func WriteHeader(w io.Writer, src, dst peer.ID) error {
	// write header to w.
	mhw := mh.NewWriter(w)
	if err := mhw.WriteMultihash(mh.Multihash(src)); err != nil {
		return fmt.Errorf("failed to write relay header: %s -- %s", dst, err)
	}
	if err := mhw.WriteMultihash(mh.Multihash(dst)); err != nil {
		return fmt.Errorf("failed to write relay header: %s -- %s", dst, err)
	}

	return nil
}

func (bs *blockstore) Get(k key.Key) (*blocks.Block, error) {
	maybeData, err := bs.datastore.Get(k.DsKey())
	if err == ds.ErrNotFound {
		return nil, ErrNotFound
	}
	if err != nil {
		return nil, err
	}
	bdata, ok := maybeData.([]byte)
	if !ok {
		return nil, ValueTypeMismatch
	}

	return blocks.NewBlockWithHash(bdata, mh.Multihash(k))
}

func TestKey(t *testing.T) {

	h1, err := mh.Sum([]byte("beep boop"), mh.SHA2_256, -1)
	if err != nil {
		t.Error(err)
	}

	k1 := Key(h1)
	h2 := mh.Multihash(k1)
	k2 := Key(h2)

	if !bytes.Equal(h1, h2) {
		t.Error("Multihashes not equal.")
	}

	if k1 != k2 {
		t.Error("Keys not equal.")
	}
}

// modifyDag writes the data in 'data' over the data in 'node' starting at 'offset'
// returns the new key of the passed in node and whether or not all the data in the reader
// has been consumed.
func (dm *DagModifier) modifyDag(node *mdag.Node, offset uint64, data io.Reader) (key.Key, bool, error) {
	f, err := ft.FromBytes(node.Data)
	if err != nil {
		return "", false, err
	}

	// If we've reached a leaf node.
	if len(node.Links) == 0 {
		n, err := data.Read(f.Data[offset:])
		if err != nil && err != io.EOF {
			return "", false, err
		}

		// Update newly written node..
		b, err := proto.Marshal(f)
		if err != nil {
			return "", false, err
		}

		nd := &mdag.Node{Data: b}
		k, err := dm.dagserv.Add(nd)
		if err != nil {
			return "", false, err
		}

		// Hey look! we're done!
		var done bool
		if n < len(f.Data[offset:]) {
			done = true
		}

		return k, done, nil
	}

	var cur uint64
	var done bool
	for i, bs := range f.GetBlocksizes() {
		// We found the correct child to write into
		if cur+bs > offset {
			child, err := node.Links[i].GetNode(dm.ctx, dm.dagserv)
			if err != nil {
				return "", false, err
			}
			k, sdone, err := dm.modifyDag(child, offset-cur, data)
			if err != nil {
				return "", false, err
			}

			offset += bs
			node.Links[i].Hash = mh.Multihash(k)

			// Recache serialized node
			_, err = node.Encoded(true)
			if err != nil {
				return "", false, err
			}

			if sdone {
				// No more bytes to write!
				done = true
				break
			}
			offset = cur + bs
		}
		cur += bs
	}

	k, err := dm.dagserv.Add(node)
	return k, done, err
}

func (k Key) ToMultihash() mh.Multihash {
	return mh.Multihash(k)
}