Add a cidlink.Memory storage option (#266)

* Add a cidlink.Memory storage option

linking/cid/memorystorage.go

@@ -0,0 +1,56 @@
+package cidlink
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/ipld/go-ipld-prime/datamodel"
+	"github.com/ipld/go-ipld-prime/linking"
+)
+
+// Memory is a simple in-memory storage for cidlinks. It's the same as `storage.Memory`
+// but uses typical multihash semantics used when reading/writing cidlinks.
+//
+// Using multihash as the storage key rather than the whole CID will remove the
+// distinction between CIDv0 and their CIDv1 counterpart. It also removes the
+// distinction between CIDs where the multihash is the same but the codec is
+// different, e.g. `dag-cbor` and a `raw` version of the same data.
+type Memory struct {
+	Bag map[string][]byte
+}
+
+func (store *Memory) beInitialized() {
+	if store.Bag != nil {
+		return
+	}
+	store.Bag = make(map[string][]byte)
+}
+
+func (store *Memory) OpenRead(lnkCtx linking.LinkContext, lnk datamodel.Link) (io.Reader, error) {
+	store.beInitialized()
+	cl, ok := lnk.(Link)
+	if !ok {
+		return nil, fmt.Errorf("incompatible link type: %T", lnk)
+	}
+	data, exists := store.Bag[string(cl.Hash())]
+	if !exists {
+		return nil, os.ErrNotExist
+	}
+	return bytes.NewReader(data), nil
+}
+
+func (store *Memory) OpenWrite(lnkCtx linking.LinkContext) (io.Writer, linking.BlockWriteCommitter, error) {
+	store.beInitialized()
+	buf := bytes.Buffer{}
+	return &buf, func(lnk datamodel.Link) error {
+		cl, ok := lnk.(Link)
+		if !ok {
+			return fmt.Errorf("incompatible link type: %T", lnk)
+		}
+
+		store.Bag[string(cl.Hash())] = buf.Bytes()
+		return nil
+	}, nil
+}