csusage-core 0.0.2

use jiff::{Span, civil::Date as JiffDate};

use crate::{cli::WeekDay, week_start};

/// The calendar unit a report groups rows by, and therefore the unit `--last`
/// counts.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum PeriodUnit {
    Day,
    Week,
    Month,
}

/// Compact `YYYYMMDD` start of the window covering the most recent `count`
/// periods, the last of which is the one `today` falls in.
///
/// `today` is a `YYYY-MM-DD` date already resolved in the report timezone, so
/// the caller decides what "now" means. Weeks start on `start_of_week` to match
/// the bucketing the report itself uses; months always start on the first.
pub fn last_periods_since(
    unit: PeriodUnit,
    count: u32,
    today: &str,
    start_of_week: WeekDay,
) -> Option<String> {
    let earlier = i64::from(count.max(1) - 1);
    let start = match unit {
        PeriodUnit::Day => parse_date(today)?
            .checked_sub(Span::new().days(earlier))
            .ok()?,
        PeriodUnit::Week => parse_date(&week_start(today, start_of_week)?)?
            .checked_sub(Span::new().weeks(earlier))
            .ok()?,
        PeriodUnit::Month => parse_date(today)?
            .first_of_month()
            .checked_sub(Span::new().months(earlier))
            .ok()?,
    };
    Some(format!(
        "{:04}{:02}{:02}",
        start.year(),
        start.month(),
        start.day()
    ))
}

fn parse_date(date: &str) -> Option<JiffDate> {
    date.parse().ok()
}

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

    #[test]
    fn covers_only_today_for_a_single_day() {
        assert_eq!(
            last_periods_since(PeriodUnit::Day, 1, "2026-07-27", WeekDay::Monday).as_deref(),
            Some("20260727")
        );
    }

    #[test]
    fn counts_days_inclusively_across_a_month_boundary() {
        assert_eq!(
            last_periods_since(PeriodUnit::Day, 3, "2026-08-01", WeekDay::Monday).as_deref(),
            Some("20260730")
        );
    }

    #[test]
    fn starts_a_single_week_at_the_configured_week_start() {
        // 2026-07-27 is a Monday.
        assert_eq!(
            last_periods_since(PeriodUnit::Week, 1, "2026-07-29", WeekDay::Monday).as_deref(),
            Some("20260727")
        );
        assert_eq!(
            last_periods_since(PeriodUnit::Week, 1, "2026-07-29", WeekDay::Sunday).as_deref(),
            Some("20260726")
        );
    }

    #[test]
    fn counts_whole_weeks_back_from_the_current_one() {
        assert_eq!(
            last_periods_since(PeriodUnit::Week, 2, "2026-07-29", WeekDay::Monday).as_deref(),
            Some("20260720")
        );
    }

    #[test]
    fn starts_months_on_the_first_and_wraps_the_year() {
        assert_eq!(
            last_periods_since(PeriodUnit::Month, 1, "2026-07-27", WeekDay::Monday).as_deref(),
            Some("20260701")
        );
        assert_eq!(
            last_periods_since(PeriodUnit::Month, 3, "2026-01-15", WeekDay::Monday).as_deref(),
            Some("20251101")
        );
    }

    #[test]
    fn treats_a_zero_count_as_the_current_period() {
        assert_eq!(
            last_periods_since(PeriodUnit::Day, 0, "2026-07-27", WeekDay::Monday).as_deref(),
            Some("20260727")
        );
    }

    #[test]
    fn rejects_dates_that_are_not_iso_days() {
        assert!(last_periods_since(PeriodUnit::Day, 1, "not-a-date", WeekDay::Monday).is_none());
    }
}