linux-ext4 - algorithm for half-md4 used in htree directories

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Date:   Fri, 1 Oct 2021 14:49:37 +0300
From:   Avi Deitcher <avi@...tcher.net>
To:     linux-ext4@...r.kernel.org
Subject: algorithm for half-md4 used in htree directories

Hi,

I have been trying to understand the algorithm used for the "half-md4"
in htree-structured directories. Going through the code (and trying
not to get into reverse engineering), it looks like it is part of md4
but not entirely? Yet any subset I take doesn't quite line up with
what I see in an actual sample.

What is the algorithm it is using to turn an entry of, e.g., "file125"
into the appropriate hash. I did run a live sample, and try to get
some form of correlation between the actual md4 hash (16 bytes) of the
above to the actual entry (4 bytes) shown by debugfs, without much
luck.

What basic thing am I missing?

Separately, how does the seed play into it?

Thanks
Avi