tiny-counter 0.1.0

use chrono::{DateTime, Duration, Utc};
#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

use crate::{Error, Result};

/// Time unit definitions for interval configuration.
///
/// Represents different time granularities for event counting.
///
/// # Calendar-Aligned Buckets vs Uniform Buckets
///
/// **Default (calendar-aligned):**
/// - Buckets snap to calendar boundaries in local time
/// - `Days` rotate at local midnight (12:00 AM)
/// - `Weeks` rotate Monday at midnight
/// - `Months` rotate on the 1st
/// - `Years` rotate January 1st
///
/// **Uniform buckets** (disable `calendar` feature):
/// - Buckets align to January 1st of the current year at 00:00 UTC
/// - `Days` = 24-hour periods from that point
/// - `Weeks` = 7-day periods
/// - `Months` = 30-day periods (not calendar months)
/// - `Years` = 365-day periods (ignores leap days)
/// - All counters of the same time unit have aligned start times
///
/// # Why Calendar-Aligned by Default?
///
/// Best for client-side and user-facing use cases:
/// - Aligns to how users talk: "today", "this week", "this month"
/// - Day boundaries at local midnight (not arbitrary 24-hour windows)
/// - Matches how users think: "I used the app 3 times today"
/// - Works with daily/weekly/monthly goals and limits
///
/// Use uniform buckets (disable `calendar` feature) when you need:
/// - Consistent bucket sizes for statistical analysis
/// - Predictable memory usage (30 days ≈ 1 month)
/// - Industry-standard "30-day rolling window" (backend analytics)
/// - Better year approximation: 12 × 30 = 360 days (1.4% error vs 365)
///
/// TimeUnits are ordered from smallest to largest:
/// Minutes < Hours < Days < Weeks < Months < Years < Ever
///
/// `Ever` is a special variant that represents "use the longest configured
/// time unit for this event." It is resolved lazily during query execution.
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub enum TimeUnit {
    Seconds,
    Minutes,
    Hours,
    Days,   // Calendar days at local midnight (or 24-hour periods without `calendar`)
    Weeks,  // Calendar weeks on Monday (or 7-day periods without `calendar`)
    Months, // Calendar months on 1st (or 30-day periods without `calendar`)
    Years,  // Calendar years on Jan 1 (or 365-day periods without `calendar`)
    Ever,   // Special: resolved to longest configured unit during query
}

/// Represents a time window for rate limiting constraints.
///
/// This type enables flexible expression of time windows in rate limits:
/// - `TimeUnit` - defaults to 1 unit (e.g., `TimeUnit::Days` means 1 day)
/// - `(usize, TimeUnit)` - explicit count (e.g., `(7, TimeUnit::Days)` means 7 days)
/// - `Duration` - converted to best matching TimeUnit
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct TimeWindow {
    pub count: usize,
    pub time_unit: TimeUnit,
}

impl From<TimeUnit> for TimeWindow {
    fn from(unit: TimeUnit) -> Self {
        TimeWindow {
            count: 1,
            time_unit: unit,
        }
    }
}

impl From<(usize, TimeUnit)> for TimeWindow {
    fn from(window: (usize, TimeUnit)) -> Self {
        TimeWindow {
            count: window.0,
            time_unit: window.1,
        }
    }
}

impl From<Duration> for TimeWindow {
    fn from(dur: Duration) -> Self {
        // Handle negative durations by taking absolute value
        let dur = dur.abs();

        // Pick best matching unit based on duration
        if dur.num_days() > 0 {
            TimeWindow {
                count: dur.num_days() as usize,
                time_unit: TimeUnit::Days,
            }
        } else if dur.num_hours() > 0 {
            TimeWindow {
                count: dur.num_hours() as usize,
                time_unit: TimeUnit::Hours,
            }
        } else if dur.num_minutes() > 0 {
            TimeWindow {
                count: dur.num_minutes() as usize,
                time_unit: TimeUnit::Minutes,
            }
        } else {
            TimeWindow {
                count: dur.num_seconds() as usize,
                time_unit: TimeUnit::Minutes,
            }
        }
    }
}

/// Helper functions for calendar-aligned time unit operations.
#[cfg(feature = "calendar")]
mod calendar {
    use chrono::{DateTime, Datelike, Local, NaiveDate, TimeZone, Utc};

    /// Convert a local date to UTC at midnight.
    pub(super) fn local_midnight_to_utc(date: NaiveDate) -> DateTime<Utc> {
        Local
            .from_local_datetime(&date.and_hms_opt(0, 0, 0).unwrap())
            .earliest()
            .unwrap()
            .with_timezone(&Utc)
    }

    /// Convert a local date and time to UTC.
    pub(super) fn local_time_to_utc(date: NaiveDate, hour: u32) -> DateTime<Utc> {
        Local
            .from_local_datetime(&date.and_hms_opt(hour, 0, 0).unwrap())
            .earliest()
            .unwrap()
            .with_timezone(&Utc)
    }

    /// Find Monday of the week containing the given date.
    pub(super) fn find_monday(date: NaiveDate) -> NaiveDate {
        let days_since_monday = date.weekday().num_days_from_monday();
        date - chrono::Days::new(days_since_monday as u64)
    }

    /// Add months to a (year, month) pair, handling overflow/underflow.
    ///
    /// Returns (final_year, final_month) where month is 1-based (1-12).
    pub(super) fn add_months(year: i32, month: u32, delta: i32) -> (i32, u32) {
        let total_month = month as i32 + delta;
        // Convert to 0-based months (0-11) for proper modulo arithmetic
        let month0 = total_month - 1;
        let year_offset = month0.div_euclid(12);
        let final_year = year + year_offset;
        let final_month = (month0.rem_euclid(12) + 1) as u32;
        (final_year, final_month)
    }
}

impl TimeUnit {
    /// Returns the duration represented by this time unit.
    ///
    /// Note: Months use a 30-day approximation, Years use 365 days.
    ///
    /// # Panics
    ///
    /// Panics if called on `TimeUnit::Ever`, which has no fixed duration.
    /// `Ever` must be resolved to a concrete time unit before calling this method.
    pub fn duration(&self) -> Duration {
        match self {
            Self::Seconds => Duration::seconds(1),
            Self::Minutes => Duration::minutes(1),
            Self::Hours => Duration::hours(1),
            Self::Days => Duration::days(1),
            Self::Weeks => Duration::weeks(1),
            Self::Months => Duration::days(30),
            Self::Years => Duration::days(365),
            Self::Ever => {
                panic!("TimeUnit::Ever has no fixed duration. Resolve to concrete time unit first.")
            }
        }
    }

