blame: make diff algorithm configurable
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>
Antonin Delpeuch committed
Nov 17, 2025 at 08:04 UTC
ffffb987fcd3b3d6b88aceed87000ef4a5b6114e
6 files changed
+278
-21
Documentation/diff-algorithm-option.adoc
new
+20
@@ -0,0 +1,20 @@
1
+`--diff-algorithm=(patience|minimal|histogram|myers)`::
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+ Choose a diff algorithm. The variants are as follows:
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++
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+--
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+ `default`;;
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+ `myers`;;
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+ The basic greedy diff algorithm. Currently, this is the default.
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+ `minimal`;;
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+ Spend extra time to make sure the smallest possible diff is
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+ produced.
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+ `patience`;;
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+ Use "patience diff" algorithm when generating patches.
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+ `histogram`;;
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+ This algorithm extends the patience algorithm to "support
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+ low-occurrence common elements".
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+--
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++
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+For instance, if you configured the `diff.algorithm` variable to a
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+non-default value and want to use the default one, then you
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+have to use `--diff-algorithm=default` option.
Documentation/diff-options.adoc
+1
-20
@@ -197,26 +197,7 @@ and starts with _<text>_, this algorithm attempts to prevent it from
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appearing as a deletion or addition in the output. It uses the "patience
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diff" algorithm internally.
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-`--diff-algorithm=(patience|minimal|histogram|myers)`::
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- Choose a diff algorithm. The variants are as follows:
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-+
203
---
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- `default`;;
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- `myers`;;
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- The basic greedy diff algorithm. Currently, this is the default.
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- `minimal`;;
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- Spend extra time to make sure the smallest possible diff is
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- produced.
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- `patience`;;
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- Use "patience diff" algorithm when generating patches.
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- `histogram`;;
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- This algorithm extends the patience algorithm to "support
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- low-occurrence common elements".
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---
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-+
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-For instance, if you configured the `diff.algorithm` variable to a
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-non-default value and want to use the default one, then you
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-have to use `--diff-algorithm=default` option.
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+include::diff-algorithm-option.adoc[]
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`--stat[=<width>[,<name-width>[,<count>]]]`::
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Generate a diffstat. By default, as much space as necessary
Documentation/git-blame.adoc
+2
@@ -85,6 +85,8 @@ include::blame-options.adoc[]
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Ignore whitespace when comparing the parent's version and
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the child's to find where the lines came from.
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+include::diff-algorithm-option.adoc[]
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+
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--abbrev=<n>::
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Instead of using the default 7+1 hexadecimal digits as the
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abbreviated object name, use <m>+1 digits, where <m> is at
builtin/blame.c
+51
-1
@@ -779,6 +779,19 @@ static int git_blame_config(const char *var, const char *value,
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}
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}
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+ if (!strcmp(var, "diff.algorithm")) {
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+ long diff_algorithm;
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+ if (!value)
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+ return config_error_nonbool(var);
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+ diff_algorithm = parse_algorithm_value(value);
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+ if (diff_algorithm < 0)
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+ return error(_("unknown value for config '%s': %s"),
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+ var, value);
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+ xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
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+ xdl_opts |= diff_algorithm;
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+ return 0;
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+ }
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+
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if (git_diff_heuristic_config(var, value, cb) < 0)
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return -1;
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if (userdiff_config(var, value) < 0)
@@ -824,6 +837,38 @@ static int blame_move_callback(const struct option *option, const char *arg, int
837
return 0;
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}
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+static int blame_diff_algorithm_minimal(const struct option *option,
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+ const char *arg, int unset)
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+{
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+ int *opt = option->value;
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+
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+ BUG_ON_OPT_ARG(arg);
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+
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+ *opt &= ~XDF_DIFF_ALGORITHM_MASK;
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+ if (!unset)
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+ *opt |= XDF_NEED_MINIMAL;
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+
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+ return 0;
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+}
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+
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+static int blame_diff_algorithm_callback(const struct option *option,
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+ const char *arg, int unset)
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+{
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+ int *opt = option->value;
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+ long value = parse_algorithm_value(arg);
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+
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+ BUG_ON_OPT_NEG(unset);
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+
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+ if (value < 0)
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+ return error(_("option diff-algorithm accepts \"myers\", "
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+ "\"minimal\", \"patience\" and \"histogram\""));
