github.com/ltltlt/go-source-code@v0.0.0-20190830023027-95be009773aa/encoding/base64/base64.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package base64 implements base64 encoding as specified by RFC 4648.
package base64

import (
	"encoding/binary"
	"io"
	"strconv"
)

/*
 * Encodings
 */

// An Encoding is a radix 64 encoding/decoding scheme, defined by a
// 64-character alphabet. The most common encoding is the "base64"
// encoding defined in RFC 4648 and used in MIME (RFC 2045) and PEM
// (RFC 1421).  RFC 4648 also defines an alternate encoding, which is
// the standard encoding with - and _ substituted for + and /(for url).
type Encoding struct {
	encode    [64]byte
	decodeMap [256]byte // use array and not map to save space
	padChar   rune
	strict    bool
}

const (
	StdPadding rune = '=' // Standard padding character
	NoPadding  rune = -1  // No padding
)

const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
const encodeURL = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"

// NewEncoding returns a new padded Encoding defined by the given alphabet,
// which must be a 64-byte string that does not contain the padding character
// or CR / LF ('\r', '\n').
// The resulting Encoding uses the default padding character ('='),
// which may be changed or disabled via WithPadding.
// 支持自定义编码转换，只要传入的encoder长度为64
func NewEncoding(encoder string) *Encoding {
	if len(encoder) != 64 {
		panic("encoding alphabet is not 64-bytes long")
	}
	for i := 0; i < len(encoder); i++ {
		if encoder[i] == '\n' || encoder[i] == '\r' {
			panic("encoding alphabet contains newline character")
		}
	}

	e := new(Encoding)
	e.padChar = StdPadding
	copy(e.encode[:], encoder) // have to copy, since encode is array

	for i := 0; i < len(e.decodeMap); i++ {
		e.decodeMap[i] = 0xFF
	}
	for i := 0; i < len(encoder); i++ {
		e.decodeMap[encoder[i]] = byte(i)
	}
	return e
}

// WithPadding creates a new encoding identical to enc except
// with a specified padding character, or NoPadding to disable padding.
// The padding character must not be '\r' or '\n', must not
// be contained in the encoding's alphabet and must be a rune equal or
// below '\xff'.
func (enc Encoding) WithPadding(padding rune) *Encoding {
	if padding == '\r' || padding == '\n' || padding > 0xff {
		panic("invalid padding")
	}

	for i := 0; i < len(enc.encode); i++ {
		if rune(enc.encode[i]) == padding {
			panic("padding contained in alphabet")
		}
	}

	enc.padChar = padding
	return &enc
}

// Strict creates a new encoding identical to enc except with
// strict decoding enabled. In this mode, the decoder requires that
// trailing padding bits are zero, as described in RFC 4648 section 3.5.
// 严格模式解码要求尾部填充位为0（不可省略=？）
func (enc Encoding) Strict() *Encoding {
	enc.strict = true
	return &enc
}

// StdEncoding is the standard base64 encoding, as defined in
// RFC 4648.
var StdEncoding = NewEncoding(encodeStd)

// URLEncoding is the alternate base64 encoding defined in RFC 4648.
// It is typically used in URLs and file names.
var URLEncoding = NewEncoding(encodeURL)

// RawStdEncoding is the standard raw, unpadded base64 encoding,
// as defined in RFC 4648 section 3.2.
// This is the same as StdEncoding but omits padding characters.
var RawStdEncoding = StdEncoding.WithPadding(NoPadding)

// RawURLEncoding is the unpadded alternate base64 encoding defined in RFC 4648.
// It is typically used in URLs and file names.
// This is the same as URLEncoding but omits padding characters.
var RawURLEncoding = URLEncoding.WithPadding(NoPadding)

/*
 * Encoder
 */

// Encode encodes src using the encoding enc, writing
// EncodedLen(len(src)) bytes to dst.
//
// The encoding pads the output to a multiple of 4 bytes,
// so Encode is not appropriate for use on individual blocks
// of a large data stream. Use NewEncoder() instead.
// 不进行append处理，dst完全使用index访问，当dst长度不足时可能panic: index out of range
// 这样速度更快，缺点是需要提前计算dst的长度并分配
func (enc *Encoding) Encode(dst, src []byte) {
	if len(src) == 0 {
		return
	}

	di, si := 0, 0 // destinition pointer, source pointer
	n := (len(src) / 3) * 3
	for si < n {
		// Convert 3x 8bit source bytes into 4 bytes
		// 3字节整数存入四字节
		val := uint(src[si+0])<<16 | uint(src[si+1])<<8 | uint(src[si+2])

