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From: Junio C Hamano <gitster@pobox.com>
To: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: git@vger.kernel.org
Subject: Re: [PATCH 2/2] git-format-patch: Document format for binary patch
Date: Wed, 24 Mar 2021 10:53:32 -0700	[thread overview]
Message-ID: <xmqqa6qszbdv.fsf@gitster.g> (raw)
In-Reply-To: <20210324123027.29460-3-bagasdotme@gmail.com> (Bagas Sanjaya's message of "Wed, 24 Mar 2021 19:30:27 +0700")

Bagas Sanjaya <bagasdotme@gmail.com> writes:

> Document binary file patch formats that are different from text file
> patch.
>
> Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
> ---
>  Documentation/git-format-patch.txt | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
> index 247033f8fc..8de172b1f4 100644
> --- a/Documentation/git-format-patch.txt
> +++ b/Documentation/git-format-patch.txt
> @@ -725,6 +725,28 @@ diff format is described as below:
>  
>  include::diff-generate-patch.txt[]
>  
> +Binary Files
> +~~~~~~~~~~~~
> +For binary files, the diff format have some differences compared to text
> +files:

I do not think this is specific to 'format-patch'.  If we need to
describe 'git diff --binary', it should be done there, so that
readers of "git diff --help" would also be able to learn the format.

> +1. Object hashes in index header line (`index <hash>..<hash> <mode>`)

s/Object hash/Object name/;

> +   are always given in full form, as binary patch is designed to be
> +   applied only to an exact copy of original file. This is to ensure
> +   that such patch don't apply to file with similar name but different
> +   hash.

... with similar but different object name.

cf. Documentation/glossary-contents.txt tells you what "object name" is.

> +2. There are additional extended header lines specific to binary files:
> +
> +        GIT binary patch
> +        delta <bytes>
> +        literal <bytes>
> +
> +3. The diff body can be either delta or full (literal) content,
> +   whichever is the smallest size. It is encoded with base85 algorithm,
> +   and emitted in 64 characters each line. All but the last line in
> +   the body are prefixed with `z`.

I do not think this is all that useful; it clutters the description
for a reader who is not interested in reimplementing an encoder or a
decoder from the document.

And it is way too insufficient for a reader who wants to reimplement
an encoder or a decoder.  For example,

 - It does not say anything about what the delta is and how it is
   computed.

 - The 'z' is redundant; the more important is to say that the first
   byte signals how many bytes are on that line and it is a mere
   artifact that we cram up to 52 bytes on a line.

 - It does not say anything about how the binary patch ensures that
   it is reversible (i.e. can be given to "git apply -R").

Thanks.

  reply	other threads:[~2021-03-24 17:54 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-24 12:30 [PATCH 0/2] Diff format documentation for git-format-patch Bagas Sanjaya
2021-03-24 12:30 ` [PATCH 1/2] git-format-patch: Include diff-generate-patch documentation Bagas Sanjaya
2021-03-24 17:40   ` Junio C Hamano
2021-03-24 12:30 ` [PATCH 2/2] git-format-patch: Document format for binary patch Bagas Sanjaya
2021-03-24 17:53   ` Junio C Hamano [this message]
2021-03-25  6:22     ` Bagas Sanjaya
2021-03-25 18:25       ` Junio C Hamano

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