    /// Counts bucket boundaries crossed between two times.
    ///
    /// Counts boundary crossings, not complete periods elapsed:
    /// - Days: midnight boundaries in local timezone
    /// - Weeks: Monday boundaries (ISO weeks)
    /// - Months: first-of-month boundaries
    ///
    /// Each boundary crossing rotates the ring buffer once.
    ///
    /// Returns:
    /// - Positive: `now` is after `then` (past events)
    /// - Zero: `now` and `then` are in the same bucket
    /// - Negative: `now` is before `then` (future events)
    pub(crate) fn num_rotations(&self, then: DateTime<Utc>, now: DateTime<Utc>) -> i64 {
        #[cfg(feature = "calendar")]
        {
            use chrono::{Datelike, Local};

            match self {
                TimeUnit::Days => {
                    let then_local = then.with_timezone(&Local).date_naive();
                    let now_local = now.with_timezone(&Local).date_naive();
                    return (now_local - then_local).num_days();
                }
                TimeUnit::Weeks => {
                    let then_local = then.with_timezone(&Local).date_naive();
                    let now_local = now.with_timezone(&Local).date_naive();
                    let week_diff =
                        now_local.iso_week().week() as i64 - then_local.iso_week().week() as i64;
                    // Use iso_week().year() not year() to handle weeks at year boundaries correctly
                    // e.g. Dec 31, 2024 is in ISO Week 1 of 2025
                    let year_diff = (now_local.iso_week().year() as i64
                        - then_local.iso_week().year() as i64)
                        * 52;
                    return week_diff + year_diff;
                }
                TimeUnit::Months => {
                    let then_local = then.with_timezone(&Local);
                    let now_local = now.with_timezone(&Local);
                    let then_months = then_local.year() as i64 * 12 + then_local.month() as i64;
                    let now_months = now_local.year() as i64 * 12 + now_local.month() as i64;
                    return now_months - then_months;
                }
                TimeUnit::Years => {
                    let then_local = then.with_timezone(&Local);
                    let now_local = now.with_timezone(&Local);
                    return (now_local.year() - then_local.year()) as i64;
                }
                _ => {} // Fall through to fixed-duration logic
            }
        }

        // Fixed-duration units (Seconds, Minutes, Hours) use simple arithmetic
        let duration = now.signed_duration_since(then);
        let unit_duration = self.duration();
        duration.num_seconds() / unit_duration.num_seconds()
    }

    pub(crate) fn rotate_start_interval(
        &self,
        interval_start: DateTime<Utc>,
        rotations: i64,
    ) -> DateTime<Utc> {
        #[cfg(feature = "calendar")]
        {
            use chrono::{Datelike, Local, TimeZone};

            match self {
                TimeUnit::Days => {
                    let local = interval_start.with_timezone(&Local);
                    let target_date = local.date_naive() + chrono::Days::new(rotations as u64);
                    return calendar::local_midnight_to_utc(target_date);
                }
                TimeUnit::Weeks => {
                    let local = interval_start.with_timezone(&Local);
                    let this_monday = calendar::find_monday(local.date_naive());
                    let target_monday = this_monday + chrono::Days::new((rotations * 7) as u64);
                    return calendar::local_midnight_to_utc(target_monday);
                }
                TimeUnit::Months => {
                    let local = interval_start.with_timezone(&Local);
                    let (final_year, final_month) =
                        calendar::add_months(local.year(), local.month(), rotations as i32);
                    return Local
                        .with_ymd_and_hms(final_year, final_month, 1, 0, 0, 0)
                        .earliest()
                        .unwrap()
                        .with_timezone(&Utc);
                }
                TimeUnit::Years => {
                    let local = interval_start.with_timezone(&Local);
                    let target_year = local.year() + rotations as i32;
                    return Local
                        .with_ymd_and_hms(target_year, 1, 1, 0, 0, 0)
                        .earliest()
                        .unwrap()
                        .with_timezone(&Utc);
                }
                _ => {} // Fall through to fixed-duration logic
            }
        }

        // Fixed-duration units (Seconds, Minutes, Hours) use simple arithmetic
        let duration = self.duration();
        interval_start + duration * rotations as i32
    }

    pub(crate) fn bucket_idx(
        &self,
        interval_start: DateTime<Utc>,
        time: DateTime<Utc>,
    ) -> Result<usize> {
        let rotations = self.num_rotations(time, interval_start);

        // Step 2: Bucket selection logic
        if rotations < 0 {
            // Event is in the future
            return Err(Error::FutureEvent);
        }
        Ok(if rotations == 0 {
            // Same interval as starting_instant
            if time >= interval_start {
                0 // Current interval, after instant
            } else {
                1 // Current interval, before instant
            }
        } else {
            // Past intervals - use rotations directly as bucket index
            rotations as usize
        })
    }

    /// Returns the end time of a bucket.
    ///
    /// - bucket_idx=0: returns `now` (current bucket ends at now)
    /// - bucket_idx>0: returns the end time of that historical bucket
    pub(crate) fn bucket_end(&self, now: DateTime<Utc>, bucket_idx: usize) -> DateTime<Utc> {
        if bucket_idx == 0 {
            return now;
        }

        #[cfg(feature = "calendar")]
        {
            use chrono::{Datelike, Local, TimeZone};

            match self {
                TimeUnit::Days => {
                    let now_local = now.with_timezone(&Local);
                    let target_date = now_local.date_naive() - chrono::Days::new(bucket_idx as u64);
                    return calendar::local_midnight_to_utc(target_date);
                }
                TimeUnit::Weeks => {
                    let now_local = now.with_timezone(&Local);
                    let this_monday = calendar::find_monday(now_local.date_naive());
                    let target_monday = this_monday - chrono::Days::new((bucket_idx * 7) as u64);
                    return calendar::local_midnight_to_utc(target_monday);
                }
                TimeUnit::Months => {
                    let now_local = now.with_timezone(&Local);
                    let (final_year, final_month) = calendar::add_months(
                        now_local.year(),
                        now_local.month(),
                        -(bucket_idx as i32),
                    );
                    return Local
                        .with_ymd_and_hms(final_year, final_month, 1, 0, 0, 0)
                        .earliest()
                        .unwrap()
                        .with_timezone(&Utc);
                }
                TimeUnit::Years => {
                    let now_local = now.with_timezone(&Local);
                    let target_year = now_local.year() - bucket_idx as i32;
                    return Local
                        .with_ymd_and_hms(target_year, 1, 1, 0, 0, 0)
                        .earliest()
                        .unwrap()
                        .with_timezone(&Utc);
                }
                _ => {} // Fall through to fixed-duration logic
            }
        }

        // Fixed-duration units (Seconds, Minutes, Hours) use simple arithmetic
        now - (self.duration() * bucket_idx as i32)
    }

    /// Returns the start time of a bucket.
    pub(crate) fn bucket_start(&self, now: DateTime<Utc>, bucket_idx: usize) -> DateTime<Utc> {
        self.bucket_end(now, bucket_idx + 1)
    }