		dst[di+0] = enc.encode[val>>18&0x3F]
		dst[di+1] = enc.encode[val>>12&0x3F]
		dst[di+2] = enc.encode[val>>6&0x3F]
		dst[di+3] = enc.encode[val&0x3F]

		si += 3
		di += 4
	}

	remain := len(src) - si // 多出的字节, 1 or 2
	if remain == 0 {
		return
	}
	// Add the remaining small block
	val := uint(src[si+0]) << 16
	if remain == 2 {
		val |= uint(src[si+1]) << 8
	}

	dst[di+0] = enc.encode[val>>18&0x3F]
	dst[di+1] = enc.encode[val>>12&0x3F]

	switch remain {
	case 2:
		dst[di+2] = enc.encode[val>>6&0x3F]
		if enc.padChar != NoPadding {
			dst[di+3] = byte(enc.padChar)
		}
	case 1:
		if enc.padChar != NoPadding {
			dst[di+2] = byte(enc.padChar)
			dst[di+3] = byte(enc.padChar)
		}
	}
}

// EncodeToString returns the base64 encoding of src.
// 一般直接call this， 而不是直接使用Encode
func (enc *Encoding) EncodeToString(src []byte) string {
	buf := make([]byte, enc.EncodedLen(len(src)))
	enc.Encode(buf, src)
	return string(buf)
}

// EncodedLen returns the length in bytes of the base64 encoding
// of an input buffer of length n.
func (enc *Encoding) EncodedLen(n int) int {
	if enc.padChar == NoPadding {
		return (n*8 + 5) / 6 // minimum # chars at 6 bits per char
	}
	return (n + 2) / 3 * 4 // minimum # 4-char quanta, 3 bytes each
}

// encoder is streaming base64 encoder
type encoder struct {
	err  error
	enc  *Encoding
	w    io.Writer
	buf  [3]byte    // buffered data waiting to be encoded
	nbuf int        // number of bytes in buf
	out  [1024]byte // output buffer
}

// NewEncoder returns a new base64 stream encoder. Data written to
// the returned writer will be encoded using enc and then written to w.
// Base64 encodings operate in 4-byte blocks; when finished
// writing, the caller must Close the returned encoder to flush any
// partially written blocks.
// 结束写后Close是必须操作，否则buf可能有残留数据未flush
func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser {
	return &encoder{enc: enc, w: w}
}

func (e *encoder) Write(p []byte) (n int, err error) {
	if e.err != nil {
		return 0, e.err
	}

	// Leading fringe.
	// “起始外围”
	if e.nbuf > 0 {
		var i int
		for i = 0; i < len(p) && e.nbuf < 3; i++ {
			e.buf[e.nbuf] = p[i]
			e.nbuf++
		}
		n += i
		p = p[i:]
		if e.nbuf < 3 {
			return
		}
		e.enc.Encode(e.out[:], e.buf[:])
		if _, e.err = e.w.Write(e.out[:4]); e.err != nil {
			return n, e.err
		}
		e.nbuf = 0
	}

	// Large interior chunks.
	// “内陆”
	for len(p) >= 3 {
		nn := len(e.out) / 4 * 3 // 一次能处理多少字节
		if nn > len(p) {
			nn = len(p)
			nn -= nn % 3
		}
		e.enc.Encode(e.out[:], p[:nn])
		if _, e.err = e.w.Write(e.out[0 : nn/3*4]); e.err != nil {
			return n, e.err
		}
		n += nn
		p = p[nn:]
	}

	// Trailing fringe.
	// “尾部外围”
	for i := 0; i < len(p); i++ {
		e.buf[i] = p[i]
	}
	e.nbuf = len(p)
	n += len(p)
	return
}

// Close flushes any pending output from the encoder.
// It is an error to call Write after calling Close.
func (e *encoder) Close() error {
	// If there's anything left in the buffer, flush it out
	if e.err == nil && e.nbuf > 0 {
		e.enc.Encode(e.out[:], e.buf[:e.nbuf])
		_, e.err = e.w.Write(e.out[:e.enc.EncodedLen(e.nbuf)])
		e.nbuf = 0
	}
	return e.err
}

/*
 * Decoder
 */

type CorruptInputError int64

func (e CorruptInputError) Error() string {
	return "illegal base64 data at input byte " + strconv.FormatInt(int64(e), 10)
}

