Subject: Re: [PATCH v4 2/2] blame: make diff algorithm configurable

From: Phillip Wood <[email-masked]>
Date: Mon, 3 Nov 2025 14:32:39 +0000

Hi Antonin

Thanks for re-rolling, this is looking pretty sound, I've left a couple 
of fairly minor comments below.

On 01/11/2025 21:57, Antonin Delpeuch via GitGitGadget wrote:
> +static int blame_diff_algorithm_minimal(const struct option *option,
> +					const char *arg, int unset)
> +{
> +	int *opt = option->value;
> +
> +	BUG_ON_OPT_ARG(arg);
> +
> +	*opt &= ~XDF_DIFF_ALGORITHM_MASK;
> +	if (!unset)
> +		*opt |= XDF_NEED_MINIMAL;
One thing I'd not thought about before was the interaction between 
"--no-minimal" and "--diff-algorithm" The code above makes 
"--no-minimal" behave like "diff-algorithm=myers" which is consistent 
with the current behavior where the only options for the diff algorithm 
are "minimal" or "myers". An alternative would be for "--no-minimal" to 
just clear XDF_NEED_MINIMAL and behave like a no-op if it is given after 
"--diff-algorithm=patience" or "--diff-algorithm=histogram". I don't 
really have a strong preference either way.

> +static int blame_diff_algorithm_callback(const struct option *option,
> +					 const char *arg, int unset)
> +{
> +	int *opt = option->value;
> +	long value = parse_algorithm_value(arg);
> +
> +	BUG_ON_OPT_NEG(unset);
> +
> +	if (value < 0)
> +		return error(_("option diff-algorithm accepts \"myers\", "
> +			       "\"minimal\", \"patience\" and \"histogram\""));
> +
> +	*opt &= ~(XDF_NEED_MINIMAL | XDF_DIFF_ALGORITHM_MASK);

We can just use XDF_DIFF_ALGORITHM_MASK now that we've added 
XDF_NEED_MINMAL to it in the last commit.

> @@ -915,11 +960,16 @@ int cmd_blame(int argc,
>   		OPT_BIT('s', NULL, &output_option, N_("suppress author name and timestamp (Default: off)"), OUTPUT_NO_AUTHOR),
>   		OPT_BIT('e', "show-email", &output_option, N_("show author email instead of name (Default: off)"), OUTPUT_SHOW_EMAIL),
>   		OPT_BIT('w', NULL, &xdl_opts, N_("ignore whitespace differences"), XDF_IGNORE_WHITESPACE),
> +		OPT_CALLBACK_F(0, "diff-algorithm", &xdl_opts, N_("<algorithm>"),
> +			       N_("choose a diff algorithm"),
> +			       PARSE_OPT_NONEG, blame_diff_algorithm_callback),
>   		OPT_STRING_LIST(0, "ignore-rev", &ignore_rev_list, N_("rev"), N_("ignore <rev> when blaming")),
>   		OPT_STRING_LIST(0, "ignore-revs-file", &ignore_revs_file_list, N_("file"), N_("ignore revisions from <file>")),
>   		OPT_BIT(0, "color-lines", &output_option, N_("color redundant metadata from previous line differently"), OUTPUT_COLOR_LINE),
>   		OPT_BIT(0, "color-by-age", &output_option, N_("color lines by age"), OUTPUT_SHOW_AGE_WITH_COLOR),
> -		OPT_BIT(0, "minimal", &xdl_opts, N_("spend extra cycles to find better match"), XDF_NEED_MINIMAL),
> +		OPT_CALLBACK_F(0, "minimal", &xdl_opts, NULL,
> +			       N_("spend extra cycles to find a better match"),
> +			       PARSE_OPT_NOARG, blame_diff_algorithm_minimal),

Given the potential for confusing interactions between "--no-minimal" 
and "--diff-algorithm" I think it would be worth adding OPT_HIDDEN here.

> diff --git a/t/t8015-blame-diff-algorithm.sh b/t/t8015-blame-diff-algorithm.sh
> new file mode 100755
> index 0000000000..5318e18cb3
> --- /dev/null
> +++ b/t/t8015-blame-diff-algorithm.sh
> @@ -0,0 +1,203 @@
> + [...]
> +	git blame file.c > output &&
> +	sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output > without_varying_parts &&
> +	sed -e "s/ *$//g" without_varying_parts > actual &&

This would be more efficient if it was written as

	sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" \
	    -e "s/ *$//g" output >actual

Our test suite is really slow on windows so it is worth trying to avoid 
creating unnecessary processes.

