Subject: Re: [PATCH 3/3] abbrev: auto size the default abbreviation

From: Junio C Hamano <[email-masked]>
Date: Tue, 01 Nov 2016 18:33:38 -0700

Jeff King <[email-masked]> writes:

> On Fri, Sep 30, 2016 at 05:19:37PM -0700, Junio C Hamano wrote:
>
>> Introduce a mechanism, where we estimate the number of objects in
>> the repository upon the first request to abbreviate an object name
>> with the default setting and come up with a sane default for the
>> repository.  Based on the expectation that we would see collision in
>> a repository with 2^(2N) objects when using object names shortened
>> to first N bits, use sufficient number of hexdigits to cover the
>> number of objects in the repository.  Each hexdigit (4-bits) we add
>> to the shortened name allows us to have four times (2-bits) as many
>> objects in the repository.

I was idly browsing the draft release notes and then documentation
and noticed that, even though the new default is to auto-scale,
there is no mention of that in the documentation and there is no way
to explicitly ask for auto-scaling.

I wonder if we want to have something like this.  I actually am
inclined to drop the change to config.c and remove the new mention
of "auto" in the documentation.

 Documentation/config.txt |  9 +++++----
 config.c                 | 14 ++++++++++----
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index a0ab66aae7..b02f8a4025 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -783,10 +783,11 @@ core.sparseCheckout::
 	linkgit:git-read-tree[1] for more information.
 
 core.abbrev::
-	Set the length object names are abbreviated to.  If unspecified,
-	many commands abbreviate to 7 hexdigits, which may not be enough
-	for abbreviated object names to stay unique for sufficiently long
-	time.
+	Set the length object names are abbreviated to.  If
+	unspecified or set to "auto", an appropriate value is
+	computed based on the approximate number of packed objects
+	in your repository, which hopefully is enough for
+	abbreviated object names to stay unique for some time.
 
 add.ignoreErrors::
 add.ignore-errors (deprecated)::
diff --git a/config.c b/config.c
index 83fdecb1bc..c363cca4a9 100644
--- a/config.c
+++ b/config.c
@@ -834,10 +834,16 @@ static int git_default_core_config(const char *var, const char *value)
 	}
 
 	if (!strcmp(var, "core.abbrev")) {
-		int abbrev = git_config_int(var, value);
-		if (abbrev < minimum_abbrev || abbrev > 40)
-			return -1;
-		default_abbrev = abbrev;
+		if (!value)
+			return config_error_nonbool(var);
+		if (!strcasecmp(value, "auto"))
+			default_abbrev = -1;
+		else {
+			int abbrev = git_config_int(var, value);
+			if (abbrev < minimum_abbrev || abbrev > 40)
+				return -1;
+			default_abbrev = abbrev;
+		}
 		return 0;
 	}