Re: [PATCH v2 2/4] Add structure representing hash algorithm

From: Stefan Beller <sbeller@google.com>
Date: Oct 30, 2017, 16:14 UTC
Message-ID: <CAGZ79kb-+DFz2m-8arh+RDJibBMc_9vjTC5ZoXbojBBcThrBdQ@mail.gmail.com>
In-Reply-To: <20171028181239.59458-3-sandals@crustytoothpaste.net>

On Sat, Oct 28, 2017 at 11:12 AM, brian m. carlson <sandals@crustytoothpaste.net> wrote:

Show 22 quoted lines

> Since in the future we want to support an additional hash algorithm, add
> a structure that represents a hash algorithm and all the data that must
> go along with it.  Add a constant to allow easy enumeration of hash
> algorithms.  Implement function typedefs to create an abstract API that
> can be used by any hash algorithm, and wrappers for the existing SHA1
> functions that conform to this API.
>
> Expose a value for hex size as well as binary size.  While one will
> always be twice the other, the two values are both used extremely
> commonly throughout the codebase and providing both leads to improved
> readability.
>
> Don't include an entry in the hash algorithm structure for the null
> object ID.  As this value is all zeros, any suitably sized all-zero
> object ID can be used, and there's no need to store a given one on a
> per-hash basis.
>
> The current hash function transition plan envisions a time when we will
> accept input from the user that might be in SHA-1 or in the NewHash
> format.  Since we cannot know which the user has provided, add a
> constant representing the unknown algorithm to allow us to indicate that
> we must look the correct value up.

Cool.

Show 13 quoted lines

> +
> +const struct git_hash_algo hash_algos[GIT_HASH_NALGOS] = {
> +       {
> +               NULL,
> +               0x00000000,
> +               0,
> +               0,
> +               0,
> +               NULL,
> +               NULL,
> +               NULL,
> +               NULL,
> +               NULL,

If we are fancy we could provide an appropriate die() call as the function pointers. That way if you call these functions by accident, you get a well worded warning instead of a segfault.