diff --git a/base.go b/base.go
index b8383bd..e8322bc 100644
--- a/base.go
+++ b/base.go
@@ -2,6 +2,7 @@ package unibase2n
 
 import (
 	"errors"
+	"math/bits"
 )
 
 // Base has an encoding buffer thus should not be copied.
@@ -46,11 +47,14 @@ func (bs Base) check() error {
 	if offe > 0x10000 {
 		return ErrOffsetOverflow
 	}
+	if bits.OnesCount8(bit) == 1 { // 2的幂永无offset, 无需管til (1/2/4/8)
+		return nil
+	}
 	tile := uint32(til) // [til, tile)
-	if bit > 8 && bit%2 == 0 {
-		tile += uint32(bit / 2)
+	if bit > 4 && bit%2 == 0 {
+		tile += uint32(bit / 2) // 6 10 12 14
 	} else {
-		tile += uint32(bit)
+		tile += uint32(bit) // 3 5 7 9 11 13 15
 	}
 	if tile > 0x10000 {
 		return ErrTailOverflow
diff --git a/decode.go b/decode.go
index 1051725..58e2023 100644
--- a/decode.go
+++ b/decode.go
@@ -2,14 +2,58 @@ package unibase2n
 
 import (
 	"encoding/binary"
+	"math/bits"
 )
 
-func (bs Base) Decode(data []byte) []byte {
-	return nil
+func (bs Base) Decode(data []byte) (out []byte) {
+	if len(data) == 0 || len(data)%2 != 0 {
+		return nil
+	}
+	if bits.OnesCount8(bs.bit) == 1 { // 2的幂永无offset, 无需管til (1/2/4/8)
+		out = make([]byte, bs.DecodeLen(len(data), 0))
+		switch bs.bit {
+		case 8:
+			dec16blk8(bs.off, data, out)
+		case 4:
+			mask := uint32(bs.off) | uint32(bs.off)<<16
+			dec32blk4(mask, data, out)
+		case 2:
+			mask := uint64(bs.off) | uint64(bs.off)<<16 | uint64(bs.off)<<32 | uint64(bs.off)<<48
+			dec64blk2(mask, data, out)
+		case 1:
+			mask64 := uint64(bs.off) | uint64(bs.off)<<16 | uint64(bs.off)<<32 | uint64(bs.off)<<48
+			mask := uint128be{a: mask64, b: mask64}
+			dec128blk1(mask, data, out)
+		}
+		return
+	}
+	offset := 0
+	tile := uint32(bs.til) // [til, tile)
+	if bs.bit > 4 && bs.bit%2 == 0 {
+		tile += uint32(bs.bit / 2) // 6 10 12 14
+	} else {
+		tile += uint32(bs.bit) // 3 5 7 9 11 13 15
+	}
+	realtail := binary.BigEndian.Uint16(data[len(data)-2:])
+	if realtail >= bs.til && uint32(realtail) < tile {
+		offset = int(realtail - bs.til)
+	}
+	out = make([]byte, bs.DecodeLen(len(data), offset))
+	switch bs.bit {
+	case 3, 5, 7, 9, 11, 13, 15:
+		mask64 := uint64(bs.off) | uint64(bs.off)<<16 | uint64(bs.off)<<32 | uint64(bs.off)<<48
+		mask := uint128be{a: mask64, b: mask64}
+		dec128blk(mask, bs.bit, data, out)
+	case 6, 10, 12, 14:
+		mask := uint64(bs.off) | uint64(bs.off)<<16 | uint64(bs.off)<<32 | uint64(bs.off)<<48
+		dec64blk(mask, bs.bit, data, out)
+	}
+	return
 }
 
 // dec128blk1 for bit 1
 //    len(in)>0, len(in)%16==0, len(out)==len(in)/16
+//go:nosplit
 func dec128blk1(mask uint128be, in, out []byte) {
 	for i := range out {
 		c := i * 16
@@ -43,21 +87,183 @@ func dec128blk1(mask uint128be, in, out []byte) {
 }
 