    /// Returns the duration of a bucket.
    #[allow(dead_code)] // Used in calendar feature only
    pub(crate) fn bucket_duration(&self, now: DateTime<Utc>, bucket_idx: usize) -> Duration {
        let bucket_start = self.bucket_start(now, bucket_idx);
        let bucket_end = self.bucket_end(now, bucket_idx);
        bucket_end - bucket_start
    }

    /// Returns the midpoint time of a bucket.
    pub(crate) fn bucket_midway(
        &self,
        clock_now: DateTime<Utc>,
        interval_start: DateTime<Utc>,
        bucket_idx: usize,
    ) -> DateTime<Utc> {
        if bucket_idx == 0 {
            let elapsed = clock_now - interval_start;
            return interval_start + (elapsed / 2);
        }

        #[cfg(feature = "calendar")]
        {
            use chrono::Local;

            match self {
                TimeUnit::Days => {
                    let now_local = clock_now.with_timezone(&Local);
                    let target_date = now_local.date_naive() - chrono::Days::new(bucket_idx as u64);
                    return calendar::local_time_to_utc(target_date, 12);
                }
                TimeUnit::Weeks | TimeUnit::Months | TimeUnit::Years => {
                    let bucket_start = self.bucket_start(clock_now, bucket_idx);
                    let duration = self.bucket_duration(clock_now, bucket_idx);
                    return bucket_start + (duration / 2);
                }
                _ => {} // Fall through to fixed-duration logic
            }
        }

        // Fixed-duration units (Seconds, Minutes, Hours) use simple arithmetic
        let duration = self.duration();
        interval_start - (duration * bucket_idx as i32 + duration / 2)
    }

    /// Returns the earliest moment tracked by all buckets.
    pub(crate) fn first_moment_ever(
        &self,
        now: DateTime<Utc>,
        bucket_count: usize,
    ) -> DateTime<Utc> {
        #[cfg(feature = "calendar")]
        {
            use chrono::{Datelike, Local, TimeZone};

            match self {
                TimeUnit::Days => {
                    let now_local = now.with_timezone(&Local);
                    let target_date =
                        now_local.date_naive() - chrono::Days::new(bucket_count as u64);
                    return calendar::local_midnight_to_utc(target_date);
                }
                TimeUnit::Weeks => {
                    let now_local = now.with_timezone(&Local);
                    let this_monday = calendar::find_monday(now_local.date_naive());
                    let target_monday = this_monday - chrono::Days::new((bucket_count * 7) as u64);
                    return calendar::local_midnight_to_utc(target_monday);
                }
                TimeUnit::Months => {
                    let now_local = now.with_timezone(&Local);
                    let (final_year, final_month) = calendar::add_months(
                        now_local.year(),
                        now_local.month(),
                        -(bucket_count as i32),
                    );
                    return Local
                        .with_ymd_and_hms(final_year, final_month, 1, 0, 0, 0)
                        .earliest()
                        .unwrap()
                        .with_timezone(&Utc);
                }
                TimeUnit::Years => {
                    let now_local = now.with_timezone(&Local);
                    let target_year = now_local.year() - bucket_count as i32;
                    return Local
                        .with_ymd_and_hms(target_year, 1, 1, 0, 0, 0)
                        .earliest()
                        .unwrap()
                        .with_timezone(&Utc);
                }
                _ => {} // Fall through to fixed-duration logic
            }
        }

        // Fixed-duration units (Seconds, Minutes, Hours) use simple arithmetic
        now - (self.duration() * bucket_count as i32)
    }

    /// Returns a representative time within a bucket (used for conversion).
    ///
    /// This is similar to bucket_midway but with different arithmetic for historical buckets.
    pub(crate) fn bucket_time(
        &self,
        clock_now: DateTime<Utc>,
        interval_start: DateTime<Utc>,
        bucket_idx: usize,
    ) -> DateTime<Utc> {
        if bucket_idx == 0 {
            let elapsed = clock_now - interval_start;
            return interval_start + (elapsed / 2);
        }

        #[cfg(feature = "calendar")]
        {
            use chrono::Local;

            match self {
                TimeUnit::Days => {
                    let now_local = clock_now.with_timezone(&Local);
                    let target_date = now_local.date_naive() - chrono::Days::new(bucket_idx as u64);
                    return calendar::local_time_to_utc(target_date, 12);
                }
                TimeUnit::Weeks | TimeUnit::Months | TimeUnit::Years => {
                    let bucket_start = self.bucket_start(clock_now, bucket_idx);
                    let duration = self.bucket_duration(clock_now, bucket_idx);
                    return bucket_start + (duration / 2);
                }
                _ => {} // Fall through to fixed-duration logic
            }
        }

        // Fixed-duration units (Seconds, Minutes, Hours) use simple arithmetic
        let duration = self.duration();
        interval_start - (duration * bucket_idx as i32) + duration / 2
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use chrono::TimeZone;

    #[cfg(feature = "calendar")]
    mod calendar_helpers {
        use super::*;
        use chrono::Local;

        /// Calculate how many calendar days are crossed between two UTC times
        /// in the local timezone.
        pub fn calendar_days_between(then: DateTime<Utc>, now: DateTime<Utc>) -> i64 {
            // Convert to local dates
            let then_local = then.with_timezone(&Local).date_naive();
            let now_local = now.with_timezone(&Local).date_naive();

            // Count calendar day boundaries crossed
            (now_local - then_local).num_days()
        }
    }

    #[test]
    fn test_duration_minutes() {
        assert_eq!(TimeUnit::Minutes.duration(), Duration::minutes(1));
    }

    #[test]
    fn test_duration_hours() {
        assert_eq!(TimeUnit::Hours.duration(), Duration::hours(1));
    }

    #[test]
    fn test_duration_days() {
        // Note: In calendar mode, Days don't have fixed duration (23-25 hours due to DST)
        // but duration() still returns 24 hours for fallback/non-calendar units
        assert_eq!(TimeUnit::Days.duration(), Duration::days(1));
    }

    #[test]
    fn test_duration_weeks() {
        assert_eq!(TimeUnit::Weeks.duration(), Duration::weeks(1));
    }

    #[test]
    fn test_duration_months() {
        // Note: In calendar mode, Months don't have fixed duration (28-31 days)
        // but duration() still returns 30 days for fallback/non-calendar units
        assert_eq!(TimeUnit::Months.duration(), Duration::days(30));
    }

    #[test]
    fn test_duration_years() {
        // Years are approximated as 365 days
        assert_eq!(TimeUnit::Years.duration(), Duration::days(365));
    }

    #[test]
    fn test_num_rotations_same_time() {
        let time = Utc::now();
        assert_eq!(TimeUnit::Days.num_rotations(time, time), 0);
    }

    #[test]
    fn test_num_rotations_past_time() {
        let now = Utc.with_ymd_and_hms(2025, 1, 10, 12, 0, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 5, 12, 0, 0).unwrap();

