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

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.