 // dec64blk2 for bit 2
-//    len(in)!=0, len(out)==len(in)/8
+//    len(in)>0, len(in)%8==0, len(out)==len(in)/8
+//go:nosplit
 func dec64blk2(mask uint64, in, out []byte) {
-	for i, n := range in {
+	for i := range out {
 		c := i * 8
-		x := (uint64(n)<<42 | uint64(n)<<28 | uint64(n)<<14 | uint64(n)) & 0x00030003_00030003
-		binary.BigEndian.PutUint64(out[c:c+8], x+mask)
+		n := binary.BigEndian.Uint64(in[c:c+8]) - mask
+		sum := n & 0x03
+		n >>= 16 - 2
+		sum |= n & 0x0c
+		n >>= 16 - 2
+		sum |= n & 0x30
+		n >>= 16 - 2
+		sum |= n & 0xc0
+		out[i] = uint8(sum)
 	}
 }
 
 // dec32blk4 for bit 4
-//    len(in)!=0, len(out)==len(in)/4
+//    len(in)>0, len(in)%4==0, len(out)==len(in)/4
+//go:nosplit
 func dec32blk4(mask uint32, in, out []byte) {
-	for i, n := range in {
+	for i := range out {
 		c := i * 4
-		x := (uint32(n)<<12 | uint32(n)) & 0x000f000f
-		binary.BigEndian.PutUint32(out[c:c+4], x+mask)
+		n := binary.BigEndian.Uint32(in[c:c+4]) - mask
+		sum := n & 0x0f
+		n >>= 16 - 4
+		sum |= n & 0xf0
+		out[i] = uint8(sum)
+	}
+}
+
+// dec16blk8 for bit 8
+//    len(in)>0, len(in)%2==0, len(out)==len(in)/2
+//go:nosplit
+func dec16blk8(mask uint16, in, out []byte) {
+	for i := range out {
+		c := i * 2
+		n := binary.BigEndian.Uint16(in[c:c+2]) - mask
+		out[i] = uint8(n)
+	}
+}
+
+// dec128blk for bit 3 5 6 7 9 11 13 15
+//    len(in)>0, len(in)%2==0, len(out)==len(in)/16*bit+offset
+//go:nosplit
+func dec128blk(mask uint128be, bit uint8, in, out []byte) {
+	var tmp [16]byte
+	i := 0
+	n := 0
+	c := 16 - bit
+	for ; i <= len(out)-int(bit); n += 16 {
+		m := uint128be{a: (uint64(1)<<bit - 1) << (64 - bit)}
+		shift := readuint128be(in[n:]).sub(mask)
+		shift.shleq(c)
+		sum := shift.and(m)
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		if len(out[i:]) < 16 {
+			sum.write(tmp[:])
+			copy(out[i:], tmp[:])
+		} else {
+			sum.write(out[i:])
+		}
+		i += int(bit)
+	}
+	if n < len(in)-2 {
+		m := uint128be{a: (uint64(1)<<bit - 1) << (64 - bit)}
+		copy(tmp[:], in[n:])
+		msk := uint16(mask.a & 0xffff)
+		for j := 0; j < 8; j++ {
+			x := binary.BigEndian.Uint16(tmp[j*2:])
+			if x < msk {
+				binary.BigEndian.PutUint16(tmp[j*2:], msk)
+			}
+		}
+		shift := readuint128be(tmp[:]).sub(mask)
+		shift.shleq(c)
+		sum := shift.and(m)
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		shift.shleq(c)
+		m.shreq(bit)
+		sum.oreq(shift.and(m))
+		sum.write(tmp[:])
+		copy(out[i:], tmp[:])
+	}
+}
+
+// dec64blk for bit 6 10 12 14
+//    len(in)>0, len(in)%2==0, len(out)==len(in)/16*bit+offset
+//go:nosplit
+func dec64blk(mask uint64, bit uint8, in, out []byte) {
+	var tmp [8]byte
+	i := 0
+	s := int(bit) >> 1
+	n := 0
+	c := 16 - bit
+	for ; i <= len(out)-s; n += 8 {
+		m := (uint64(1)<<bit - 1) << (64 - bit)
+		shift := binary.BigEndian.Uint64(in[n:]) - mask
+		shift <<= c
+		sum := shift & m
+		shift <<= c
+		m >>= bit
+		sum |= shift & m
+		shift <<= c
+		m >>= bit
+		sum |= shift & m
+		shift <<= c
+		m >>= bit
+		sum |= shift & m
+		if len(out[i:]) < 8 {
+			binary.BigEndian.PutUint64(tmp[:], sum)
+			copy(out[i:], tmp[:])
+		} else {
+			binary.BigEndian.PutUint64(out[i:], sum)
+		}
+		i += s
+	}
+	if n < len(in)-2 {
+		m := (uint64(1)<<bit - 1) << (64 - bit)
+		copy(tmp[:], in[n:])
+		msk := uint16(mask & 0xffff)
+		for j := 0; j < 4; j++ {
+			x := binary.BigEndian.Uint16(tmp[j*2:])
+			if x < msk {
+				binary.BigEndian.PutUint16(tmp[j*2:], msk)
+			}
+		}
+		shift := binary.BigEndian.Uint64(tmp[:]) - mask
+		shift <<= c
+		sum := shift & m
+		shift <<= c
+		m >>= bit
+		sum |= shift & m
+		shift <<= c
+		m >>= bit
+		sum |= shift & m
+		shift <<= c
+		m >>= bit
+		sum |= shift & m
+		binary.BigEndian.PutUint64(tmp[:], sum)
+		copy(out[i:], tmp[:])
 	}
 }
diff --git a/decode_test.go b/decode_test.go
index 96ffbe5..318ffe5 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -1,12 +1,33 @@
 package unibase2n
 