        // 5 days have passed
        assert_eq!(TimeUnit::Days.num_rotations(then, now), 5);
    }

    #[test]
    fn test_num_rotations_future_time() {
        let now = Utc.with_ymd_and_hms(2025, 1, 5, 12, 0, 0).unwrap();
        let future = Utc.with_ymd_and_hms(2025, 1, 10, 12, 0, 0).unwrap();

        // Event is 5 days in the future (negative)
        assert_eq!(TimeUnit::Days.num_rotations(future, now), -5);
    }

    #[test]
    fn test_num_rotations_hours() {
        let now = Utc.with_ymd_and_hms(2025, 1, 1, 15, 0, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 1, 10, 0, 0).unwrap();

        // 5 hours have passed
        assert_eq!(TimeUnit::Hours.num_rotations(then, now), 5);
    }

    #[test]
    fn test_num_rotations_seconds() {
        let now = Utc.with_ymd_and_hms(2025, 1, 1, 10, 0, 45).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 1, 10, 0, 0).unwrap();

        // 45 seconds have passed
        assert_eq!(TimeUnit::Seconds.num_rotations(then, now), 45);
    }

    #[test]
    fn test_num_rotations_minutes() {
        let now = Utc.with_ymd_and_hms(2025, 1, 1, 10, 30, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 1, 10, 0, 0).unwrap();

        // 30 minutes have passed
        assert_eq!(TimeUnit::Minutes.num_rotations(then, now), 30);
    }

    #[test]
    fn test_num_rotations_weeks() {
        let now = Utc.with_ymd_and_hms(2025, 1, 22, 0, 0, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 1, 0, 0, 0).unwrap();

        // 3 weeks have passed (21 days)
        assert_eq!(TimeUnit::Weeks.num_rotations(then, now), 3);
    }

    #[test]
    #[cfg(not(feature = "calendar"))]
    fn test_num_rotations_months() {
        let now = Utc.with_ymd_and_hms(2025, 4, 1, 0, 0, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 1, 0, 0, 0).unwrap();

        // 90 days = 3 complete 30-day months (fixed-duration mode)
        let days_diff = (now - then).num_days();
        assert_eq!(days_diff, 90);
        assert_eq!(TimeUnit::Months.num_rotations(then, now), 3);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_num_rotations_months_calendar_year_boundary() {
        // Test that months calculation handles year boundaries correctly
        // Dec 31, 2024 → March 1, 2025
        let then = Utc.with_ymd_and_hms(2024, 12, 31, 12, 0, 0).unwrap();
        let now = Utc.with_ymd_and_hms(2025, 3, 1, 12, 0, 0).unwrap();

        let rotations = TimeUnit::Months.num_rotations(then, now);

        // Calculation:
        //   then_months = 2024 * 12 + 12 = 24300
        //   now_months = 2025 * 12 + 3 = 24303
        //   diff = 3 months
        // Counts month number difference: December (12) → March (3) = 3 months
        assert_eq!(
            rotations, 3,
            "Should be 3 months from Dec 31, 2024 to March 1, 2025"
        );
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_num_rotations_months_calendar_multi_year_span() {
        // Test spanning multiple years with same month
        // Dec 15, 2023 → Dec 1, 2025 should be 24 months
        let then = Utc.with_ymd_and_hms(2023, 12, 15, 0, 0, 0).unwrap();
        let now = Utc.with_ymd_and_hms(2025, 12, 1, 0, 0, 0).unwrap();

        let rotations = TimeUnit::Months.num_rotations(then, now);

        // Calculation:
        //   then_months = 2023 * 12 + 12 = 24288
        //   now_months = 2025 * 12 + 12 = 24312
        //   diff = 24 months
        assert_eq!(
            rotations, 24,
            "Should be 24 months from Dec 2023 to Dec 2025"
        );
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_num_rotations_months_calendar() {
        let now = Utc.with_ymd_and_hms(2025, 4, 1, 0, 0, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 1, 0, 0, 0).unwrap();

        // Jan 1 to Apr 1 = 3 calendar months (Jan→Feb→Mar→Apr)
        assert_eq!(TimeUnit::Months.num_rotations(then, now), 3);

        // Test non-boundary dates
        let now = Utc.with_ymd_and_hms(2025, 4, 15, 12, 0, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 15, 12, 0, 0).unwrap();

        // Jan 15 to Apr 15 = still 3 calendar months
        assert_eq!(TimeUnit::Months.num_rotations(then, now), 3);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_num_rotations_weeks_calendar_year_boundary() {
        use chrono::Datelike;

        // Critical edge case: Dec 31, 2024 is in ISO Week 1 of 2025
        // Dec 31, 2024 is a Tuesday
        // The week containing it: Mon Dec 30, 2024 → Sun Jan 5, 2025
        // This week contains the first Thursday of 2025 (Jan 2), so it's ISO Week 1 of 2025
        let then = Utc.with_ymd_and_hms(2024, 12, 31, 12, 0, 0).unwrap();
        let now = Utc.with_ymd_and_hms(2025, 3, 1, 12, 0, 0).unwrap();

        // Verify ISO week properties
        use chrono::Local;
        let then_local = then.with_timezone(&Local).date_naive();
        let now_local = now.with_timezone(&Local).date_naive();

        // Dec 31, 2024: calendar year = 2024, ISO week year = 2025, ISO week = 1
        assert_eq!(
            then_local.iso_week().year(),
            2025,
            "Dec 31, 2024 should be in ISO week year 2025"
        );
        assert_eq!(
            then_local.iso_week().week(),
            1,
            "Dec 31, 2024 should be in ISO week 1"
        );

        // March 1, 2025: ISO week = 9
        assert_eq!(
            now_local.iso_week().week(),
            9,
            "March 1, 2025 should be in ISO week 9"
        );

        let rotations = TimeUnit::Weeks.num_rotations(then, now);

        // Should be 8 weeks (not 60 weeks!)
        // If we incorrectly used calendar year instead of ISO week year:
        //   year_diff = (2025 - 2024) * 52 = 52
        //   week_diff = 9 - 1 = 8
        //   WRONG total = 60 weeks
        //
        // Correct calculation using ISO week year:
        //   year_diff = (2025 - 2025) * 52 = 0
        //   week_diff = 9 - 1 = 8
        //   CORRECT total = 8 weeks
        assert_eq!(
            rotations, 8,
            "Should be 8 ISO weeks from Dec 31, 2024 to March 1, 2025"
        );
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_num_rotations_weeks_calendar_multi_year_span() {
        use chrono::Datelike;

