//! Cron math shared by the scheduler loop and the `routine.schedule` tool.

use croner::Cron;
use time::OffsetDateTime;

#[derive(Debug, thiserror::Error)]
pub enum ScheduleError {
    #[error("invalid cron pattern: {0}")]
    Pattern(String),
}

/// The next firing strictly after `after` for a 5-field cron pattern.
pub fn next_occurrence(
    pattern: &str,
    after: OffsetDateTime,
) -> Result<OffsetDateTime, ScheduleError> {
    let cron = Cron::new(pattern)
        .parse()
        .map_err(|e| ScheduleError::Pattern(e.to_string()))?;
    let chrono_after = chrono::DateTime::from_timestamp(after.unix_timestamp(), 0)
        .ok_or_else(|| ScheduleError::Pattern("timestamp out of range".into()))?;
    let next = cron
        .find_next_occurrence(&chrono_after, false)
        .map_err(|e| ScheduleError::Pattern(e.to_string()))?;
    OffsetDateTime::from_unix_timestamp(next.timestamp())
        .map_err(|e| ScheduleError::Pattern(e.to_string()))
}