//! System time-zone resolution for recurrence.
//!
//! GoingsOn stores instants in UTC and has no per-user time-zone preference, so
//! "the user's zone" is the operating system's zone. Recurrence advances dates in
//! this zone (see `goingson_core::recurrence`) so a fixed local time-of-day, "every
//! day at 09:00", survives daylight-saving transitions instead of drifting an hour.

use chrono::{DateTime, NaiveDateTime, TimeZone, Utc};
use chrono_tz::Tz;

/// The user's IANA time zone, resolved from the OS. Falls back to UTC if the OS
/// zone can't be read or doesn't parse as a known IANA name.
pub fn system_tz() -> Tz {
    iana_time_zone::get_timezone()
        .ok()
        .and_then(|name| name.parse().ok())
        .unwrap_or(Tz::UTC)
}

/// Interpret a civil (wall-clock) datetime as being in the user's system zone
/// and convert it to the corresponding UTC instant.
///
/// Window queries (weekly/monthly review) work with civil dates like "the start
/// of this week"; those midnights are local, not UTC, so stamping them as UTC
/// misattributes edge-of-period rows by the zone offset. On a DST spring-forward
/// gap the civil time doesn't exist, so fall back to treating it as UTC rather
/// than panicking.
pub fn local_civil_to_utc(civil: NaiveDateTime) -> DateTime<Utc> {
    system_tz()
        .from_local_datetime(&civil)
        .earliest()
        .map_or_else(
            || DateTime::<Utc>::from_naive_utc_and_offset(civil, Utc),
            |dt| dt.with_timezone(&Utc),
        )
}