        // Test case that would fail with "anniversary" approach but passes with ISO week year
        // Dec 30, 2024 is a Monday (ISO Week 1 of 2025)
        // Dec 29, 2025 is a Monday (ISO Week 1 of 2026)
        // From Dec 30 to Dec 29 next year = 364 days = 52 weeks
        let then = Utc.with_ymd_and_hms(2024, 12, 30, 0, 0, 0).unwrap();
        let now = Utc.with_ymd_and_hms(2025, 12, 29, 0, 0, 0).unwrap();

        use chrono::Local;
        let then_local = then.with_timezone(&Local).date_naive();
        let now_local = now.with_timezone(&Local).date_naive();

        // Verify ISO week properties
        assert_eq!(
            then_local.iso_week().year(),
            2025,
            "Dec 30, 2024 should be in ISO week year 2025"
        );
        assert_eq!(
            then_local.iso_week().week(),
            1,
            "Dec 30, 2024 should be in ISO week 1"
        );

        assert_eq!(
            now_local.iso_week().year(),
            2026,
            "Dec 29, 2025 should be in ISO week year 2026"
        );
        assert_eq!(
            now_local.iso_week().week(),
            1,
            "Dec 29, 2025 should be in ISO week 1"
        );

        let rotations = TimeUnit::Weeks.num_rotations(then, now);

        // ISO week year approach (CORRECT):
        //   year_diff = (2026 - 2025) * 52 = 52
        //   week_diff = 1 - 1 = 0
        //   total = 52 weeks ✓
        //
        // Alternative "anniversary" approach (WRONG):
        //   year_diff = (2025 - 2024) = 1
        //   Since now (Dec 29) < then (Dec 30), decrement: year_diff = 0
        //   week_diff = 1 - 1 = 0
        //   WRONG total = 0 weeks (should be 52!)
        assert_eq!(
            rotations, 52,
            "Should be 52 ISO weeks from Dec 30, 2024 to Dec 29, 2025"
        );
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_num_rotations_days_calendar() {
        use calendar_helpers::calendar_days_between;

        // Test that Days counts calendar day boundaries (local midnight), not 24-hour periods
        // then: Jan 1, 2025, 23:00 UTC (11 PM)
        // now:  Jan 5, 2025, 01:00 UTC (1 AM)
        // UTC elapsed: 3 days, 2 hours
        let then = Utc.with_ymd_and_hms(2025, 1, 1, 23, 0, 0).unwrap();
        let now = Utc.with_ymd_and_hms(2025, 1, 5, 1, 0, 0).unwrap();

        // Calculate expected calendar days in local timezone
        let expected_days = calendar_days_between(then, now);

        let rotations = TimeUnit::Days.num_rotations(then, now);

        // Should match exact calendar days crossed in local timezone
        assert_eq!(
            rotations, expected_days,
            "Expected {} calendar days (local timezone), got {}",
            expected_days, rotations
        );
    }

    #[test]
    fn test_num_rotations_years() {
        let now = Utc.with_ymd_and_hms(2027, 1, 1, 0, 0, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 1, 0, 0, 0).unwrap();

        // 730 days = 2 years (365 days each)
        assert_eq!(TimeUnit::Years.num_rotations(then, now), 2);
    }

    #[test]
    fn test_num_rotations_partial_unit() {
        let now = Utc.with_ymd_and_hms(2025, 1, 1, 12, 30, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2025, 1, 1, 12, 0, 0).unwrap();

        // 30 minutes = 0 complete hours
        assert_eq!(TimeUnit::Hours.num_rotations(then, now), 0);
    }

    // Property test: num_rotations is transitive
    #[test]
    fn test_num_rotations_transitive() {
        let a = Utc.with_ymd_and_hms(2025, 1, 1, 0, 0, 0).unwrap();
        let b = Utc.with_ymd_and_hms(2025, 1, 5, 0, 0, 0).unwrap();
        let c = Utc.with_ymd_and_hms(2025, 1, 10, 0, 0, 0).unwrap();

        let ab = TimeUnit::Days.num_rotations(a, b);
        let bc = TimeUnit::Days.num_rotations(b, c);
        let ac = TimeUnit::Days.num_rotations(a, c);

        assert_eq!(ab + bc, ac);
    }

    #[test]
    fn test_time_unit_is_copy() {
        let unit = TimeUnit::Days;
        let _unit2 = unit; // Should compile (Copy)
        let _unit3 = unit; // Should still work
    }

    #[test]
    fn test_time_unit_is_eq() {
        assert_eq!(TimeUnit::Days, TimeUnit::Days);
        assert_ne!(TimeUnit::Days, TimeUnit::Hours);
    }

    #[test]
    fn test_time_unit_is_hash() {
        use std::collections::HashSet;
        let mut set = HashSet::new();
        set.insert(TimeUnit::Days);
        assert!(set.contains(&TimeUnit::Days));
    }

    #[test]
    fn test_large_time_jump() {
        let now = Utc.with_ymd_and_hms(2025, 12, 31, 0, 0, 0).unwrap();
        let then = Utc.with_ymd_and_hms(2020, 1, 1, 0, 0, 0).unwrap();

        // Many days have passed
        let rotations = TimeUnit::Days.num_rotations(then, now);
        assert!(rotations > 2000); // At least 5+ years worth of days
    }

    // TimeWindow conversion tests
    #[test]
    fn test_time_window_from_time_unit() {
        let window: TimeWindow = TimeUnit::Days.into();
        assert_eq!(window.count, 1);
        assert_eq!(window.time_unit, TimeUnit::Days);
    }

    #[test]
    fn test_time_window_from_tuple() {
        let window: TimeWindow = (7, TimeUnit::Days).into();
        assert_eq!(window.count, 7);
        assert_eq!(window.time_unit, TimeUnit::Days);
    }

    #[test]
    fn test_time_window_from_duration_days() {
        let window: TimeWindow = Duration::days(7).into();
        assert_eq!(window.count, 7);
        assert_eq!(window.time_unit, TimeUnit::Days);
    }

    #[test]
    fn test_time_window_from_duration_hours() {
        let window: TimeWindow = Duration::hours(12).into();
        assert_eq!(window.count, 12);
        assert_eq!(window.time_unit, TimeUnit::Hours);
    }

    #[test]
    fn test_time_window_from_duration_minutes() {
        let window: TimeWindow = Duration::minutes(30).into();
        assert_eq!(window.count, 30);
        assert_eq!(window.time_unit, TimeUnit::Minutes);
    }

    #[test]
    fn test_time_window_from_duration_seconds() {
        let window: TimeWindow = Duration::seconds(45).into();
        // Seconds are converted to minutes (0 minutes)
        assert_eq!(window.count, 45);
        assert_eq!(window.time_unit, TimeUnit::Minutes);
    }

    // Ord implementation tests
    #[test]
    fn test_time_unit_ord() {
        assert!(TimeUnit::Minutes < TimeUnit::Hours);
        assert!(TimeUnit::Hours < TimeUnit::Days);
        assert!(TimeUnit::Days < TimeUnit::Weeks);
        assert!(TimeUnit::Weeks < TimeUnit::Months);
        assert!(TimeUnit::Months < TimeUnit::Years);
    }

