fm-rds-tx/cmd/wmdecode/main.go

// cmd/wmdecode — STFT-domain spread-spectrum watermark decoder.
//
// Extracts the embedded fingerprint from an FM broadcast recording
// without needing to know the license key. Optionally verifies
// against supplied keys.
//
// Usage:
//
//	wmdecode <file.wav>                    Extract fingerprint
//	wmdecode <file.wav> [key ...]          Extract + verify keys
package main

import (
	"encoding/binary"
	"fmt"
	"math"
	"math/cmplx"
	"os"
	"sort"
	"time"

	"github.com/jan/fm-rds-tx/internal/dsp"
	"github.com/jan/fm-rds-tx/internal/watermark"
)

func main() {
	if len(os.Args) < 2 {
		fmt.Fprintln(os.Stderr, "usage: wmdecode <file.wav> [key ...]")
		os.Exit(1)
	}

	t0 := time.Now()

	samples, recRate, err := readMonoWAV(os.Args[1])
	if err != nil {
		fmt.Fprintf(os.Stderr, "read WAV: %v\n", err)
		os.Exit(1)
	}
	rms := rmsLevel(samples)
	fmt.Printf("WAV:   %d samples @ %.0f Hz = %.2fs, RMS %.1f dBFS\n",
		len(samples), recRate, float64(len(samples))/recRate, 20*math.Log10(rms+1e-9))

	// Step 1: Decimate to WMRate (12 kHz)
	wmRate := float64(watermark.WMRate)
	decimFactor := int(recRate / wmRate)
	if decimFactor < 1 {
		decimFactor = 1
	}
	actualRate := recRate / float64(decimFactor)
	fmt.Printf("Downsample: %d:1 (%.0f Hz → %.0f Hz)\n", decimFactor, recRate, actualRate)

	lpfCoeffs := designLPF8(5500, recRate)
	filtered := applyIIR(samples, lpfCoeffs)

	nDown := len(filtered) / decimFactor
	down := make([]float64, nDown)
	for i := 0; i < nDown; i++ {
		down[i] = filtered[i*decimFactor]
	}

	// Step 2: STFT + cepstrum filtering
	fftSize := watermark.FFTSize
	hop := watermark.FFTHop
	nFrames := (nDown - fftSize) / hop
	if nFrames <= 0 {
		fmt.Fprintln(os.Stderr, "Recording too short")
		os.Exit(1)
	}

	var window [watermark.FFTSize]float64
	dsp.HannWindow(window[:])
	fmt.Printf("STFT:  %d frames (%.1fs required, %.1fs available)\n",
		nFrames, float64(watermark.SamplesPerWM)/wmRate, float64(nDown)/wmRate)

	type stftMag [watermark.FFTSize / 2]float64
	frameMags := make([]stftMag, nFrames)
	for f := 0; f < nFrames; f++ {
		offset := f * hop
		var buf [watermark.FFTSize]complex128
		for i := 0; i < fftSize; i++ {
			buf[i] = complex(down[offset+i]*window[i], 0)
		}
		dsp.FFT(buf[:])
		for bin := 0; bin < fftSize/2; bin++ {
			mag := cmplx.Abs(buf[bin])
			if mag < 1e-12 {
				mag = 1e-12
			}
			frameMags[f][bin] = 20 * math.Log10(mag)
		}
		cepstrumFilter(frameMags[f][:], 8)
	}

	// Step 3: Cycle-offset search (key-independent — fixed PN)
	det := watermark.NewSTFTDetector()

	totalGroups := watermark.TotalGroups
	timeRep := watermark.TimeRep
	framesPerWM := watermark.FramesPerWM
	numBins := watermark.NumBins
	binLow := watermark.BinLow
	centerRep := timeRep / 2

	bestMetric := -1.0
	var bestCorrs [watermark.PayloadBits]float64
	bestCycleOff := 0
	bestRepOff := 0

