Subject: Re: [[PATCH v2] 4/4] rebase: avoid computing unnecessary patch IDs

From: Junio C Hamano <[email-masked]>
Date: Fri, 29 Jul 2016 14:46:16 -0700

Kevin Willford <[email-masked]> writes:

>  static int patch_id_cmp(struct patch_id *a,
>  			struct patch_id *b,
> -			void *keydata)
> +			struct diff_options *opt)
>  {
> +	if (is_null_sha1(a->patch_id) &&
> +	    commit_patch_id(a->commit, opt, a->patch_id, 0))
> +		return error("Could not get patch ID for %s",
> +			oid_to_hex(&a->commit->object.oid));
> +	if (is_null_sha1(b->patch_id) &&
> +	    commit_patch_id(b->commit, opt, b->patch_id, 0))
> +		return error("Could not get patch ID for %s",
> +			oid_to_hex(&b->commit->object.oid));
>  	return hashcmp(a->patch_id, b->patch_id);
>  }

These error returns initially looks slightly iffy in that in general
the caller of any_cmp_fn() wants to know how a/b compares, but by
returning error(), it always says "a is smaller than b".  This
however may be OK because the callers in hashmap_get* implementation
only wants to know "are they equal?", and we are saying "no they
cannot possibly be equal" here.  The original that ran a full
commit_patch_id() in now-removed add_commit() helper function didn't
even diagnose this error and silently omitted the commit from the
candidate list, so this may be even seen as an improvement.

The idea of using the two level hash, computing the more expensive
one only when the hashmap hashes of the result of the cheaper hash
function collide, is excellent.  Thanks.