diff: teach diff to read algorithm from diff driver

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 parsing to indicate the diff algorithm was set via command
line args.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 4b36d51beb66f08bff4576ed3afcbc9a6d78d63a..966e9696311967d705955061f91de2b7a4adf7c3 100644 (file)
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -758,6 +758,37 @@ 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 per path. For example,
+one may want to use the `minimal` diff algorithm for .json files, and the
+`histogram` for .c files, and so on without having to pass in the algorithm
+through the command line 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 `myers`, `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 `diff.<name>.command` is defined for path with the
+`diff=<name>` attribute, it is executed as an external diff driver
+(see above), and adding `diff.<name>.algorithm` has no effect, as the
+algorithm is not passed to the external diff driver.
 
 Defining a custom hunk-header
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

diff --git a/diff.c b/diff.c
index 3b97f5eca2f4cc7886929ff2bf6106bcb1663f14..b006467dc227855895049769823f2f47d060aa52 100644 (file)
--- a/diff.c
+++ b/diff.c
@@ -4460,15 +4460,13 @@ 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) {
                /*
@@ -4485,12 +4483,16 @@ 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 (!o->ignore_driver_algorithm && drv && 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)
@@ -4587,6 +4589,14 @@ 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,
@@ -5140,6 +5150,8 @@ 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;
 }
 
@@ -5155,6 +5167,8 @@ 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;
 }
 
@@ -5295,6 +5309,7 @@ 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 fd33caeb25dc84b53dbac18d59898bcd3cc36a7d..7b1fe25a53cf0fc2e027720a9843758cfbe2b691 100644 (file)
--- 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 8d1e408bb58f5e097f242d5c1fd2c7bb036deb6e..a8f5d3274a5a6007ca9acb3317752c6c1c7982a0 100644 (file)
--- 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 myers &&
+               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 151d9a52784b1f4a149913d38739c91e389d95d6..8719a5111cb77c94d9d4ae80924831c0c9a42aaf 100644 (file)
--- 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
@@ -393,6 +393,8 @@ 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 aee91bc77e6d592021d8597da7fce0123c779a32..24419db6973f5cd9bdfc6b9f300d5ce472c15cfd 100644 (file)
--- 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;