mercurial/pure/mpatch.py

rust-index: find_gca_candidates bit sets genericization

This allows to use arbitratry size of inputs in `find_gca_candidates()`.
We're genericizing so that the common case of up to 63 inputs can be
treated with the efficient implementation backed by `u64`.

Some complications with the borrow checker came, because arbitrary sized
bit sets will not be `Copy`, hence mutating them keeps a mut ref on the `seen`
vector. This is solved by some cloning, most of which can be avoided,
preferably in a follow-up after proof that this works (hence after exposition
to Python layer).

As far as performance is concerned, calling `clone()` on a `Copy` object
(good case when number of revs is less than 64) should end up just doing a
copy, according to this excerpt of the `Clone` trait documentation:

  Types that are Copy should have a trivial implementation of Clone.
  More formally: if T: Copy, x: T, and y: &T, then let x = y.clone();
  is equivalent to let x = *y;.
  Manual implementations should be careful to uphold this invariant;
  however, unsafe code must not rely on it to ensure memory safety.

We kept the general structure, hence why there are some double negations.
This also could be made nicer in a follow-up.

The `NonStaticPoisonableBitSet` is included to ensure that the
`PoisonableBitSet` trait is general enough (had to correct `vec_of_empty()` for
instance). Moving the genericization one level to encompass the `seen`
vector and not its elements would be better for performance, if worth it.

# mpatch.py - Python implementation of mpatch.c
#
# Copyright 2009 Olivia Mackall <[e-mail]> 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.


import io
import struct

from typing import (
    List,
    Tuple,
)


stringio = io.BytesIO


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: List[Tuple[int, int]], src: List[Tuple[int, int]], l: int
) -> None:  # 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: stringio, dest: int, src: int, count: int) -> None:
    """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: stringio, buf: int, list: List[Tuple[int, int]]
) -> Tuple[int, int]:
    start = buf
    for l, p in reversed(list):
        _move(m, buf, p, l)
        buf += l
    return (buf - start, start)


def patches(a: bytes, bins: List[bytes]) -> bytes:
    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(b">lll", m.read(12))
            except struct.error:
                raise mpatchError(b"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: int, delta: bytes) -> int:
    outlen, last, bin = 0, 0, 0
    binend = len(delta)
    data = 12

    while data <= binend:
        decode = delta[bin : bin + 12]
        start, end, length = struct.unpack(b">lll", decode)
        if start > end:
            break
        bin = data + length
        data = bin + 12
        outlen += start - last
        last = end
        outlen += length

    if bin != binend:
        raise mpatchError(b"patch cannot be decoded")

    outlen += orig - last
    return outlen