# sidedata.py - Logic around store extra data alongside revlog revisions

#

# Copyright 2019 Pierre-Yves David <pierre-yves.david@octobus.net)

#

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

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

"""core code for "sidedata" support

The "sidedata" are stored alongside the revision without actually being part of

its content and not affecting its hash. It's main use cases is to cache

important information related to a changesets.

The current implementation is experimental and subject to changes. Do not rely

on it in production.

Sidedata are stored in the revlog itself, withing the revision rawtext. They

are inserted, removed from it using the flagprocessors mechanism. The following

format is currently used::

    initial header:

        <number of sidedata; 2 bytes>

    sidedata (repeated N times):

        <sidedata-key; 2 bytes>

        <sidedata-entry-length: 4 bytes>

        <sidedata-content-sha1-digest: 20 bytes>

        <sidedata-content; X bytes>

    normal raw text:

        <all bytes remaining in the rawtext>

This is a simple and effective format. It should be enought to experiment with

the concept.

"""

from __future__ import absolute_import

import hashlib

import struct

from .. import error

SIDEDATA_HEADER = struct.Struct('>H')

SIDEDATA_ENTRY = struct.Struct('>HL20s')

def sidedatareadprocessor(rl, text):

    sidedata = {}

    offset = 0

    nbentry, = SIDEDATA_HEADER.unpack(text[:SIDEDATA_HEADER.size])

    offset += SIDEDATA_HEADER.size

    dataoffset = SIDEDATA_HEADER.size + (SIDEDATA_ENTRY.size * nbentry)

    for i in range(nbentry):

        nextoffset = offset + SIDEDATA_ENTRY.size

        key, size, storeddigest = SIDEDATA_ENTRY.unpack(text[offset:nextoffset])

        offset = nextoffset

        # read the data associated with that entry

        nextdataoffset = dataoffset + size

        entrytext = text[dataoffset:nextdataoffset]

        readdigest = hashlib.sha1(entrytext).digest()

        if storeddigest != readdigest:

            raise error.SidedataHashError(key, storeddigest, readdigest)

        sidedata[key] = entrytext

        dataoffset = nextdataoffset

    text = text[dataoffset:]

    return text, True, sidedata