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[PATCH v3 2/2] diff: teach diff to read algorithm from diff driver

From
John Cai via GitGitGadget <gitgitgadget@gmail.com>
Date
Feb 17, 2023, 20:21 UTC
Message-ID
<b330222ce83bdf03c20085ff10fcff8a090474d5.1676665285.git.gitgitgadget@gmail.com>
In-Reply-To
<pull.1452.v3.git.git.1676665285.gitgitgadget@gmail.com>
From: John Cai <johncai86@gmail.com>

It can be useful to specify diff algorithms per file type. For example, one may want to use the minimal diff algorithm for .json files, another for .c files, etc.

The diff machinery already checks attributes for a diff driver. Teach the diff driver parser a new type "algorithm" to look for in the config, which will be used if a driver has been specified through the attributes.

Enforce precedence of the diff algorithm by favoring the command line option, then looking at the driver attributes & config combination, then finally the diff.algorithm config.

To enforce precedence order, use a new `ignore_driver_algorithm` member during options pasing to indicate the diff algorithm was set via command line args.

Signed-off-by: John Cai <johncai86@gmail.com>
---
 Documentation/gitattributes.txt | 37 ++++++++++++++++++++++++++++++++
 diff.c                          | 33 ++++++++++++++++++++--------
 diff.h                          |  1 +
 t/lib-diff-alternative.sh       | 38 ++++++++++++++++++++++++++++++++-
 userdiff.c                      |  4 +++-
 userdiff.h                      |  1 +
 6 files changed, 103 insertions(+), 11 deletions(-)
diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index c19e64ea0ef..f212079a131 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -758,6 +758,43 @@ with the above configuration, i.e. `j-c-diff`, with 7
 parameters, just like `GIT_EXTERNAL_DIFF` program is called.
 See linkgit:git[1] for details.
 
+Setting the internal diff algorithm
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The diff algorithm can be set through the `diff.algorithm` config key, but
+sometimes it may be helpful to set the diff algorithm by path. For example, one
+might wish to set a diff algorithm automatically for all `.json` files such that
+the user would not need to pass in a separate command line `--diff-algorithm`
+flag each time.
+
+First, in `.gitattributes`, assign the `diff` attribute for paths.
+
+------------------------
+*.json diff=<name>
+------------------------
+
+Then, define a "diff.<name>.algorithm" configuration to specify the diff
+algorithm, choosing from `meyers`, `patience`, `minimal`, or `histogram`.
+
+----------------------------------------------------------------
+[diff "<name>"]
+  algorithm = histogram
+----------------------------------------------------------------
+
+This diff algorithm applies to user facing diff output like git-diff(1),
+git-show(1) and is used for the `--stat` output as well. The merge machinery
+will not use the diff algorithm set through this method.
+
+NOTE: If the `command` key also exists, then Git will treat this as an external
+diff and attempt to use the value set for `command` as an external program. For
+instance, the following config, combined with the above `.gitattributes` file,
+will result in `command` favored over `algorithm`.
+
+----------------------------------------------------------------
+[diff "<name>"]
+  command = j-c-diff
+  algorithm = histogram
+----------------------------------------------------------------
 
 Defining a custom hunk-header
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/diff.c b/diff.c
index 5efc22ca06b..04469da6d34 100644
--- a/diff.c
+++ b/diff.c
@@ -4456,15 +4456,13 @@ static void run_diff_cmd(const char *pgm,
 	const char *xfrm_msg = NULL;
 	int complete_rewrite = (p->status == DIFF_STATUS_MODIFIED) && p->score;
 	int must_show_header = 0;
+	struct userdiff_driver *drv = NULL;
 
