reader.go

/*
This file is based on the source code available at https://github.com/pkg/json under BSD-2-Clause License

Copyright (c) 2020, Dave Cheney <dave@cheney.net>
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright notice,
  this list of conditions and the following disclaimer in the documentation
  and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

package gosax

import "io"

// A byteReader implements a sliding window over an io.Reader.
type byteReader struct {
	data   []byte
	offset int
	r      io.Reader
	err    error
}

// release discards n bytes from the front of the window.
func (b *byteReader) release(n int) {
	b.offset += n
}

// window returns the current window.
// The window is invalidated by calls to release or extend.
func (b *byteReader) window() []byte {
	return b.data[b.offset:]
}

// tuning constants for byteReader.extend.
const (
	newBufferSize = 4096
	minReadSize   = newBufferSize >> 2
)

// extend extends the window with data from the underlying reader.
func (b *byteReader) extend() int {
	if b.err != nil {
		return 0
	}

	remaining := len(b.data) - b.offset
	if remaining == 0 {
		b.data = b.data[:0]
		b.offset = 0
	}
	if cap(b.data)-len(b.data) >= minReadSize {
		// nothing to do, enough space exists between len and cap.
	} else if cap(b.data)-remaining >= minReadSize {
		// buffer has enough space if we move the data to the front.
		b.compact()
	} else {
		// otherwise, we must allocate/extend a new buffer
		b.grow()
	}
	remaining += b.offset
	n, err := b.r.Read(b.data[remaining:cap(b.data)])
	// reduce length to the existing plus the data we read.
	b.data = b.data[:remaining+n]
	b.err = err
	return n
}

// grow grows the buffer, moving the active data to the front.
func (b *byteReader) grow() {
	buf := make([]byte, max(cap(b.data)*2, newBufferSize))
	copy(buf, b.data[b.offset:])
	b.data = buf
	b.offset = 0
}

// compact moves the active data to the front of the buffer.
func (b *byteReader) compact() {
	copy(b.data, b.data[b.offset:])
	b.offset = 0
}