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+
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+ *opt &= ~XDF_DIFF_ALGORITHM_MASK;
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+ *opt |= value;
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+
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+ return 0;
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+}
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+
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static int is_a_rev(const char *name)
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{
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struct object_id oid;
@@ -915,11 +960,16 @@ int cmd_blame(int argc,
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OPT_BIT('s', NULL, &output_option, N_("suppress author name and timestamp (Default: off)"), OUTPUT_NO_AUTHOR),
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OPT_BIT('e', "show-email", &output_option, N_("show author email instead of name (Default: off)"), OUTPUT_SHOW_EMAIL),
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OPT_BIT('w', NULL, &xdl_opts, N_("ignore whitespace differences"), XDF_IGNORE_WHITESPACE),
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+ OPT_CALLBACK_F(0, "diff-algorithm", &xdl_opts, N_("<algorithm>"),
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+ N_("choose a diff algorithm"),
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+ PARSE_OPT_NONEG, blame_diff_algorithm_callback),
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OPT_STRING_LIST(0, "ignore-rev", &ignore_rev_list, N_("rev"), N_("ignore <rev> when blaming")),
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OPT_STRING_LIST(0, "ignore-revs-file", &ignore_revs_file_list, N_("file"), N_("ignore revisions from <file>")),
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OPT_BIT(0, "color-lines", &output_option, N_("color redundant metadata from previous line differently"), OUTPUT_COLOR_LINE),
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OPT_BIT(0, "color-by-age", &output_option, N_("color lines by age"), OUTPUT_SHOW_AGE_WITH_COLOR),
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- OPT_BIT(0, "minimal", &xdl_opts, N_("spend extra cycles to find better match"), XDF_NEED_MINIMAL),
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+ OPT_CALLBACK_F(0, "minimal", &xdl_opts, NULL,
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+ N_("spend extra cycles to find a better match"),
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+ PARSE_OPT_NOARG | PARSE_OPT_HIDDEN, blame_diff_algorithm_minimal),
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OPT_STRING('S', NULL, &revs_file, N_("file"), N_("use revisions from <file> instead of calling git-rev-list")),
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OPT_STRING(0, "contents", &contents_from, N_("file"), N_("use <file>'s contents as the final image")),
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OPT_CALLBACK_F('C', NULL, &opt, N_("score"), N_("find line copies within and across files"), PARSE_OPT_OPTARG, blame_copy_callback),
t/meson.build
+1
@@ -955,6 +955,7 @@ integration_tests = [
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't8012-blame-colors.sh',
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't8013-blame-ignore-revs.sh',
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't8014-blame-ignore-fuzzy.sh',
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+ 't8015-blame-diff-algorithm.sh',
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't8020-last-modified.sh',
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't9001-send-email.sh',
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't9002-column.sh',
t/t8015-blame-diff-algorithm.sh
new
+203
@@ -0,0 +1,203 @@
1
+#!/bin/sh
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+
3
+test_description='git blame with specific diff algorithm'
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+
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+. ./test-lib.sh
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+
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+test_expect_success setup '
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+ cat >file.c <<-\EOF &&
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+ int f(int x, int y)
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+ {
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+ if (x == 0)
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+ {
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+ return y;
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+ }
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+ return x;
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+ }
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+
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+ int g(size_t u)
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+ {
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+ while (u < 30)
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+ {
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+ u++;
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+ }
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+ return u;
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+ }
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+ EOF
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+ test_write_lines x x x x >file.txt &&
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+ git add file.c file.txt &&
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+ GIT_AUTHOR_NAME=Commit_1 git commit -m Commit_1 &&
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+
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+ cat >file.c <<-\EOF &&
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+ int g(size_t u)
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+ {
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+ while (u < 30)
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+ {
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+ u++;
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+ }
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+ return u;
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+ }
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+
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+ int h(int x, int y, int z)
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+ {
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+ if (z == 0)
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+ {
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+ return x;
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+ }
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+ return y;
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+ }
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+ EOF
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+ test_write_lines x x x A B C D x E F G >file.txt &&
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+ git add file.c file.txt &&
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+ GIT_AUTHOR_NAME=Commit_2 git commit -m Commit_2
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+'
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+
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+test_expect_success 'blame uses Myers diff algorithm by default' '
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+ cat >expected <<-\EOF &&
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+ Commit_2 int g(size_t u)
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+ Commit_1 {
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+ Commit_2 while (u < 30)
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+ Commit_1 {
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+ Commit_2 u++;
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+ Commit_1 }
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+ Commit_2 return u;
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+ Commit_1 }
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+ Commit_1
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+ Commit_2 int h(int x, int y, int z)
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+ Commit_1 {
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+ Commit_2 if (z == 0)
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+ Commit_1 {
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+ Commit_2 return x;
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+ Commit_1 }
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+ Commit_2 return y;