 import (
+	"bytes"
+	"math/bits"
 	"math/rand"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 )
 
+func TestEnDecode(t *testing.T) {
+	var buf [4096]byte
+	_, err := rand.Read(buf[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, 1, bits.OnesCount8(8))
+	bs := Base{off: 0, til: 32768, bit: 1}
+	for ; bs.bit < 16; bs.bit++ {
+		for i := 1; i <= 4096; i++ {
+			e := bs.Encode(buf[:i])
+			d := bs.Decode(e)
+			if !bytes.Equal(d, buf[:i]) {
+				t.Fatal(bs.bit, i, e, d, buf[:i])
+			}
+		}
+	}
+}
+
 func TestDec128blk1(t *testing.T) {
 	var in, tmp [32]byte
 	_, err := rand.Read(in[:])
@@ -20,3 +41,100 @@ func TestDec128blk1(t *testing.T) {
 	dec128blk1(uint128be{0x2333233323332333, 0x2333233323332333}, out[:], tmp[:])
 	assert.Equal(t, in, tmp)
 }
+
+func TestDec64blk2(t *testing.T) {
+	var in, tmp [32]byte
+	_, err := rand.Read(in[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	n, _ := Base{bit: 2}.EncodeLen(32)
+	out := make([]byte, n)
+	enc64blk2(0x2333233323332333, in[:], out)
+	t.Log(out)
+	dec64blk2(0x2333233323332333, out[:], tmp[:])
+	assert.Equal(t, in, tmp)
+}
+
+func TestDec32blk4(t *testing.T) {
+	var in, tmp [32]byte
+	_, err := rand.Read(in[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	n, _ := Base{bit: 4}.EncodeLen(32)
+	out := make([]byte, n)
+	enc32blk4(0x23332333, in[:], out)
+	t.Log(out)
+	dec32blk4(0x23332333, out[:], tmp[:])
+	assert.Equal(t, in, tmp)
+}
+
+func TestDec16blk8(t *testing.T) {
+	var in, tmp [32]byte
+	_, err := rand.Read(in[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	n, _ := Base{bit: 8}.EncodeLen(32)
+	out := make([]byte, n)
+	enc16blk8(0x2333, in[:], out)
+	t.Log(out)
+	dec16blk8(0x2333, out[:], tmp[:])
+	assert.Equal(t, in, tmp)
+}
+
+func TestDec128blk(t *testing.T) {
+	var in, tmp [32]byte
+	_, err := rand.Read(in[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	mask := uint128be{0x2333233323332333, 0x2333233323332333}
+	bs := Base{}
+	for _, bit := range [...]byte{3, 5, 6, 7, 9, 11, 13, 15} {
+		bs.bit = bit
+		n, _ := bs.EncodeLen(32)
+		out := make([]byte, n)
+		enc128blk(mask, bit, in[:], out)
+		t.Log(out)
+		dec128blk(mask, bit, out[:], tmp[:])
+		assert.Equal(t, in, tmp)
+	}
+}
+
+func TestDec64blk(t *testing.T) {
+	var in, tmp [32]byte
+	_, err := rand.Read(in[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	// 6
+	n, _ := Base{bit: 6}.EncodeLen(32)
+	out := make([]byte, n)
+	enc64blk(0x2333233323332333, 6, in[:], out)
+	t.Log(out)
+	dec64blk(0x2333233323332333, 6, out[:], tmp[:])
+	assert.Equal(t, in, tmp)
+	// 10
+	n, _ = Base{bit: 10}.EncodeLen(32)
+	out = make([]byte, n)
+	enc64blk(0x2333233323332333, 10, in[:], out)
+	t.Log(out)
+	dec64blk(0x2333233323332333, 10, out[:], tmp[:])
+	assert.Equal(t, in, tmp)
+	// 12
+	n, _ = Base{bit: 12}.EncodeLen(32)
+	out = make([]byte, n)
+	enc64blk(0x2333233323332333, 12, in[:], out)
+	t.Log(out)
+	dec64blk(0x2333233323332333, 12, out[:], tmp[:])
+	assert.Equal(t, in, tmp)
+	// 14
+	n, _ = Base{bit: 14}.EncodeLen(32)
+	out = make([]byte, n)
+	enc64blk(0x2333233323332333, 14, in[:], out)
+	t.Log(out)
+	dec64blk(0x2333233323332333, 14, out[:], tmp[:])
+	assert.Equal(t, in, tmp)
+}
diff --git a/encode.go b/encode.go
index 48062f5..7042007 100644
--- a/encode.go
+++ b/encode.go
@@ -38,6 +38,7 @@ func (bs Base) Encode(data []byte) (out []byte) {
 
