Merge branch 'ad/merge-file-diff-algo'

"git merge-file" learned to take the "--diff-algorithm" option to
use algorithm different from the default "myers" diff.

* ad/merge-file-diff-algo:
  merge-file: add --diff-algorithm option

3 files changed, 157 insertions, 1 deletions

diff --git a/Documentation/git-merge-file.txt b/Documentation/git-merge-file.txt
index 6a081eacb7..71915a00fa 100644
--- a/Documentation/git-merge-file.txt
+++ b/Documentation/git-merge-file.txt
@@ -92,6 +92,12 @@ object store and the object ID of its blob is written to standard output.
 	Instead of leaving conflicts in the file, resolve conflicts
 	favouring our (or their or both) side of the lines.
 
+--diff-algorithm={patience|minimal|histogram|myers}::
+	Use a different diff algorithm while merging. The current default is "myers",
+	but selecting more recent algorithm such as "histogram" can help
+	avoid mismerges that occur due to unimportant matching lines
+	(such as braces from distinct functions). See also
+	linkgit:git-diff[1] `--diff-algorithm`.
 
 EXAMPLES
 --------
diff --git a/builtin/merge-file.c b/builtin/merge-file.c
index 832c93d8d5..1f987334a3 100644
--- a/builtin/merge-file.c
+++ b/builtin/merge-file.c
@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "abspath.h"
+#include "diff.h"
 #include "hex.h"
 #include "object-name.h"
 #include "object-store.h"
@@ -28,6 +29,30 @@ static int label_cb(const struct option *opt, const char *arg, int unset)
 	return 0;
 }
 
+static int set_diff_algorithm(xpparam_t *xpp,
+			      const char *alg)
+{
+	long diff_algorithm = parse_algorithm_value(alg);
+	if (diff_algorithm < 0)
+		return -1;
+	xpp->flags = (xpp->flags & ~XDF_DIFF_ALGORITHM_MASK) | diff_algorithm;
+	return 0;
+}
+
+static int diff_algorithm_cb(const struct option *opt,
+				const char *arg, int unset)
+{
+	xpparam_t *xpp = opt->value;
+
+	BUG_ON_OPT_NEG(unset);
+
+	if (set_diff_algorithm(xpp, arg))
+		return error(_("option diff-algorithm accepts \"myers\", "
+			       "\"minimal\", \"patience\" and \"histogram\""));
+
+	return 0;
+}
+
 int cmd_merge_file(int argc, const char **argv, const char *prefix)
 {
 	const char *names[3] = { 0 };
@@ -48,6 +73,9 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 			    XDL_MERGE_FAVOR_THEIRS),
 		OPT_SET_INT(0, "union", &xmp.favor, N_("for conflicts, use a union version"),
 			    XDL_MERGE_FAVOR_UNION),
+		OPT_CALLBACK_F(0, "diff-algorithm", &xmp.xpp, N_("<algorithm>"),
+			     N_("choose a diff algorithm"),
+			     PARSE_OPT_NONEG, diff_algorithm_cb),
 		OPT_INTEGER(0, "marker-size", &xmp.marker_size,
 			    N_("for conflicts, use this marker size")),
 		OPT__QUIET(&quiet, N_("do not warn about conflicts")),
diff --git a/t/t6403-merge-file.sh b/t/t6403-merge-file.sh
index 2c92209eca..fb872c5a11 100755
--- a/t/t6403-merge-file.sh
+++ b/t/t6403-merge-file.sh
@@ -56,7 +56,67 @@ test_expect_success 'setup' '
 	deduxit me super semitas jusitiae,
 	EOF
 
-	printf "propter nomen suum." >>new4.txt
+	printf "propter nomen suum." >>new4.txt &&
+
+	cat >base.c <<-\EOF &&
+	int f(int x, int y)
+	{
+		if (x == 0)
+		{
+			return y;
+		}
+		return x;
+	}
+
+	int g(size_t u)
+	{
+		while (u < 30)
+		{
+			u++;
+		}
+		return u;
+	}
+	EOF
+
+	cat >ours.c <<-\EOF &&
+	int g(size_t u)
+	{
+		while (u < 30)
+		{
+			u++;
+		}
+		return u;
+	}
+
+	int h(int x, int y, int z)
+	{
+		if (z == 0)
+		{
+			return x;
+		}
+		return y;
+	}
+	EOF
+
+	cat >theirs.c <<-\EOF
+	int f(int x, int y)
+	{
+		if (x == 0)
+		{
+			return y;
+		}
+		return x;
+	}
+
+	int g(size_t u)
+	{
+		while (u > 34)
+		{
+			u--;
+		}
+		return u;
+	}
+	EOF
 '
 
 test_expect_success 'merge with no changes' '
@@ -447,4 +507,66 @@ test_expect_success '--object-id fails without repository' '
 	grep "not a git repository" err
 '
 
+test_expect_success 'merging C files with "myers" diff algorithm creates some spurious conflicts' '
+	cat >expect.c <<-\EOF &&
+	int g(size_t u)
+	{
+		while (u < 30)
+		{
+			u++;
+		}
+		return u;
+	}
+
+	int h(int x, int y, int z)
+	{
+	<<<<<<< ours.c
+		if (z == 0)
+	||||||| base.c
+		while (u < 30)
+	=======
+		while (u > 34)
+	>>>>>>> theirs.c
+		{
+	<<<<<<< ours.c
+			return x;
+	||||||| base.c
+			u++;
+	=======
+			u--;
+	>>>>>>> theirs.c
+		}
+		return y;
+	}
+	EOF
+
+	test_must_fail git merge-file -p --diff3 --diff-algorithm myers ours.c base.c theirs.c >myers_output.c &&
+	test_cmp expect.c myers_output.c
+'
+
+test_expect_success 'merging C files with "histogram" diff algorithm avoids some spurious conflicts' '
+	cat >expect.c <<-\EOF &&
+	int g(size_t u)
+	{
+		while (u > 34)
+		{
+			u--;
+		}
+		return u;
+	}
+
+	int h(int x, int y, int z)
+	{
+		if (z == 0)
+		{
+			return x;
+		}
+		return y;
+	}
+	EOF
+
+	git merge-file -p --diff3 --diff-algorithm histogram ours.c base.c theirs.c >histogram_output.c &&
+	test_cmp expect.c histogram_output.c
+'
+
 test_done