feat(pkg/scale): support for custom VaryingDataType types (#2612)

* wip decode functionality

* wip encode custom vdt

* refactor tests

* more tests, fix decode for nested vdt

* update readme, provide example of nested VDTs

* add copyright

* fix lint

pkg/scale/README.md

@@ -82,7 +82,7 @@ SCALE uses a compact encoding for variable width unsigned integers.
 ### Basic Example
 
 Basic example which encodes and decodes a `uint`.
-```
+```go
 import (
 	"fmt"
 	"github.com/ChainSafe/gossamer/pkg/scale"
@@ -111,7 +111,7 @@ func ExampleBasic() {
 
 Use the `scale` struct tag for struct fields to conform to specific encoding sequence of struct field values.  A struct tag of `"-"` will be omitted from encoding and decoding.
 
-```
+```go
 import (
 	"fmt"
 	"github.com/ChainSafe/gossamer/pkg/scale"
@@ -159,7 +159,7 @@ result := scale.NewResult(int32(0), int32(0)
 result.Set(scale.Ok, 10)
 ```
 
-```
+```go
 import (
 	"fmt"
 	"github.com/ChainSafe/gossamer/pkg/scale"
@@ -213,7 +213,7 @@ func ExampleResult() {
 A `VaryingDataType` is analogous to a Rust enum.  A `VaryingDataType` needs to be constructed using the  `NewVaryingDataType` constructor.  `VaryingDataTypeValue` is an
 interface with one `Index() uint` method that needs to be implemented.  The returned `uint` index should be unique per type and needs to be the same index as defined in the Rust enum to ensure interopability.  To set the value of the `VaryingDataType`, the `VaryingDataType.Set()` function should be called with an associated `VaryingDataTypeValue`.
 
-```
+```go
 import (
 	"fmt"
 	"github.com/ChainSafe/gossamer/pkg/scale"
@@ -323,4 +323,192 @@ func ExampleVaryingDataTypeSlice() {
 		panic(fmt.Errorf("uh oh: %+v %+v", vdts, vdts1))
 	}
 }
+```
+
+#### Nested VaryingDataType
+
+See `varying_data_type_nested_example.go` for a working example of a custom `VaryingDataType` with another custom `VaryingDataType` as a value of the parent `VaryingDataType`.  In the case of nested `VaryingDataTypes`, a custom type needs to be created for the child `VaryingDataType` because it needs to fulfill the `VaryingDataTypeValue` interface.
+
+```go
+import (
+	"fmt"
+	"reflect"
+
+	"github.com/ChainSafe/gossamer/pkg/scale"
+)
+
+// ParentVDT is a VaryingDataType that consists of multiple nested VaryingDataType
+// instances (aka. a rust enum containing multiple enum options)
+type ParentVDT scale.VaryingDataType
+
+// Set will set a VaryingDataTypeValue using the underlying VaryingDataType
+func (pvdt *ParentVDT) Set(val scale.VaryingDataTypeValue) (err error) {
+	// cast to VaryingDataType to use VaryingDataType.Set method
+	vdt := scale.VaryingDataType(*pvdt)
+	err = vdt.Set(val)
+	if err != nil {
+		return
+	}
+	// store original ParentVDT with VaryingDataType that has been set
+	*pvdt = ParentVDT(vdt)
+	return
+}
+
+// Value will return value from underying VaryingDataType
+func (pvdt *ParentVDT) Value() (val scale.VaryingDataTypeValue) {
+	vdt := scale.VaryingDataType(*pvdt)
+	return vdt.Value()
+}
+
+// NewParentVDT is constructor for ParentVDT
+func NewParentVDT() ParentVDT {
+	// use standard VaryingDataType constructor to construct a VaryingDataType
+	vdt, err := scale.NewVaryingDataType(NewChildVDT(), NewOtherChildVDT())
+	if err != nil {
+		panic(err)
+	}
+	// cast to ParentVDT
+	return ParentVDT(vdt)
+}
+
+// ChildVDT type is used as a VaryingDataTypeValue for ParentVDT
+type ChildVDT scale.VaryingDataType
+
+// Index fulfills the VaryingDataTypeValue interface.  T
+func (cvdt ChildVDT) Index() uint {
+	return 1
+}
+
+// Set will set a VaryingDataTypeValue using the underlying VaryingDataType
+func (cvdt *ChildVDT) Set(val scale.VaryingDataTypeValue) (err error) {
