fits-well 0.1.4

//! Record an observation time in a FITS header, read it back, and convert between
//! ISO-8601, Julian Date, and time scales:
//!
//! ```sh
//! cargo run --example time
//! ```

use std::fs::File;

use fits_well::{Datetime, FitsReader, FitsWriter, Header, TimeScale};

fn main() -> fits_well::Result<()> {
    let path = std::env::temp_dir().join("fits_well_time.fits");

    // A header-only HDU (NAXIS = 0) recording when an observation was taken — the
    // standard §9 time keywords an instrument writes.
    let mut header = Header::new();
    header
        .set("SIMPLE", true)
        .set("BITPIX", 8)
        .set("NAXIS", 0)
        .set("DATE-OBS", "2024-03-14T15:09:26")
        .set("MJD-OBS", 60383.631551)
        .set("TIMESYS", "UTC");
    let mut writer = FitsWriter::new(File::create(&path)?);
    writer.write_header(&header)?; // NAXIS=0 → header only, no data unit
    writer.into_inner().sync_all()?;
    println!("wrote {}", path.display());

    // Read the file and pull the time metadata from its header.
    let reader = FitsReader::open(File::open(&path)?)?;
    let header = &reader.hdus()[0].header;

    // `header.obs_mjd()` resolves the observation time (MJD-OBS, else DATE-OBS).
    println!("observation MJD = {:?}", header.obs_mjd());

    // The DATE-OBS string itself parses to a `Datetime`, then to Julian Date.
    let t = Datetime::parse(header.get_text("DATE-OBS").unwrap())?;
    println!("DATE-OBS -> JD {:.5}, MJD {:.5}", t.to_jd(), t.to_mjd());

    // Convert that instant from the header's TIMESYS (UTC) to Terrestrial Time.
    let timesys = TimeScale::parse(header.get_text("TIMESYS").unwrap());
    let jd_tt = timesys.convert(t.to_jd(), TimeScale::parse("TT"));
    println!(
        "UTC -> TT differs by {:.3} s",
        (jd_tt - t.to_jd()) * 86400.0
    );

    Ok(())
}