	for cycleOff := 0; cycleOff < framesPerWM; cycleOff += timeRep {
		for repOff := 0; repOff < timeRep; repOff++ {
			var testCorrs [watermark.PayloadBits]float64
			for f := 0; f < nFrames; f++ {
				wmFrame := ((f - cycleOff - repOff) % framesPerWM + framesPerWM) % framesPerWM
				if wmFrame%timeRep != centerRep {
					continue
				}
				g := wmFrame / timeRep
				if g >= totalGroups {
					continue
				}
				var corr float64
				for b := 0; b < numBins; b++ {
					corr += frameMags[f][binLow+b] * float64(det.PNChipAt(g, b))
				}
				testCorrs[det.GroupBit(g)] += corr
			}
			var metric float64
			for _, c := range testCorrs {
				metric += c * c
			}
			if metric > bestMetric {
				bestMetric = metric
				bestCorrs = testCorrs
				bestCycleOff = cycleOff
				bestRepOff = repOff
			}
		}
	}

	var sumAbs float64
	for _, c := range bestCorrs {
		sumAbs += math.Abs(c)
	}
	fmt.Printf("Sync:  cycleOff=%d repOff=%d (%.1fs into recording)\n",
		bestCycleOff, bestRepOff, float64(bestCycleOff*hop)/wmRate)
	fmt.Printf("Corrs: avg|c|=%.1f\n", sumAbs/128)

	// Step 4: RS decode — extract fingerprint
	var recv [watermark.RsTotalBytes]byte
	confs := make([]float64, watermark.PayloadBits)
	for i := 0; i < watermark.PayloadBits; i++ {
		confs[i] = math.Abs(bestCorrs[i])
		if bestCorrs[i] < 0 {
			recv[i/8] |= 1 << uint(7-(i%8))
		}
	}

	type bc struct{ idx int; conf float64 }
	byteConfs := make([]bc, watermark.RsTotalBytes)
	for b := 0; b < watermark.RsTotalBytes; b++ {
		minC := confs[b*8]
		for bit := 1; bit < 8; bit++ {
			if confs[b*8+bit] < minC {
				minC = confs[b*8+bit]
			}
		}
		byteConfs[b] = bc{b, minC}
	}
	sort.Slice(byteConfs, func(a, b int) bool { return byteConfs[a].conf < byteConfs[b].conf })

	var payload [watermark.RsDataBytes]byte
	decoded := false
	nErasures := 0
	for nErase := 0; nErase <= watermark.RsCheckBytes; nErase++ {
		if nErase == 0 {
			p, ok := watermark.RSDecode(recv, nil)
			if ok {
				payload = p
				decoded = true
				break
			}
			continue
		}
		erasePos := make([]int, nErase)
		for i := 0; i < nErase; i++ {
			erasePos[i] = byteConfs[i].idx
		}
		sort.Ints(erasePos)
		p, ok := watermark.RSDecode(recv, erasePos)
		if ok {
			payload = p
			nErasures = nErase
			decoded = true
			break
		}
	}

	if !decoded {
		fmt.Printf("\nWatermark: NOT DETECTED\n")
		fmt.Printf("Done in %v\n", time.Since(t0).Round(time.Millisecond))
		os.Exit(1)
	}

	fmt.Printf("\nWatermark: DETECTED (%d erasures)\n", nErasures)
	fmt.Printf("Fingerprint: %x\n", payload)

	// Optional: verify against supplied keys
	keys := os.Args[2:]
	if len(keys) > 0 {
		fmt.Println()
		for _, key := range keys {
			if watermark.KeyMatchesPayload(key, payload) {
				fmt.Printf("  ✓ MATCH: %q\n", key)
			} else {
				fmt.Printf("  ✗       %q\n", key)
			}
		}
	}

	fmt.Printf("\nDone in %v\n", time.Since(t0).Round(time.Millisecond))
}

// --- Cepstrum filter ---

func cepstrumFilter(magDB []float64, nCeps int) {
	n := len(magDB)
	if n < nCeps*2 {
		return
	}
	ceps := make([]float64, n)
	for k := 0; k < n; k++ {
		var sum float64
		for i := 0; i < n; i++ {
			sum += magDB[i] * math.Cos(math.Pi*float64(k)*(float64(i)+0.5)/float64(n))
		}
		ceps[k] = sum
	}
	for k := 0; k < nCeps; k++ {
		ceps[k] = 0
	}
	for i := 0; i < n; i++ {
		var sum float64
		for k := 0; k < n; k++ {
			w := 1.0
			if k == 0 {
				w = 0.5
			}
			sum += w * ceps[k] * math.Cos(math.Pi*float64(k)*(float64(i)+0.5)/float64(n))
		}
		magDB[i] = sum * 2.0 / float64(n)
	}
}

