chrono::{Datelike, Duration, NaiveDate, ParseResult};
use std::ops::{Add, Sub};

/// Determines if the date is Saturday or Sunday.
fn is_weekend(date: NaiveDate) -> bool {
    matches!(date.weekday(), chrono::Weekday::Sat | chrono::Weekday::Sun)
}

/// A `DateSeries` stores a list of [`NaiveDate`] values and shifts by **calendar days**.
///
/// ## Example Usage
///
/// ```
/// use chrono::NaiveDate;
/// use msyrs::core::dateseries::DateSeries;
///
/// // Create from explicit dates
/// let ds = DateSeries::new(vec![
///     NaiveDate::from_ymd_opt(2023, 7, 14).unwrap(), // a Friday
///     NaiveDate::from_ymd_opt(2023, 7, 15).unwrap(), // a Saturday
/// ]);
///
/// // Shift forward by 5 calendar days
/// let ds_plus = ds + 5;
/// // 2023-07-14 + 5 => 2023-07-19 (Wednesday)
/// // 2023-07-15 + 5 => 2023-07-20 (Thursday)
///
/// assert_eq!(ds_plus.data()[0], NaiveDate::from_ymd_opt(2023, 7, 19).unwrap());
/// assert_eq!(ds_plus.data()[1], NaiveDate::from_ymd_opt(2023, 7, 20).unwrap());
/// ```
///
#[derive(Debug, Clone)]
pub struct DateSeries {
    data: Vec<NaiveDate>,
}

impl DateSeries {
    /// Creates a new `DateSeries` from a vector of [`NaiveDate`] values.
    ///
    /// # Panics
    /// - Does not panic on invalid weekend or anything; this type accepts all valid dates.
    pub fn new(data: Vec<NaiveDate>) -> Self {
        Self { data }
    }

    /// Constructs a `DateSeries` by parsing an ISO‑8601 start/end string (YYYY‑MM‑DD)
    /// and including **every calendar date** from start to end (inclusive).
    ///
    /// # Errors
    /// - Returns a [`chrono::ParseError`](chrono::ParseError) if parsing fails.
    /// - Panics if `start` > `end` chronologically.
    ///
    /// # Examples
    ///
    /// ```
    /// use msyrs::core::dateseries::DateSeries;
    /// # fn main() -> Result<(), chrono::ParseError> {
    /// let ds = DateSeries::from_iso8601_range("2023-07-14", "2023-07-16")?;
    /// assert_eq!(ds.data().len(), 3);
    /// # Ok(())
    /// # }
    /// ```
    pub fn from_iso8601_range(start: &str, end: &str) -> ParseResult<Self> {
        let start_date = NaiveDate::parse_from_str(start, "%Y-%m-%d")?;
        let end_date = NaiveDate::parse_from_str(end, "%Y-%m-%d")?;
        assert!(
            start_date <= end_date,
            "start date cannot be after end date"
        );

        let mut dates = Vec::new();
        let mut current = start_date;
        while current <= end_date {
            dates.push(current);
            current = current
                .checked_add_signed(Duration::days(1))
                .expect("Date overflow in from_iso8601_range");
        }
        Ok(Self::new(dates))
    }

    /// Returns a reference to the underlying slice of dates.
    pub fn data(&self) -> &[NaiveDate] {
        &self.data
    }

    /// Internal helper applying a function to each date.
    fn apply<F>(&self, op: F) -> Self
    where
        F: Fn(NaiveDate) -> NaiveDate,
    {
        let new_data = self.data.iter().map(|&date| op(date)).collect();
        Self { data: new_data }
    }
}

/// Implements adding calendar days to each `NaiveDate`.
///
/// If the shifted date goes out of chrono's valid range, it panics.
impl Add<i64> for DateSeries {
    type Output = Self;

    fn add(self, rhs: i64) -> Self::Output {
        self.apply(|date| {
            date.checked_add_signed(Duration::days(rhs))
                .expect("Overflow in date addition")
        })
    }
}

/// Implements subtracting calendar days from each `NaiveDate`.
///
/// If the shifted date goes out of chrono's valid range, it panics.
impl Sub<i64> for DateSeries {
    type Output = Self;

    fn sub(self, rhs: i64) -> Self::Output {
        self.apply(|date| {
            date.checked_sub_signed(Duration::days(rhs))
                .expect("Overflow in date subtraction")
        })
    }
}

