encoding/protobuf/textproto: add decoder implementation

Note that this is incredibly buggy. Not much I can do,
as there just doesn't seem to be any good textproto parser
for Go, and this one is the recommended "gold standard".

Issue #5

Change-Id: Ieab0910dc4ea6072d9dc50e2947d8a7fb33ba7ef

encoding/protobuf/pbinternal/attribute.go

@@ -0,0 +1,142 @@
+// Copyright 2021 CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package pbinternal
+
+import (
+	"strings"
+
+	"cuelang.org/go/cue"
+)
+
+type CompositeType int
+
+const (
+	Normal CompositeType = iota
+	List
+	Map
+)
+
+type ValueType int
+
+const (
+	Unknown ValueType = iota
+	Message
+	Int
+	Float
+	String
+	Bytes
+	Bool
+)
+
+type Info struct {
+	Name    string
+	CUEName string
+	Attr    cue.Attribute
+	Value   cue.Value
+
+	CompositeType CompositeType
+	ValueType     ValueType
+	Type          string
+
+	// For maps only
+	KeyType       ValueType // only for maps
+	KeyTypeString string
+}
+
+func FromIter(i *cue.Iterator) (info Info, err error) {
+	return FromValue(i.Label(), i.Value())
+}
+
+func FromValue(name string, v cue.Value) (info Info, err error) {
+	a := v.Attribute("protobuf")
+
+	info.Name = name
+	info.CUEName = name
+
+	if a.Err() == nil {
+		info.Attr = a
+
+		s, ok, err := a.Lookup(1, "name")
+		if err != nil {
+			return info, err
+		}
+		if ok {
+			info.Name = strings.TrimSpace(s)
+		}
+
+		info.Type, err = a.String(1)
+		if err != nil {
+			return info, err
+		}
+	}
+
+	switch v.IncompleteKind() {
+	case cue.ListKind:
+		info.CompositeType = List
+		e, _ := v.Elem()
+		if e.Exists() {
+			v = e
+		} else {
+			for i, _ := v.List(); i.Next(); {
+				v = i.Value()
+				break
+			}
+		}
+
+	case cue.StructKind:
+		if strings.HasPrefix(info.Type, "map[") {
+			a := strings.SplitN(info.Type[len("map["):], ",", 2)
+			info.KeyTypeString = strings.TrimSpace(a[0])
+			switch info.KeyTypeString {
+			case "string":
+				info.KeyType = String
+			case "bytes":
+				info.KeyType = Bytes
+			case "double", "float":
+				info.KeyType = Float
+			case "bool":
+				info.KeyType = Bool
+			default:
+				info.KeyType = Int // Assuming
+			}
+			info.CompositeType = Map
+			v, _ = v.Elem()
+		}
+	}
+
+	info.Value = v
+
+	switch v.IncompleteKind() {
+	case cue.StructKind:
+		info.ValueType = Message
+
+	case cue.StringKind:
+		info.ValueType = String
+
+	case cue.BytesKind:
+		info.ValueType = Bytes
+
+	case cue.BoolKind:
+		info.ValueType = Bool
+
+	case cue.IntKind:
+		info.ValueType = Int
+
+	case cue.FloatKind, cue.NumberKind:
+		info.ValueType = Float
+	}
+
+	return info, nil
+}