// decodeQuantum decodes up to 4 base64 bytes. It takes for parameters
// the destination buffer dst, the source buffer src and an index in the
// source buffer si.
// It returns the number of bytes read from src, the number of bytes written
// to dst, and an error, if any.
func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err error) {
	// Decode quantum using the base64 alphabet
	var dbuf [4]byte
	dinc, dlen := 3, 4

	for j := 0; j < len(dbuf); j++ {
		if len(src) == si {
			switch {
			case j == 0:
				return si, 0, nil
			case j == 1, enc.padChar != NoPadding:
				return si, 0, CorruptInputError(si - j)
			}
			dinc, dlen = j-1, j
			break
		}
		in := src[si]
		si++

		out := enc.decodeMap[in]
		if out != 0xff {
			dbuf[j] = out
			continue
		}

		if in == '\n' || in == '\r' {
			j--
			continue
		}

		if rune(in) != enc.padChar {
			return si, 0, CorruptInputError(si - 1)
		}

		// We've reached the end and there's padding
		switch j {
		case 0, 1:
			// incorrect padding
			return si, 0, CorruptInputError(si - 1)
		case 2:
			// "==" is expected, the first "=" is already consumed.
			// skip over newlines
			for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
				si++
			}
			if si == len(src) {
				// not enough padding
				return si, 0, CorruptInputError(len(src))
			}
			if rune(src[si]) != enc.padChar {
				// incorrect padding
				return si, 0, CorruptInputError(si - 1)
			}

			si++
		}

		// skip over newlines
		for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
			si++
		}
		if si < len(src) {
			// trailing garbage
			err = CorruptInputError(si)
		}
		dinc, dlen = 3, j
		break
	}

	// Convert 4x 6bit source bytes into 3 bytes
	val := uint(dbuf[0])<<18 | uint(dbuf[1])<<12 | uint(dbuf[2])<<6 | uint(dbuf[3])
	dbuf[2], dbuf[1], dbuf[0] = byte(val>>0), byte(val>>8), byte(val>>16)
	switch dlen {
	case 4:
		dst[2] = dbuf[2]
		dbuf[2] = 0
		fallthrough
	case 3:
		dst[1] = dbuf[1]
		if enc.strict && dbuf[2] != 0 {
			return si, 0, CorruptInputError(si - 1)
		}
		dbuf[1] = 0
		fallthrough
	case 2:
		dst[0] = dbuf[0]
		if enc.strict && (dbuf[1] != 0 || dbuf[2] != 0) {
			return si, 0, CorruptInputError(si - 2)
		}
	}
	dst = dst[dinc:]

	return si, dlen - 1, err
}

// DecodeString returns the bytes represented by the base64 string s.
func (enc *Encoding) DecodeString(s string) ([]byte, error) {
	dbuf := make([]byte, enc.DecodedLen(len(s)))
	n, err := enc.Decode(dbuf, []byte(s))
	return dbuf[:n], err
}

type decoder struct {
	err     error
	readErr error // error from r.Read
	enc     *Encoding
	r       io.Reader
	buf     [1024]byte // leftover input
	nbuf    int
	out     []byte // leftover decoded output
	outbuf  [1024 / 4 * 3]byte
}

func (d *decoder) Read(p []byte) (n int, err error) {
	// Use leftover decoded output from last read.
	if len(d.out) > 0 {
		n = copy(p, d.out)
		d.out = d.out[n:]
		return n, nil
	}

	if d.err != nil {
		return 0, d.err
	}

	// This code assumes that d.r strips supported whitespace ('\r' and '\n').

	// Refill buffer.
	for d.nbuf < 4 && d.readErr == nil {
		nn := len(p) / 3 * 4
		if nn < 4 {
			nn = 4
		}
		if nn > len(d.buf) {
			nn = len(d.buf)
		}
		nn, d.readErr = d.r.Read(d.buf[d.nbuf:nn])
		d.nbuf += nn
	}

	if d.nbuf < 4 {
		if d.enc.padChar == NoPadding && d.nbuf > 0 {
			// Decode final fragment, without padding.
			var nw int
			nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:d.nbuf])
			d.nbuf = 0
			d.out = d.outbuf[:nw]
			n = copy(p, d.out)
			d.out = d.out[n:]
			if n > 0 || len(p) == 0 && len(d.out) > 0 {
				return n, nil
			}
			if d.err != nil {
				return 0, d.err
			}
		}
		d.err = d.readErr
		if d.err == io.EOF && d.nbuf > 0 {
			d.err = io.ErrUnexpectedEOF
		}
		return 0, d.err
	}

	// Decode chunk into p, or d.out and then p if p is too small.
	nr := d.nbuf / 4 * 4
	nw := d.nbuf / 4 * 3
	if nw > len(p) {
		nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:nr])
		d.out = d.outbuf[:nw]
		n = copy(p, d.out)
		d.out = d.out[n:]
	} else {
		n, d.err = d.enc.Decode(p, d.buf[:nr])
	}
	d.nbuf -= nr
	copy(d.buf[:d.nbuf], d.buf[nr:])
	return n, d.err
}

