diff --git a/README.md b/README.md
index 50d6727..70e45fa 100644
--- a/README.md
+++ b/README.md
@@ -234,11 +234,10 @@ $ go test -cpuprofile cpu.prof -memprofile mem.prof -bench . -benchmem
 ```
 ```
 goos: linux
-goarch: amd64
+goarch: arm64
 pkg: github.com/encodingx/binary
-cpu: Intel(R) Core(TM) i5-8259U CPU @ 2.30GHz
-BenchmarkMarshal   	 2145660	       552.5 ns/op	      64 B/op	       6 allocs/op
-BenchmarkUnmarshal 	 1878840	       655.3 ns/op	      64 B/op	       8 allocs/op
+BenchmarkMarshal-2     	 3435181	       349.9 ns/op	      64 B/op	       6 allocs/op
+BenchmarkUnmarshal-2   	 3004425	       396.4 ns/op	      64 B/op	       8 allocs/op
 PASS
-ok  	github.com/encodingx/binary	3.873s
+ok  	github.com/encodingx/binary	3.288s
 ```
diff --git a/binary.go b/binary.go
index 0a08e4c..39461b4 100644
--- a/binary.go
+++ b/binary.go
@@ -5,15 +5,12 @@ import (
 	"fmt"
 	"io"
 
+	"github.com/encodingx/binary/internal/codecs"
 	"github.com/encodingx/binary/internal/validation"
 )
 
-const (
-	wordLengthUpperLimitBytes = 8
-)
-
 var (
-	defaultCodec = newCodec()
+	defaultCodec codecs.Codec = codecs.NewCodec()
 )
 
 func Marshal(iface interface{}) (bytes []byte, e error) {
@@ -22,7 +19,7 @@ func Marshal(iface interface{}) (bytes []byte, e error) {
 	)
 
 	var (
-		operation codecOperation
+		operation codecs.CodecOperation
 	)
 
 	defer func() {
@@ -39,12 +36,12 @@ func Marshal(iface interface{}) (bytes []byte, e error) {
 		return
 	}()
 
-	operation, e = defaultCodec.newOperation(iface)
+	operation, e = defaultCodec.NewOperation(iface)
 	if e != nil {
 		return
 	}
 
-	bytes, e = operation.marshal()
+	bytes, e = operation.Marshal()
 	if e != nil {
 		return
 	}
@@ -58,7 +55,7 @@ func Unmarshal(bytes []byte, iface interface{}) (e error) {
 	)
 
 	var (
-		operation codecOperation
+		operation codecs.CodecOperation
 	)
 
 	defer func() {
@@ -75,12 +72,12 @@ func Unmarshal(bytes []byte, iface interface{}) (e error) {
 		return
 	}()
 
-	operation, e = defaultCodec.newOperation(iface)
+	operation, e = defaultCodec.NewOperation(iface)
 	if e != nil {
 		return
 	}
 
-	e = operation.unmarshal(bytes)
+	e = operation.Unmarshal(bytes)
 	if e != nil {
 		return
 	}
diff --git a/codec.go b/internal/codecs/codec.go
similarity index 52%
rename from codec.go
rename to internal/codecs/codec.go
index bad36b9..8aec8ef 100644
--- a/codec.go
+++ b/internal/codecs/codec.go
@@ -1,25 +1,26 @@
-package binary
+package codecs
 
 import (
 	"reflect"
 
+	"github.com/encodingx/binary/internal/codecs/metadata"
 	"github.com/encodingx/binary/internal/validation"
 )
 
-type codec struct {
-	formatMetadataCache map[reflect.Type]formatMetadata
+type Codec struct {
+	formatMetadataCache map[reflect.Type]metadata.FormatMetadata
 }
 
-func newCodec() (c codec) {
-	c = codec{
-		formatMetadataCache: make(map[reflect.Type]formatMetadata),
+func NewCodec() (c Codec) {
+	c = Codec{
+		formatMetadataCache: make(map[reflect.Type]metadata.FormatMetadata),
 	}
 
 	return
 }
 
-func (c codec) formatMetadataFromTypeReflection(reflection reflect.Type) (
-	format formatMetadata, e error,
+func (c Codec) formatMetadataFromTypeReflection(reflection reflect.Type) (
+	format metadata.FormatMetadata, e error,
 ) {
 	var (
 		inCache bool
@@ -43,7 +44,7 @@ func (c codec) formatMetadataFromTypeReflection(reflection reflect.Type) (
 		return
 	}
 
-	format, e = newFormatMetadataFromTypeReflection(
+	format, e = metadata.NewFormatMetadataFromTypeReflection(
 		reflection.Elem(),
 	)
 	if e != nil {
@@ -55,8 +56,8 @@ func (c codec) formatMetadataFromTypeReflection(reflection reflect.Type) (
 	return
 }
 
-func (c codec) newOperation(iface interface{}) (
-	operation codecOperation, e error,
+func (c Codec) NewOperation(iface interface{}) (
+	operation CodecOperation, e error,
 ) {
 	operation.format, e = c.formatMetadataFromTypeReflection(
