diff: add --ignore-blank-lines option

The goal of the patch is to introduce the GNU diff
-B/--ignore-blank-lines as closely as possible. The short option is not
available because it's already used for "break-rewrites".

When this option is used, git-diff will not create hunks that simply
add or remove empty lines, but will still show empty lines
addition/suppression if they are close enough to "valuable" changes.

There are two differences between this option and GNU diff -B option:
- GNU diff doesn't have "--inter-hunk-context", so this must be handled
- The following sequence looks like a bug (context is displayed twice):

    $ seq 5 >file1
    $ cat <<EOF >file2
    change
    1
    2

    3
    4
    5
    change
    EOF
    $ diff -u -B file1 file2
    --- file1	2013-06-08 22:13:04.471517834 +0200
    +++ file2	2013-06-08 22:13:23.275517855 +0200
    @@ -1,5 +1,7 @@
    +change
     1
     2
    +
     3
     4
     5
    @@ -3,3 +5,4 @@
     3
     4
     5
    +change

So here is a more thorough description of the option:
- real changes are interesting
- blank lines that are close enough (less than context size) to
interesting changes are considered interesting (recursive definition)
- "context" lines are used around each hunk of interesting changes
- If two hunks are separated by less than "inter-hunk-context", they
will be merged into one.

The implementation does the "interesting changes selection" in a single
pass.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: apelisse

Documentation/diff-options.txt

@@ -439,6 +439,9 @@ endif::git-format-patch[]
 	differences even if one line has whitespace where the other
 	line has none.
 
+--ignore-blank-lines::
+	Ignore changes whose lines are all blank.
+
 --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.

diff.c

@@ -3593,6 +3593,8 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		DIFF_XDL_SET(options, IGNORE_WHITESPACE_CHANGE);
 	else if (!strcmp(arg, "--ignore-space-at-eol"))
 		DIFF_XDL_SET(options, IGNORE_WHITESPACE_AT_EOL);
+	else if (!strcmp(arg, "--ignore-blank-lines"))
+		DIFF_XDL_SET(options, IGNORE_BLANK_LINES);
 	else if (!strcmp(arg, "--patience"))
 		options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
 	else if (!strcmp(arg, "--histogram"))

t/t4015-diff-whitespace.sh

@@ -142,6 +142,351 @@ EOF
 git diff --ignore-space-at-eol > out
 test_expect_success 'another test, with --ignore-space-at-eol' 'test_cmp expect out'
 
