#!/usr/bin/env python3

import sys, os
import argparse
import subprocess

def existing_filename(f):
    if not os.path.exists(f):
        raise ValueError("No such file or directory.")
    return f

def existing_dirname_or_dash(f):
    if f == '-':
        pass
    elif not os.path.exists(f):
        raise ValueError("No such file or directory.")
    elif not os.path.isdir(f):
        raise ValueError("Not a directory: %s" % f)
    return f

def int_1plus(s):
    i = int(s)
    if i < 1:
        raise ValueError("Index must be greater 0.")
    return i

def parse_args(argv):
    parser = argparse.ArgumentParser(prog=argv[0])
    parser.add_argument("image", help="image file", type=existing_filename)
    parser.add_argument("mountpoint", help="mount point/loop device ['-' uses next available]", type=existing_dirname_or_dash)
    parser.add_argument("-p", help="partition index, 1-based [default: 1]", type=int_1plus, default=1, metavar="index", dest="part")
    parser.add_argument("-l", help="Don't mount, just setup the loop device.", action="store_true", default=False)
    parser.add_argument("-t", help="filesystem type [default: auto]", metavar="vfstype")
    parser.add_argument("-o", help="mount options [default: none]", metavar="options")
    parser.add_argument("-parted", help="parted executable [default: parted]", default="parted")
    parser.add_argument("-mount", help="mount executable [default: mount]", default="mount")
    parser.add_argument("-losetup", help="losetup executable [default: losetup]", default="losetup")
    return parser.parse_args(argv[1:])

def main(argv):
    args = parse_args(argv)

    parted = subprocess.check_output([args.parted, args.image, "-s", "-m", "u", "b", "p"])
    partitions = parted.split(b";\n")[2:]

    try:
        part = partitions[args.part-1]
    except IndexError:
        raise SystemExit("Partition index out of range: %i, has %i" % (args.part, len(partitions)))

    offset = part.split(b':')[1].rstrip(b'B').decode("ascii")

    if args.l:
        if args.mountpoint == '-':
            args.mountpoint = "-f"
        losetup = [args.losetup, "--show", "--offset", offset, args.mountpoint, args.image]
        subprocess.check_call(losetup)
    else:
        if args.mountpoint == '-':
            raise ValueError("Mountpoint cannot be '-' unless -l (only setup loop device) is also given")
        options = "offset=%s" % offset
        if args.o:
            options += ',' + args.o
        mount = [args.mount, args.image, args.mountpoint, "-o", options]
        if args.t:
            mount.append("-t")
            mount.append(args.t)
        subprocess.check_call(mount)

if __name__ == "__main__":
    import sys
    sys.exit(main(sys.argv))