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+ Commit_1 }
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+ EOF
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+
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+ git blame file.c >output &&
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+ sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output >without_varying_parts &&
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+ sed -e "s/ *$//g" without_varying_parts >actual &&
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+ test_cmp expected actual
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+'
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+
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+test_expect_success 'blame honors --diff-algorithm option' '
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+ cat >expected <<-\EOF &&
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+ Commit_1 int g(size_t u)
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+ Commit_1 {
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+ Commit_1 while (u < 30)
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+ Commit_1 {
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+ Commit_1 u++;
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+ Commit_1 }
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+ Commit_1 return u;
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+ Commit_1 }
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+ Commit_2
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+ Commit_2 int h(int x, int y, int z)
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+ Commit_2 {
95
+ Commit_2 if (z == 0)
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+ Commit_2 {
97
+ Commit_2 return x;
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+ Commit_2 }
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+ Commit_2 return y;
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+ Commit_2 }
101
+ EOF
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+
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+ git blame file.c --diff-algorithm histogram >output &&
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+ sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output >without_varying_parts &&
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+ sed -e "s/ *$//g" without_varying_parts >actual &&
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+ test_cmp expected actual
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+'
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+
109
+test_expect_success 'blame honors diff.algorithm config variable' '
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+ cat >expected <<-\EOF &&
111
+ Commit_1 int g(size_t u)
112
+ Commit_1 {
113
+ Commit_1 while (u < 30)
114
+ Commit_1 {
115
+ Commit_1 u++;
116
+ Commit_1 }
117
+ Commit_1 return u;
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+ Commit_1 }
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+ Commit_2
120
+ Commit_2 int h(int x, int y, int z)
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+ Commit_2 {
122
+ Commit_2 if (z == 0)
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+ Commit_2 {
124
+ Commit_2 return x;
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+ Commit_2 }
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+ Commit_2 return y;
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+ Commit_2 }
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+ EOF
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+
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+ git -c diff.algorithm=histogram blame file.c >output &&
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+ sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" \
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+ -e "s/ *$//g" output >actual &&
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+ test_cmp expected actual
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+'
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+
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+test_expect_success 'blame gives priority to --diff-algorithm over diff.algorithm' '
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+ cat >expected <<-\EOF &&
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+ Commit_1 int g(size_t u)
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+ Commit_1 {
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+ Commit_1 while (u < 30)
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+ Commit_1 {
142
+ Commit_1 u++;
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+ Commit_1 }
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+ Commit_1 return u;
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+ Commit_1 }
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+ Commit_2
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+ Commit_2 int h(int x, int y, int z)
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+ Commit_2 {
149
+ Commit_2 if (z == 0)
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+ Commit_2 {
151
+ Commit_2 return x;
152
+ Commit_2 }
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+ Commit_2 return y;
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+ Commit_2 }
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+ EOF
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+
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+ git -c diff.algorithm=myers blame file.c --diff-algorithm histogram >output &&
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+ sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" \
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+ -e "s/ *$//g" output >actual &&
160
+ test_cmp expected actual
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+'
162
+
163
+test_expect_success 'blame honors --minimal option' '
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+ cat >expected <<-\EOF &&
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+ Commit_1 x
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+ Commit_1 x
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+ Commit_1 x
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+ Commit_2 A
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+ Commit_2 B
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+ Commit_2 C
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+ Commit_2 D
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+ Commit_1 x
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+ Commit_2 E
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+ Commit_2 F
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+ Commit_2 G
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+ EOF
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+
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+ git blame file.txt --minimal >output &&
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+ sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output >actual &&
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+ test_cmp expected actual
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+'
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+
183
+test_expect_success 'blame respects the order of diff options' '
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+ cat >expected <<-\EOF &&
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+ Commit_1 x
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+ Commit_1 x
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+ Commit_1 x
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+ Commit_2 A
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+ Commit_2 B
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+ Commit_2 C
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+ Commit_2 D
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+ Commit_2 x
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+ Commit_2 E
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+ Commit_2 F
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+ Commit_2 G
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+ EOF
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+
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+ git blame file.txt --minimal --diff-algorithm myers >output &&
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+ sed -e "s/^[^ ]* (\([^ ]*\) [^)]*)/\1/g" output >actual &&
200
+ test_cmp expected actual
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+'
202
+
203
+test_done