Use array instead of hash for backend.ID

Since backend.ID is always a slice of constant length, use an array
instead of a slice. Mostly, arrays behave as slices, except that an
array cannot be nil, so use `*backend.ID` insteaf of `backend.ID` in
places where the absence of an ID is possible (e.g. for the Subtree of a
Node, which may not present when the node is a file node).

This change allows to directly use backend.ID as the the key for a map,
so that arbitrary data structures (e.g. a Set implemented as a
map[backend.ID]struct{}) can easily be formed.

backend/generic.go

@@ -23,10 +23,7 @@ const hashSize = sha256.Size
 
 // Hash returns the ID for data.
 func Hash(data []byte) ID {
-	h := hashData(data)
-	id := make([]byte, IDSize)
-	copy(id, h[:])
-	return id
+	return hashData(data)
 }
 
 // Find loads the list of all blobs of type t and searches for names which

backend/id.go

@@ -11,39 +11,55 @@ import (
 const IDSize = hashSize
 
 // ID references content within a repository.
-type ID []byte
+type ID [IDSize]byte
 
 // ParseID converts the given string to an ID.
 func ParseID(s string) (ID, error) {
 	b, err := hex.DecodeString(s)
 
 	if err != nil {
-		return nil, err
+		return ID{}, err
 	}
 
 	if len(b) != IDSize {
-		return nil, errors.New("invalid length for hash")
+		return ID{}, errors.New("invalid length for hash")
 	}
 
-	return ID(b), nil
+	id := ID{}
+	copy(id[:], b)
+
+	return id, nil
 }
 
 func (id ID) String() string {
-	return hex.EncodeToString(id)
+	return hex.EncodeToString(id[:])
 }
 
 const shortStr = 4
 
-func (id ID) Str() string {
+// Str returns the shortened string version of id.
+func (id *ID) Str() string {
 	if id == nil {
 		return "[nil]"
 	}
+
+	if id.IsNull() {
+		return "[null]"
+	}
+
 	return hex.EncodeToString(id[:shortStr])
 }
 
+// IsNull returns true iff id only consists of null bytes.
+func (id ID) IsNull() bool {
+	var nullID ID
+
+	return id == nullID
+}
+
 // Equal compares an ID to another other.
 func (id ID) Equal(other ID) bool {
-	return bytes.Equal(id, other)
+	return id == other
 }
 
 // EqualString compares this ID to another one, given as a string.
@@ -53,12 +69,15 @@ func (id ID) EqualString(other string) (bool, error) {
 		return false, err
 	}
 
-	return id.Equal(ID(s)), nil
+	id2 := ID{}
+	copy(id2[:], s)
+
+	return id == id2, nil
 }
 
 // Compare compares this ID to another one, returning -1, 0, or 1.
 func (id ID) Compare(other ID) int {
-	return bytes.Compare(other, id)
+	return bytes.Compare(other[:], id[:])
 }
 
 func (id ID) MarshalJSON() ([]byte, error) {
@@ -72,8 +91,7 @@ func (id *ID) UnmarshalJSON(b []byte) error {
 		return err
 	}
 
-	*id = make([]byte, IDSize)
-	_, err = hex.Decode(*id, []byte(s))
+	_, err = hex.Decode(id[:], []byte(s))
 	if err != nil {
 		return err
 	}
@@ -82,10 +100,7 @@ func (id *ID) UnmarshalJSON(b []byte) error {
 }
 
 func IDFromData(d []byte) ID {
-	hash := hashData(d)
-	id := make([]byte, IDSize)
-	copy(id, hash[:])
-	return id
+	return hashData(d)
 }
 
 type IDs []ID

backend/id_int_test.go

@@ -0,0 +1,16 @@
+package backend
+
+import "testing"
+
+func TestIDMethods(t *testing.T) {
+	var id ID
+
+	if id.Str() != "[null]" {
+		t.Errorf("ID.Str() returned wrong value, want %v, got %v", "[null]", id.Str())
+	}
+
+	var pid *ID
+	if pid.Str() != "[nil]" {
+		t.Errorf("ID.Str() returned wrong value, want %v, got %v", "[nil]", pid.Str())
+	}
+}