    #[test]
    fn test_time_unit_ord_transitive() {
        assert!(TimeUnit::Minutes < TimeUnit::Days);
        assert!(TimeUnit::Hours < TimeUnit::Weeks);
        assert!(TimeUnit::Days < TimeUnit::Years);
    }

    #[test]
    fn test_time_unit_ord_reflexive() {
        assert!(TimeUnit::Minutes <= TimeUnit::Minutes);
        assert!(TimeUnit::Hours >= TimeUnit::Hours);
    }

    #[test]
    fn test_time_unit_ever_is_largest() {
        assert!(TimeUnit::Years < TimeUnit::Ever);
        assert!(TimeUnit::Minutes < TimeUnit::Ever);
        assert!(TimeUnit::Days < TimeUnit::Ever);
    }

    #[test]
    #[should_panic(expected = "TimeUnit::Ever has no fixed duration")]
    fn test_time_unit_ever_duration_panics() {
        TimeUnit::Ever.duration();
    }

    #[test]
    fn test_time_window_from_negative_duration_days() {
        let window: TimeWindow = Duration::days(-7).into();
        // Should convert absolute value
        assert_eq!(window.count, 7);
        assert_eq!(window.time_unit, TimeUnit::Days);
    }

    #[test]
    fn test_time_window_from_negative_duration_hours() {
        let window: TimeWindow = Duration::hours(-12).into();
        assert_eq!(window.count, 12);
        assert_eq!(window.time_unit, TimeUnit::Hours);
    }

    #[test]
    fn test_time_window_from_negative_duration_minutes() {
        let window: TimeWindow = Duration::minutes(-30).into();
        assert_eq!(window.count, 30);
        assert_eq!(window.time_unit, TimeUnit::Minutes);
    }

    #[test]
    fn test_time_window_from_negative_duration_seconds() {
        let window: TimeWindow = Duration::seconds(-45).into();
        assert_eq!(window.count, 45);
        assert_eq!(window.time_unit, TimeUnit::Minutes);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_end_days_calendar() {
        use chrono::{Local, Timelike};

        let now = Utc.with_ymd_and_hms(2025, 1, 5, 14, 30, 0).unwrap();

        // bucket_idx=0 should return now
        assert_eq!(TimeUnit::Days.bucket_end(now, 0), now);

        // bucket_idx=1 should return local midnight of yesterday
        let bucket_end = TimeUnit::Days.bucket_end(now, 1);
        let local_bucket = bucket_end.with_timezone(&Local);

        // Should be at midnight
        assert_eq!(local_bucket.hour(), 0);
        assert_eq!(local_bucket.minute(), 0);
        assert_eq!(local_bucket.second(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_end_months_calendar() {
        use chrono::{Datelike, Local, Timelike};

        let now = Utc.with_ymd_and_hms(2025, 4, 15, 14, 30, 0).unwrap();

        // bucket_idx=0 should return now
        assert_eq!(TimeUnit::Months.bucket_end(now, 0), now);

        // bucket_idx=1 should return local midnight of 1st of March
        let bucket_end = TimeUnit::Months.bucket_end(now, 1);
        let local_bucket = bucket_end.with_timezone(&Local);

        // Should be March 1st at midnight
        assert_eq!(local_bucket.month(), 3);
        assert_eq!(local_bucket.day(), 1);
        assert_eq!(local_bucket.hour(), 0);
        assert_eq!(local_bucket.minute(), 0);
        assert_eq!(local_bucket.second(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_start_calendar() {
        let now = Utc.with_ymd_and_hms(2025, 4, 15, 14, 30, 0).unwrap();

        // bucket_start should be bucket_end of next bucket
        let bucket_0_start = TimeUnit::Months.bucket_start(now, 0);
        let bucket_1_end = TimeUnit::Months.bucket_end(now, 1);

        assert_eq!(bucket_0_start, bucket_1_end);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_start_months_calendar() {
        use chrono::{Datelike, Local, Timelike};

        let now = Utc.with_ymd_and_hms(2025, 4, 15, 14, 30, 0).unwrap();

        // Bucket 1 start should be bucket 2 end (Feb 1 at midnight)
        let bucket_1_start = TimeUnit::Months.bucket_start(now, 1);
        let local_start = bucket_1_start.with_timezone(&Local);

        // Should be Feb 1 at midnight
        assert_eq!(local_start.month(), 2);
        assert_eq!(local_start.day(), 1);
        assert_eq!(local_start.hour(), 0);
        assert_eq!(local_start.minute(), 0);
        assert_eq!(local_start.second(), 0);

        // Verify relationship: bucket_start(n) == bucket_end(n+1)
        let bucket_2_end = TimeUnit::Months.bucket_end(now, 2);
        assert_eq!(bucket_1_start, bucket_2_end);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_rotate_start_interval_days_calendar() {
        use chrono::{Datelike, Local, Timelike};

        let start = Utc.with_ymd_and_hms(2025, 1, 1, 0, 0, 0).unwrap();

        // Rotate forward 3 days
        let rotated = TimeUnit::Days.rotate_start_interval(start, 3);
        let local_rotated = rotated.with_timezone(&Local);

        // Should be Jan 4 at local midnight
        assert_eq!(local_rotated.day(), 4);
        assert_eq!(local_rotated.hour(), 0);
        assert_eq!(local_rotated.minute(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_rotate_start_interval_months_calendar() {
        use chrono::{Datelike, Local, Timelike};

        let start = Utc.with_ymd_and_hms(2025, 1, 1, 0, 0, 0).unwrap();

        // Rotate forward 3 months
        let rotated = TimeUnit::Months.rotate_start_interval(start, 3);
        let local_rotated = rotated.with_timezone(&Local);

        // Should be April 1 at local midnight
        assert_eq!(local_rotated.month(), 4);
        assert_eq!(local_rotated.day(), 1);
        assert_eq!(local_rotated.hour(), 0);
        assert_eq!(local_rotated.minute(), 0);

        // Test with year boundary
        let start = Utc.with_ymd_and_hms(2025, 11, 1, 0, 0, 0).unwrap();
        let rotated = TimeUnit::Months.rotate_start_interval(start, 3);
        let local_rotated = rotated.with_timezone(&Local);

        // Should be Feb 1, 2026 at local midnight
        assert_eq!(local_rotated.year(), 2026);
        assert_eq!(local_rotated.month(), 2);
        assert_eq!(local_rotated.day(), 1);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_midway_days_calendar() {
        use chrono::{Local, Timelike};

        let now = Utc.with_ymd_and_hms(2025, 1, 5, 14, 30, 0).unwrap();
        let interval_start = Utc.with_ymd_and_hms(2025, 1, 5, 0, 0, 0).unwrap();

