# 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

from . import policy, pycompat
stringio = pycompat.stringio
modulepolicy = policy.policy
policynocffi = policy.policynocffi

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

    m = stringio()

    # 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

if modulepolicy not in policynocffi:
    try:
        from _mpatch_cffi import ffi, lib
    except ImportError:
        if modulepolicy == 'cffi': # strict cffi import
            raise
    else:
        @ffi.def_extern()
        def cffi_get_next_item(arg, pos):
            all, bins = ffi.from_handle(arg)
            container = ffi.new("struct mpatch_flist*[1]")
            to_pass = ffi.new("char[]", str(bins[pos]))
            all.append(to_pass)
            r = lib.mpatch_decode(to_pass, len(to_pass) - 1, container)
            if r < 0:
                return ffi.NULL
            return container[0]

        def patches(text, bins):
            lgt = len(bins)
            all = []
            if not lgt:
                return text
            arg = (all, bins)
            patch = lib.mpatch_fold(ffi.new_handle(arg),
                                    lib.cffi_get_next_item, 0, lgt)
            if not patch:
                raise mpatchError("cannot decode chunk")
            outlen = lib.mpatch_calcsize(len(text), patch)
            if outlen < 0:
                lib.mpatch_lfree(patch)
                raise mpatchError("inconsistency detected")
            buf = ffi.new("char[]", outlen)
            if lib.mpatch_apply(buf, text, len(text), patch) < 0:
                lib.mpatch_lfree(patch)
                raise mpatchError("error applying patches")
            res = ffi.buffer(buf, outlen)[:]
            lib.mpatch_lfree(patch)
            return res