4 files changed, 219 insertions, 4 deletions
diff --git a/objects/id.go b/objects/id.go
new file mode 100644
index 0000000..22eeaa2
--- /dev/null
+++ b/objects/id.go
@@ -0,0 +1,111 @@
+package objects
+
+import (
+	"bytes"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"golang.org/x/crypto/sha3"
+	"hash"
+	"io"
+	"strings"
+)
+
+type ObjectIdAlgo string
+
+const (
+	OIdAlgoSHA3_256 ObjectIdAlgo = "sha3-256"
+	OIdAlgoDefault               = OIdAlgoSHA3_256
+)
+
+var allowedAlgos = map[ObjectIdAlgo]struct{}{OIdAlgoSHA3_256: {}}
+
+func (algo ObjectIdAlgo) checkAlgo() bool {
+	_, ok := allowedAlgos[algo]
+	return ok
+}
+
+func (algo ObjectIdAlgo) sumLength() int {
+	if algo != OIdAlgoSHA3_256 {
+		panic("Only sha3-256 is implemented!")
+	}
+
+	return 32
+}
+
+// ObjectId identifies an object using a cryptogrpahic hash
+type ObjectId struct {
+	Algo ObjectIdAlgo
+	Sum  []byte
+}
+
+func (oid ObjectId) wellformed() bool {
+	return oid.Algo.checkAlgo() && len(oid.Sum) == oid.Algo.sumLength()
+}
+
+func (oid ObjectId) String() string {
+	return fmt.Sprintf("%s:%s", oid.Algo, hex.EncodeToString(oid.Sum))
+}
+
+func ParseObjectId(s string) (oid ObjectId, err error) {
+	parts := strings.SplitN(s, ":", 2)
+	if len(parts) != 2 {
+		err = errors.New("Invalid ObjectId format")
+		return
+	}
+
+	oid.Algo = ObjectIdAlgo(parts[0])
+
+	oid.Sum, err = hex.DecodeString(parts[1])
+	if err != nil {
+		return
+	}
+
+	if !oid.wellformed() {
+		err = errors.New("Object ID is malformed")
+	}
+
+	return
+}
+
+func (a ObjectId) Equals(b ObjectId) bool {
+	return a.Algo == b.Algo && bytes.Equal(a.Sum, b.Sum)
+}
+
+// ObjectIdGenerator generates an ObjectId from the binary representation of an object
+type ObjectIdGenerator interface {
+	io.Writer // Must not fail
+	GetId() ObjectId
+}
+
+func (algo ObjectIdAlgo) Generator() ObjectIdGenerator {
+	if algo != OIdAlgoSHA3_256 {
+		panic("Only sha3-256 is implemented!")
+	}
+
+	return hashObjectIdGenerator{
+		algo: algo,
+		Hash: sha3.New256(),
+	}
+}
+
+type hashObjectIdGenerator struct {
+	algo ObjectIdAlgo
+	hash.Hash
+}
+
+func (h hashObjectIdGenerator) GetId() ObjectId {
+	return ObjectId{
+		Algo: h.algo,
+		Sum:  h.Sum([]byte{}),
+	}
+}
+
+func (oid ObjectId) VerifyObject(o Object) bool {
+	gen := oid.Algo.Generator()
+	if err := o.Serialize(gen); err != nil {
+		panic(err)
+	}
+
+	return gen.GetId().Equals(oid)
+}
diff --git a/objects/id_test.go b/objects/id_test.go
new file mode 100644
index 0000000..361798d
--- /dev/null
+++ b/objects/id_test.go
@@ -0,0 +1,77 @@
+package objects
+
+import (
+	"testing"
+)
+
+func TestParseValidId(t *testing.T) {
+	have, err := ParseObjectId("sha3-256:000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f")
+	if err != nil {
+		t.Fatalf("Unexpected error: %s", err)
+	}
+
+	want := ObjectId{
+		Algo: OIdAlgoSHA3_256,
+		Sum: []byte{
+			0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+			0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+		},
+	}
+
+	if !have.Equals(want) {
+		t.Errorf("unexpected result, want: '%s', have: '%s'", want, have)
+	}
+}
+
+func TestParseMalformedIds(t *testing.T) {
+	malformed := []string{
+		"",                // Empty not permitted
+		"sha3-256",        // Missing :
+		"sha3-256:",       // Missing hex sum
+		":abcdef",         // Missing algo
+		"foobar:abcdef",   // Basic format ok, but unknown algo
+		"sha3-256:foobar", // Not hexadecimal
+		"sha3-256:abcdef", // sum length mismatch
+	}
+
+	for _, s := range malformed {
+		oid, err := ParseObjectId(s)
+		if err == nil {
+			t.Errorf("'%s' resulted in valid id (%s), expected error", s, oid)
+		}
+	}
+}
+
+func TestParseSerialize(t *testing.T) {
+	want := ObjectId{
+		Algo: OIdAlgoSHA3_256,
+		Sum: []byte{
+			0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+			0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+		},
+	}
+
+	have, err := ParseObjectId(want.String())
+	if err != nil {
+		t.Fatalf("Unexpected error: %s", err)
+	}
+
+	if !have.Equals(want) {
+		t.Errorf("unexpected result, want: '%s', have: '%s'", want, have)
+	}
+}
+
+func TestSerializeParse(t *testing.T) {
+	want := "sha3-256:000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f"
+
+	oid, err := ParseObjectId(want)
+	if err != nil {
+		t.Fatalf("Unexpected error: %s", err)
+	}
+
+	have := oid.String()
+
+	if have != want {
+		t.Errorf("unexpected result, want: '%s', have: '%s'", want, have)
+	}
+}
diff --git a/objects/object.go b/objects/object.go
index 2bc3a53..a1d2335 100644
--- a/objects/object.go
+++ b/objects/object.go
@@ -7,10 +7,6 @@ import (
 	"strings"
 )
 