        // Bucket 0 (current): midpoint between interval_start and now
        let midway_0 = TimeUnit::Days.bucket_midway(now, interval_start, 0);
        let expected_0 = interval_start + (now - interval_start) / 2;
        assert_eq!(midway_0, expected_0);

        // Bucket 1 (yesterday): should be noon of yesterday
        let midway_1 = TimeUnit::Days.bucket_midway(now, interval_start, 1);
        let local_midway = midway_1.with_timezone(&Local);

        // Should be at noon (12:00)
        assert_eq!(local_midway.hour(), 12);
        assert_eq!(local_midway.minute(), 0);
        assert_eq!(local_midway.second(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_midway_months_calendar() {
        let now = Utc.with_ymd_and_hms(2025, 4, 15, 14, 30, 0).unwrap();
        let interval_start = Utc.with_ymd_and_hms(2025, 4, 1, 0, 0, 0).unwrap();

        // Bucket 1 (last month): midpoint should be in middle of March
        let midway_1 = TimeUnit::Months.bucket_midway(now, interval_start, 1);

        // Get bucket boundaries
        let bucket_start = TimeUnit::Months.bucket_start(now, 1);
        let bucket_end = TimeUnit::Months.bucket_end(now, 1);
        let expected = bucket_start + (bucket_end - bucket_start) / 2;

        assert_eq!(midway_1, expected);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_first_moment_ever_days_calendar() {
        use chrono::{Datelike, Local, Timelike};

        let now = Utc.with_ymd_and_hms(2025, 1, 10, 14, 30, 0).unwrap();

        // 5 buckets means 5 days back
        let first = TimeUnit::Days.first_moment_ever(now, 5);
        let local_first = first.with_timezone(&Local);

        // Should be 5 days ago at local midnight
        let now_local = now.with_timezone(&Local);
        assert_eq!(local_first.day(), now_local.day() - 5);
        assert_eq!(local_first.hour(), 0);
        assert_eq!(local_first.minute(), 0);
        assert_eq!(local_first.second(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_first_moment_ever_months_calendar() {
        use chrono::{Datelike, Local, Timelike};

        let now = Utc.with_ymd_and_hms(2025, 4, 15, 14, 30, 0).unwrap();

        // 3 buckets means 3 months back
        let first = TimeUnit::Months.first_moment_ever(now, 3);
        let local_first = first.with_timezone(&Local);

        // Should be January 1 at local midnight
        assert_eq!(local_first.month(), 1);
        assert_eq!(local_first.day(), 1);
        assert_eq!(local_first.hour(), 0);
        assert_eq!(local_first.minute(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_time_days_calendar() {
        use chrono::{Local, Timelike};

        let now = Utc.with_ymd_and_hms(2025, 1, 5, 14, 30, 0).unwrap();
        let interval_start = Utc.with_ymd_and_hms(2025, 1, 5, 0, 0, 0).unwrap();

        // Bucket 1 (yesterday): representative time should be noon
        let bucket_time_1 = TimeUnit::Days.bucket_time(now, interval_start, 1);
        let local_time = bucket_time_1.with_timezone(&Local);

        // Should be at noon (12:00)
        assert_eq!(local_time.hour(), 12);
        assert_eq!(local_time.minute(), 0);
        assert_eq!(local_time.second(), 0);
    }

    // ========================================================================
    // Comprehensive Week Tests
    // ========================================================================

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_end_weeks_calendar() {
        use chrono::{Datelike, Local, Timelike, Weekday};

        // Jan 15, 2025 is a Wednesday
        let now = Utc.with_ymd_and_hms(2025, 1, 15, 14, 30, 0).unwrap();

        // bucket_idx=0 should return now
        assert_eq!(TimeUnit::Weeks.bucket_end(now, 0), now);

        // bucket_idx=1 should return local midnight of Monday of last week
        let bucket_end = TimeUnit::Weeks.bucket_end(now, 1);
        let local_bucket = bucket_end.with_timezone(&Local);

        // Should be Monday at midnight
        assert_eq!(local_bucket.weekday(), Weekday::Mon);
        assert_eq!(local_bucket.hour(), 0);
        assert_eq!(local_bucket.minute(), 0);
        assert_eq!(local_bucket.second(), 0);

        // Should be Monday Jan 13 minus 7 days = Monday Jan 6
        let now_local = now.with_timezone(&Local);
        let this_monday = calendar::find_monday(now_local.date_naive());
        let expected_monday = this_monday - chrono::Days::new(7);

        assert_eq!(local_bucket.date_naive(), expected_monday);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_midway_weeks_calendar() {
        // Jan 15, 2025 is a Wednesday
        let now = Utc.with_ymd_and_hms(2025, 1, 15, 14, 30, 0).unwrap();
        let interval_start = Utc.with_ymd_and_hms(2025, 1, 13, 0, 0, 0).unwrap(); // Monday

        // Bucket 1 (last week): midpoint should be Thursday noon
        let midway_1 = TimeUnit::Weeks.bucket_midway(now, interval_start, 1);

        // Calculate expected: bucket_start + duration/2
        let bucket_start = TimeUnit::Weeks.bucket_start(now, 1);
        let bucket_end = TimeUnit::Weeks.bucket_end(now, 1);
        let expected = bucket_start + (bucket_end - bucket_start) / 2;

        assert_eq!(
            midway_1, expected,
            "Week midpoint should be bucket_start + duration/2"
        );
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_rotate_start_interval_weeks_calendar() {
        use chrono::{Datelike, Local, Timelike, Weekday};

        // Start at Monday Jan 6, 2025 at midnight
        let start = Utc.with_ymd_and_hms(2025, 1, 6, 0, 0, 0).unwrap();

        // Rotate forward 2 weeks
        let rotated = TimeUnit::Weeks.rotate_start_interval(start, 2);
        let local_rotated = rotated.with_timezone(&Local);

        // Should be Monday Jan 20 at local midnight
        assert_eq!(local_rotated.weekday(), Weekday::Mon);
        assert_eq!(local_rotated.day(), 20);
        assert_eq!(local_rotated.hour(), 0);
        assert_eq!(local_rotated.minute(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_first_moment_ever_weeks_calendar() {
        use chrono::{Datelike, Local, Timelike, Weekday};

        let now = Utc.with_ymd_and_hms(2025, 1, 15, 14, 30, 0).unwrap();

        // 3 buckets means 3 weeks back from this Monday
        let first = TimeUnit::Weeks.first_moment_ever(now, 3);
        let local_first = first.with_timezone(&Local);

        // Should be Monday 3 weeks ago at local midnight
        assert_eq!(local_first.weekday(), Weekday::Mon);
        assert_eq!(local_first.hour(), 0);
        assert_eq!(local_first.minute(), 0);