+	// cast to VaryingDataType to use VaryingDataType.Set method
+	vdt := scale.VaryingDataType(*cvdt)
+	err = vdt.Set(val)
+	if err != nil {
+		return
+	}
+	// store original ParentVDT with VaryingDataType that has been set
+	*cvdt = ChildVDT(vdt)
+	return
+}
+
+// Value will return value from underying VaryingDataType
+func (cvdt *ChildVDT) Value() (val scale.VaryingDataTypeValue) {
+	vdt := scale.VaryingDataType(*cvdt)
+	return vdt.Value()
+}
+
+// NewChildVDT is constructor for ChildVDT
+func NewChildVDT() ChildVDT {
+	// use standard VaryingDataType constructor to construct a VaryingDataType
+	// constarined to types ChildInt16, ChildStruct, and ChildString
+	vdt, err := scale.NewVaryingDataType(ChildInt16(0), ChildStruct{}, ChildString(""))
+	if err != nil {
+		panic(err)
+	}
+	// cast to ParentVDT
+	return ChildVDT(vdt)
+}
+
+// OtherChildVDT type is used as a VaryingDataTypeValue for ParentVDT
+type OtherChildVDT scale.VaryingDataType
+
+// Index fulfills the VaryingDataTypeValue interface.
+func (ocvdt OtherChildVDT) Index() uint {
+	return 2
+}
+
+// Set will set a VaryingDataTypeValue using the underlying VaryingDataType
+func (cvdt *OtherChildVDT) Set(val scale.VaryingDataTypeValue) (err error) {
+	// cast to VaryingDataType to use VaryingDataType.Set method
+	vdt := scale.VaryingDataType(*cvdt)
+	err = vdt.Set(val)
+	if err != nil {
+		return
+	}
+	// store original ParentVDT with VaryingDataType that has been set
+	*cvdt = OtherChildVDT(vdt)
+	return
+}
+
+// NewOtherChildVDT is constructor for OtherChildVDT
+func NewOtherChildVDT() OtherChildVDT {
+	// use standard VaryingDataType constructor to construct a VaryingDataType
+	// constarined to types ChildInt16 and ChildStruct
+	vdt, err := scale.NewVaryingDataType(ChildInt16(0), ChildStruct{}, ChildString(""))
+	if err != nil {
+		panic(err)
+	}
+	// cast to ParentVDT
+	return OtherChildVDT(vdt)
+}
+
+// ChildInt16 is used as a VaryingDataTypeValue for ChildVDT and OtherChildVDT
+type ChildInt16 int16
+
+// Index fulfills the VaryingDataTypeValue interface.  The ChildVDT type is used as a
+// VaryingDataTypeValue for ParentVDT
+func (ci ChildInt16) Index() uint {
+	return 1
+}
+
+// ChildStruct is used as a VaryingDataTypeValue for ChildVDT and OtherChildVDT
+type ChildStruct struct {
+	A string
+	B bool
+}
+
+// Index fulfills the VaryingDataTypeValue interface
+func (cs ChildStruct) Index() uint {
+	return 2
+}
+
+// ChildString is used as a VaryingDataTypeValue for ChildVDT and OtherChildVDT
+type ChildString string
+
+// Index fulfills the VaryingDataTypeValue interface
+func (cs ChildString) Index() uint {
+	return 3
+}
+
+func ExampleNestedVaryingDataType() {
+	parent := NewParentVDT()
+
+	// populate parent with ChildVDT
+	child := NewChildVDT()
+	child.Set(ChildInt16(888))
+	err := parent.Set(child)
+	if err != nil {
+		panic(err)
+	}
+
+	// validate ParentVDT.Value()
+	fmt.Printf("parent.Value(): %+v\n", parent.Value())
+	// should cast to ChildVDT, since that was set earlier
+	valChildVDT := parent.Value().(ChildVDT)
+	// validate ChildVDT.Value() as ChildInt16(888)
+	fmt.Printf("child.Value(): %+v\n", valChildVDT.Value())
+
+	// marshal into scale encoded bytes
+	bytes, err := scale.Marshal(parent)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Printf("bytes: % x\n", bytes)
+
+	// unmarshal into another ParentVDT
+	dstParent := NewParentVDT()
+	err = scale.Unmarshal(bytes, &dstParent)
+	if err != nil {
+		panic(err)
+	}
+	// assert both ParentVDT instances are the same
+	fmt.Println(reflect.DeepEqual(parent, dstParent))
+
+	// Output:
+	// parent.Value(): {value:888 cache:map[1:0 2:{A: B:false} 3:]}
+	// child.Value(): 888
+	// bytes: 01 01 78 03
+	// true
+}
 ```