// Decode decodes src using the encoding enc. It writes at most
// DecodedLen(len(src)) bytes to dst and returns the number of bytes
// written. If src contains invalid base64 data, it will return the
// number of bytes successfully written and CorruptInputError.
// New line characters (\r and \n) are ignored.
func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
	if len(src) == 0 {
		return 0, nil
	}

	si := 0
	ilen := len(src)
	olen := len(dst)
	for strconv.IntSize >= 64 && ilen-si >= 8 && olen-n >= 8 {
		if ok := enc.decode64(dst[n:], src[si:]); ok {
			n += 6
			si += 8
		} else {
			var ninc int
			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
			n += ninc
			if err != nil {
				return n, err
			}
		}
	}

	for ilen-si >= 4 && olen-n >= 4 {
		if ok := enc.decode32(dst[n:], src[si:]); ok {
			n += 3
			si += 4
		} else {
			var ninc int
			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
			n += ninc
			if err != nil {
				return n, err
			}
		}
	}

	for si < len(src) {
		var ninc int
		si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
		n += ninc
		if err != nil {
			return n, err
		}
	}
	return n, err
}

// decode32 tries to decode 4 base64 char into 3 bytes.
// len(dst) and len(src) must both be >= 4.
// Returns true if decode succeeded.
// 保证解码中间出错不影响dst, 只有src中的4个byte对应的字符均不是0xff时才写dst
func (enc *Encoding) decode32(dst, src []byte) bool {
	var dn, n uint32
	if n = uint32(enc.decodeMap[src[0]]); n == 0xff {
		return false
	}
	dn |= n << 26
	if n = uint32(enc.decodeMap[src[1]]); n == 0xff {
		return false
	}
	dn |= n << 20
	if n = uint32(enc.decodeMap[src[2]]); n == 0xff {
		return false
	}
	dn |= n << 14
	if n = uint32(enc.decodeMap[src[3]]); n == 0xff {
		return false
	}
	dn |= n << 8

	binary.BigEndian.PutUint32(dst, dn)
	return true
}

// decode64 tries to decode 8 base64 char into 6 bytes.
// len(dst) and len(src) must both be >= 8.
// Returns true if decode succeeded.
func (enc *Encoding) decode64(dst, src []byte) bool {
	var dn, n uint64
	if n = uint64(enc.decodeMap[src[0]]); n == 0xff {
		return false
	}
	dn |= n << 58
	if n = uint64(enc.decodeMap[src[1]]); n == 0xff {
		return false
	}
	dn |= n << 52
	if n = uint64(enc.decodeMap[src[2]]); n == 0xff {
		return false
	}
	dn |= n << 46
	if n = uint64(enc.decodeMap[src[3]]); n == 0xff {
		return false
	}
	dn |= n << 40
	if n = uint64(enc.decodeMap[src[4]]); n == 0xff {
		return false
	}
	dn |= n << 34
	if n = uint64(enc.decodeMap[src[5]]); n == 0xff {
		return false
	}
	dn |= n << 28
	if n = uint64(enc.decodeMap[src[6]]); n == 0xff {
		return false
	}
	dn |= n << 22
	if n = uint64(enc.decodeMap[src[7]]); n == 0xff {
		return false
	}
	dn |= n << 16

	binary.BigEndian.PutUint64(dst, dn)
	return true
}

type newlineFilteringReader struct {
	wrapped io.Reader
}

func (r *newlineFilteringReader) Read(p []byte) (int, error) {
	n, err := r.wrapped.Read(p)
	for n > 0 {
		offset := 0
		for i, b := range p[:n] { // left shift, watch closely
			if b != '\r' && b != '\n' {
				if i != offset {
					p[offset] = b
				}
				offset++
			}
		}
		if offset > 0 {
			return offset, err
		}
		// Previous buffer entirely LR or CR, read again
		n, err = r.wrapped.Read(p)
	}
	return n, err
}

// NewDecoder constructs a new base64 stream decoder.
// 其会自动过滤\n和\r
func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
	return &decoder{enc: enc, r: &newlineFilteringReader{r}}
}

// DecodedLen returns the maximum length in bytes of the decoded data
// corresponding to n bytes of base64-encoded data.
func (enc *Encoding) DecodedLen(n int) int {
	if enc.padChar == NoPadding {
		// Unpadded data may end with partial block of 2-3 characters.
		return n * 6 / 8
	}
	// Padded base64 should always be a multiple of 4 characters in length.
	return n / 4 * 3
}