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gitea/vendor/github.com/philhofer/fwd/reader.go
2018-05-19 20:49:46 +08:00

384 lines
9.2 KiB
Go

// The `fwd` package provides a buffered reader
// and writer. Each has methods that help improve
// the encoding/decoding performance of some binary
// protocols.
//
// The `fwd.Writer` and `fwd.Reader` type provide similar
// functionality to their counterparts in `bufio`, plus
// a few extra utility methods that simplify read-ahead
// and write-ahead. I wrote this package to improve serialization
// performance for http://github.com/tinylib/msgp,
// where it provided about a 2x speedup over `bufio` for certain
// workloads. However, care must be taken to understand the semantics of the
// extra methods provided by this package, as they allow
// the user to access and manipulate the buffer memory
// directly.
//
// The extra methods for `fwd.Reader` are `Peek`, `Skip`
// and `Next`. `(*fwd.Reader).Peek`, unlike `(*bufio.Reader).Peek`,
// will re-allocate the read buffer in order to accommodate arbitrarily
// large read-ahead. `(*fwd.Reader).Skip` skips the next `n` bytes
// in the stream, and uses the `io.Seeker` interface if the underlying
// stream implements it. `(*fwd.Reader).Next` returns a slice pointing
// to the next `n` bytes in the read buffer (like `Peek`), but also
// increments the read position. This allows users to process streams
// in arbitrary block sizes without having to manage appropriately-sized
// slices. Additionally, obviating the need to copy the data from the
// buffer to another location in memory can improve performance dramatically
// in CPU-bound applications.
//
// `fwd.Writer` only has one extra method, which is `(*fwd.Writer).Next`, which
// returns a slice pointing to the next `n` bytes of the writer, and increments
// the write position by the length of the returned slice. This allows users
// to write directly to the end of the buffer.
//
package fwd
import "io"
const (
// DefaultReaderSize is the default size of the read buffer
DefaultReaderSize = 2048
// minimum read buffer; straight from bufio
minReaderSize = 16
)
// NewReader returns a new *Reader that reads from 'r'
func NewReader(r io.Reader) *Reader {
return NewReaderSize(r, DefaultReaderSize)
}
// NewReaderSize returns a new *Reader that
// reads from 'r' and has a buffer size 'n'
func NewReaderSize(r io.Reader, n int) *Reader {
rd := &Reader{
r: r,
data: make([]byte, 0, max(minReaderSize, n)),
}
if s, ok := r.(io.Seeker); ok {
rd.rs = s
}
return rd
}
// Reader is a buffered look-ahead reader
type Reader struct {
r io.Reader // underlying reader
// data[n:len(data)] is buffered data; data[len(data):cap(data)] is free buffer space
data []byte // data
n int // read offset
state error // last read error
// if the reader past to NewReader was
// also an io.Seeker, this is non-nil
rs io.Seeker
}
// Reset resets the underlying reader
// and the read buffer.
func (r *Reader) Reset(rd io.Reader) {
r.r = rd
r.data = r.data[0:0]
r.n = 0
r.state = nil
if s, ok := rd.(io.Seeker); ok {
r.rs = s
} else {
r.rs = nil
}
}
// more() does one read on the underlying reader
func (r *Reader) more() {
// move data backwards so that
// the read offset is 0; this way
// we can supply the maximum number of
// bytes to the reader
if r.n != 0 {
if r.n < len(r.data) {
r.data = r.data[:copy(r.data[0:], r.data[r.n:])]
} else {
r.data = r.data[:0]
}
r.n = 0
}
var a int
a, r.state = r.r.Read(r.data[len(r.data):cap(r.data)])
if a == 0 && r.state == nil {
r.state = io.ErrNoProgress
return
} else if a > 0 && r.state == io.EOF {
// discard the io.EOF if we read more than 0 bytes.
// the next call to Read should return io.EOF again.
r.state = nil
}
r.data = r.data[:len(r.data)+a]
}
// pop error
func (r *Reader) err() (e error) {
e, r.state = r.state, nil
return
}
// pop error; EOF -> io.ErrUnexpectedEOF
func (r *Reader) noEOF() (e error) {
e, r.state = r.state, nil
if e == io.EOF {
e = io.ErrUnexpectedEOF
}
return
}
// buffered bytes
func (r *Reader) buffered() int { return len(r.data) - r.n }
// Buffered returns the number of bytes currently in the buffer
func (r *Reader) Buffered() int { return len(r.data) - r.n }
// BufferSize returns the total size of the buffer
func (r *Reader) BufferSize() int { return cap(r.data) }
// Peek returns the next 'n' buffered bytes,
// reading from the underlying reader if necessary.
// It will only return a slice shorter than 'n' bytes
// if it also returns an error. Peek does not advance
// the reader. EOF errors are *not* returned as
// io.ErrUnexpectedEOF.
func (r *Reader) Peek(n int) ([]byte, error) {
// in the degenerate case,
// we may need to realloc
// (the caller asked for more
// bytes than the size of the buffer)
if cap(r.data) < n {
old := r.data[r.n:]
r.data = make([]byte, n+r.buffered())
r.data = r.data[:copy(r.data, old)]
r.n = 0
}
// keep filling until
// we hit an error or
// read enough bytes
for r.buffered() < n && r.state == nil {
r.more()
}
// we must have hit an error
if r.buffered() < n {
return r.data[r.n:], r.err()
}
return r.data[r.n : r.n+n], nil
}
// Skip moves the reader forward 'n' bytes.
