feat(cid): add FromDigest for previously computed hash

cid.go

@@ -573,8 +573,15 @@ func (p Prefix) Sum(data []byte) (Cid, error) {
 
 		return Undef, fmt.Errorf("invalid v0 prefix")
 	}
+	digest, err := mh.Digest(data, p.MhType, length)
+	if err != nil {
+		return Undef, err
+	}
+	return p.FromDigest(digest)
+}
 
-	hash, err := mh.Sum(data, p.MhType, length)
+func (p Prefix) FromDigest(digest []byte) (Cid, error) {
+	hash, err := mh.Encode(digest, p.MhType)
 	if err != nil {
 		return Undef, err
 	}

cid_test.go

@@ -105,7 +105,37 @@ func TestPrefixSum(t *testing.T) {
 		}
 	}
 }
+func TestPrefixDigest(t *testing.T) {
+	// Test creating CIDs both manually and with Prefix.
+	// Tests: https://github.com/ipfs/go-cid/issues/83
+	for _, hashfun := range []uint64{
+		mh.ID, mh.SHA3, mh.SHA2_256,
+	} {
+		h1, err := mh.Sum([]byte("TEST"), hashfun, -1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		c1 := NewCidV1(Raw, h1)
 
+		h2, err := mh.Sum([]byte("foobar"), hashfun, -1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		c2 := NewCidV1(Raw, h2)
+
+		digest, err := mh.Digest([]byte("foobar"), hashfun, -1)
+		if err != nil {
+			t.Fatal(err)
+		}
+		c3, err := c1.Prefix().FromDigest(digest)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !c2.Equals(c3) {
+			t.Fatal("expected CIDs to be equal")
+		}
+	}
+}
 func TestBasicMarshaling(t *testing.T) {
 	h, err := mh.Sum([]byte("TEST"), mh.SHA3, 4)
 	if err != nil {

go.mod

@@ -2,7 +2,7 @@ module github.com/ipfs/go-cid
 
 require (
 	github.com/multiformats/go-multibase v0.0.3
-	github.com/multiformats/go-multihash v0.0.14
+	github.com/multiformats/go-multihash v0.0.15-0.20210226003459-69a4fed1d6f2
 	github.com/multiformats/go-varint v0.0.6
 )
 

go.sum

@@ -15,8 +15,11 @@ github.com/multiformats/go-multibase v0.0.3/go.mod h1:5+1R4eQrT3PkYZ24C3W2Ue2tPw
 github.com/multiformats/go-multihash v0.0.13 h1:06x+mk/zj1FoMsgNejLpy6QTvJqlSt/BhLEy87zidlc=
 github.com/multiformats/go-multihash v0.0.13/go.mod h1:VdAWLKTwram9oKAatUcLxBNUjdtcVwxObEQBtRfuyjc=
 github.com/multiformats/go-multihash v0.0.14/go.mod h1:VdAWLKTwram9oKAatUcLxBNUjdtcVwxObEQBtRfuyjc=
+github.com/multiformats/go-multihash v0.0.15-0.20210226003459-69a4fed1d6f2 h1:0DZ3ybel2YfQ4L9GrglYgfXxdXVT8CVkJt//crxSQDM=
+github.com/multiformats/go-multihash v0.0.15-0.20210226003459-69a4fed1d6f2/go.mod h1:VdAWLKTwram9oKAatUcLxBNUjdtcVwxObEQBtRfuyjc=
 github.com/multiformats/go-varint v0.0.5 h1:XVZwSo04Cs3j/jS0uAEPpT3JY6DzMcVLLoWOSnCxOjg=
 github.com/multiformats/go-varint v0.0.5/go.mod h1:3Ls8CIEsrijN6+B7PbrXRPxHRPuXSrVKRY101jdMZYE=
+github.com/multiformats/go-varint v0.0.6 h1:gk85QWKxh3TazbLxED/NlDVv8+q+ReFJk7Y2W/KhfNY=
 github.com/multiformats/go-varint v0.0.6/go.mod h1:3Ls8CIEsrijN6+B7PbrXRPxHRPuXSrVKRY101jdMZYE=
 github.com/spaolacci/murmur3 v1.1.0 h1:7c1g84S4BPRrfL5Xrdp6fOJ206sU9y293DDHaoy0bLI=
 github.com/spaolacci/murmur3 v1.1.0/go.mod h1:JwIasOWyU6f++ZhiEuf87xNszmSA2myDM2Kzu9HwQUA=