1 files changed, 111 insertions, 0 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)
+}