pkg/scale/decode.go

@@ -149,7 +149,12 @@ func (ds *decodeState) unmarshal(dstv reflect.Value) (err error) {
 		case reflect.Ptr:
 			err = ds.decodePointer(dstv)
 		case reflect.Struct:
-			err = ds.decodeStruct(dstv)
+			ok := reflect.ValueOf(in).CanConvert(reflect.TypeOf(VaryingDataType{}))
+			if ok {
+				err = ds.decodeCustomVaryingDataType(dstv)
+			} else {
+				err = ds.decodeStruct(dstv)
+			}
 		case reflect.Array:
 			err = ds.decodeArray(dstv)
 		case reflect.Slice:
@@ -344,6 +349,19 @@ func (ds *decodeState) decodeVaryingDataTypeSlice(dstv reflect.Value) (err error
 	return
 }
 
+func (ds *decodeState) decodeCustomVaryingDataType(dstv reflect.Value) (err error) {
+	initialType := dstv.Type()
+	converted := dstv.Convert(reflect.TypeOf(VaryingDataType{}))
+	tempVal := reflect.New(converted.Type())
+	tempVal.Elem().Set(converted)
+	err = ds.decodeVaryingDataType(tempVal.Elem())
+	if err != nil {
+		return
+	}
+	dstv.Set(tempVal.Elem().Convert(initialType))
+	return
+}
+
 func (ds *decodeState) decodeVaryingDataType(dstv reflect.Value) (err error) {
 	var b byte
 	b, err = ds.ReadByte()
@@ -358,12 +376,13 @@ func (ds *decodeState) decodeVaryingDataType(dstv reflect.Value) (err error) {
 		return
 	}
 
-	tempVal := reflect.New(reflect.TypeOf(val)).Elem()
-	err = ds.unmarshal(tempVal)
+	tempVal := reflect.New(reflect.TypeOf(val))
+	tempVal.Elem().Set(reflect.ValueOf(val))
+	err = ds.unmarshal(tempVal.Elem())
 	if err != nil {
 		return
 	}
-	err = vdt.Set(tempVal.Interface().(VaryingDataTypeValue))
+	err = vdt.Set(tempVal.Elem().Interface().(VaryingDataTypeValue))
 	if err != nil {
 		return
 	}

pkg/scale/encode.go

@@ -73,7 +73,12 @@ func (es *encodeState) marshal(in interface{}) (err error) {
 				err = es.marshal(elem.Interface())
 			}
 		case reflect.Struct:
-			err = es.encodeStruct(in)
+			ok := reflect.ValueOf(in).CanConvert(reflect.TypeOf(VaryingDataType{}))
+			if ok {
+				err = es.encodeCustomVaryingDataType(in)
+			} else {
+				err = es.encodeStruct(in)
+			}
 		case reflect.Array:
 			err = es.encodeArray(in)
 		case reflect.Slice:
@@ -148,6 +153,11 @@ func (es *encodeState) encodeResult(res Result) (err error) {
 	return
 }
 
+func (es *encodeState) encodeCustomVaryingDataType(in interface{}) (err error) {
+	vdt := reflect.ValueOf(in).Convert(reflect.TypeOf(VaryingDataType{})).Interface().(VaryingDataType)
+	return es.encodeVaryingDataType(vdt)
+}
+
 func (es *encodeState) encodeVaryingDataType(vdt VaryingDataType) (err error) {
 	err = es.WriteByte(byte(vdt.value.Index()))
 	if err != nil {