-
-	if (o->flags.allow_external) {
-		struct userdiff_driver *drv;
-
+	if (o->flags.allow_external || !o->ignore_driver_algorithm)
 		drv = userdiff_find_by_path(o->repo->index, attr_path);
-		if (drv && drv->external)
-			pgm = drv->external;
-	}
+
+	if (o->flags.allow_external && drv && drv->external)
+		pgm = drv->external;
 
 	if (msg) {
 		/*
@@ -4481,12 +4479,16 @@ static void run_diff_cmd(const char *pgm,
 		run_external_diff(pgm, name, other, one, two, xfrm_msg, o);
 		return;
 	}
-	if (one && two)
+	if (one && two) {
+		if (drv && !o->ignore_driver_algorithm && drv->algorithm)
+			set_diff_algorithm(o, drv->algorithm);
+
 		builtin_diff(name, other ? other : name,
 			     one, two, xfrm_msg, must_show_header,
 			     o, complete_rewrite);
-	else
+	} else {
 		fprintf(o->file, "* Unmerged path %s\n", name);
+	}
 }
 
 static void diff_fill_oid_info(struct diff_filespec *one, struct index_state *istate)
@@ -4583,6 +4585,14 @@ static void run_diffstat(struct diff_filepair *p, struct diff_options *o,
 	const char *name;
 	const char *other;
 
+	if (!o->ignore_driver_algorithm) {
+		struct userdiff_driver *drv = userdiff_find_by_path(o->repo->index, p->one->path);
+
+		if (drv && drv->algorithm) {
+			set_diff_algorithm(o, drv->algorithm);
+		}
+	}
+
 	if (DIFF_PAIR_UNMERGED(p)) {
 		/* unmerged */
 		builtin_diffstat(p->one->path, NULL, NULL, NULL,
@@ -5130,6 +5140,8 @@ static int diff_opt_diff_algorithm(const struct option *opt,
 		return error(_("option diff-algorithm accepts \"myers\", "
 			       "\"minimal\", \"patience\" and \"histogram\""));
 
+	options->ignore_driver_algorithm = 1;
+
 	return 0;
 }
 
@@ -5145,6 +5157,8 @@ static int diff_opt_diff_algorithm_no_arg(const struct option *opt,
 		BUG("available diff algorithms include \"myers\", "
 			       "\"minimal\", \"patience\" and \"histogram\"");
 
+	options->ignore_driver_algorithm = 1;
+
 	return 0;
 }
 
@@ -5285,6 +5299,7 @@ static int diff_opt_patience(const struct option *opt,
 	for (i = 0; i < options->anchors_nr; i++)
 		free(options->anchors[i]);
 	options->anchors_nr = 0;
+	options->ignore_driver_algorithm = 1;
 
 	return set_diff_algorithm(options, "patience");
 }
diff --git a/diff.h b/diff.h
index 41eb2c3d428..8d770b1d579 100644
--- a/diff.h
+++ b/diff.h
@@ -333,6 +333,7 @@ struct diff_options {
 	int prefix_length;
 	const char *stat_sep;
 	int xdl_opts;
+	int ignore_driver_algorithm;
 
 	/* see Documentation/diff-options.txt */
 	char **anchors;
diff --git a/t/lib-diff-alternative.sh b/t/lib-diff-alternative.sh
index 8d1e408bb58..2dc02bca873 100644
--- a/t/lib-diff-alternative.sh
+++ b/t/lib-diff-alternative.sh
@@ -105,10 +105,46 @@ index $file1..$file2 100644
  }
 EOF
 
+	cat >expect_diffstat <<EOF
+ file1 => file2 | 21 ++++++++++-----------
+ 1 file changed, 10 insertions(+), 11 deletions(-)
+EOF
+
 	STRATEGY=$1
 