Thanks

Phillip

> +	test_cmp expected actual
> +'
> +
> +test_expect_success 'blame honors --diff-algorithm option' '
> +	cat >expected <<-\EOF &&
> +	Commit_1 int g(size_t u)
> +	Commit_1 {
> +	Commit_1   while (u < 30)
> +	Commit_1   {
> +	Commit_1     u++;
> +	Commit_1   }
> +	Commit_1   return u;
> +	Commit_1 }
> +	Commit_2
> +	Commit_2 int h(int x, int y, int z)
> +	Commit_2 {
> +	Commit_2   if (z == 0)
> +	Commit_2   {
> +	Commit_2     return x;
> +	Commit_2   }
> +	Commit_2   return y;
> +	Commit_2 }
> +	EOF
> +
> +	git blame file.c --diff-algorithm histogram > output &&
> +	sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output > without_varying_parts &&
> +	sed -e "s/ *$//g" without_varying_parts > actual &&
> +	test_cmp expected actual
> +'
> +
> +test_expect_success 'blame honors diff.algorithm config variable' '
> +	cat >expected <<-\EOF &&
> +	Commit_1 int g(size_t u)
> +	Commit_1 {
> +	Commit_1   while (u < 30)
> +	Commit_1   {
> +	Commit_1     u++;
> +	Commit_1   }
> +	Commit_1   return u;
> +	Commit_1 }
> +	Commit_2
> +	Commit_2 int h(int x, int y, int z)
> +	Commit_2 {
> +	Commit_2   if (z == 0)
> +	Commit_2   {
> +	Commit_2     return x;
> +	Commit_2   }
> +	Commit_2   return y;
> +	Commit_2 }
> +	EOF
> +
> +	git -c diff.algorithm=histogram blame file.c > output &&
> +	sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output > without_varying_parts &&
> +	sed -e "s/ *$//g" without_varying_parts > actual &&
> +	test_cmp expected actual
> +'
> +
> +test_expect_success 'blame gives priority to --diff-algorithm over diff.algorithm' '
> +	cat >expected <<-\EOF &&
> +	Commit_1 int g(size_t u)
> +	Commit_1 {
> +	Commit_1   while (u < 30)
> +	Commit_1   {
> +	Commit_1     u++;
> +	Commit_1   }
> +	Commit_1   return u;
> +	Commit_1 }
> +	Commit_2
> +	Commit_2 int h(int x, int y, int z)
> +	Commit_2 {
> +	Commit_2   if (z == 0)
> +	Commit_2   {
> +	Commit_2     return x;
> +	Commit_2   }
> +	Commit_2   return y;
> +	Commit_2 }
> +	EOF
> +
> +	git -c diff.algorithm=myers blame file.c --diff-algorithm histogram &&
> +	sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output > without_varying_parts &&
> +	sed -e "s/ *$//g" without_varying_parts > actual &&
> +	test_cmp expected actual
> +'
> +
> +test_expect_success 'blame honors --minimal option' '
> +	cat >expected <<-\EOF &&
> +	Commit_1 x
> +	Commit_1 x
> +	Commit_1 x
> +	Commit_2 A
> +	Commit_2 B
> +	Commit_2 C
> +	Commit_2 D
> +	Commit_1 x
> +	Commit_2 E
> +	Commit_2 F
> +	Commit_2 G
> +	EOF
> +
> +	git blame file.txt --minimal > output &&
> +	sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output > actual &&
> +	test_cmp expected actual
> +'
> +
> +test_expect_success 'blame respects the order of diff options' '
> +	cat >expected <<-\EOF &&
> +	Commit_1 x
> +	Commit_1 x
> +	Commit_1 x
> +	Commit_2 A
> +	Commit_2 B
> +	Commit_2 C
> +	Commit_2 D
> +	Commit_2 x
> +	Commit_2 E
> +	Commit_2 F
> +	Commit_2 G
> +	EOF
> +
> +	git blame file.txt --minimal --diff-algorithm myers > output &&
> +	sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output > actual &&
> +	test_cmp expected actual
> +'
> +
> +test_done