#!/usr/bin/env python
#
# Send a sparse file more space-efficiently.
# See recv-hole for a description of the protocol.
#
# Note that xfer-hole requires a version of Linux with support for
# SEEK_DATA and SEEK_HOLE.
#
#
# Copyright (C) 2014-2015  Codethink Limited
#
# 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; version 2 of the License.
#
# 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-2 =*=



import errno
import os
import sys


SEEK_DATA = 3
SEEK_HOLE = 4


filename = sys.argv[1]
fd = os.open(filename, os.O_RDONLY)
pos = 0


DATA = 'data'
HOLE = 'hole'
EOF = 'eof'


def safe_lseek(fd, pos, whence):
    try:
        return os.lseek(fd, pos, whence)
    except OSError as e:
        if e.errno == errno.ENXIO:
            return -1
        raise


def current_data_or_pos(fd, pos):
    length = safe_lseek(fd, 0, os.SEEK_END)
    next_data = safe_lseek(fd, pos, SEEK_DATA)
    next_hole = safe_lseek(fd, pos, SEEK_HOLE)

    if pos == length:
        return EOF, pos
    elif pos == next_data:
        return DATA, pos
    elif pos == next_hole:
        return HOLE, pos
    else:
        assert False, \
            ("Do not understand: pos=%d next_data=%d next_hole=%d" %
             (pos, next_data, next_hole))


def next_data_or_hole(fd, pos):
    length = safe_lseek(fd, 0, os.SEEK_END)
    next_data = safe_lseek(fd, pos, SEEK_DATA)
    next_hole = safe_lseek(fd, pos, SEEK_HOLE)

    if pos == length:
        return EOF, pos
    elif pos == next_data:
        # We are at data.
        if next_hole == -1 or next_hole == length:
            return EOF, length
        else:
            return HOLE, next_hole
    elif pos == next_hole:
        # We are at a hole.
        if next_data == -1 or next_data == length:
            return EOF, length
        else:
            return DATA, next_data
    else:
        assert False, \
            ("Do not understand: pos=%d next_data=%d next_hole=%d" %
             (pos, next_data, next_hole))


def find_data_and_holes(fd):
    pos = safe_lseek(fd, 0, os.SEEK_CUR)

    kind, pos = current_data_or_pos(fd, pos)
    while kind != EOF:
        yield kind, pos
        kind, pos = next_data_or_hole(fd, pos)
    yield kind, pos


def make_xfer_instructions(fd):
    prev_kind = None
    prev_pos = None
    for kind, pos in find_data_and_holes(fd):
        if prev_kind == DATA:
            yield (DATA, prev_pos, pos)
        elif prev_kind == HOLE:
            yield (HOLE, prev_pos, pos)
        prev_kind = kind
        prev_pos = pos


def copy_slice_from_file(to, fd, start, end):
    safe_lseek(fd, start, os.SEEK_SET)
    nbytes = end - start
    max_at_a_time = 1024**2
    while nbytes > 0:
        data = os.read(fd, min(nbytes, max_at_a_time))
        if not data:
            break
        to.write(data)
        nbytes -= len(data)
    

for kind, start, end in make_xfer_instructions(fd):
    if kind == HOLE:
        sys.stdout.write('HOLE\n%d\n' % (end - start))
    elif kind == DATA:
        sys.stdout.write('DATA\n%d\n' % (end - start))
        copy_slice_from_file(sys.stdout, fd, start, end)