av/codec/codecutil/lex.go

/*
NAME
  lex.go

AUTHOR
  Trek Hopton <trek@ausocean.org>

LICENSE
  This file is Copyright (C) 2019 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 in gpl.txt.
  If not, see [GNU licenses](http://www.gnu.org/licenses).
*/

package codecutil

import (
	"fmt"
	"io"
	"time"
)

// ByteLexer is used to lex bytes using a buffer size which is configured upon construction.
type ByteLexer struct {
	bufSize int
}

// NewByteLexer returns a pointer to a ByteLexer with the given buffer size.
func NewByteLexer(s int) (*ByteLexer, error) {
	if s <= 0 {
		return nil, fmt.Errorf("invalid buffer size: %v", s)
	}
	return &ByteLexer{bufSize: s}, nil
}

// zeroTicks can be used to create an instant ticker.
var zeroTicks chan time.Time

func init() {
	zeroTicks = make(chan time.Time)
	close(zeroTicks)
}

// Lex reads l.bufSize bytes from src and writes them to dst every d seconds.
func (l *ByteLexer) Lex(dst io.Writer, src io.Reader, d time.Duration) error {
	if d < 0 {
		return fmt.Errorf("invalid delay: %v", d)
	}

	var ticker *time.Ticker
	if d == 0 {
		ticker = &time.Ticker{C: zeroTicks}
	} else {
		ticker = time.NewTicker(d)
		defer ticker.Stop()
	}

	buf := make([]byte, l.bufSize)
	for {
		<-ticker.C
		off, err := src.Read(buf)
		// The only error that will stop the lexer is an EOF.
		if err == io.EOF {
			return err
		} else if err != nil {
			continue
		}
		_, err = dst.Write(buf[:off])
		if err != nil {
			return err
		}
	}
}

// Noop assumes that for writing to src is blocked until the entire previous
// write to src is read, i.e. src is expected to connected to a pipe-like structure.
func Noop(dst io.Writer, src io.Reader, d time.Duration) error {
	if d < 0 {
		return fmt.Errorf("invalid delay: %v", d)
	}

	if d == 0 {
		d = 40 * time.Millisecond
	}
	ticker := time.NewTicker(d)
	defer ticker.Stop()

	const checkDur = 500 * time.Millisecond
	rateChkTicker := time.NewTicker(checkDur)
	frameCh := make(chan []byte, 1000)
	errCh := make(chan error)
	go func() {
		for {
			toWrite := <-frameCh
			_, err := dst.Write(toWrite)
			if err != nil {
				errCh <- fmt.Errorf("could not write to dst: %w", err)
			}

			<-ticker.C
			select {
			case <-rateChkTicker.C:
				var adj int
				const equilibrium = 500
				if len(frameCh) > equilibrium {
					adj = -2
				} else if len(frameCh) < equilibrium {
					adj = 2
				}
				d += time.Millisecond * time.Duration(adj)
				ticker.Reset(d)
			default:
			}
		}
	}()

	const maxFrameSize = 250000 // = 20kB
	buf := make([]byte, maxFrameSize)
	for {
		n, err := src.Read(buf)
		if err != nil {
			return fmt.Errorf("could not read src: %w", err)
		}
		newFrame := make([]byte, n)
		copy(newFrame, buf[:n])
		frameCh <- newFrame
		select {
		case err := <-errCh:
			return fmt.Errorf("error from output routine: %w", err)
		default:
		}
	}
}