Anki/rs/ankirs/src/sched.rs

use chrono::{DateTime, FixedOffset, Local, TimeZone, Timelike};

pub struct SchedTimingToday {
    /// The number of days that have passed since the collection was created.
    pub days_elapsed: u32,
    /// Timestamp of the next day rollover.
    pub next_day_at: i64,
}

/// Timing information for the current day.
/// - created is the collection creation time
/// - now is the current UTC timestamp
/// - minutes_west is relative to the local timezone (eg UTC+10 hours is -600)
/// - rollover_hour is the hour of the day the rollover happens (eg 4 for 4am)
pub fn sched_timing_today(
    created: i64,
    now: i64,
    minutes_west: i32,
    rollover_hour: i8,
) -> SchedTimingToday {
    let rollover_today = rollover_for_today(now, minutes_west, rollover_hour).timestamp();

    SchedTimingToday {
        days_elapsed: days_elapsed(created, now, rollover_today),
        next_day_at: rollover_today + 86_400,
    }
}

/// Convert timestamp to the local timezone, with the provided rollover hour.
fn rollover_for_today(
    timestamp: i64,
    minutes_west: i32,
    rollover_hour: i8,
) -> DateTime<FixedOffset> {
    let local_offset = fixed_offset_from_minutes(minutes_west);
    let rollover_hour = normalized_rollover_hour(rollover_hour);
    local_offset
        .timestamp(timestamp, 0)
        .with_hour(rollover_hour as u32)
        .unwrap()
}

/// The number of times the day rolled over between two timestamps.
fn days_elapsed(start: i64, end: i64, rollover_today: i64) -> u32 {
    // get the number of full days that have elapsed
    let secs = (end - start).max(0);
    let days = (secs / 86_400) as u32;

    // minus one if today's cutoff hasn't passed
    if days > 0 && end < rollover_today {
        days - 1
    } else {
        days
    }
}

/// Negative rollover hours are relative to the next day, eg -1 = 23.
/// Cap hour to 23.
fn normalized_rollover_hour(hour: i8) -> u8 {
    let capped_hour = hour.max(-23).min(23);
    if capped_hour < 0 {
        (24 + capped_hour) as u8
    } else {
        capped_hour as u8
    }
}

/// Build a FixedOffset struct, capping minutes_west if out of bounds.
fn fixed_offset_from_minutes(minutes_west: i32) -> FixedOffset {
    let bounded_minutes = minutes_west.max(-23 * 60).min(23 * 60);
    FixedOffset::west(bounded_minutes * 60)
}

/// Relative to the local timezone, the number of minutes UTC differs by.
/// eg, Australia at +10 hours is -600.
/// Includes the daylight savings offset if applicable.
#[allow(dead_code)]
fn utc_minus_local_mins() -> i32 {
    Local::now().offset().utc_minus_local() / 60
}

#[cfg(test)]
mod test {
    use crate::sched::{
        fixed_offset_from_minutes, normalized_rollover_hour, sched_timing_today,
        utc_minus_local_mins,
    };
    use chrono::{FixedOffset, TimeZone};

    #[test]
    fn test_rollover() {
        assert_eq!(normalized_rollover_hour(4), 4);
        assert_eq!(normalized_rollover_hour(23), 23);
        assert_eq!(normalized_rollover_hour(24), 23);
        assert_eq!(normalized_rollover_hour(-1), 23);
        assert_eq!(normalized_rollover_hour(-2), 22);
        assert_eq!(normalized_rollover_hour(-23), 1);
        assert_eq!(normalized_rollover_hour(-24), 1);
    }

    #[test]
    fn test_fixed_offset() {
        let offset = fixed_offset_from_minutes(-600);
        assert_eq!(offset.utc_minus_local(), -600 * 60);
    }

    // helper
    fn elap(start: i64, end: i64, west: i32, rollhour: i8) -> u32 {
        let today = sched_timing_today(start, end, west, rollhour);
        today.days_elapsed
    }

    #[test]
    fn test_days_elapsed() {
        let offset = utc_minus_local_mins();

        let created_dt = FixedOffset::west(offset * 60)
            .ymd(2019, 12, 1)
            .and_hms(2, 0, 0);
        let crt = created_dt.timestamp();

        // days can't be negative
        assert_eq!(elap(crt, crt, offset, 4), 0);
        assert_eq!(elap(crt, crt - 86_400, offset, 4), 0);

        // 2am the next day is still the same day
        assert_eq!(elap(crt, crt + 24 * 3600, offset, 4), 0);

        // day rolls over at 4am
        assert_eq!(elap(crt, crt + 26 * 3600, offset, 4), 1);

        // the longest extra delay is +23, or 19 hours past the 4 hour default
        assert_eq!(elap(crt, crt + (26 + 18) * 3600, offset, 23), 0);
        assert_eq!(elap(crt, crt + (26 + 19) * 3600, offset, 23), 1);

        // a collection created @ midnight in MDT in the past
        let mdt = FixedOffset::west(6 * 60 * 60);
        let mst = FixedOffset::west(7 * 60 * 60);
        let crt = mdt.ymd(2018, 8, 6).and_hms(0, 0, 0).timestamp();
        // with the current time being MST
        let now = mst.ymd(2019, 12, 26).and_hms(20, 0, 0).timestamp();
        let offset = mst.utc_minus_local() / 60;
        assert_eq!(elap(crt, now, offset, 4), 507);
        // the previous implementation generated a diferent elapsed number of days with a change
        // to DST, but the number shouldn't change
        let offset = mdt.utc_minus_local() / 60;
        assert_eq!(elap(crt, now, offset, 4), 507);
    }
}