// --- LPF ---

type biquad struct{ b0, b1, b2, a1, a2 float64 }
type iirCoeffs []biquad

func designLPF8(cutoffHz, sampleRate float64) iirCoeffs {
	angles := []float64{math.Pi / 16, 3 * math.Pi / 16, 5 * math.Pi / 16, 7 * math.Pi / 16}
	coeffs := make(iirCoeffs, 4)
	for i, angle := range angles {
		q := 1.0 / (2 * math.Cos(angle))
		omega := 2 * math.Pi * cutoffHz / sampleRate
		cosW := math.Cos(omega)
		sinW := math.Sin(omega)
		alpha := sinW / (2 * q)
		a0 := 1 + alpha
		coeffs[i] = biquad{
			b0: (1 - cosW) / 2 / a0, b1: (1 - cosW) / a0, b2: (1 - cosW) / 2 / a0,
			a1: (-2 * cosW) / a0, a2: (1 - alpha) / a0,
		}
	}
	return coeffs
}

func applyIIR(samples []float64, coeffs iirCoeffs) []float64 {
	out := make([]float64, len(samples))
	copy(out, samples)
	for _, bq := range coeffs {
		var z1, z2 float64
		for i, x := range out {
			y := bq.b0*x + z1
			z1 = bq.b1*x - bq.a1*y + z2
			z2 = bq.b2*x - bq.a2*y
			out[i] = y
		}
	}
	return out
}

// --- WAV reader ---

func rmsLevel(s []float64) float64 {
	var acc float64
	for _, v := range s {
		acc += v * v
	}
	return math.Sqrt(acc / float64(len(s)))
}

func readMonoWAV(path string) ([]float64, float64, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, 0, err
	}
	if len(data) < 44 || string(data[0:4]) != "RIFF" || string(data[8:12]) != "WAVE" {
		return nil, 0, fmt.Errorf("not a RIFF/WAVE file")
	}
	var channels, bitsPerSample uint16
	var sampleRate uint32
	var dataStart, dataLen int
	i := 12
	for i+8 <= len(data) {
		id := string(data[i : i+4])
		sz := int(binary.LittleEndian.Uint32(data[i+4 : i+8]))
		i += 8
		switch id {
		case "fmt ":
			if sz >= 16 {
				channels = binary.LittleEndian.Uint16(data[i+2 : i+4])
				sampleRate = binary.LittleEndian.Uint32(data[i+4 : i+8])
				bitsPerSample = binary.LittleEndian.Uint16(data[i+14 : i+16])
			}
		case "data":
			dataStart, dataLen = i, sz
		}
		i += sz
		if sz%2 != 0 {
			i++
		}
		if dataStart > 0 && channels > 0 {
			break
		}
	}
	if dataStart == 0 || bitsPerSample != 16 || channels == 0 {
		return nil, 0, fmt.Errorf("unsupported WAV")
	}
	if dataStart+dataLen > len(data) {
		dataLen = len(data) - dataStart
	}
	step := int(channels) * 2
	nFrames := dataLen / step
	out := make([]float64, nFrames)
	for j := 0; j < nFrames; j++ {
		off := dataStart + j*step
		l := float64(int16(binary.LittleEndian.Uint16(data[off : off+2])))
		r := l
		if channels >= 2 {
			r = float64(int16(binary.LittleEndian.Uint16(data[off+2 : off+4])))
		}
		out[j] = (l + r) / 2.0 / 32768.0
	}
	return out, float64(sampleRate), nil
}