Re: just curious: what influences a commit hash?

[Matt Enright <awickedshimmy@gmail.com>]

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On Thu, 2009-03-05 at 07:36 +0100, stoecher@gmx.at wrote:
> Hi,
> 
> being new to git I did some experiments with commits looking at the hashes. What I observed:
> * The same commit (same file, same committer, same message) into different empty repositories (git init) gives different hashes. So I assume that also the time of the commit influences the hash. Is this intended? For what reason?
> * Having created two repositories exactly the same way (the history is the same except for the commit times and hashes) I applied the same patch (using git am) and again I got different hashes for these commits. So in some way also the repository/branch influences the hash of a commit!?

This should be expected if the initial hashes in the history are
different. The hash of a commit is based also on the hashes of all
parent commits - in this way git 'protects' the repository history by
guaranteeing that if two objects have the same hash, they will come from
the same history.
So the second issue is a consequence of the first, though I am not
certain why the first occurs (if the file contents and size are the
same, I would expect the hash for the blob/tree to be the same - maybe
due to git's special handling of initial commits?)

> From reading the Git user's manual, chapter 10, object storage format, I was not expecting this. Can someone explain or give a link to a more detailed description?
> 
> thank you,
> 
> Wolfgang
> 

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