+	test_expect_success "$STRATEGY diff from attributes" '
+		echo "file* diff=driver" >.gitattributes &&
+		git config diff.driver.algorithm "$STRATEGY" &&
+		test_must_fail git diff --no-index file1 file2 > output &&
+		cat expect &&
+		cat output &&
+		test_cmp expect output
+	'
+
+	test_expect_success "$STRATEGY diff from attributes has valid diffstat" '
+		echo "file* diff=driver" >.gitattributes &&
+		git config diff.driver.algorithm "$STRATEGY" &&
+		test_must_fail git diff --stat --no-index file1 file2 > output &&
+		test_cmp expect_diffstat output
+	'
+
 	test_expect_success "$STRATEGY diff" '
-		test_must_fail git diff --no-index "--$STRATEGY" file1 file2 > output &&
+		test_must_fail git diff --no-index "--diff-algorithm=$STRATEGY" file1 file2 > output &&
+		test_cmp expect output
+	'
+
+	test_expect_success "$STRATEGY diff command line precedence before attributes" '
+		echo "file* diff=driver" >.gitattributes &&
+		git config diff.driver.algorithm meyers &&
+		test_must_fail git diff --no-index "--diff-algorithm=$STRATEGY" file1 file2 > output &&
+		test_cmp expect output
+	'
+
+	test_expect_success "$STRATEGY diff attributes precedence before config" '
+		git config diff.algorithm default &&
+		echo "file* diff=driver" >.gitattributes &&
+		git config diff.driver.algorithm "$STRATEGY" &&
+		test_must_fail git diff --no-index file1 file2 > output &&
 		test_cmp expect output
 	'
 
diff --git a/userdiff.c b/userdiff.c
index d71b82feb74..ff25cfc4b4c 100644
--- a/userdiff.c
+++ b/userdiff.c
@@ -293,7 +293,7 @@ PATTERNS("scheme",
 	 "|([^][)(}{[ \t])+"),
 PATTERNS("tex", "^(\\\\((sub)*section|chapter|part)\\*{0,1}\\{.*)$",
 	 "\\\\[a-zA-Z@]+|\\\\.|[a-zA-Z0-9\x80-\xff]+"),
-{ "default", NULL, -1, { NULL, 0 } },
+{ "default", NULL, NULL, -1, { NULL, 0 } },
 };
 #undef PATTERNS
 #undef IPATTERN
@@ -394,6 +394,8 @@ int userdiff_config(const char *k, const char *v)
 		return parse_bool(&drv->textconv_want_cache, k, v);
 	if (!strcmp(type, "wordregex"))
 		return git_config_string(&drv->word_regex, k, v);
+	if (!strcmp(type, "algorithm"))
+		return git_config_string(&drv->algorithm, k, v);
 
 	return 0;
 }
diff --git a/userdiff.h b/userdiff.h
index aee91bc77e6..24419db6973 100644
--- a/userdiff.h
+++ b/userdiff.h
@@ -14,6 +14,7 @@ struct userdiff_funcname {
 struct userdiff_driver {
 	const char *name;
 	const char *external;
+	const char *algorithm;
 	int binary;
 	struct userdiff_funcname funcname;
 	const char *word_regex;

