feat(graphql): use a composite cursor

the implementation is now an hexadecimal representation of a byte array
with a length of 20, the first 8 bytes contains the validator points
and the 12 others the validator id.

app/nemeton/cursor.go

@@ -0,0 +1,51 @@
+package nemeton
+
+import (
+	"encoding/binary"
+	"encoding/hex"
+	"fmt"
+
+	"go.mongodb.org/mongo-driver/bson/primitive"
+)
+
+type Cursor struct {
+	points   uint64
+	objectID primitive.ObjectID
+}
+
+func (c *Cursor) Hex() string {
+	return hex.EncodeToString(c.MarshalBinary())
+}
+
+func (c *Cursor) FromHex(enc string) error {
+	bin, err := hex.DecodeString(enc)
+	if err != nil {
+		return err
+	}
+
+	return c.UnmarshalBinary(bin)
+}
+
+func (c *Cursor) MarshalBinary() []byte {
+	var enc [20]byte
+
+	binary.LittleEndian.PutUint64(enc[:8], c.points)
+
+	rawID := [12]byte(c.objectID)
+	copy(enc[8:], rawID[:])
+
+	return enc[:]
+}
+
+func (c *Cursor) UnmarshalBinary(enc []byte) error {
+	if len(enc) != 20 {
+		return fmt.Errorf("expected a 20 length byte array")
+	}
+
+	c.points = binary.LittleEndian.Uint64(enc[:8])
+
+	var decID [12]byte
+	copy(decID[:], enc[8:])
+	c.objectID = decID
+	return nil
+}

graphql/scalar/cursor.go

@@ -4,21 +4,25 @@ import (
 	"fmt"
 	"io"
 
+	"okp4/nemeton-leaderboard/app/nemeton"
+
 	"github.com/99designs/gqlgen/graphql"
-	"go.mongodb.org/mongo-driver/bson/primitive"
 )
 
-func UnmarshalCursor(v interface{}) (primitive.ObjectID, error) {
+func UnmarshalCursor(v interface{}) (*nemeton.Cursor, error) {
 	strCursor, ok := v.(string)
 	if !ok {
-		return primitive.ObjectID{}, fmt.Errorf("expect type 'string' and got type '%T'", v)
+		return nil, fmt.Errorf("expect type 'string' and got type '%T'", v)
 	}
 
-	return primitive.ObjectIDFromHex(strCursor)
+	c := &nemeton.Cursor{}
+	return c, c.FromHex(strCursor)
 }
 
-func MarshalCursor(c primitive.ObjectID) graphql.Marshaler {
+func MarshalCursor(c *nemeton.Cursor) graphql.Marshaler {
 	return graphql.WriterFunc(func(w io.Writer) {
+		_, _ = w.Write([]byte(`"`))
 		_, _ = w.Write([]byte(c.Hex()))
+		_, _ = w.Write([]byte(`"`))
 	})
 }