#!/usr/bin/env python3

# Dependencies:
# - python3.7 or later
# - gs (Ghostscript)

import argparse, os, re, subprocess, sys

def main(args):
    d=args.dimensions
    o=args.output
    if not o:
        o=f'empty-{d}.pdf'
    if os.path.exists(o):
        die(f'Output file {o} already exists')
    dm=m("^([0-9.]+)(pts|pt|in|cm|mm)x([0-9.]+)(pts|pt|in|cm|mm)$", d)
    w=None
    h=None
    try:
        w=int(float(dm[1])*({'pts':1,'pt':1,'in':72,'cm':28.3,'mm':2.83}[dm[2]]))
        h=int(float(dm[3])*({'pts':1,'pt':1,'in':72,'cm':28.3,'mm':2.83}[dm[4]]))
    except:
        die('Invalid dimensions')
    subprocess.run([
        'gs', '-dBATCH', '-dNOPAUSE', '-dSAFER', '-dQUIET',
        f'-sOutputFile={o}',
        '-sDEVICE=pdfwrite',
        f'-dDEVICEWIDTHPOINTS={w}', f'-dDEVICEHEIGHTPOINTS={h}',
    ], check=True)
    print(f'Wrote a pdf file with dimensions {w}x{h} pts to {o}')

def m(pattern, string):
    match = re.match(pattern, string, re.DOTALL)
    if not match:
        return None
    if(match.group(0) != string):
        die(f'Pattern /${pattern}/ matched only part of string')
    ret = []
    for index in range((match.lastindex or 0)+1):
        ret.append(match.group(index))
    return ret

def die(reason):
    print(reason, file=sys.stderr)
    sys.exit(2)

parser = argparse.ArgumentParser(description='Create an empty, transparent PDF file.')
parser.add_argument('-o', '--output', metavar='FILE', type=str, help='Output PDF file. (default: empty-DIMENSIONS.pdf)')
parser.add_argument('-d', '--dimensions', type=str, default='3inx2in', help='The page dimensions of the file to create. Supports units pts, in, cm, and mm. (default: 3inx2in)')

main(parser.parse_args(sys.argv[1:]))