Subject: Re: [PATCH v2 1/2] diff: consolidate diff algorithm option parsing

From: Junio C Hamano <[email-masked]>
Date: Tue, 14 Feb 2023 18:38:20 -0800

"John Cai via GitGitGadget" <[email-masked]> writes:

> From: John Cai <[email-masked]>
>
> The diff option parsing for --minimal, --patience, --histgoram can all
> be consolidated into one function. This is a preparatory step for the
> subsequent commit which teaches diff to keep track of whether or not a
> diff algorithm has been set via the command line.

Everybody other than patience used to be just a bit-op but now
everybody is a callback?

> diff --git a/diff.c b/diff.c
> index 329eebf16a0..92a0eab942e 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -3437,6 +3437,22 @@ static int diff_filepair_is_phoney(struct diff_filespec *one,
>  	return !DIFF_FILE_VALID(one) && !DIFF_FILE_VALID(two);
>  }
>  
> +static int set_diff_algorithm(struct diff_options *opts,
> +			      const char *alg)
> +{
> +	long value = parse_algorithm_value(alg);
> +
> +	if (value < 0)
> +		return 1;
> +
> +	/* clear out previous settings */
> +	DIFF_XDL_CLR(opts, NEED_MINIMAL);
> +	opts->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
> +	opts->xdl_opts |= value;
> +
> +	return 0;
> +}

The above is a faithful copy of diff_opt_diff_algorithm(), except
that it returns 1 (not -1) on failure, which is unexpected in this
codebase, and should be corrected if this patch gets rerolled.

>  static void builtin_diff(const char *name_a,
>  			 const char *name_b,
>  			 struct diff_filespec *one,
> @@ -5107,17 +5123,40 @@ static int diff_opt_diff_algorithm(const struct option *opt,
>  				   const char *arg, int unset)
>  {
>  	struct diff_options *options = opt->value;
> -	long value = parse_algorithm_value(arg);
>  
>  	BUG_ON_OPT_NEG(unset);
> -	if (value < 0)
> +
> +	if (set_diff_algorithm(options, arg))
>  		return error(_("option diff-algorithm accepts \"myers\", "
>  			       "\"minimal\", \"patience\" and \"histogram\""));
>  
> -	/* clear out previous settings */
> -	DIFF_XDL_CLR(options, NEED_MINIMAL);
> -	options->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
> -	options->xdl_opts |= value;
> +	return 0;
> +}

This version of diff_opt_diff_algorithm() behaves identically from
the version before this patch, which is excellent.

> +static int diff_opt_diff_algorithm_no_arg(const struct option *opt,
> +				   const char *arg, int unset)
> +{
> +	struct diff_options *options = opt->value;
> +
> +	BUG_ON_OPT_NEG(unset);
> +	BUG_ON_OPT_ARG(arg);
> +
> +	if (!strcmp(opt->long_name, "patience")) {
> +		size_t i;
> +		/*
> +		 * Both --patience and --anchored use PATIENCE_DIFF
> +		 * internally, so remove any anchors previously
> +		 * specified.
> +		 */
> +		for (i = 0; i < options->anchors_nr; i++)
> +			free(options->anchors[i]);
> +		options->anchors_nr = 0;
> +	}
> +
> +	if (set_diff_algorithm(options, opt->long_name))
> +		BUG("available diff algorithms include \"myers\", "
> +			       "\"minimal\", \"patience\" and \"histogram\"");
> +
>  	return 0;
>  }

Calling this instead of diff_opt_patience() would make "--patience"
parsed identically as before without this patch, which is excellent.

> @@ -5562,9 +5581,10 @@ struct option *add_diff_options(const struct option *opts,
>  			    N_("prevent rename/copy detection if the number of rename/copy targets exceeds given limit")),
>  
>  		OPT_GROUP(N_("Diff algorithm options")),
> -		OPT_BIT(0, "minimal", &options->xdl_opts,
> -			N_("produce the smallest possible diff"),
> -			XDF_NEED_MINIMAL),
> +		OPT_CALLBACK_F(0, "minimal", options, NULL,
> +			       N_("produce the smallest possible diff"),
> +			       PARSE_OPT_NONEG | PARSE_OPT_NOARG,
> +			       diff_opt_diff_algorithm_no_arg),

I offhand cannot say that these two are equivalent, even though they
ought to be (otherwise this patch would break things).  The callback
seems to do much more than just a simple "flip the NEED_MINIMAL bit
on".

> -		OPT_BITOP(0, "histogram", &options->xdl_opts,
> -			  N_("generate diff using the \"histogram diff\" algorithm"),
> -			  XDF_HISTOGRAM_DIFF, XDF_DIFF_ALGORITHM_MASK),
> +			       diff_opt_diff_algorithm_no_arg),
> +		OPT_CALLBACK_F(0, "histogram", options, NULL,
> +			       N_("generate diff using the \"histogram diff\" algorithm"),
> +			       PARSE_OPT_NONEG | PARSE_OPT_NOARG,
> +			       diff_opt_diff_algorithm_no_arg),

Likewise.

By nature, patience (and anchored) needs to do much more than
everybody else, so it almost feels that it is OK (and preferable,
even) to leave it a special case to make the distinction stand out.
Consolidating everybody else who are much simpler to share the
more complex callback does not look like a good change to me, at
least at the first glance.

Thanks.