/// A “Business Date Series” for Monday–Friday only.
///
/// 1. The constructor disallows weekend dates (panics if any date is Sat/Sun).  
/// 2. Adding or subtracting an `i64` interprets that integer as *business days*, skipping weekends.
///    For example, adding 1 to a Friday yields the following Monday.
///
/// ## Example Usage
///
/// ```
/// use chrono::NaiveDate;
/// use msyrs::core::dateseries::BDateSeries;
///
/// // Friday
/// let friday = NaiveDate::from_ymd_opt(2023, 7, 14).unwrap();
/// let mut bds = BDateSeries::new(vec![friday]);
///
/// // Adding 1 “business day” => next Monday, 2023-07-17
/// bds = bds + 1;
/// assert_eq!(bds.data()[0], NaiveDate::from_ymd_opt(2023, 7, 17).unwrap());
/// ```
#[derive(Debug, Clone)]
pub struct BDateSeries {
    data: Vec<NaiveDate>,
}

impl BDateSeries {
    /// Creates a new `BDateSeries`, panicking if any of the supplied dates is on Saturday/Sunday.
    pub fn new(data: Vec<NaiveDate>) -> Self {
        for &d in &data {
            if is_weekend(d) {
                panic!("BDateSeries cannot contain weekend dates: {}", d);
            }
        }
        Self { data }
    }

    /// Constructs a `BDateSeries` by parsing an ISO‑8601 start/end string (YYYY‑MM‑DD).
    ///
    /// Only Monday–Friday dates within `[start, end]` are included in the series.  
    ///
    /// # Errors
    /// - Returns a [`chrono::ParseError`](chrono::ParseError) if parsing fails.
    /// - Panics if `start` > `end` chronologically.
    ///
    /// # Examples
    ///
    /// ```
    /// use msyrs::core::dateseries::BDateSeries;
    /// # fn main() -> Result<(), chrono::ParseError> {
    /// let bds = BDateSeries::from_iso8601_range("2023-07-14", "2023-07-18")?;
    /// // 2023-07-14 (Friday), 2023-07-15 (Saturday) => skipped,
    /// // 2023-07-16 (Sunday) => skipped,
    /// // 2023-07-17 (Monday), 2023-07-18 (Tuesday)
    /// // so total 3 valid business days
    /// assert_eq!(bds.data().len(), 3);
    /// # Ok(())
    /// # }
    /// ```
    pub fn from_iso8601_range(start: &str, end: &str) -> ParseResult<Self> {
        let start_date = NaiveDate::parse_from_str(start, "%Y-%m-%d")?;
        let end_date = NaiveDate::parse_from_str(end, "%Y-%m-%d")?;
        assert!(
            start_date <= end_date,
            "start date cannot be after end date"
        );

        let mut dates = Vec::new();
        let mut current = start_date;
        while current <= end_date {
            if !is_weekend(current) {
                dates.push(current);
            }
            current = current
                .checked_add_signed(Duration::days(1))
                .expect("Date overflow in from_iso8601_range");
        }
        Ok(Self::new(dates))
    }

    /// Returns a reference to the underlying slice of dates.
    pub fn data(&self) -> &[NaiveDate] {
        &self.data
    }

    /// Internal helper that tries to shift a date forward or backward by one day at a time,
    /// skipping weekends, for a total of `delta` business days.
    fn shift_business_days(date: NaiveDate, delta: i64) -> NaiveDate {
        if delta == 0 {
            return date;
        }

        let step = if delta > 0 { 1 } else { -1 };
        let abs_delta = delta.abs();

        let mut new_date = date;
        for _ in 0..abs_delta {
            // Move by 1 day in the correct direction
            new_date = new_date
                .checked_add_signed(Duration::days(step))
                .expect("Overflow in BDateSeries add/sub");
            // If we land on weekend, keep moving until Monday..Friday
            while is_weekend(new_date) {
                new_date = new_date
                    .checked_add_signed(Duration::days(step))
                    .expect("Overflow in BDateSeries skipping weekend");
            }
        }
        new_date
    }

    /// Internal helper to apply a shift of `delta` business days to each date.
    fn apply(&self, delta: i64) -> Self {
        let new_data = self
            .data
            .iter()
            .map(|&date| Self::shift_business_days(date, delta))
            .collect();
        Self { data: new_data }
    }
}

/// Implement *business day* addition for `BDateSeries`.
///
/// # Panics
/// - If the resulting date(s) overflow `NaiveDate` range.
/// - `BDateSeries` is guaranteed to remain Monday..Friday after the shift.
impl Add<i64> for BDateSeries {
    type Output = Self;

    fn add(self, rhs: i64) -> Self::Output {
        self.apply(rhs)
    }
}

/// Implement *business day* subtraction for `BDateSeries`.
///
/// # Panics
/// - If the resulting date(s) overflow `NaiveDate`.
/// - `BDateSeries` is guaranteed to remain Monday..Friday after the shift.
impl Sub<i64> for BDateSeries {
    type Output = Self;

    fn sub(self, rhs: i64) -> Self::Output {
        self.apply(-rhs)
    }
}
