av/codec/mjpeg/lex.go

/*
NAME
  lex.go

DESCRIPTION
  lex.go provides a lexer to extract separate JPEG images from a MJPEG stream.

AUTHOR
  Dan Kortschak <dan@ausocean.org>

LICENSE
  lex.go is Copyright (C) 2017 the Australian Ocean Lab (AusOcean)

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
  for more details.

  You should have received a copy of the GNU General Public License
  along with revid in gpl.txt. If not, see http://www.gnu.org/licenses.
*/

// lex.go provides a lexer to extract separate JPEG images from a MJPEG stream.

package mjpeg

import (
	"bufio"
	"bytes"
	"fmt"
	"io"
	"time"
)

var noDelay = make(chan time.Time)

func init() {
	close(noDelay)
}

// Lex parses MJPEG frames read from src into separate writes to dst with
// successive writes being performed not earlier than the specified delay.
func Lex(dst io.Writer, src io.Reader, delay time.Duration) error {
	var tick <-chan time.Time
	if delay == 0 {
		tick = noDelay
	} else {
		ticker := time.NewTicker(delay)
		defer ticker.Stop()
		tick = ticker.C
	}

	r := bufio.NewReader(src)
	for {
		buf := make([]byte, 2, 4<<10)
		n, err := r.Read(buf)
		if n < 2 {
			return nil
		}
		if err != nil {
			return err
		}
		if !bytes.Equal(buf, []byte{0xff, 0xd8}) {
			return fmt.Errorf("parser: not MJPEG frame start: %#v", buf)
		}
		var last byte
		for {
			b, err := r.ReadByte()
			if err != nil {
				return err
			}
			buf = append(buf, b)
			if last == 0xff && b == 0xd9 {
				break
			}
			last = b
		}
		<-tick
		_, err = dst.Write(buf)
		if err != nil {
			return err
		}
	}
}

// scanner is a byte scanner.
type scanner struct {
	buf []byte
	off int

	// r is the source of data for the scanner.
	r io.Reader
}

// newScanner returns a scanner initialised with an io.Reader and a read buffer.
func newScanner(r io.Reader, buf []byte) *scanner {
	return &scanner{r: r, buf: buf[:0]}
}

// scanUntilZeroInto scans the scanner's underlying io.Reader until a zero byte
// has been read, appending all read bytes to dst. The resulting appended data,
// the last read byte and whether the last read byte was zero are returned.
func (c *scanner) scanUntilZeroInto(dst []byte) (res []byte, b byte, err error) {
outer:
	for {
		var i int
		for i, b = range c.buf[c.off:] {
			if b != 0x0 {
				continue
			}
			dst = append(dst, c.buf[c.off:c.off+i+1]...)
			c.off += i + 1
			break outer
		}
		dst = append(dst, c.buf[c.off:]...)
		err = c.reload()
		if err != nil {
			break
		}
	}
	return dst, b, err
}

// readByte is an unexported ReadByte.
func (c *scanner) readByte() (byte, error) {
	if c.off >= len(c.buf) {
		err := c.reload()
		if err != nil {
			return 0, err
		}
	}
	b := c.buf[c.off]
	c.off++
	return b, nil
}

// reload re-fills the scanner's buffer.
func (c *scanner) reload() error {
	n, err := c.r.Read(c.buf[:cap(c.buf)])
	c.buf = c.buf[:n]
	if err != nil {
		if err != io.EOF {
			return err
		}
		if n == 0 {
			return io.EOF
		}
	}
	c.off = 0
	return nil
}