Add a command to read a cid from an array of bytes

cid.go

@@ -290,36 +290,16 @@ func uvError(read int) error {
 // Please use decode when parsing a regular Cid string, as Cast does not
 // expect multibase-encoded data. Cast accepts the output of Cid.Bytes().
 func Cast(data []byte) (Cid, error) {
-	if len(data) == 34 && data[0] == 18 && data[1] == 32 {
-		h, err := mh.Cast(data)
-		if err != nil {
-			return Undef, err
-		}
-
-		return NewCidV0(h), nil
-	}
-
-	vers, n := binary.Uvarint(data)
-	if err := uvError(n); err != nil {
-		return Undef, err
-	}
-
-	if vers != 1 {
-		return Undef, fmt.Errorf("expected 1 as the cid version number, got: %d", vers)
-	}
-
-	_, cn := binary.Uvarint(data[n:])
-	if err := uvError(cn); err != nil {
-		return Undef, err
-	}
-
-	rest := data[n+cn:]
-	h, err := mh.Cast(rest)
+	nr, c, err := CidFromBytes(data)
 	if err != nil {
 		return Undef, err
 	}
 
-	return Cid{string(data[0 : n+cn+len(h)])}, nil
+	if nr != len(data) {
+		return Undef, fmt.Errorf("trailing bytes in data buffer passed to cid Cast")
+	}
+
+	return c, nil
 }
 
 // UnmarshalBinary is equivalent to Cast(). It implements the
@@ -601,3 +581,42 @@ func PrefixFromBytes(buf []byte) (Prefix, error) {
 		MhLength: int(mhlen),
 	}, nil
 }
+
+func CidFromBytes(data []byte) (int, Cid, error) {
+	if len(data) > 2 && data[0] == 18 && data[1] == 32 {
+		if len(data) < 34 {
+			return 0, Undef, fmt.Errorf("not enough bytes for cid v0")
+		}
+
+		h, err := mh.Cast(data[:34])
+		if err != nil {
+			return 0, Undef, err
+		}
+
+		return 34, NewCidV0(h), nil
+	}
+
+	vers, n := binary.Uvarint(data)
+	if err := uvError(n); err != nil {
+		return 0, Undef, err
+	}
+
+	if vers != 1 {
+		return 0, Undef, fmt.Errorf("expected 1 as the cid version number, got: %d", vers)
+	}
+
+	_, cn := binary.Uvarint(data[n:])
+	if err := uvError(cn); err != nil {
+		return 0, Undef, err
+	}
+
+	mhr := mh.NewReader(bytes.NewReader(data[n+cn:]))
+	h, err := mhr.ReadMultihash()
+	if err != nil {
+		return 0, Undef, err
+	}
+
+	l := n + cn + len(h)
+
+	return l, Cid{string(data[0:l])}, nil
+}