SwiftFITS

About

FITS Image Library for Swift.

This library provides a simple interface to read FITS files in Swift, based on the FITS 4.0 standard.

Status

SwiftFITS is currently read-only: it parses existing FITS files into their header/data structure. Write and serialization support is planned but not yet implemented.

Conformance & Limitations

SwiftFITS targets the base FITS 4.0 standard. The following properties are intentional, not latent surprises:

• Keyword names may contain only A–Z, 0–9, _ and -, and must be left-justified in the 8-byte keyword field (a leading space is rejected).
• Long-keyword conventions are out of scope: HIERARCH and similar ESO conventions are treated as ordinary 8-byte keywords, not expanded. Only CONTINUE/HISTORY/COMMENT multi-record merging is supported.
• Mandatory keywords (SIMPLE/XTENSION, BITPIX, NAXIS, NAXISn, and PCOUNT/GCOUNT for extensions) must appear in their exact standard order and index — this is enforced even in lenient mode.
• Section layout is geometry-driven: each header's declared geometry (|BITPIX|/8 × GCOUNT × (PCOUNT + ∏ NAXISn), with random groups handled) determines exactly how many data blocks follow, rather than guessing from byte content. Trailing all-blank padding is preserved.
• END is excluded from a section's properties but is retained in the raw bytes, so a parsed file round-trips byte-for-byte through FITSFile.data.
• Strict vs. lenient: .strict rejects technically-noncompliant input, while .lenient (the default) tolerates common real-world deviations (unknown value types, trailing characters after a string's closing quote, non-printable header text, data-length mismatches, a missing space after the = value indicator, lowercase e/d float exponents, NUL-padded headers, a truncated trailing partial block, and non-blank records after the END marker).
• Not thread-safe: FITSFile, FITSSection and FITSBlock carry mutable state and cache structural facts lazily on read, so they are not Sendable and must not be shared across threads without external synchronization.

Requirements & Portability

SwiftFITS is written in pure Swift and depends only on Foundation — no third-party dependencies. It is developed, built and tested on macOS (see the CI badge above). Being Foundation-only, the library is expected to be portable to Linux; Linux CI and SwiftPM-on-Linux verification are not yet in place and are tracked as future work.

Swift Package Manager

SwiftFITS ships a Package.swift and can be consumed as a Swift package. Add it to your dependencies:

.package( url: "https://github.com/macmade/SwiftFITS.git", branch: "main" )

The Xcode project (SwiftFITS.xcodeproj) is also provided for development.

Cloning

This project uses submodules.
To clone it, use the following command:

git clone --recursive https://github.com/macmade/SwiftFITS.git

Example Usage

import Foundation
import SwiftFITS

do
{
    let file = try FITSFile( url: URL( fileURLWithPath: "/path/to/file.fits" ), options: .lenient )
    
    if let header = file.header
    {
        print( header.properties )
    }
}
catch // SwiftFITS.FITSError
{
    print( error )
}

License

Project is released under the terms of the MIT License.

Repository Infos

Owner:          Jean-David Gadina - XS-Labs
Web:            www.xs-labs.com
Blog:           www.noxeos.com
Twitter:        @macmade
GitHub:         github.com/macmade
LinkedIn:       ch.linkedin.com/in/macmade/
StackOverflow:  stackoverflow.com/users/182676/macmade