time-local 0.4.1

use std::sync::OnceLock;

use time::UtcOffset;

pub trait UtcOffsetExt {
    /// Cached result of [`time::UtcOffset::current_local_offset`].
    ///
    /// # Panics
    ///
    /// Panics if [`crate::init`] has not been called with a succesful return value.
    fn cached_local_offset() -> time::UtcOffset;
}

static UTC_OFFSET: OnceLock<time::UtcOffset> = OnceLock::new();

impl UtcOffsetExt for time::UtcOffset {
    fn cached_local_offset() -> time::UtcOffset {
        *UTC_OFFSET.get().unwrap_or_else(|| utc_offset_init_error())
    }
}

pub trait OffsetDateTimeExt {
    /// Convenience method that calls [`time::OffsetDateTime::to_offset`] with the [`time::UtcOffset`] for `self`.
    fn to_local(self) -> Result<time::OffsetDateTime, time::error::IndeterminateOffset>;
}

impl OffsetDateTimeExt for time::OffsetDateTime {
    fn to_local(self) -> Result<time::OffsetDateTime, time::error::IndeterminateOffset> {
        Ok(self.to_offset(time::UtcOffset::local_offset_at(self)?))
    }
}

/// Call this function before your program spawns threads and before you use [`UtcOffsetExt::cached_local_offset`].
pub fn init() -> Result<(), time::error::IndeterminateOffset> {
    let utc_offset = UtcOffset::current_local_offset()?;
    UTC_OFFSET
        .set(utc_offset)
        .unwrap_or_else(|_| utc_offset_init_error());
    Ok(())
}

fn utc_offset_init_error() -> ! {
    panic!(
        "call `time_local::init()` once during application initialization before spawning threads"
    )
}