func (enc *Encoding) EncodeToString(src []byte) string {
	buf := make([]byte, enc.base.EncodedLen(len(src)))
	enc.Encode(buf, src)
	return string(buf)
}
Encode encodes src using the encoding enc, writing
EncodedLen(len(src)) bytes to dst.
package base64
import (
	"encoding/base64"
	"github.com/segmentio/asm/cpu"
	"github.com/segmentio/asm/internal/unsafebytes"
)
type Encoding struct {
	enc    func(dst []byte, src []byte, lut *int8) (int, int)
	enclut [32]int8
	dec    func(dst []byte, src []byte, lut *int8) (int, int)
	declut [48]int8
	base *base64.Encoding
}
const (
	minEncodeLen = 28
	minDecodeLen = 45
)
func newEncoding(encoder string) *Encoding {
	e := &Encoding{base: base64.NewEncoding(encoder)}
	if cpu.X86.Has(cpu.AVX2) {
		e.enableEncodeAVX2(encoder)
		e.enableDecodeAVX2(encoder)
	}
	return e
}
func (e *Encoding) enableEncodeAVX2(encoder string) {
	tab := [32]int8{int8(encoder[0]), int8(encoder[letterRange]) - letterRange}
	for i, ch := range encoder[2*letterRange:] {
		tab[2+i] = int8(ch) - 2*letterRange - int8(i)
	}
	e.enc = encodeAVX2
	e.enclut = tab
}
func (e *Encoding) enableDecodeAVX2(encoder string) {
	c62, c63 := int8(encoder[62]), int8(encoder[63])
	url := c63 == '_'
	if url {
		c63 = '/'
	}
	tab := [48]int8{
		0, 63 - c63, 62 - c62, 4, -65, -65, -71, -71,
		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
		0x15, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11,
		0x11, 0x11, 0x13, 0x1B, 0x1B, 0x1B, 0x1B, 0x1B,
	}
	tab[(c62&15)+16] = 0x1A
	tab[(c63&15)+16] = 0x1A
	if url {
		e.dec = decodeAVX2URI
	} else {
		e.dec = decodeAVX2
	}
	e.declut = tab
}
func (enc Encoding) WithPadding(padding rune) *Encoding {
	enc.base = enc.base.WithPadding(padding)
	return &enc
}
func (enc Encoding) Strict() *Encoding {
	enc.base = enc.base.Strict()
	return &enc
}
func (enc *Encoding) Encode(dst, src []byte) {
	if len(src) >= minEncodeLen && enc.enc != nil {
		d, s := enc.enc(dst, src, &enc.enclut[0])
		dst = dst[d:]
		src = src[s:]
	}
	enc.base.Encode(dst, src)
}