 // enc16blk1 for bit 1 (actual enc128blk1)
 //    len(in)!=0, len(out)==len(in)*16
+//go:nosplit
 func enc16blk1(mask uint16, in, out []byte) {
 	for i, n := range in {
 		c := i * 16
@@ -54,6 +55,7 @@ func enc16blk1(mask uint16, in, out []byte) {
 
 // enc64blk2 for bit 2
 //    len(in)!=0, len(out)==len(in)*8
+//go:nosplit
 func enc64blk2(mask uint64, in, out []byte) {
 	for i, n := range in {
 		c := i * 8
@@ -64,6 +66,7 @@ func enc64blk2(mask uint64, in, out []byte) {
 
 // enc32blk4 for bit 4
 //    len(in)!=0, len(out)==len(in)*4
+//go:nosplit
 func enc32blk4(mask uint32, in, out []byte) {
 	for i, n := range in {
 		c := i * 4
@@ -74,6 +77,7 @@ func enc32blk4(mask uint32, in, out []byte) {
 
 // enc16blk8 for bit 8
 //    len(in)!=0, len(out)==len(in)*2
+//go:nosplit
 func enc16blk8(mask uint16, in, out []byte) {
 	for i, n := range in {
 		c := i * 2
@@ -83,6 +87,7 @@ func enc16blk8(mask uint16, in, out []byte) {
 
 // enc128blk for bit 3 5 6 7 9 11 13 15
 //    len(in)>0, len(out)==len(in)/bit*16
+//go:nosplit
 func enc128blk(mask uint128be, bit uint8, in, out []byte) {
 	// 由于最后一次处理有读取越界, 因此作扩展
 	var buf [16]byte
@@ -158,6 +163,7 @@ func enc128blk(mask uint128be, bit uint8, in, out []byte) {
 
 // enc64blk for bit 6 10 12 14
 //    len(in)>0, len(out)==len(in)/bit*16
+//go:nosplit
 func enc64blk(mask uint64, bit uint8, in, out []byte) {
 	// 由于最后一次处理有读取越界, 因此作扩展
 	var buf [8]byte