/*
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 (
	"errors"
	"fmt"
	"io"
	"math"
	"time"
	"context"
)

// 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
		}
	}
}

var errBuffChanReceiveTimeout = errors.New("buffer chan receive timeout")

// Noop reads media "frames" from src, queues and then writes to dst at intervals,
// maintaining a steady number of frames stored in the queue (channel). This ensures frames
// are outputted at a consistent rate; useful if reads occur from src in blocks (a
// side effect if src is connected to an input that receives packets containing
// multiple frames at intervals e.g. MPEG-TS over HTTP).
// Noop assumes that writing to the input connected to src is blocked until the
// entire previous write is read, i.e. src is expected to be connected to
// a pipe-like structure.
func Noop(dst io.Writer, src io.Reader, d time.Duration) error {
	// Controller tuning constants.
	const (
		target        = 500                   // Target channel size to maintain.
		coef          = 0.05                  // Proportional controller coefficient.
		minDelay      = 1                     // Minimum delay between writes (micro s).
		maxDelay      = 1000000               // Maximum delay between writes (micro s).
		defaultDelay  = 40 * time.Millisecond // Default delay between writes, equivalent to ~25fps.
		bufferTimeout = 1 * time.Minute       // Buffer read/write timeout.
	)

	// Ring buffer tuning.
	const (
		ringCap      = 1000   // Ring buffer capacity.
		ringElemSize = 250000 // Ring buffer element size i.e. max h264 frame size.
	)

	if d < 0 {
		return fmt.Errorf("invalid delay: %v", d)
	}

	if d == 0 {
		d = defaultDelay
	}

	var (
		delay = time.NewTicker(d)                                   // Ticker based on delay between frames.
		errCh = make(chan error)                                    // Used by the output routine to signal errors to the main loop.
		rb    = newRingBuffer(ringElemSize, ringCap, bufferTimeout) // Use a ring buffer to reduce allocation and GC load.
	)
	defer delay.Stop()

	// This routine is responsible for frame output to rest of the pipeline.
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()
	go func() {
		for {
			err := rb.writeTo(dst)
			if err != nil && !errors.Is(err, errBuffChanReceiveTimeout) {
				errCh <- fmt.Errorf("could not write to dst: %w", err)
			}

			select {
			case <-delay.C:
			case <-ctx.Done():
				return
			}

			// Adjust delay using proportional controller.
			adj := coef * float64(target-rb.len())
			dMicro := float64(d.Microseconds()) + adj
			d = time.Microsecond * time.Duration(math.Min(math.Max(dMicro, minDelay), maxDelay))
			delay.Reset(d)
		}
	}()

	// This loop is responsible for reading frames and checking any errors from
	// the output routine.
	for {
		err := rb.readFrom(src)
		if err != nil {
			return fmt.Errorf("could not read src: %w", err)
		}
		select {
		case err := <-errCh:
			return fmt.Errorf("error from output routine: %w", err)
		default:
		}
	}
}

// ringBuffer is a basic concurrency safe ring buffer. Concurrency safety is
// achieved using a channel between read and write methods i.e. overwrite/dropping
// behaviour is absent and blocking will occur.
type ringBuffer struct {
	n       int           // Num. of elements.
	i       int           // Current index in underlying buffer.
	buf     [][]byte      // Underlying buffer.
	ch      chan []byte   // ch will act as our concurrency safe queue.
	timeout time.Duration // readFrom and writeTo timeout.
}

// newRingBuffer returns a new ringBuffer with sz as the element size in bytes
// and cap as the number of elements.
func newRingBuffer(sz, cap int, timeout time.Duration) *ringBuffer {
	rb := &ringBuffer{
		buf:     make([][]byte, cap),
		n:       cap,
		ch:      make(chan []byte, cap),
		timeout: timeout,
	}
	for i := range rb.buf {
		rb.buf[i] = make([]byte, sz)
	}
	return rb
}

// readFrom gets the next []byte from the buffer and uses it to read from r.
// This data is then stored in the buffer channel ready for writeTo to retreive.
// readFrom will block if the buffer channel is filled, at least within the
// timeout, otherwise an error is returned.
func (b *ringBuffer) readFrom(r io.Reader) error {
	buf := b.buf[b.i]
	b.i++
	if b.i == b.n {
		b.i = 0
	}
	n, err := r.Read(buf)
	if err != nil {
		return err
	}
	timeout := time.NewTimer(b.timeout)
	select {
	case b.ch <- buf[:n]:
	case <-timeout.C:
		return errors.New("buffer chan send timeout")
	}
	return nil
}

// writeTo tries to get a []byte from the buffer channel within the timeout
// and then writes to w if successful, otherwise an error is returned.
func (b *ringBuffer) writeTo(w io.Writer) error {
	timeout := time.NewTimer(b.timeout)
	select {
	case p := <-b.ch:
		_, err := w.Write(p)
		if err != nil {
			return err
		}
	case <-timeout.C:
		return errBuffChanReceiveTimeout
	}
	return nil
}

func (b *ringBuffer) len() int {
	return len(b.ch)
}