add 'Prefix' object and some more helper routines

cid.go

@@ -174,3 +174,76 @@ func (c *Cid) Loggable() map[string]interface{} {
 		"cid": c,
 	}
 }
+
+func (c *Cid) Prefix() Prefix {
+	dec, _ := mh.Decode(c.hash) // assuming we got a valid multiaddr, this will not error
+	return Prefix{
+		MhType:   dec.Code,
+		MhLength: dec.Length,
+		Version:  c.version,
+		Codec:    c.codec,
+	}
+}
+
+// Prefix represents all the metadata of a cid, minus any actual content information
+type Prefix struct {
+	Version  uint64
+	Codec    uint64
+	MhType   int
+	MhLength int
+}
+
+func (p Prefix) Sum(data []byte) (*Cid, error) {
+	hash, err := mh.Sum(data, p.MhType, p.MhLength)
+	if err != nil {
+		return nil, err
+	}
+
+	switch p.Version {
+	case 0:
+		return NewCidV0(hash), nil
+	case 1:
+		return NewCidV1(p.Codec, hash), nil
+	default:
+		return nil, fmt.Errorf("invalid cid version")
+	}
+}
+
+func (p Prefix) Bytes() []byte {
+	buf := make([]byte, 16)
+	n := binary.PutUvarint(buf, p.Version)
+	n += binary.PutUvarint(buf[n:], p.Codec)
+	n += binary.PutUvarint(buf[n:], uint64(p.MhType))
+	n += binary.PutUvarint(buf[n:], uint64(p.MhLength))
+	return buf[:n]
+}
+
+func PrefixFromBytes(buf []byte) (Prefix, error) {
+	r := bytes.NewReader(buf)
+	vers, err := binary.ReadUvarint(r)
+	if err != nil {
+		return Prefix{}, err
+	}
+
+	codec, err := binary.ReadUvarint(r)
+	if err != nil {
+		return Prefix{}, err
+	}
+
+	mhtype, err := binary.ReadUvarint(r)
+	if err != nil {
+		return Prefix{}, err
+	}
+
+	mhlen, err := binary.ReadUvarint(r)
+	if err != nil {
+		return Prefix{}, err
+	}
+
+	return Prefix{
+		Version:  vers,
+		Codec:    codec,
+		MhType:   int(mhtype),
+		MhLength: int(mhlen),
+	}, nil
+}

cid_test.go

@@ -79,3 +79,32 @@ func TestV0ErrorCases(t *testing.T) {
 		t.Fatal("should have failed to decode that ref")
 	}
 }
+
+func TestPrefixRoundtrip(t *testing.T) {
+	data := []byte("this is some test content")
+	hash, _ := mh.Sum(data, mh.SHA2_256, -1)
+	c := NewCidV1(CBOR, hash)
+
+	pref := c.Prefix()
+
+	c2, err := pref.Sum(data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !c.Equals(c2) {
+		t.Fatal("output didnt match original")
+	}
+
+	pb := pref.Bytes()
+
+	pref2, err := PrefixFromBytes(pb)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if pref.Version != pref2.Version || pref.Codec != pref2.Codec ||
+		pref.MhType != pref2.MhType || pref.MhLength != pref2.MhLength {
+		t.Fatal("input prefix didnt match output")
+	}
+}

set.go

@@ -26,7 +26,7 @@ func (s *Set) Len() int {
 }
 
 func (s *Set) Keys() []*Cid {
-	var out []*Cid
+	out := make([]*Cid, 0, len(s.set))
 	for k, _ := range s.set {
 		c, _ := Cast([]byte(k))
 		out = append(out, c)
@@ -42,3 +42,14 @@ func (s *Set) Visit(c *Cid) bool {
 
 	return false
 }
+
+func (s *Set) ForEach(f func(c *Cid) error) error {
+	for cs, _ := range s.set {
+		c, _ := Cast([]byte(cs))
+		err := f(c)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}