-- 
gitgitgadget
Previous: Elijah NewrenNext: Junio C Hamano
Message 59 of 78 in “Teach diff to honor diff algorithms set through git attributes”
  1. 0/2 Teach diff to honor diff algorithms set through git attributesJohn Cai via GitGitGadget, Feb 5, 2023
  2. 1/2 diff: consolidate diff algorithm option parsingJohn Cai via GitGitGadget, Feb 5, 2023
  3. Phillip WoodFeb 6, 2023
  4. 2/2 diff: teach diff to read gitattribute diff-algorithmJohn Cai via GitGitGadget, Feb 5, 2023
  5. Eric SunshineFeb 5, 2023
  6. John CaiFeb 6, 2023
  7. Phillip WoodFeb 6, 2023
  8. Eric SunshineFeb 6, 2023
  9. John CaiFeb 6, 2023
  10. Elijah NewrenFeb 9, 2023
  11. "bad" diffs (was: [PATCH 2/2] diff: teach diff to read gitattribute diff-algorithm)Ævar Arnfjörð Bjarmason, Feb 9, 2023
  12. John CaiFeb 9, 2023
  13. Ævar Arnfjörð BjarmasonFeb 6, 2023
  14. John CaiFeb 6, 2023
  15. Phillip WoodFeb 7, 2023
  16. John CaiFeb 7, 2023
  17. Elijah NewrenFeb 9, 2023
  18. Phillip WoodFeb 9, 2023
  19. Elijah NewrenFeb 10, 2023
  20. Phillip WoodFeb 11, 2023
  21. Jeff KingFeb 11, 2023
  22. Elijah NewrenFeb 15, 2023
  23. Jeff KingFeb 15, 2023
  24. Junio C HamanoFeb 15, 2023
  25. Phillip WoodFeb 15, 2023
  26. Jeff KingFeb 15, 2023
  27. Ævar Arnfjörð BjarmasonFeb 7, 2023
  28. Phillip WoodFeb 15, 2023
  29. Elijah NewrenFeb 9, 2023
  30. John CaiFeb 14, 2023
  31. Elijah NewrenFeb 15, 2023
  32. Elijah NewrenFeb 9, 2023
  33. Ævar Arnfjörð BjarmasonFeb 9, 2023
  34. Jeff KingFeb 11, 2023
  35. Jeff KingFeb 7, 2023
  36. Ævar Arnfjörð BjarmasonFeb 7, 2023
  37. Junio C HamanoFeb 7, 2023
  38. Ævar Arnfjörð BjarmasonFeb 7, 2023
  39. Junio C HamanoFeb 7, 2023
  40. Ævar Arnfjörð BjarmasonFeb 7, 2023
  41. John CaiFeb 9, 2023
  42. Jeff KingFeb 11, 2023
  43. 0/2 Teach diff to honor diff algorithms set through git attributesJohn Cai via GitGitGadget, Feb 14, 2023
  44. 1/2 diff: consolidate diff algorithm option parsingJohn Cai via GitGitGadget, Feb 14, 2023
  45. Junio C HamanoFeb 15, 2023
  46. John CaiFeb 15, 2023
  47. Junio C HamanoFeb 15, 2023
  48. Jeff KingFeb 16, 2023
  49. Junio C HamanoFeb 16, 2023
  50. John CaiFeb 16, 2023
  51. 2/2 diff: teach diff to read gitattribute diff-algorithmJohn Cai via GitGitGadget, Feb 14, 2023
  52. Junio C HamanoFeb 15, 2023
  53. Junio C HamanoFeb 15, 2023
  54. John CaiFeb 16, 2023
  55. 0/2 Teach diff to honor diff algorithms set through git attributesJohn Cai via GitGitGadget, Feb 17, 2023
  56. 1/2 diff: consolidate diff algorithm option parsingJohn Cai via GitGitGadget, Feb 17, 2023
  57. Junio C HamanoFeb 17, 2023
  58. Elijah NewrenFeb 18, 2023
  59. 2/2 diff: teach diff to read algorithm from diff driverJohn Cai via GitGitGadget, Feb 17, 2023
  60. Junio C HamanoFeb 17, 2023
  61. Elijah NewrenFeb 18, 2023
  62. John CaiFeb 20, 2023
  63. Elijah NewrenFeb 20, 2023
  64. John CaiFeb 20, 2023
  65. Elijah NewrenFeb 20, 2023
  66. John CaiFeb 20, 2023
  67. Jeff KingFeb 22, 2023
  68. John CaiFeb 24, 2023
  69. Elijah NewrenFeb 18, 2023
  70. John CaiFeb 20, 2023
  71. 0/2 Teach diff to honor diff algorithms set through git attributesJohn Cai via GitGitGadget, Feb 20, 2023
  72. 1/2 diff: consolidate diff algorithm option parsingJohn Cai via GitGitGadget, Feb 20, 2023
  73. 2/2 diff: teach diff to read algorithm from diff driverJohn Cai via GitGitGadget, Feb 20, 2023
  74. Junio C HamanoFeb 21, 2023
  75. Elijah NewrenFeb 21, 2023
  76. Junio C HamanoFeb 21, 2023
  77. John CaiFeb 21, 2023
  78. Elijah NewrenFeb 21, 2023

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