easy way to demonstrate length of colliding SHA-1 prefixes?

>   as part of an upcoming git class i'm delivering, i thought it would
> be amusing to demonstrate the maximum length of colliding SHA-1
> prefixes in a repository (in my case, i use the linux kernel git repo
> for most of my examples).
>
>   is there a way to display the objects in the object database that
> clash in the longest object name SHA-1 prefix; i mean, short of
> manually listing all object names, running that through cut and sort
> and uniq and ... you get the idea.
>
>   is there a cute way to do that? thanks.

> On Sun, Dec 02 2018, Robert P. J. Day wrote:
>
> >   as part of an upcoming git class i'm delivering, i thought it
> > would be amusing to demonstrate the maximum length of colliding
> > SHA-1 prefixes in a repository (in my case, i use the linux kernel
> > git repo for most of my examples).
> >
> >   is there a way to display the objects in the object database
> > that clash in the longest object name SHA-1 prefix; i mean, short
> > of manually listing all object names, running that through cut and
> > sort and uniq and ... you get the idea.
> >
> >   is there a cute way to do that? thanks.
>
> You'll always need to list them all. It's inherently an operation
> where for each SHA-1 you need to search for other ones with that
> prefix up to a given length.

> Perhaps you've missed that you can use --abbrev=N for this, and just
> grep for things that are loger than that N, e.g. for linux.git:
>
>     git log --oneline --abbrev=10 --pretty=format:%h |
>     grep -E -v '^.{10}$' |
>     perl -pe 's/^(.{10}).*/$1/'
>
> This will list the 4 objects that need more than 10 characters to be
> shown unambiguously. If you then "git cat-file -t" them you'll get
> the disambiguation help.

> 
> On Sun, Dec 02 2018, Robert P. J. Day wrote:
> 
>>    as part of an upcoming git class i'm delivering, i thought it would
>> be amusing to demonstrate the maximum length of colliding SHA-1
>> prefixes in a repository (in my case, i use the linux kernel git repo
>> for most of my examples).
>>
>>    is there a way to display the objects in the object database that
>> clash in the longest object name SHA-1 prefix; i mean, short of
>> manually listing all object names, running that through cut and sort
>> and uniq and ... you get the idea.
>>
>>    is there a cute way to do that? thanks.
> 

> You'll always need to list them all. It's inherently an operation where
> for each SHA-1 you need to search for other ones with that prefix up to
> a given length.
> 
> Perhaps you've missed that you can use --abbrev=N for this, and just
> grep for things that are loger than that N, e.g. for linux.git:
> 
>      git log --oneline --abbrev=10 --pretty=format:%h |
>      grep -E -v '^.{10}$' |
>      perl -pe 's/^(.{10}).*/$1/'

> Here is a one-liner to do it. It is Perl line noise, so it's not very cute,
> thought that is subjective. The output shown below is for the Git project
> (not Linux) repository as I've currently synced it:
> 
> $ git rev-list --objects HEAD | sort | perl -anE 'BEGIN { $prev = ""; $long
> = "" } $n = $F[0]; for my $i (reverse 1..40) {last if $i < length($long); if
> (substr($prev, 0, $i) eq substr($n, 0, $i)) {$long = substr($prev, 0, $i);
> last} } $prev = $n; END {say $long}'

> > You'll always need to list them all. It's inherently an operation where
> > for each SHA-1 you need to search for other ones with that prefix up to
> > a given length.
> > 
> > Perhaps you've missed that you can use --abbrev=N for this, and just
> > grep for things that are loger than that N, e.g. for linux.git:
> > 
> >      git log --oneline --abbrev=10 --pretty=format:%h |
> >      grep -E -v '^.{10}$' |
> >      perl -pe 's/^(.{10}).*/$1/'
> 
> I think the goal was to search all object hashes, not just commits. And git
> rev-list --objects will do that.