# mpatch.py - Python implementation of mpatch.c

#

# Copyright 2009 Matt Mackall <mpm@selenic.com> and others

#

# This software may be used and distributed according to the terms of the

# GNU General Public License version 2 or any later version.

from __future__ import absolute_import

import struct

class mpatchError(Exception):

    """error raised when a delta cannot be decoded

    """

# This attempts to apply a series of patches in time proportional to

# the total size of the patches, rather than patches * len(text). This

# means rather than shuffling strings around, we shuffle around

# pointers to fragments with fragment lists.

#

# When the fragment lists get too long, we collapse them. To do this

# efficiently, we do all our operations inside a buffer created by

# mmap and simply use memmove. This avoids creating a bunch of large

# temporary string buffers.

def _pull(dst, src, l): # pull l bytes from src

    while l:

        f = src.pop()

        if f[0] > l: # do we need to split?

            src.append((f[0] - l, f[1] + l))

            dst.append((l, f[1]))

            return

        dst.append(f)

        l -= f[0]

def _move(m, dest, src, count):

    """move count bytes from src to dest

    The file pointer is left at the end of dest.

    """

    m.seek(src)

    buf = m.read(count)

    m.seek(dest)

    m.write(buf)

def _collect(m, buf, list):

    start = buf

    for l, p in reversed(list):

        _move(m, buf, p, l)

        buf += l

    return (buf - start, start)

def patches(a, bins):

    if not bins:

        return a

    plens = [len(x) for x in bins]

    pl = sum(plens)

    bl = len(a) + pl

    tl = bl + bl + pl # enough for the patches and two working texts

    b1, b2 = 0, bl

    if not tl:

        return a

    # load our original text

    m.write(a)

    frags = [(len(a), b1)]

    # copy all the patches into our segment so we can memmove from them

    pos = b2 + bl

    m.seek(pos)

    for p in bins: m.write(p)

    for plen in plens:

        # if our list gets too long, execute it

        if len(frags) > 128:

            b2, b1 = b1, b2

            frags = [_collect(m, b1, frags)]

        new = []

        end = pos + plen

        last = 0

        while pos < end:

            m.seek(pos)

            try:

                p1, p2, l = struct.unpack(">lll", m.read(12))

            except struct.error:

                raise mpatchError("patch cannot be decoded")

            _pull(new, frags, p1 - last) # what didn't change

            _pull([], frags, p2 - p1)    # what got deleted

            new.append((l, pos + 12))   # what got added

            pos += l + 12

            last = p2

        frags.extend(reversed(new))     # what was left at the end

    t = _collect(m, b2, frags)

    m.seek(t[1])

    return m.read(t[0])

def patchedsize(orig, delta):

    outlen, last, bin = 0, 0, 0

    binend = len(delta)

    data = 12

    while data <= binend:

        decode = delta[bin:bin + 12]

        start, end, length = struct.unpack(">lll", decode)

        if start > end:

            break

        bin = data + length

        data = bin + 12

        outlen += start - last

        last = end

        outlen += length

    if bin != binend:

        raise mpatchError("patch cannot be decoded")

    outlen += orig - last

    return outlen