Simple script for constructing small XAR files (License: CC0-1.0)

xar-generator.py

"""
Simple script for constructing small XAR files.
License: CC0-1.0
"""

import datetime
import gzip
import hashlib
import io
import struct
import xml.etree.ElementTree as ET
import zlib


def main():
    with open("a.pkg", "wb") as f:
        f.truncate()
        f.write(b"xar!")

        xar = ET.Element("xar")
        toc = ET.SubElement(xar, "toc")
        ET.SubElement(toc, "creation-time").text = datetime.datetime.now().strftime("%Y-%m-%dT%H:%M:%S")
        checksum = ET.SubElement(toc, "checksum")
        checksum.attrib["style"] = "sha1"
        ET.SubElement(checksum, "offset").text = "0"
        ET.SubElement(checksum, "size").text = "20"

        toc_data = io.BytesIO()
        heap_data = io.BytesIO()
        heap_data.write(b"\xFF" * 20)  # Make space for SHA1 checksum, we fill it in later.

        file_ids = 0
        files = [("a", b"x" * 100), ("b", b"x" * 100), ("c", b"x" * 100), ("d/1/2/3/a", b"x" * 10)]
        for filename, data in files:
            compressed_data = gzip.compress(data)

            # Create subdirectories, if necessary.
            root_el = toc
            if "/" in filename:
                *parts, filename = filename.split("/")
                for part in parts:
                    part_el = ET.SubElement(root_el, "file")
                    part_el.attrib["id"] = str(file_ids)
                    file_ids += 1
                    ET.SubElement(part_el, "type").text = "directory"
                    ET.SubElement(part_el, "name").text = part
                    root_el = part_el

            file = ET.SubElement(root_el, "file")
            file.attrib["id"] = str(file_ids)
            file_ids += 1
            ET.SubElement(file, "type").text = "file"
            ET.SubElement(file, "name").text = filename
            file_data = ET.SubElement(file, "data")
            ET.SubElement(file_data, "length").text = str(len(compressed_data))
            ET.SubElement(file_data, "offset").text = str(heap_data.tell())
            ET.SubElement(file_data, "size").text = str(len(data))
            ET.SubElement(file_data, "encoding").attrib["style"] = "application/x-gzip"
            ext_checksum = ET.SubElement(file_data, "extracted-checksum")
            ext_checksum.attrib["style"] = "sha1"
            ext_checksum.text = hashlib.sha1(data).hexdigest()
            arc_checksum = ET.SubElement(file_data, "archived-checksum")
            arc_checksum.attrib["style"] = "sha1"
            arc_checksum.text = hashlib.sha1(compressed_data).hexdigest()

            heap_data.write(compressed_data)

        xar_tree = ET.ElementTree(xar)
        xar_tree.write(toc_data)
        toc_bytes = toc_data.getvalue()
        toc_bytes_compressed = zlib.compress(toc_bytes)

        heap_data.seek(0, 0)
        heap_data.write(hashlib.sha1(toc_bytes_compressed).digest())
        heap_data.seek(2, 0)

        header_length = 28
        format_version = 1
        toc_compressed_length = len(toc_bytes_compressed)
        toc_uncompressed_length = len(toc_bytes)
        checksum_alg = 1

        header_bytes = struct.pack(
            ">HHQQI",
            header_length,
            format_version,
            toc_compressed_length,
            toc_uncompressed_length,
            checksum_alg
        )
        assert len(header_bytes) + 4 == header_length

        f.write(header_bytes)
        f.write(toc_bytes_compressed)
        f.write(heap_data.getvalue())


if __name__ == "__main__":
    main()