-type ObjectId interface {
-	fmt.Stringer
-}
-
 type ObjectType string
 
 const (
@@ -25,6 +21,7 @@ type Object struct {
 	Payload []byte
 }
 
+// SerialiteObject writes the binary representation of an object to a io.Writer
 func (o Object) Serialize(w io.Writer) error {
 	if _, err := fmt.Fprintf(w, "%s %d\n", o.Type, len(o.Payload)); err != nil {
 		return err
@@ -34,6 +31,16 @@ func (o Object) Serialize(w io.Writer) error {
 	return err
 }
 
+func (o Object) SerializeAndId(w io.Writer, algo ObjectIdAlgo) (ObjectId, error) {
+	gen := algo.Generator()
+
+	if err := o.Serialize(io.MultiWriter(w, gen)); err != nil {
+		return ObjectId{}, err
+	}
+
+	return gen.GetId(), nil
+}
+
 type UnserializeError struct {
 	Reason error
 }
@@ -65,6 +72,8 @@ func (br *bytewiseReader) ReadByte() (b byte, err error) {
 	return
 }
 
+// UnserializeObject attempts to read an object from a stream.
+// It is advisable to pass a buffered reader, if feasible.
 func UnserializeObject(r io.Reader) (Object, error) {
 	br := newBytewiseReader(r)
 
diff --git a/objects/object_test.go b/objects/object_test.go
index c596b39..54b33db 100644
--- a/objects/object_test.go
+++ b/objects/object_test.go
@@ -86,3 +86,21 @@ func TestSerialize(t *testing.T) {
 		t.Errorf("Unexpected serialization result: %v", b)
 	}
 }
+
+func TestSerializeAndId(t *testing.T) {
+	o := Object{
+		Type:    OTBlob,
+		Payload: []byte("foo bar\nbaz"),
+	}
+
+	buf := new(bytes.Buffer)
+	id, err := o.SerializeAndId(buf, OIdAlgoDefault)
+
+	if err != nil {
+		t.Fatalf("Serialization failed:%s", err)
+	}
+
+	if !id.VerifyObject(o) {
+		t.Errorf("Verification failed")
+	}
+}