Allow encoding/decoding uint struct tag keys

decode.go

@@ -175,6 +175,15 @@ func (d *Decoder) UsePreallocateValues(on bool) {
 	}
 }
 
+// UseUIntStructKeys enables support for decoding uint struct tag keys to their corresponding struct field.
+func (d *Decoder) UseUIntStructKeys(on bool) {
+	if on {
+		d.flags |= useUIntStructKeysFlag
+	} else {
+		d.flags &= ^useUIntStructKeysFlag
+	}
+}
+
 // DisableAllocLimit enables fully allocating slices/maps when the size is known
 func (d *Decoder) DisableAllocLimit(on bool) {
 	if on {

decode_map.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"reflect"
+	"strconv"
 
 	"github.com/vmihailenco/msgpack/v5/msgpcode"
 )
@@ -332,9 +333,30 @@ func (d *Decoder) decodeStruct(v reflect.Value, n int) error {
 
 	fields := structs.Fields(v.Type(), d.structTag)
 	for i := 0; i < n; i++ {
-		name, err := d.decodeStringTemp()
-		if err != nil {
-			return err
+		var name string
+		var err error
+
+		// Try to decode the key as a uint if useUIntStructKeysFlag is set
+		if d.flags&useUIntStructKeysFlag != 0 {
+			code, err := d.PeekCode()
+			if err != nil {
+				return err
+			}
+			if msgpcode.IsUint(code) {
+				intKey, err := d.DecodeUint()
+				if err != nil {
+					return err
+				}
+				name = strconv.FormatUint(uint64(intKey), 10)
+			}
+		}
+
+		// if useUIntStructKeysFlag is not set or if the current key is not a uint, fallback to a string key
+		if name == "" {
+			name, err = d.decodeStringTemp()
+			if err != nil {
+				return err
+			}
 		}
 
 		if f := fields.Map[name]; f != nil {

encode.go

@@ -17,6 +17,7 @@ const (
 	useCompactFloatsFlag
 	useInternedStringsFlag
 	omitEmptyFlag
+	useUIntStructKeysFlag
 )
 
 type writer interface {
@@ -196,6 +197,15 @@ func (e *Encoder) UseInternedStrings(on bool) {
 	}
 }
 
+// UseUIntStructKeys causes the Encoder to encode struct fields that have a valid uint tag key as uints.
+func (e *Encoder) UseUIntStructKeys(on bool) {
+	if on {
+		e.flags |= useUIntStructKeysFlag
+	} else {
+		e.flags &= ^useUIntStructKeysFlag
+	}
+}
+
 func (e *Encoder) Encode(v interface{}) error {
 	switch v := v.(type) {
 	case nil:

encode_map.go

@@ -4,6 +4,7 @@ import (
 	"math"
 	"reflect"
 	"sort"
+	"strconv"
 
 	"github.com/vmihailenco/msgpack/v5/msgpcode"
 )
@@ -201,8 +202,22 @@ func encodeStructValue(e *Encoder, strct reflect.Value) error {
 	}
 
 	for _, f := range fields {
-		if err := e.EncodeString(f.name); err != nil {
-			return err
+		uintKeyEncoded := false
+		if e.flags&useUIntStructKeysFlag != 0 {
+			// Try to encode the key as a uint if useUIntStructKeysFlag is set
+			uintKey, err := strconv.ParseUint(f.name, 10, 0)
+			if err == nil {
+				if err := e.EncodeUint(uintKey); err != nil {
+					return err
+				}
+				uintKeyEncoded = true
+			}
+		}
+		// If useUIntStructKeysFlag is not set or if the key was not encoded as a uint, encode it as a string
+		if !uintKeyEncoded {
+			if err := e.EncodeString(f.name); err != nil {
+				return err
+			}
 		}
 		if err := f.EncodeValue(e, strct); err != nil {
 			return err

example_test.go

@@ -26,6 +26,41 @@ func ExampleMarshal() {
 	// Output: bar
 }
 
+func ExampleMarshal_intKeys() {
+	type Item struct {
+		Foo string `msgpack:"100"`
+		Bar string `msgpack:"200"`
+		Baz string `msgpack:"not_int_key"`
+	}
+
+	buf := new(bytes.Buffer)
+	enc := msgpack.NewEncoder(buf)
+	enc.UseUIntStructKeys(true)
+	err := enc.Encode(&Item{
+		Foo: "foo",
+		Bar: "bar",
+		Baz: "baz",
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	var item Item
+	dec := msgpack.NewDecoder(buf)
+	dec.UseUIntStructKeys(true)
+	err = dec.Decode(&item)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(item.Foo)
+	fmt.Println(item.Bar)
+	fmt.Println(item.Baz)
+	// Output:
+	// foo
+	// bar
+	// baz
+}
+
 func ExampleMarshal_mapStringInterface() {
 	in := map[string]interface{}{"foo": 1, "hello": "world"}
 	b, err := msgpack.Marshal(in)

msgpcode/msgpcode.go

@@ -86,3 +86,7 @@ func IsFixedExt(c byte) bool {
 func IsExt(c byte) bool {
 	return IsFixedExt(c) || c == Ext8 || c == Ext16 || c == Ext32
 }
+
+func IsUint(c byte) bool {
+	return c <= PosFixedNumHigh || c == Uint8 || c == Uint16 || c == Uint32 || c == Uint64
+}