From 72a5b561fc1c4286bc7c5b0693afc076af261e1f Mon Sep 17 00:00:00 2001
From: Junio C Hamano <gitster@pobox.com>
Date: Thu, 28 Oct 2010 14:39:13 -0700
Subject: [PATCH] core.abbrevguard: Ensure short object names stay unique a bit
 longer

Even though git makes sure that it uses enough hexdigits to show an
abbreviated object name unambiguously, as more objects are added to the
repository over time, a short name that used to be unique will stop being
unique.  Git uses this many extra hexdigits that are more than necessary
to make the object name currently unique, in the hope that its output will
stay unique a bit longer.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/config.txt | 9 +++++++++
 cache.h                  | 1 +
 config.c                 | 7 +++++++
 environment.c            | 1 +
 sha1_name.c              | 4 +++-
 5 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 538ebb5e2e..6994338c90 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -374,6 +374,15 @@ core.warnAmbiguousRefs::
 	If true, git will warn you if the ref name you passed it is ambiguous
 	and might match multiple refs in the .git/refs/ tree. True by default.
 
+core.abbrevguard::
+	Even though git makes sure that it uses enough hexdigits to show
+	an abbreviated object name unambiguously, as more objects are
+	added to the repository over time, a short name that used to be
+	unique will stop being unique.  Git uses this many extra hexdigits
+	that are more than necessary to make the object name currently
+	unique, in the hope that its output will stay unique a bit longer.
+	Defaults to 0.
+
 core.compression::
 	An integer -1..9, indicating a default compression level.
 	-1 is the zlib default. 0 means no compression,
diff --git a/cache.h b/cache.h
index 33decd942d..931fb597ee 100644
--- a/cache.h
+++ b/cache.h
@@ -545,6 +545,7 @@ extern int assume_unchanged;
 extern int prefer_symlink_refs;
 extern int log_all_ref_updates;
 extern int warn_ambiguous_refs;
+extern int unique_abbrev_extra_length;
 extern int shared_repository;
 extern const char *apply_default_whitespace;
 extern const char *apply_default_ignorewhitespace;
diff --git a/config.c b/config.c
index 4b0a82040e..1aa72c2c41 100644
--- a/config.c
+++ b/config.c
@@ -489,6 +489,13 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.abbrevguard")) {
+		unique_abbrev_extra_length = git_config_int(var, value);
+		if (unique_abbrev_extra_length < 0)
+			unique_abbrev_extra_length = 0;
+		return 0;
+	}
+
 	if (!strcmp(var, "core.bare")) {
 		is_bare_repository_cfg = git_config_bool(var, value);
 		return 0;
diff --git a/environment.c b/environment.c
index de5581fe51..92e16b19b2 100644
--- a/environment.c
+++ b/environment.c
@@ -21,6 +21,7 @@ int prefer_symlink_refs;
 int is_bare_repository_cfg = -1; /* unspecified */
 int log_all_ref_updates = -1; /* unspecified */
 int warn_ambiguous_refs = 1;
+int unique_abbrev_extra_length;
 int repository_format_version;
 const char *git_commit_encoding;
 const char *git_log_output_encoding;
diff --git a/sha1_name.c b/sha1_name.c
index 484081de82..4a226adece 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -206,7 +206,9 @@ const char *find_unique_abbrev(const unsigned char *sha1, int len)
 		if (exists
 		    ? !status
 		    : status == SHORT_NAME_NOT_FOUND) {
-			hex[len] = 0;
+			int cut_at = len + unique_abbrev_extra_length;
+			cut_at = (cut_at < 40) ? cut_at : 40;
+			hex[cut_at] = 0;
 			return hex;
 		}
 		len++;