Re: [PATCH] doc: clarify that most --abbrev[=<n>] are about minimum length

[Derrick Stolee <stolee@gmail.com>]

On 11/3/2020 8:44 PM, Junio C Hamano wrote:
> Early text written in 2006 explains the "--abbrev=<n>" option to
> "show only a partial prefix", without saying that the length of the
> partial prefix is not necessarily the number given to the option to
> ensure that the output names the object uniquely.
> 
> Luckily, the text written for "git describe" in 2011 explains that
> the output is made to name the object uniquely using at least <n>
> hexdigits much clearly.
> 
> Model the explanation and update documentation for the diff family
> of commands, "blame", "branch --verbose", "ls-files" and "ls-tree".

Thanks for being thorough here! Your patch is already an
improvement on what we have. I hope that my suggestions
below help to improve it further.

> --- a/Documentation/diff-options.txt
> +++ b/Documentation/diff-options.txt
> @@ -446,7 +446,8 @@ endif::git-format-patch[]
>  --abbrev[=<n>]::
>  	Instead of showing the full 40-byte hexadecimal object
>  	name in diff-raw format output and diff-tree header
> -	lines, show only a partial prefix.
> +	lines, show <n> hexadecimal digits, or as many digits
> +	as needed to form a unique object name.

A reoccurring theme in this patch is that the --abbrev option
provides a lower bound, even if fewer digits are needed for a
unique object name. Using these two phrases with an "or" has
some ambiguity about which is chosen.

Perhaps we should try to describe it as "<n> is the default,
but can grow as needed". Here is an attempt:

-	lines, show only a partial prefix.
+	lines, show at least <n> hexadecimal digits, increasing the
+	prefix length as needed to uniquely identify an object.

If you prefer wording like this, then there are a few very
similar snippets later in git-branch.txt and git-ls-files.txt.

> --- a/Documentation/git-blame.txt
> +++ b/Documentation/git-blame.txt
> @@ -87,7 +87,9 @@ include::blame-options.txt[]
>  
>  --abbrev=<n>::
>  	Instead of using the default 7+1 hexadecimal digits as the
> -	abbreviated object name, use <n>+1 digits. Note that 1 column
> +	abbreviated object name, use <m>+1 digits, where <m> is at
> +	least <n> but ensures the commit object names are unique.
> +	Note that 1 column
>  	is used for a caret to mark the boundary commit.

This captures the "at least <n>" pretty well, especially considering
the complications around "+1".

> ---abbrev=<length>::
> -	Alter the sha1's minimum display length in the output listing.
> +--abbrev=<n>::
> +	In the verbose listing that show the commit object name,
> +	use <n>, or as many hexdigits as needed to form a unique
> +	object name.

Adapting from your wording here made me think that this might
be a simpler way to phrase it:

+--abbrev=<n>::
+	In the verbose listing that show the commit object name,
+	use at least <n> hexdigits to form a unique object name.

> diff --git a/Documentation/pretty-options.txt b/Documentation/pretty-options.txt
> index 17c5aac4b7..da6ab0de0c 100644
> --- a/Documentation/pretty-options.txt
> +++ b/Documentation/pretty-options.txt
> @@ -16,7 +16,8 @@ configuration (see linkgit:git-config[1]).
>  
>  --abbrev-commit::
>  	Instead of showing the full 40-byte hexadecimal commit object
> -	name, show only a partial prefix.  Non default number of
> +	name, show only a partial prefix that names the object uniquely.

Perhaps "show a prefix that uniquely identifies the object" ?

> +	Non default number of
>  	digits can be specified with "--abbrev=<n>" (which also modifies
>  	diff output, if it is displayed).

I know this is in the existing docs, but this last sentence should
probably start with "A non-default number of digits..." Alternatively,
we could rephrase the entire thing to use more active voice:

+ The `--abbrev=<n>` option modifies the minimum number of digits
+ for these prefixes, including objects reported in displayed diff
+ output.

Thanks,
-Stolee