1// Copyright 2014 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package http2
6
7import (
8	"errors"
9)
10
11// fixedBuffer is an io.ReadWriter backed by a fixed size buffer.
12// It never allocates, but moves old data as new data is written.
13type fixedBuffer struct {
14	buf  []byte
15	r, w int
16}
17
18var (
19	errReadEmpty = errors.New("read from empty fixedBuffer")
20	errWriteFull = errors.New("write on full fixedBuffer")
21)
22
23// Read copies bytes from the buffer into p.
24// It is an error to read when no data is available.
25func (b *fixedBuffer) Read(p []byte) (n int, err error) {
26	if b.r == b.w {
27		return 0, errReadEmpty
28	}
29	n = copy(p, b.buf[b.r:b.w])
30	b.r += n
31	if b.r == b.w {
32		b.r = 0
33		b.w = 0
34	}
35	return n, nil
36}
37
38// Len returns the number of bytes of the unread portion of the buffer.
39func (b *fixedBuffer) Len() int {
40	return b.w - b.r
41}
42
43// Write copies bytes from p into the buffer.
44// It is an error to write more data than the buffer can hold.
45func (b *fixedBuffer) Write(p []byte) (n int, err error) {
46	// Slide existing data to beginning.
47	if b.r > 0 && len(p) > len(b.buf)-b.w {
48		copy(b.buf, b.buf[b.r:b.w])
49		b.w -= b.r
50		b.r = 0
51	}
52
53	// Write new data.
54	n = copy(b.buf[b.w:], p)
55	b.w += n
56	if n < len(p) {
57		err = errWriteFull
58	}
59	return n, err
60}
61