fm-rds-tx/internal/audio/stream.go

package audio

import (
	"encoding/binary"
	"fmt"
	"io"
	"sync/atomic"
)

// StreamSource is a lock-free SPSC (single-producer, single-consumer) ring buffer
// for real-time audio streaming. One goroutine writes PCM frames, the DSP
// goroutine reads them via NextFrame(). Returns silence on underrun.
//
// Zero allocations in steady state. No mutex in the read or write path.
type StreamSource struct {
	ring       []Frame
	size       int
	mask       int // size-1, for fast modulo (size must be power of 2)
	SampleRate int

	writePos atomic.Int64
	readPos  atomic.Int64

	Underruns atomic.Uint64
	Overflows atomic.Uint64
	Written   atomic.Uint64
	highWatermark atomic.Int64
}

// NewStreamSource creates a ring buffer with the given capacity (rounded up
// to next power of 2) and input sample rate.
func NewStreamSource(capacity, sampleRate int) *StreamSource {
	// Round up to power of 2
	size := 1
	for size < capacity {
		size <<= 1
	}
	return &StreamSource{
		ring:       make([]Frame, size),
		size:       size,
		mask:       size - 1,
		SampleRate: sampleRate,
	}
}

// WriteFrame pushes a single frame into the ring buffer.
// Returns false if the buffer is full (overflow).
func (s *StreamSource) WriteFrame(f Frame) bool {
	wp := s.writePos.Load()
	rp := s.readPos.Load()
	if wp-rp >= int64(s.size) {
		s.Overflows.Add(1)
		return false
	}
	s.ring[int(wp)&s.mask] = f
	s.writePos.Add(1)
	s.Written.Add(1)
	s.updateHighWatermark()
	return true
}

// WritePCM decodes interleaved S16LE stereo PCM bytes and writes frames
// to the ring buffer. Returns the number of frames written.
func (s *StreamSource) WritePCM(data []byte) int {
	frames := len(data) / 4 // 2 channels × 2 bytes per sample
	written := 0
	for i := 0; i < frames; i++ {
		off := i * 4
		l := int16(binary.LittleEndian.Uint16(data[off:]))
		r := int16(binary.LittleEndian.Uint16(data[off+2:]))
		f := NewFrame(
			Sample(float64(l)/32768.0),
			Sample(float64(r)/32768.0),
		)
		if !s.WriteFrame(f) {
			break
		}
		written++
	}
	return written
}

// ReadFrame consumes one frame from the ring buffer.
// Returns silence (0,0) on underrun.
func (s *StreamSource) ReadFrame() Frame {
	rp := s.readPos.Load()
	wp := s.writePos.Load()
	if rp >= wp {
		s.Underruns.Add(1)
		return NewFrame(0, 0)
	}
	f := s.ring[int(rp)&s.mask]
	s.readPos.Add(1)
	return f
}

// NextFrame implements the frameSource interface.
func (s *StreamSource) NextFrame() Frame {
	return s.ReadFrame()
}

// Available returns the number of frames currently buffered.
func (s *StreamSource) Available() int {
	return int(s.writePos.Load() - s.readPos.Load())
}

// Buffered returns the fill ratio (0.0 = empty, 1.0 = full).
func (s *StreamSource) Buffered() float64 {
	return float64(s.Available()) / float64(s.size)
}

// Stats returns diagnostic counters.
func (s *StreamSource) Stats() StreamStats {
	available := s.Available()
	buffered := 0.0
	if s.size > 0 {
		buffered = float64(available) / float64(s.size)
	}
	highWatermark := int(s.highWatermark.Load())
	return StreamStats{
		Available:               available,
		Capacity:                s.size,
		Buffered:                buffered,
		BufferedDurationSeconds: s.bufferedDurationSeconds(available),
		HighWatermark:           highWatermark,
		HighWatermarkDurationSeconds: s.bufferedDurationSeconds(highWatermark),
		Written:                 s.Written.Load(),
		Underruns:               s.Underruns.Load(),
		Overflows:               s.Overflows.Load(),
	}
}

// StreamStats exposes runtime telemetry for the stream buffer.
type StreamStats struct {
	Available               int     `json:"available"`
	Capacity                int     `json:"capacity"`
	Buffered                float64 `json:"buffered"`
	BufferedDurationSeconds float64 `json:"bufferedDurationSeconds"`
	HighWatermark           int     `json:"highWatermark"`
	HighWatermarkDurationSeconds float64 `json:"highWatermarkDurationSeconds"`
	Written                 uint64  `json:"written"`
	Underruns               uint64  `json:"underruns"`
	Overflows               uint64  `json:"overflows"`
}

func (s *StreamSource) bufferedDurationSeconds(available int) float64 {
	if s.SampleRate <= 0 {
		return 0
	}
	return float64(available) / float64(s.SampleRate)
}

func (s *StreamSource) updateHighWatermark() {
	available := s.Available()
	for {
		prev := s.highWatermark.Load()
		if int64(available) <= prev {
			return
		}
		if s.highWatermark.CompareAndSwap(prev, int64(available)) {
			return
		}
	}
}

// --- StreamResampler ---

// StreamResampler wraps a StreamSource and rate-converts from the stream's
// native sample rate to the target output rate using linear interpolation.
// Consumes input frames on demand — no buffering beyond the ring buffer.
type StreamResampler struct {
	src   *StreamSource
	ratio float64 // inputRate / outputRate (< 1 when upsampling)
	pos   float64
	prev  Frame
	curr  Frame
}

// NewStreamResampler creates a streaming resampler.
func NewStreamResampler(src *StreamSource, outputRate float64) *StreamResampler {
	if src == nil || outputRate <= 0 || src.SampleRate <= 0 {
		return &StreamResampler{src: src, ratio: 1.0}
	}
	return &StreamResampler{
		src:   src,
		ratio: float64(src.SampleRate) / outputRate,
	}
}

// NextFrame returns the next interpolated frame at the output rate.
// Implements the frameSource interface.
func (r *StreamResampler) NextFrame() Frame {
	if r.src == nil {
		return NewFrame(0, 0)
	}

	// Consume input samples as the fractional position advances
	for r.pos >= 1.0 {
		r.prev = r.curr
		r.curr = r.src.ReadFrame()
		r.pos -= 1.0
	}

	frac := r.pos
	l := float64(r.prev.L)*(1-frac) + float64(r.curr.L)*frac
	ri := float64(r.prev.R)*(1-frac) + float64(r.curr.R)*frac
	r.pos += r.ratio
	return NewFrame(Sample(l), Sample(ri))
}

// --- Ingest helpers ---

// IngestReader continuously reads S16LE stereo PCM from an io.Reader into
// a StreamSource. Blocks until the reader returns an error or io.EOF.
// Designed to run as a goroutine.
func IngestReader(r io.Reader, dst *StreamSource) error {
	buf := make([]byte, 16384) // 4096 frames per read (16KB)
	for {
		n, err := r.Read(buf)
		if n > 0 {
			dst.WritePCM(buf[:n])
		}
		if err != nil {
			if err == io.EOF {
				return nil
			}
			return fmt.Errorf("audio ingest: %w", err)
		}
	}
}