// Returns the number of bytes skipped and any
// errors encountered. It is analogous to Seek(n, 1).
// If the underlying reader implements io.Seeker, then
// that method will be used to skip forward.
//
// If the reader encounters
// an EOF before skipping 'n' bytes, it
// returns io.ErrUnexpectedEOF. If the
// underlying reader implements io.Seeker, then
// those rules apply instead. (Many implementations
// will not return `io.EOF` until the next call
// to Read.)
func (r *Reader) Skip(n int) (int, error) {
// fast path
if r.buffered() >= n {
r.n += n
return n, nil
}
// use seeker implementation
// if we can
if r.rs != nil {
return r.skipSeek(n)
}
// loop on filling
// and then erasing
o := n
for r.buffered() < n && r.state == nil {
r.more()
// we can skip forward
// up to r.buffered() bytes
step := min(r.buffered(), n)
r.n += step
n -= step
}
// at this point, n should be
// 0 if everything went smoothly
return o - n, r.noEOF()
}
// Next returns the next 'n' bytes in the stream.
// Unlike Peek, Next advances the reader position.
// The returned bytes point to the same
// data as the buffer, so the slice is
// only valid until the next reader method call.
// An EOF is considered an unexpected error.
// If an the returned slice is less than the
// length asked for, an error will be returned,
// and the reader position will not be incremented.
func (r *Reader) Next(n int) ([]byte, error) {
// in case the buffer is too small
if cap(r.data) < n {
old := r.data[r.n:]
r.data = make([]byte, n+r.buffered())
r.data = r.data[:copy(r.data, old)]
r.n = 0
}
// fill at least 'n' bytes
for r.buffered() < n && r.state == nil {
r.more()
}
if r.buffered() < n {
return r.data[r.n:], r.noEOF()
}
out := r.data[r.n : r.n+n]
r.n += n
return out, nil
}
// skipSeek uses the io.Seeker to seek forward.
// only call this function when n > r.buffered()
func (r *Reader) skipSeek(n int) (int, error) {
o := r.buffered()
// first, clear buffer
n -= o
r.n = 0
r.data = r.data[:0]
// then seek forward remaning bytes
i, err := r.rs.Seek(int64(n), 1)
return int(i) + o, err
}
// Read implements `io.Reader`
func (r *Reader) Read(b []byte) (int, error) {
// if we have data in the buffer, just
// return that.
if r.buffered() != 0 {
x := copy(b, r.data[r.n:])
r.n += x
return x, nil
}
var n int
// we have no buffered data; determine
// whether or not to buffer or call
// the underlying reader directly
if len(b) >= cap(r.data) {
n, r.state = r.r.Read(b)
} else {
r.more()
n = copy(b, r.data)
r.n = n
}
if n == 0 {
return 0, r.err()
}
return n, nil
}
// ReadFull attempts to read len(b) bytes into
// 'b'. It returns the number of bytes read into
// 'b', and an error if it does not return len(b).
// EOF is considered an unexpected error.
func (r *Reader) ReadFull(b []byte) (int, error) {
var n int // read into b
var nn int // scratch
l := len(b)
// either read buffered data,
// or read directly for the underlying
// buffer, or fetch more buffered data.
for n < l && r.state == nil {
if r.buffered() != 0 {
nn = copy(b[n:], r.data[r.n:])
n += nn
r.n += nn
} else if l-n > cap(r.data) {
nn, r.state = r.r.Read(b[n:])
n += nn
} else {
r.more()
}
}
if n < l {
return n, r.noEOF()
}
return n, nil
}
// ReadByte implements `io.ByteReader`
func (r *Reader) ReadByte() (byte, error) {
for r.buffered() < 1 && r.state == nil {
r.more()
}
if r.buffered() < 1 {
return 0, r.err()
}
b := r.data[r.n]
r.n++
return b, nil
}
// WriteTo implements `io.WriterTo`
func (r *Reader) WriteTo(w io.Writer) (int64, error) {
var (
i int64
ii int
err error
)
// first, clear buffer
if r.buffered() > 0 {
ii, err = w.Write(r.data[r.n:])
i += int64(ii)
if err != nil {
return i, err
}
r.data = r.data[0:0]
r.n = 0
}
for r.state == nil {
// here we just do
// 1:1 reads and writes
r.more()
if r.buffered() > 0 {
ii, err = w.Write(r.data)
i += int64(ii)
if err != nil {
return i, err
}
r.data = r.data[0:0]
r.n = 0
}
}
if r.state != io.EOF {
return i, r.err()
}
return i, nil
}
func min(a int, b int) int {
if a < b {
return a
}
return b
}
func max(a int, b int) int {
if a < b {
return b
}
return a
}