[PATCH v3] difftool.c: learn a new way start from specified file

["ZheNing Hu via GitGitGadget" <gitgitgadget@gmail.com>]

From: ZheNing Hu <adlternative@gmail.com>

`git difftool` only allow us to select file to view in turn.
If there is a commit with many files and we exit in the search,
We will have to traverse list again to get the file diff which
we want to see. Therefore, here is a new method: user can use
`git difftool --start-from=<filename>` to start viewing from
the specified file. This will improve the user experience.
At the same time, turn bit field constants into bit shift format
in `diff.h`.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>
---
    difftool.c: learn a new way start at specified file
    
    git user may should travel the diff list to choice file diff to view, if
    they exit in midway,they must travel it again. By starting from the
    specified file method, provides a possibility for this user-friendly
    solution.
    
    this patch's origin discuss is here:
    https://lore.kernel.org/git/gOXOaoqn-E9A2ob7ykWEcDc7ZxmSwAjcP5CCFKfr5ejCOWZQ1lfAUZcbgYT9AyQCcDgJvCrnrtziXiels-Hxol3xlkGTVHk24SvAdaSUtKQ=@rtzoeller.com/
    
    Maybe this patch is more like skip to in Junio's original thread than
    the previous versions.
    
    Thanks!

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-870%2Fadlternative%2Fdifftool_save_point-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-870/adlternative/difftool_save_point-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/870

Range-diff vs v2:

 1:  2468eaff322b < -:  ------------ git-difftool-helper.sh: learn a new way go back to last save point
 -:  ------------ > 1:  29fc6b4bc08f difftool.c: learn a new way start from specified file


 Documentation/git-difftool.txt |  3 +++
 builtin/difftool.c             |  7 ++++++-
 diff.c                         |  9 +++++++++
 diff.h                         | 20 ++++++++++----------
 t/t7800-difftool.sh            | 12 ++++++++++++
 5 files changed, 40 insertions(+), 11 deletions(-)

diff --git a/Documentation/git-difftool.txt b/Documentation/git-difftool.txt
index 484c485fd06c..552be097dfea 100644
--- a/Documentation/git-difftool.txt
+++ b/Documentation/git-difftool.txt
@@ -34,6 +34,9 @@ OPTIONS
 	This is the default behaviour; the option is provided to
 	override any configuration settings.
 
+--start-from::
+	Start viewing diff from the specified file.
+
 -t <tool>::
 --tool=<tool>::
 	Use the diff tool specified by <tool>.  Valid values include
diff --git a/builtin/difftool.c b/builtin/difftool.c
index 6e18e623fddf..67d2befa1210 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -690,7 +690,7 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 {
 	int use_gui_tool = 0, dir_diff = 0, prompt = -1, symlinks = 0,
 	    tool_help = 0, no_index = 0;
-	static char *difftool_cmd = NULL, *extcmd = NULL;
+	static char *difftool_cmd = NULL, *extcmd = NULL, *start_file = NULL;
 	struct option builtin_difftool_options[] = {
 		OPT_BOOL('g', "gui", &use_gui_tool,
 			 N_("use `diff.guitool` instead of `diff.tool`")),
@@ -714,6 +714,8 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 		OPT_STRING('x', "extcmd", &extcmd, N_("command"),
 			   N_("specify a custom command for viewing diffs")),
 		OPT_ARGUMENT("no-index", &no_index, N_("passed to `diff`")),
+		OPT_STRING(0, "start-from", &start_file, N_("start-from"),
+			   N_("start viewing diff from the specified file")),
 		OPT_END()
 	};
 
@@ -724,6 +726,9 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 			     builtin_difftool_usage, PARSE_OPT_KEEP_UNKNOWN |
 			     PARSE_OPT_KEEP_DASHDASH);
 
