File: mksparse

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#!/usr/bin/env python
# Copyright 2013-2014  Lars Wirzenius
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#
# =*= License: GPL-3+ =*=


description = '''Create a sparse file.

The first command line argument is the name of the output file. The
second argument is a specification for how the file is to be made
sparse: a sequence of "data" and "hole" words, which may be
interspersed with spaces, commas, or the word "a", all of which are
ignored, except that the "data" and "hole" words must have something
in between them.

'''


import cliapp
import os


class CreateSparseFile(cliapp.Application):

    data_size = 1024
    hole_size = 1024**2

    def process_args(self, args):
        if len(args) != 2:
            raise cliapp.AppException('Usage!')

        output_filename = args[0]
        spec = self.parse_spec(args[1])

        with open(output_filename, 'w') as f:
            for word in spec:
                if word == 'hole':
                    self.append_hole(f)
                else:
                    assert word == 'data'
                    self.append_data(f)

    def parse_spec(self, arg):
        # Remove commas.
        arg = ' '.join(arg.split(','))

        # Split into words.
        words = arg.split()

        # Remove any words that are not "data" or "hole".
        spec = [x for x in words if x in ('data', 'hole')]

        return spec

    def append_data(self, f):
        f.write('x' * self.data_size)
        f.flush()

    def append_hole(self, f):
        fd = f.fileno()
        pos = os.lseek(fd, self.hole_size, os.SEEK_CUR)
        os.ftruncate(fd, pos)


CreateSparseFile(description=description).run()