diff: add option to show context between close hunks

Merge two hunks if there is only the specified number of otherwise unshown
context between them.  For --inter-hunk-context=1, the resulting patch has
the same number of lines but shows uninterrupted context instead of a
context header line in between.

Patches generated with this option are easier to read but are also more
likely to conflict if the file to be patched contains other changes.

This patch keeps the default for this option at 0.  It is intended to just
make the feature available in order to see its advantages and downsides.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

7 files changed, 106 insertions, 1 deletions

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index c62b45cdba..c7fcc8007a 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -215,6 +215,10 @@ endif::git-format-patch[]
 -w::
 	Shorthand for "--ignore-all-space".
 
+--inter-hunk-context=<lines>::
+	Show the context between diff hunks, up to the specified number
+	of lines, thereby fusing hunks that are close to each other.
+
 --exit-code::
 	Make the program exit with codes similar to diff(1).
 	That is, it exits with 1 if there were differences and
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index e00454983e..a046441974 100755
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -776,6 +776,7 @@ _git_diff ()
 			--no-ext-diff
 			--no-prefix --src-prefix= --dst-prefix=
 			--base --ours --theirs
+			--inter-hunk-context=
 			"
 		return
 		;;
@@ -967,6 +968,7 @@ _git_log ()
 			--color-words --walk-reflogs
 			--parents --children --full-history
 			--merge
+			--inter-hunk-context=
 			"
 		return
 		;;
diff --git a/diff.c b/diff.c
index 0484601f42..56b80f9609 100644
--- a/diff.c
+++ b/diff.c
@@ -1469,6 +1469,7 @@ static void builtin_diff(const char *name_a,
 		ecbdata.file = o->file;
 		xpp.flags = XDF_NEED_MINIMAL | o->xdl_opts;
 		xecfg.ctxlen = o->context;
+		xecfg.interhunkctxlen = o->interhunkcontext;
 		xecfg.flags = XDL_EMIT_FUNCNAMES;
 		if (pe)
 			xdiff_set_find_func(&xecfg, pe->pattern, pe->cflags);
@@ -2538,6 +2539,9 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		options->b_prefix = arg + 13;
 	else if (!strcmp(arg, "--no-prefix"))
 		options->a_prefix = options->b_prefix = "";
+	else if (opt_arg(arg, '\0', "inter-hunk-context",
+			 &options->interhunkcontext))
+		;
 	else if (!prefixcmp(arg, "--output=")) {
 		options->file = fopen(arg + strlen("--output="), "w");
 		options->close_file = 1;
diff --git a/diff.h b/diff.h
index 42582edee6..4d5a32781d 100644
--- a/diff.h
+++ b/diff.h
@@ -78,6 +78,7 @@ struct diff_options {
 	const char *a_prefix, *b_prefix;
 	unsigned flags;
 	int context;
+	int interhunkcontext;
 	int break_opt;
 	int detect_rename;
 	int skip_stat_unmatch;
diff --git a/t/t4032-diff-inter-hunk-context.sh b/t/t4032-diff-inter-hunk-context.sh
new file mode 100755
index 0000000000..e4e3e28fc7
--- /dev/null
+++ b/t/t4032-diff-inter-hunk-context.sh
@@ -0,0 +1,92 @@
+#!/bin/sh
+
+test_description='diff hunk fusing'
+
+. ./test-lib.sh
+
+f() {
+	echo $1
+	i=1
+	while test $i -le $2
+	do
+		echo $i
+		i=$(expr $i + 1)
+	done
+	echo $3
+}
+
+t() {
+	case $# in
+	4) hunks=$4; cmd="diff -U$3";;
+	5) hunks=$5; cmd="diff -U$3 --inter-hunk-context=$4";;
+	esac
+	label="$cmd, $1 common $2"
+	file=f$1
+	expected=expected.$file.$3.$hunks
+
+	if ! test -f $file
+	then
+		f A $1 B >$file
+		git add $file
+		git commit -q -m. $file
+		f X $1 Y >$file
+	fi
+
+	test_expect_success "$label: count hunks ($hunks)" "
+		test $(git $cmd $file | grep '^@@ ' | wc -l) = $hunks
+	"
+
+	test -f $expected &&
+	test_expect_success "$label: check output" "
+		git $cmd $file | grep -v '^index ' >actual &&
+		test_cmp $expected actual
+	"
+}
+
+cat <<EOF >expected.f1.0.1 || exit 1
+diff --git a/f1 b/f1
+--- a/f1
++++ b/f1
+@@ -1,3 +1,3 @@
+-A
++X
+ 1
+-B
++Y
+EOF
+
+cat <<EOF >expected.f1.0.2 || exit 1
+diff --git a/f1 b/f1
+--- a/f1
++++ b/f1
+@@ -1 +1 @@
+-A
++X
+@@ -3 +3 @@ A
+-B
++Y
+EOF
+
+# common lines	ctx	intrctx	hunks
+t 1 line	0		2
+t 1 line	0	0	2
+t 1 line	0	1	1
+t 1 line	0	2	1
+t 1 line	1		1
+
+t 2 lines	0		2
+t 2 lines	0	0	2
+t 2 lines	0	1	2
+t 2 lines	0	2	1
+t 2 lines	1		1
+
+t 3 lines	1		2
+t 3 lines	1	0	2
+t 3 lines	1	1	1
+t 3 lines	1	2	1
+
+t 9 lines	3		2
+t 9 lines	3	2	2
+t 9 lines	3	3	1
+
+test_done
diff --git a/xdiff/xdiff.h b/xdiff/xdiff.h
index 84fff583e2..361f802319 100644
--- a/xdiff/xdiff.h
+++ b/xdiff/xdiff.h
@@ -84,6 +84,7 @@ typedef long (*find_func_t)(const char *line, long line_len, char *buffer, long
 
 typedef struct s_xdemitconf {
 	long ctxlen;
+	long interhunkctxlen;
 	unsigned long flags;
 	find_func_t find_func;
 	void *find_func_priv;
diff --git a/xdiff/xemit.c b/xdiff/xemit.c
index 4625c1b421..05bfa41f10 100644
--- a/xdiff/xemit.c
+++ b/xdiff/xemit.c
@@ -59,9 +59,10 @@ static int xdl_emit_record(xdfile_t *xdf, long ri, char const *pre, xdemitcb_t *
  */
 xdchange_t *xdl_get_hunk(xdchange_t *xscr, xdemitconf_t const *xecfg) {
 	xdchange_t *xch, *xchp;
+	long max_common = 2 * xecfg->ctxlen + xecfg->interhunkctxlen;
 
 	for (xchp = xscr, xch = xscr->next; xch; xchp = xch, xch = xch->next)
-		if (xch->i1 - (xchp->i1 + xchp->chg1) > 2 * xecfg->ctxlen)
+		if (xch->i1 - (xchp->i1 + xchp->chg1) > max_common)
 			break;
 
 	return xchp;