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blame: make diff algorithm configurable

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The diff algorithm used in 'git-blame(1)' is set to 'myers',
without the possibility to change it aside from the `--minimal` option.

There has been long-standing interest in changing the default diff
algorithm to "histogram", and Git 3.0 was floated as a possible occasion
for taking some steps towards that:

https://lore.kernel.org/git/xmqqed873vgn.fsf@gitster.g/

As a preparation for this move, it is worth making sure that the diff
algorithm is configurable where useful.

Make it configurable in the `git-blame(1)` command by introducing the
`--diff-algorithm` option and make honor the `diff.algorithm` config
variable. Keep Myers diff as the default.

Signed-off-by: Antonin Delpeuch <antonin@delpeuch.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Antonin Delpeuch
2025-11-17 08:04:32 +00:00
committed by Junio C Hamano
parent 881793c4f7
commit ffffb987fc
6 changed files with 278 additions and 21 deletions
Download Patch File Download Diff File Expand all files Collapse all files

20
Documentation/diff-algorithm-option.adoc Normal file
View File

@@ -0,0 +1,20 @@
`--diff-algorithm=(patience|minimal|histogram|myers)`::
Choose a diff algorithm. The variants are as follows:
+
--
`default`;;
`myers`;;
The basic greedy diff algorithm. Currently, this is the default.
`minimal`;;
Spend extra time to make sure the smallest possible diff is
produced.
`patience`;;
Use "patience diff" algorithm when generating patches.
`histogram`;;
This algorithm extends the patience algorithm to "support
low-occurrence common elements".
--
+
For instance, if you configured the `diff.algorithm` variable to a
non-default value and want to use the default one, then you
have to use `--diff-algorithm=default` option.

21
Documentation/diff-options.adoc
View File

@@ -197,26 +197,7 @@ and starts with _<text>_, this algorithm attempts to prevent it from
appearing as a deletion or addition in the output. It uses the "patience
diff" algorithm internally.
`--diff-algorithm=(patience|minimal|histogram|myers)`::
Choose a diff algorithm. The variants are as follows:
+
--
`default`;;
`myers`;;
The basic greedy diff algorithm. Currently, this is the default.
`minimal`;;
Spend extra time to make sure the smallest possible diff is
produced.
`patience`;;
Use "patience diff" algorithm when generating patches.
`histogram`;;
This algorithm extends the patience algorithm to "support
low-occurrence common elements".
--
+
For instance, if you configured the `diff.algorithm` variable to a
non-default value and want to use the default one, then you
have to use `--diff-algorithm=default` option.
include::diff-algorithm-option.adoc[]
`--stat[=<width>[,<name-width>[,<count>]]]`::
Generate a diffstat. By default, as much space as necessary

2
Documentation/git-blame.adoc
View File

@@ -85,6 +85,8 @@ include::blame-options.adoc[]
Ignore whitespace when comparing the parent's version and
the child's to find where the lines came from.
include::diff-algorithm-option.adoc[]
--abbrev=<n>::
Instead of using the default 7+1 hexadecimal digits as the
abbreviated object name, use <m>+1 digits, where <m> is at

52
builtin/blame.c
View File

@@ -779,6 +779,19 @@ static int git_blame_config(const char *var, const char *value,
}
}
if (!strcmp(var, "diff.algorithm")) {
long diff_algorithm;
if (!value)
return config_error_nonbool(var);
diff_algorithm = parse_algorithm_value(value);
if (diff_algorithm < 0)
return error(_("unknown value for config '%s': %s"),
var, value);
xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
xdl_opts |= diff_algorithm;
return 0;
}
if (git_diff_heuristic_config(var, value, cb) < 0)
return -1;
if (userdiff_config(var, value) < 0)
@@ -824,6 +837,38 @@ static int blame_move_callback(const struct option *option, const char *arg, int
return 0;
}
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;
return 0;
}
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_DIFF_ALGORITHM_MASK;
*opt |= value;
return 0;
}
static int is_a_rev(const char *name)
{
struct object_id oid;
@@ -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 | PARSE_OPT_HIDDEN, blame_diff_algorithm_minimal),
OPT_STRING('S', NULL, &revs_file, N_("file"), N_("use revisions from <file> instead of calling git-rev-list")),
OPT_STRING(0, "contents", &contents_from, N_("file"), N_("use <file>'s contents as the final image")),
OPT_CALLBACK_F('C', NULL, &opt, N_("score"), N_("find line copies within and across files"), PARSE_OPT_OPTARG, blame_copy_callback),

1
t/meson.build
View File

@@ -955,6 +955,7 @@ integration_tests = [
't8012-blame-colors.sh',
't8013-blame-ignore-revs.sh',
't8014-blame-ignore-fuzzy.sh',
't8015-blame-diff-algorithm.sh',
't8020-last-modified.sh',
't9001-send-email.sh',
't9002-column.sh',

203
t/t8015-blame-diff-algorithm.sh Executable file
View File

@@ -0,0 +1,203 @@
#!/bin/sh
test_description='git blame with specific diff algorithm'
. ./test-lib.sh
test_expect_success setup '
cat >file.c <<-\EOF &&
int f(int x, int y)
{
if (x == 0)
{
return y;
}
return x;
}
int g(size_t u)
{
while (u < 30)
{
u++;
}
return u;
}
EOF
test_write_lines x x x x >file.txt &&
git add file.c file.txt &&
GIT_AUTHOR_NAME=Commit_1 git commit -m Commit_1 &&
cat >file.c <<-\EOF &&
int g(size_t u)
{
while (u < 30)
{
u++;
}
return u;
}
int h(int x, int y, int z)
{
if (z == 0)
{
return x;
}
return y;
}
EOF
test_write_lines x x x A B C D x E F G >file.txt &&
git add file.c file.txt &&
GIT_AUTHOR_NAME=Commit_2 git commit -m Commit_2
'
test_expect_success 'blame uses Myers diff algorithm by default' '
cat >expected <<-\EOF &&
Commit_2 int g(size_t u)
Commit_1 {
Commit_2 while (u < 30)
Commit_1 {
Commit_2 u++;
Commit_1 }
Commit_2 return u;
Commit_1 }
Commit_1
Commit_2 int h(int x, int y, int z)
Commit_1 {
Commit_2 if (z == 0)
Commit_1 {
Commit_2 return x;
Commit_1 }
Commit_2 return y;
Commit_1 }
EOF
git 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 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" \
-e "s/ *$//g" output >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 >output &&
sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" \
-e "s/ *$//g" output >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