+	if (start_file)
+		setenv("START_FILE", start_file, 1);
+
 	if (tool_help)
 		return print_tool_help();
 
diff --git a/diff.c b/diff.c
index 69e3bc00ed8f..cdad26f99063 100644
--- a/diff.c
+++ b/diff.c
@@ -4249,6 +4249,7 @@ static void run_external_diff(const char *pgm,
 			      const char *xfrm_msg,
 			      struct diff_options *o)
 {
+	const char *start_file = NULL;
 	struct strvec argv = STRVEC_INIT;
 	struct strvec env = STRVEC_INIT;
 	struct diff_queue_struct *q = &diff_queued_diff;
@@ -4272,9 +4273,17 @@ static void run_external_diff(const char *pgm,
 
 	diff_free_filespec_data(one);
 	diff_free_filespec_data(two);
+
+	start_file = xstrdup_or_null(getenv("START_FILE"));
+	if (start_file) {
+		if (strcmp(start_file, name))
+			goto finish;
+		unsetenv("START_FILE");
+	}
 	if (run_command_v_opt_cd_env(argv.v, RUN_USING_SHELL, NULL, env.v))
 		die(_("external diff died, stopping at %s"), name);
 
+finish:
 	remove_tempfile();
 	strvec_clear(&argv);
 	strvec_clear(&env);
diff --git a/diff.h b/diff.h
index 2ff2b1c7f2ca..f67c43f5af95 100644
--- a/diff.h
+++ b/diff.h
@@ -86,18 +86,18 @@ typedef void (*diff_format_fn_t)(struct diff_queue_struct *q,
 
 typedef struct strbuf *(*diff_prefix_fn_t)(struct diff_options *opt, void *data);
 
-#define DIFF_FORMAT_RAW		0x0001
-#define DIFF_FORMAT_DIFFSTAT	0x0002
-#define DIFF_FORMAT_NUMSTAT	0x0004
-#define DIFF_FORMAT_SUMMARY	0x0008
-#define DIFF_FORMAT_PATCH	0x0010
-#define DIFF_FORMAT_SHORTSTAT	0x0020
-#define DIFF_FORMAT_DIRSTAT	0x0040
+#define DIFF_FORMAT_RAW		(1U<<0)
+#define DIFF_FORMAT_DIFFSTAT	(1U<<1)
+#define DIFF_FORMAT_NUMSTAT	(1U<<2)
+#define DIFF_FORMAT_SUMMARY	(1U<<3)
+#define DIFF_FORMAT_PATCH	(1U<<4)
+#define DIFF_FORMAT_SHORTSTAT	(1U<<5)
+#define DIFF_FORMAT_DIRSTAT	(1U<<6)
 
 /* These override all above */
-#define DIFF_FORMAT_NAME	0x0100
-#define DIFF_FORMAT_NAME_STATUS	0x0200
-#define DIFF_FORMAT_CHECKDIFF	0x0400
+#define DIFF_FORMAT_NAME	(1U<<8)
+#define DIFF_FORMAT_NAME_STATUS	(1U<<9)
+#define DIFF_FORMAT_CHECKDIFF	(1U<<10)
 
 /* Same as output_format = 0 but we know that -s flag was given
  * and we should not give default value to output_format.
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index 9662abc1e784..74baac96a23f 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -762,4 +762,16 @@ test_expect_success 'difftool --gui, --tool and --extcmd are mutually exclusive'
 	test_must_fail git difftool --gui --tool=test-tool --extcmd=cat
 '
 
+test_expect_success 'difftool --start-from' '
+	difftool_test_setup &&
+	test_when_finished git reset --hard &&
+	echo 1 >1 &&
+	echo 2 >2 &&
+	echo 3 >3 &&
+	git add 1 2 3 &&
+	git commit -a -m "123" &&
+	git difftool --start-from="2" HEAD^ 2>&1 >output &&
+	test_line_count = 4 output
+'
+
 test_done

base-commit: e6362826a0409539642a5738db61827e5978e2e4
-- 
gitgitgadget