        // Calculate expected Monday
        let now_local = now.with_timezone(&Local);
        let this_monday = calendar::find_monday(now_local.date_naive());
        let expected_monday = this_monday - chrono::Days::new(3 * 7);

        assert_eq!(local_first.date_naive(), expected_monday);
    }

    // ========================================================================
    // Comprehensive Years Tests
    // ========================================================================

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_end_years_calendar() {
        use chrono::{Datelike, Local, Timelike};

        let now = Utc.with_ymd_and_hms(2025, 4, 15, 14, 30, 0).unwrap();

        // bucket_idx=0 should return now
        assert_eq!(TimeUnit::Years.bucket_end(now, 0), now);

        // bucket_idx=1 should return local midnight of Jan 1, 2024
        let bucket_end = TimeUnit::Years.bucket_end(now, 1);
        let local_bucket = bucket_end.with_timezone(&Local);

        // Should be Jan 1, 2024 at midnight
        assert_eq!(local_bucket.year(), 2024);
        assert_eq!(local_bucket.month(), 1);
        assert_eq!(local_bucket.day(), 1);
        assert_eq!(local_bucket.hour(), 0);
        assert_eq!(local_bucket.minute(), 0);
        assert_eq!(local_bucket.second(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_bucket_midway_years_calendar() {
        let now = Utc.with_ymd_and_hms(2025, 6, 15, 14, 30, 0).unwrap();
        let interval_start = Utc.with_ymd_and_hms(2025, 1, 1, 0, 0, 0).unwrap();

        // Bucket 1 (last year): midpoint should be middle of 2024
        let midway_1 = TimeUnit::Years.bucket_midway(now, interval_start, 1);

        // Calculate expected: bucket_start + duration/2
        let bucket_start = TimeUnit::Years.bucket_start(now, 1);
        let bucket_end = TimeUnit::Years.bucket_end(now, 1);
        let expected = bucket_start + (bucket_end - bucket_start) / 2;

        assert_eq!(
            midway_1, expected,
            "Year midpoint should be bucket_start + duration/2"
        );
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_rotate_start_interval_years_calendar() {
        use chrono::{Datelike, Local, Timelike};

        let start = Utc.with_ymd_and_hms(2023, 1, 1, 0, 0, 0).unwrap();

        // Rotate forward 2 years
        let rotated = TimeUnit::Years.rotate_start_interval(start, 2);
        let local_rotated = rotated.with_timezone(&Local);

        // Should be Jan 1, 2025 at local midnight
        assert_eq!(local_rotated.year(), 2025);
        assert_eq!(local_rotated.month(), 1);
        assert_eq!(local_rotated.day(), 1);
        assert_eq!(local_rotated.hour(), 0);
        assert_eq!(local_rotated.minute(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_first_moment_ever_years_calendar() {
        use chrono::{Datelike, Local, Timelike};

        let now = Utc.with_ymd_and_hms(2025, 6, 15, 14, 30, 0).unwrap();

        // 3 buckets means 3 years back
        let first = TimeUnit::Years.first_moment_ever(now, 3);
        let local_first = first.with_timezone(&Local);

        // Should be January 1, 2022 at local midnight
        assert_eq!(local_first.year(), 2022);
        assert_eq!(local_first.month(), 1);
        assert_eq!(local_first.day(), 1);
        assert_eq!(local_first.hour(), 0);
        assert_eq!(local_first.minute(), 0);
    }

    // ========================================================================
    // Year Rollover Tests
    // ========================================================================

    #[test]
    #[cfg(feature = "calendar")]
    fn test_week_spanning_year_boundary() {
        use chrono::{Datelike, Local, Timelike, Weekday};

        // Dec 30, 2024 is a Monday, Jan 6, 2025 is also a Monday
        let now = Utc.with_ymd_and_hms(2025, 1, 3, 10, 0, 0).unwrap(); // Friday, Jan 3

        // bucket_idx=1 should return last week's Monday (Dec 23, 2024)
        let bucket_end = TimeUnit::Weeks.bucket_end(now, 1);
        let local_bucket = bucket_end.with_timezone(&Local);

        assert_eq!(local_bucket.year(), 2024);
        assert_eq!(local_bucket.month(), 12);
        assert_eq!(local_bucket.day(), 23);
        assert_eq!(local_bucket.weekday(), Weekday::Mon);
        assert_eq!(local_bucket.hour(), 0);
        assert_eq!(local_bucket.minute(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_month_bucket_end_crossing_year_backward() {
        use chrono::{Datelike, Local, Timelike};

        // Start in January 2025
        let now = Utc.with_ymd_and_hms(2025, 1, 15, 10, 0, 0).unwrap();

        // bucket_idx=2 should return Nov 1, 2024 at midnight
        let bucket_end = TimeUnit::Months.bucket_end(now, 2);
        let local_bucket = bucket_end.with_timezone(&Local);

        assert_eq!(local_bucket.year(), 2024);
        assert_eq!(local_bucket.month(), 11);
        assert_eq!(local_bucket.day(), 1);
        assert_eq!(local_bucket.hour(), 0);
        assert_eq!(local_bucket.minute(), 0);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_num_rotations_days_crossing_year() {
        // From Dec 28, 2024 to Jan 5, 2025 (8 calendar days)
        let then = Utc.with_ymd_and_hms(2024, 12, 28, 12, 0, 0).unwrap();
        let now = Utc.with_ymd_and_hms(2025, 1, 5, 12, 0, 0).unwrap();

        let rotations = TimeUnit::Days.num_rotations(then, now);

        // Calculate expected in local timezone
        use chrono::Local;
        let then_local = then.with_timezone(&Local).date_naive();
        let now_local = now.with_timezone(&Local).date_naive();
        let expected = (now_local - then_local).num_days();

        assert_eq!(rotations, expected);
        // Should be 8 days (but verify with actual local calculation)
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_num_rotations_months_crossing_year() {
        // From Oct 2024 to Feb 2025 (4 months)
        let then = Utc.with_ymd_and_hms(2024, 10, 15, 12, 0, 0).unwrap();
        let now = Utc.with_ymd_and_hms(2025, 2, 15, 12, 0, 0).unwrap();

        let rotations = TimeUnit::Months.num_rotations(then, now);

        // Oct → Nov → Dec → Jan → Feb = 4 month boundaries crossed
        assert_eq!(rotations, 4);
    }

    #[test]
    #[cfg(feature = "calendar")]
    fn test_num_rotations_years_multiple() {
        // From 2020 to 2025 (5 years)
        let then = Utc.with_ymd_and_hms(2020, 6, 15, 12, 0, 0).unwrap();
        let now = Utc.with_ymd_and_hms(2025, 6, 15, 12, 0, 0).unwrap();

        let rotations = TimeUnit::Years.num_rotations(then, now);

        // 2020 → 2021 → 2022 → 2023 → 2024 → 2025 = 5 year boundaries
        assert_eq!(rotations, 5);
    }
}