 		reflect.TypeOf(iface),
@@ -70,21 +71,21 @@ func (c codec) newOperation(iface interface{}) (
 	return
 }
 
-type codecOperation struct {
-	format          formatMetadata
+type CodecOperation struct {
+	format          metadata.FormatMetadata
 	valueReflection reflect.Value
 }
 
-func (c codecOperation) marshal() (bytes []byte, e error) {
-	bytes = c.format.marshal(c.valueReflection)
+func (c CodecOperation) Marshal() (bytes []byte, e error) {
+	bytes = c.format.Marshal(c.valueReflection)
 
 	return
 }
 
-func (c codecOperation) unmarshal(bytes []byte) (e error) {
-	if len(bytes) != c.format.lengthInBytes {
+func (c CodecOperation) Unmarshal(bytes []byte) (e error) {
+	if len(bytes) != c.format.LengthInBytes() {
 		e = validation.NewLengthOfByteSliceNotEqualToFormatLengthError(
-			uint(c.format.lengthInBytes),
+			uint(c.format.LengthInBytes()),
 			uint(len(bytes)),
 		)
 
@@ -95,7 +96,7 @@ func (c codecOperation) unmarshal(bytes []byte) (e error) {
 		return
 	}
 
-	c.format.unmarshal(bytes, c.valueReflection)
+	c.format.Unmarshal(bytes, c.valueReflection)
 
 	return
 }
diff --git a/metadata-bit-field.go b/internal/codecs/metadata/bit-field.go
similarity index 99%
rename from metadata-bit-field.go
rename to internal/codecs/metadata/bit-field.go
index 1712b0b..6e17388 100644
--- a/metadata-bit-field.go
+++ b/internal/codecs/metadata/bit-field.go
@@ -1,4 +1,4 @@
-package binary
+package metadata
 
 import (
 	"encoding/binary"
diff --git a/internal/codecs/metadata/constants.go b/internal/codecs/metadata/constants.go
new file mode 100644
index 0000000..eb5e073
--- /dev/null
+++ b/internal/codecs/metadata/constants.go
@@ -0,0 +1,5 @@
+package metadata
+
+const (
+	wordLengthUpperLimitBytes = 8
+)
diff --git a/metadata-format.go b/internal/codecs/metadata/format.go
similarity index 78%
rename from metadata-format.go
rename to internal/codecs/metadata/format.go
index 0bd48e6..6e959b0 100644
--- a/metadata-format.go
+++ b/internal/codecs/metadata/format.go
@@ -1,4 +1,4 @@
-package binary
+package metadata
 
 import (
 	"reflect"
@@ -6,13 +6,13 @@ import (
 	"github.com/encodingx/binary/internal/validation"
 )
 
-type formatMetadata struct {
+type FormatMetadata struct {
 	words         []wordMetadata
 	lengthInBytes int
 }
 
-func newFormatMetadataFromTypeReflection(reflection reflect.Type) (
-	format formatMetadata, e error,
+func NewFormatMetadataFromTypeReflection(reflection reflect.Type) (
+	format FormatMetadata, e error,
 ) {
 	var (
 		i int
@@ -32,7 +32,7 @@ func newFormatMetadataFromTypeReflection(reflection reflect.Type) (
 		return
 	}
 
-	format = formatMetadata{
+	format = FormatMetadata{
 		words: make([]wordMetadata,
 			reflection.NumField(),
 		),
@@ -52,7 +52,7 @@ func newFormatMetadataFromTypeReflection(reflection reflect.Type) (
 	return
 }
 
-func (m formatMetadata) marshal(reflection reflect.Value) (bytes []byte) {
+func (m FormatMetadata) Marshal(reflection reflect.Value) (bytes []byte) {
 	// Merge byte slices marshalled from words,
 	// in the order they appear in the format.
 
@@ -78,7 +78,7 @@ func (m formatMetadata) marshal(reflection reflect.Value) (bytes []byte) {
 	return
 }
 
-func (m formatMetadata) unmarshal(bytes []byte, reflection reflect.Value) {
+func (m FormatMetadata) Unmarshal(bytes []byte, reflection reflect.Value) {
 	var (
 		i    int
 		j    int
@@ -105,3 +105,7 @@ func (m formatMetadata) unmarshal(bytes []byte, reflection reflect.Value) {
 
 	return
 }
+
+func (m FormatMetadata) LengthInBytes() int {
+	return m.lengthInBytes
+}
diff --git a/metadata-word.go b/internal/codecs/metadata/word.go
similarity index 99%
rename from metadata-word.go
rename to internal/codecs/metadata/word.go
index 1baff95..fab8eb3 100644
--- a/metadata-word.go
+++ b/internal/codecs/metadata/word.go
@@ -1,4 +1,4 @@
-package binary
+package metadata
 
 import (
 	"encoding/binary"