fm-rds-tx/internal/offline/generator.go

package offline

import (
	"context"
	"encoding/binary"
	"fmt"
	"path/filepath"
	"time"

	"github.com/jan/fm-rds-tx/internal/audio"
	cfgpkg "github.com/jan/fm-rds-tx/internal/config"
	"github.com/jan/fm-rds-tx/internal/dsp"
	"github.com/jan/fm-rds-tx/internal/mpx"
	"github.com/jan/fm-rds-tx/internal/output"
	"github.com/jan/fm-rds-tx/internal/rds"
	"github.com/jan/fm-rds-tx/internal/stereo"
)

type frameSource interface {
	NextFrame() audio.Frame
}

// PreEmphasizedSource wraps an audio source and applies pre-emphasis at the
// audio input rate, before upsampling to composite rate. This is more
// efficient than filtering at composite rate and is the correct signal path.
type PreEmphasizedSource struct {
	src  frameSource
	preL *dsp.PreEmphasis
	preR *dsp.PreEmphasis
	gain float64
}

func NewPreEmphasizedSource(src frameSource, tauUS, sampleRate, gain float64) *PreEmphasizedSource {
	p := &PreEmphasizedSource{src: src, gain: gain}
	if tauUS > 0 {
		p.preL = dsp.NewPreEmphasis(tauUS, sampleRate)
		p.preR = dsp.NewPreEmphasis(tauUS, sampleRate)
	}
	return p
}

func (p *PreEmphasizedSource) NextFrame() audio.Frame {
	f := p.src.NextFrame()
	l := float64(f.L) * p.gain
	r := float64(f.R) * p.gain
	if p.preL != nil {
		l = p.preL.Process(l)
		r = p.preR.Process(r)
	}
	return audio.NewFrame(audio.Sample(l), audio.Sample(r))
}

type SourceInfo struct {
	Kind       string
	SampleRate float64
	Detail     string
}

type Generator struct {
	cfg cfgpkg.Config
}

func NewGenerator(cfg cfgpkg.Config) *Generator {
	return &Generator{cfg: cfg}
}

func (g *Generator) sourceFor(sampleRate float64) (frameSource, SourceInfo) {
	if g.cfg.Audio.InputPath != "" {
		if src, err := audio.LoadWAVSource(g.cfg.Audio.InputPath); err == nil {
			return audio.NewResampledSource(src, sampleRate), SourceInfo{Kind: "wav", SampleRate: float64(src.SampleRate), Detail: g.cfg.Audio.InputPath}
		}
		return audio.NewConfiguredToneSource(sampleRate, g.cfg.Audio.ToneLeftHz, g.cfg.Audio.ToneRightHz, g.cfg.Audio.ToneAmplitude), SourceInfo{Kind: "tone-fallback", SampleRate: sampleRate, Detail: g.cfg.Audio.InputPath}
	}
	return audio.NewConfiguredToneSource(sampleRate, g.cfg.Audio.ToneLeftHz, g.cfg.Audio.ToneRightHz, g.cfg.Audio.ToneAmplitude), SourceInfo{Kind: "tones", SampleRate: sampleRate, Detail: "generated"}
}

func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame {
	sampleRate := float64(g.cfg.FM.CompositeRateHz)
	if sampleRate <= 0 {
		sampleRate = 228000
	}
	samples := int(duration.Seconds() * sampleRate)
	if samples <= 0 {
		samples = int(sampleRate / 10)
	}

	frame := &output.CompositeFrame{
		Samples:      make([]output.IQSample, samples),
		SampleRateHz: sampleRate,
		Timestamp:    time.Now().UTC(),
		Sequence:     1,
	}

	// Audio source with pre-emphasis applied at audio rate (before stereo encoding)
	rawSource, _ := g.sourceFor(sampleRate)
	source := NewPreEmphasizedSource(rawSource, g.cfg.FM.PreEmphasisTauUS, sampleRate, g.cfg.Audio.Gain)

	// Stereo encoder (unity-normalized pilot + subcarrier)
	stereoEncoder := stereo.NewStereoEncoder(sampleRate)

	// MPX combiner — config values are direct linear injection levels, no magic numbers
	combiner := mpx.DefaultCombiner{
		MonoGain:   1.0,
		StereoGain: 1.0,
		PilotGain:  g.cfg.FM.PilotLevel,
		RDSGain:    g.cfg.FM.RDSInjection,
	}

	// RDS encoder (unity-normalized output)
	piCode, _ := cfgpkg.ParsePI(g.cfg.RDS.PI) // already validated
	rdsEnc, _ := rds.NewEncoder(rds.RDSConfig{
		PI:         piCode,
		PS:         g.cfg.RDS.PS,
		RT:         g.cfg.RDS.RadioText,
		PTY:        uint8(g.cfg.RDS.PTY),
		SampleRate: sampleRate,
	})

