From: Taylor Blau <me@ttaylorr.com>
Date: Wed, 22 Jan 2025 22:08:39 GMT
Subject: Re: [PATCH v3 1/8] pack-objects: create new name-hash function version
Message-ID: <Z5FsZ7MK6YcmeYIV@nand.local>
In-Reply-To: <68b4127580e2d475bec0d7cd0f6a9ae5e626b3c9.1734715194.git.gitgitgadget@gmail.com>

On Fri, Dec 20, 2024 at 05:19:47PM +0000, Jonathan Tan via GitGitGadget wrote:
> The first change is to be more careful about paths using non-ASCII
> characters. With these characters in mind, reverse the bits in the byte
> as the least-significant bits have the highest entropy and we want to
> maximize their influence. This is done with some bit manipulation that
> swaps the two halves, then the quarters within those halves, and then
> the bits within those quarters.

Makes sense, and seems quite reasonable.

> The second change is to perform hash composition operations at every
> level of the path. This is done by storing a 'base' hash value that
> contains the hash of the parent directory. When reaching a directory
> boundary, we XOR the current level's name-hash value with a downshift of
> the previous level's hash. This perturbation intends to create low-bit
> distinctions for paths with the same final 16 bytes but distinct parent
> directory structures.

Very clever, I love this idea.

Thanks,
Taylor