+test_expect_success 'ignore-blank-lines: only new lines' '
+	test_seq 5 >x &&
+	git update-index x &&
+	test_seq 5 | sed "/3/i \\
+" >x &&
+	git diff --ignore-blank-lines >out &&
+	>expect &&
+	test_cmp out expect
+'
+
+test_expect_success 'ignore-blank-lines: only new lines with space' '
+	test_seq 5 >x &&
+	git update-index x &&
+	test_seq 5 | sed "/3/i \ " >x &&
+	git diff -w --ignore-blank-lines >out &&
+	>expect &&
+	test_cmp out expect
+'
+
+test_expect_success 'ignore-blank-lines: after change' '
+	cat <<-\EOF >x &&
+	1
+	2
+
+	3
+	4
+	5
+
+	6
+	7
+	EOF
+	git update-index x &&
+	cat <<-\EOF >x &&
+	change
+
+	1
+	2
+	3
+	4
+	5
+	6
+
+	7
+	EOF
+	git diff --inter-hunk-context=100 --ignore-blank-lines >out.tmp &&
+	cat <<-\EOF >expected &&
+	diff --git a/x b/x
+	--- a/x
+	+++ b/x
+	@@ -1,6 +1,7 @@
+	+change
+	+
+	 1
+	 2
+	-
+	 3
+	 4
+	 5
+	EOF
+	compare_diff_patch expected out.tmp
+'
+
+test_expect_success 'ignore-blank-lines: before change' '
+	cat <<-\EOF >x &&
+	1
+	2
+
+	3
+	4
+	5
+	6
+	7
+	EOF
+	git update-index x &&
+	cat <<-\EOF >x &&
+
+	1
+	2
+	3
+	4
+	5
+
+	6
+	7
+	change
+	EOF
+	git diff --inter-hunk-context=100 --ignore-blank-lines >out.tmp &&
+	cat <<-\EOF >expected &&
+	diff --git a/x b/x
+	--- a/x
+	+++ b/x
+	@@ -4,5 +4,7 @@
+	 3
+	 4
+	 5
+	+
+	 6
+	 7
+	+change
+	EOF
+	compare_diff_patch expected out.tmp
+'
+
+test_expect_success 'ignore-blank-lines: between changes' '
+	cat <<-\EOF >x &&
+	1
+	2
+	3
+	4
+	5
+
+
+	6
+	7
+	8
+	9
+	10
+	EOF
+	git update-index x &&
+	cat <<-\EOF >x &&
+	change
+	1
+	2
+
+	3
+	4
+	5
+	6
+	7
+	8
+
+	9
+	10
+	change
+	EOF
+	git diff --ignore-blank-lines >out.tmp &&
+	cat <<-\EOF >expected &&
+	diff --git a/x b/x
+	--- a/x
+	+++ b/x
+	@@ -1,5 +1,7 @@
+	+change
+	 1
+	 2
+	+
+	 3
+	 4
+	 5
+	@@ -8,5 +8,7 @@
+	 6
+	 7
+	 8
+	+
+	 9
+	 10
+	+change
+	EOF
+	compare_diff_patch expected out.tmp
+'
+
+test_expect_success 'ignore-blank-lines: between changes (with interhunkctx)' '
+	test_seq 10 >x &&
+	git update-index x &&
+	cat <<-\EOF >x &&
+	change
+	1
+	2
+
+	3
+	4
+	5
+
+	6
+	7
+	8
+	9
+
+	10
+	change
+	EOF
+	git diff --inter-hunk-context=2 --ignore-blank-lines >out.tmp &&
+	cat <<-\EOF >expected &&
+	diff --git a/x b/x
+	--- a/x
+	+++ b/x
+	@@ -1,10 +1,15 @@
+	+change
+	 1
+	 2
+	+
+	 3
+	 4
+	 5
+	+
+	 6
+	 7
+	 8
+	 9
+	+
+	 10
+	+change
+	EOF
+	compare_diff_patch expected out.tmp
+'
+
+test_expect_success 'ignore-blank-lines: scattered spaces' '
+	test_seq 10 >x &&
+	git update-index x &&
+	cat <<-\EOF >x &&
+	change
+	1
+	2
+	3
+
+	4
+
+	5
+
+	6
+
+
+	7
+
+	8
+	9
+	10
+	change
+	EOF
+	git diff --inter-hunk-context=4 --ignore-blank-lines >out.tmp &&
+	cat <<-\EOF >expected &&
+	diff --git a/x b/x
+	--- a/x
+	+++ b/x
+	@@ -1,3 +1,4 @@
+	+change
+	 1
+	 2
+	 3
+	@@ -8,3 +15,4 @@
+	 8
+	 9
+	 10
+	+change
+	EOF
+	compare_diff_patch expected out.tmp
+'
+
+test_expect_success 'ignore-blank-lines: spaces coalesce' '
+	test_seq 6 >x &&
+	git update-index x &&
+	cat <<-\EOF >x &&
+	change
+	1
+	2
+	3
+
+	4
+
+	5
+
+	6
+	change
+	EOF
+	git diff --inter-hunk-context=4 --ignore-blank-lines >out.tmp &&
+	cat <<-\EOF >expected &&
+	diff --git a/x b/x
+	--- a/x
+	+++ b/x
+	@@ -1,6 +1,11 @@
+	+change
+	 1
+	 2
+	 3
+	+
+	 4
+	+
+	 5
+	+
+	 6
+	+change
+	EOF
+	compare_diff_patch expected out.tmp
+'
+
+test_expect_success 'ignore-blank-lines: mix changes and blank lines' '
+	test_seq 16 >x &&
+	git update-index x &&
+	cat <<-\EOF >x &&
+	change
+	1
+	2
+
+	3
+	4
+	5
+	change
+	6
+	7
+	8
+
+	9
+	10
+	11
+	change
+	12
+	13
+	14
+
+	15
+	16
+	change
+	EOF
+	git diff --ignore-blank-lines >out.tmp &&
+	cat <<-\EOF >expected &&
+	diff --git a/x b/x
+	--- a/x
+	+++ b/x
+	@@ -1,8 +1,11 @@
+	+change
+	 1
+	 2
+	+
+	 3
+	 4
+	 5
+	+change
+	 6
+	 7
+	 8
+	@@ -9,8 +13,11 @@
+	 9
+	 10
+	 11
+	+change
+	 12
+	 13
+	 14
+	+
+	 15
+	 16
+	+change
+	EOF
+	compare_diff_patch expected out.tmp
+'
+
 test_expect_success 'check mixed spaces and tabs in indent' '
 
 	# This is indented with SP HT SP.

xdiff/xdiff.h

@@ -39,6 +39,8 @@ extern "C" {
 #define XDF_DIFF_ALGORITHM_MASK (XDF_PATIENCE_DIFF | XDF_HISTOGRAM_DIFF)
 #define XDF_DIFF_ALG(x) ((x) & XDF_DIFF_ALGORITHM_MASK)
 
+#define XDF_IGNORE_BLANK_LINES (1 << 7)
+
 #define XDL_EMIT_FUNCNAMES (1 << 0)
 #define XDL_EMIT_COMMON (1 << 1)
 #define XDL_EMIT_FUNCCONTEXT (1 << 2)