Add StringOfBase function

cid.go

@@ -47,6 +47,10 @@ var (
 	// ErrCidTooShort means that the cid passed to decode was not long
 	// enough to be a valid Cid
 	ErrCidTooShort = errors.New("cid too short")
+
+	// ErrInvalidEncoding means that selected encoding is not supported
+	// by this Cid version
+	ErrInvalidEncoding = errors.New("invalid base encoding")
 )
 
 // These are multicodec-packed content types. The should match
@@ -247,6 +251,22 @@ func (c *Cid) String() string {
 	}
 }
 
+// String returns the string representation of a Cid
+// encoded is selected base
+func (c *Cid) StringOfBase(base mbase.Encoding) (string, error) {
+	switch c.version {
+	case 0:
+		if base != mbase.Base58BTC {
+			return "", ErrInvalidEncoding
+		}
+		return c.hash.B58String(), nil
+	case 1:
+		return mbase.Encode(base, c.bytesV1())
+	default:
+		panic("not possible to reach this point")
+	}
+}
+
 // Hash returns the multihash contained by a Cid.
 func (c *Cid) Hash() mh.Multihash {
 	return c.hash

cid_test.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"testing"
 
+	mbase "github.com/multiformats/go-multibase"
 	mh "github.com/multiformats/go-multihash"
 )
 
@@ -55,6 +56,60 @@ func TestBasicMarshaling(t *testing.T) {
 	assertEqual(t, cid, out2)
 }
 
+func TestBasesMarshaling(t *testing.T) {
+	h, err := mh.Sum([]byte("TEST"), mh.SHA3, 4)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cid := &Cid{
+		codec:   7,
+		version: 1,
+		hash:    h,
+	}
+
+	data := cid.Bytes()
+
+	out, err := Cast(data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	assertEqual(t, cid, out)
+
+	testBases := []mbase.Encoding {
+		mbase.Base16,
+		mbase.Base32,
+		mbase.Base32hex,
+		mbase.Base32pad,
+		mbase.Base32hexPad,
+		mbase.Base58BTC,
+		mbase.Base58Flickr,
+		mbase.Base64pad,
+		mbase.Base64urlPad,
+		mbase.Base64url,
+		mbase.Base64,
+	}
+
+	for _, b := range testBases {
+		s, err := cid.StringOfBase(b)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if s[0] != byte(b) {
+			t.Fatal("Invalid multibase header")
+		}
+
+		out2, err := Decode(s)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		assertEqual(t, cid, out2)
+	}
+}
+
 func TestEmptyString(t *testing.T) {
 	_, err := Decode("")
 	if err == nil {