difftree: simplify hash comparison with deprecated files modes

Difftree hash comparisson was quite complex because the hashes of
deprecated files were diferent from the hashes of regular files.  But
git's difftree really treat them as equal.

This patch fix this by making treenoder return the same hash for regular
files than for deprecated files; now the hash comparison function is
just a bytes.Equal call.

3 files changed, 31 insertions, 51 deletions

diff --git a/plumbing/object/difftree.go b/plumbing/object/difftree.go
index bcc78ad..3bc29cb 100644
--- a/plumbing/object/difftree.go
+++ b/plumbing/object/difftree.go
@@ -3,7 +3,6 @@ package object
 import (
 	"bytes"
 
-	"srcd.works/go-git.v4/plumbing/filemode"
 	"srcd.works/go-git.v4/utils/merkletrie"
 	"srcd.works/go-git.v4/utils/merkletrie/noder"
 )
@@ -14,6 +13,10 @@ func DiffTree(a, b *Tree) (Changes, error) {
 	from := newTreeNoder(a)
 	to := newTreeNoder(b)
 
+	hashEqual := func(a, b noder.Hasher) bool {
+		return bytes.Equal(a.Hash(), b.Hash())
+	}
+
 	merkletrieChanges, err := merkletrie.DiffTree(from, to, hashEqual)
 	if err != nil {
 		return nil, err
@@ -21,39 +24,3 @@ func DiffTree(a, b *Tree) (Changes, error) {
 
 	return newChanges(merkletrieChanges)
 }
-
-// check if the hash of the contents is different, if not, check if
-// the permissions are different (but taking into account deprecated
-// file modes).  On a treenoder, the hash of the contents is codified
-// in the first 20 bytes of the data returned by Hash() and the last
-// 4 bytes is the mode.
-func hashEqual(a, b noder.Hasher) bool {
-	hashA, hashB := a.Hash(), b.Hash()
-	contentsA, contentsB := hashA[:20], hashB[:20]
-
-	sameContents := bytes.Equal(contentsA, contentsB)
-	if !sameContents {
-		return false
-	}
-
-	modeA, modeB := hashA[20:], hashB[20:]
-
-	return equivalentMode(modeA, modeB)
-}
-
-func equivalentMode(a, b []byte) bool {
-	if isFilish(a) && isFilish(b) {
-		return true
-	}
-	return bytes.Equal(a, b)
-}
-
-var (
-	file           = filemode.Regular.Bytes()
-	fileDeprecated = filemode.Deprecated.Bytes()
-)
-
-func isFilish(b []byte) bool {
-	return bytes.Equal(b, file) ||
-		bytes.Equal(b, fileDeprecated)
-}
diff --git a/plumbing/object/difftree_test.go b/plumbing/object/difftree_test.go
index fa996c2..11f6b23 100644
--- a/plumbing/object/difftree_test.go
+++ b/plumbing/object/difftree_test.go
@@ -358,24 +358,27 @@ func (s *DiffTreeSuite) TestDiffTree(c *C) {
 }
 
 func (s *DiffTreeSuite) TestIssue279(c *C) {
-	// HashEqual should ignore files if the only change is from a 100664
-	// mode to a 100644 or vice versa.
-	from := &treeNoder{
-		hash: plumbing.NewHash("d08e895238bac36d8220586fdc28c27e1a7a76d3"),
+	// treeNoders should have the same hash when their mode is
+	// filemode.Deprecated and filemode.Regular.
+	a := &treeNoder{
+		hash: plumbing.NewHash("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"),
 		mode: filemode.Regular,
 	}
-	to := &treeNoder{
-		hash: plumbing.NewHash("d08e895238bac36d8220586fdc28c27e1a7a76d3"),
-		mode: filemode.Regular,
+	b := &treeNoder{
+		hash: plumbing.NewHash("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"),
+		mode: filemode.Deprecated,
 	}
-	c.Assert(hashEqual(from, to), Equals, true)
-	c.Assert(hashEqual(to, from), Equals, true)
+	c.Assert(a.Hash(), DeepEquals, b.Hash())
 
-	// but should detect if the contents of the file also changed.
-	to = &treeNoder{
-		hash: plumbing.NewHash("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"),
+	// yet, they should have different hashes if their contents change.
+	aa := &treeNoder{
+		hash: plumbing.NewHash("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"),
 		mode: filemode.Regular,
 	}
-	c.Assert(hashEqual(from, to), Equals, false)
-	c.Assert(hashEqual(to, from), Equals, false)
+	c.Assert(a.Hash(), Not(DeepEquals), aa.Hash())
+	bb := &treeNoder{
+		hash: plumbing.NewHash("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"),
+		mode: filemode.Deprecated,
+	}
+	c.Assert(b.Hash(), Not(DeepEquals), bb.Hash())
 }
diff --git a/plumbing/object/treenoder.go b/plumbing/object/treenoder.go
index 89fcdb1..80cd9b0 100644
--- a/plumbing/object/treenoder.go
+++ b/plumbing/object/treenoder.go
@@ -45,7 +45,17 @@ func (t *treeNoder) String() string {
 	return "treeNoder <" + t.name + ">"
 }
 
+// The hash of a treeNoder is the result of concatenating the hash of
+// its contents and its mode; that way the difftree algorithm will
+// detect changes in the contents of files and also in their mode.
+//
+// Files with Regular and Deprecated file modes are considered the same
+// for the purpose of difftree, so Regular will be used as the mode for
+// Deprecated files here.
 func (t *treeNoder) Hash() []byte {
+	if t.mode == filemode.Deprecated {
+		return append(t.hash[:], filemode.Regular.Bytes()...)
+	}
 	return append(t.hash[:], t.mode.Bytes()...)
 }