	// Limiter
	var limiter *dsp.MPXLimiter
	ceiling := g.cfg.FM.LimiterCeiling
	if ceiling <= 0 {
		ceiling = 1.0
	}
	if g.cfg.FM.LimiterEnabled {
		limiter = dsp.NewMPXLimiter(ceiling, 0.1, 50, sampleRate)
	}

	// FM modulator
	var fmMod *dsp.FMModulator
	if g.cfg.FM.FMModulationEnabled {
		fmMod = dsp.NewFMModulator(sampleRate)
		if g.cfg.FM.MaxDeviationHz > 0 {
			fmMod.MaxDeviation = g.cfg.FM.MaxDeviationHz
		}
	}

	// --- Sample loop (zero-allocation hot path) ---
	for i := 0; i < samples; i++ {
		in := source.NextFrame()

		comps := stereoEncoder.Encode(in)
		if !g.cfg.FM.StereoEnabled {
			comps.Stereo = 0
			comps.Pilot = 0
		}

		rdsValue := 0.0
		if g.cfg.RDS.Enabled {
			// RDS: biphase-coded BPSK, carrier and bit clock from pilot.
			// Carrier = sin(3 × pilotPhase × 2π) = 57 kHz phase-locked.
			// Bit clock = 1 bit per 16 pilot cycles with biphase midpoint inversion.
			pilotPhase := stereoEncoder.PilotPhase()
			rdsCarrier := stereoEncoder.RDSCarrier()
			rdsSymbol := rdsEnc.BiphaseSymbolForPilotPhase(pilotPhase)
			rdsValue = rdsSymbol * rdsCarrier
		}

		composite := combiner.Combine(comps.Mono, comps.Stereo, comps.Pilot, rdsValue)
		composite *= g.cfg.FM.OutputDrive

		if limiter != nil {
			composite = limiter.Process(composite)
			composite = dsp.HardClip(composite, ceiling) // safety net only with limiter
		}

		if fmMod != nil {
			iq_i, iq_q := fmMod.Modulate(composite)
			frame.Samples[i] = output.IQSample{I: float32(iq_i), Q: float32(iq_q)}
		} else {
			frame.Samples[i] = output.IQSample{I: float32(composite), Q: 0}
		}
	}

	return frame
}

func (g *Generator) WriteFile(path string, duration time.Duration) error {
	if path == "" {
		path = g.cfg.Backend.OutputPath
	}
	if path == "" {
		path = filepath.Join("build", "offline", "composite.iqf32")
	}
	backend, err := output.NewFileBackend(path, binary.LittleEndian, output.BackendInfo{
		Name:        "offline-file",
		Description: "offline composite file backend",
	})
	if err != nil {
		return err
	}
	defer backend.Close(context.Background())

	if err := backend.Configure(context.Background(), output.BackendConfig{
		SampleRateHz: float64(g.cfg.FM.CompositeRateHz),
		Channels:     2,
		IQLevel:      float32(g.cfg.FM.OutputDrive),
	}); err != nil {
		return err
	}

	frame := g.GenerateFrame(duration)
	if _, err := backend.Write(context.Background(), frame); err != nil {
		return err
	}
	return backend.Flush(context.Background())
}

func (g *Generator) Summary(duration time.Duration) string {
	sampleRate := float64(g.cfg.FM.CompositeRateHz)
	if sampleRate <= 0 {
		sampleRate = 228000
	}
	_, info := g.sourceFor(sampleRate)
	preemph := "off"
	if g.cfg.FM.PreEmphasisTauUS > 0 {
		preemph = fmt.Sprintf("%.0fµs", g.cfg.FM.PreEmphasisTauUS)
	}
	modMode := "composite"
	if g.cfg.FM.FMModulationEnabled {
		modMode = fmt.Sprintf("FM-IQ(±%.0fHz)", g.cfg.FM.MaxDeviationHz)
	}
	return fmt.Sprintf("offline frame: freq=%.1fMHz rate=%d duration=%s drive=%.2f stereo=%t rds=%t preemph=%s limiter=%t output=%s source=%s detail=%s",
		g.cfg.FM.FrequencyMHz, g.cfg.FM.CompositeRateHz, duration.String(),
		g.cfg.FM.OutputDrive, g.cfg.FM.StereoEnabled, g.cfg.RDS.Enabled,
		preemph, g.cfg.FM.LimiterEnabled, modMode, info.Kind, info.Detail)
}