SpotiFLAC-Mobile/referensi/backend/spectrum.go

package backend

import (
	"fmt"
	"math"
	"math/cmplx"

	"github.com/mewkiz/flac"
)

// SpectrumData contains frequency spectrum information
type SpectrumData struct {
	TimeSlices []TimeSlice `json:"time_slices"`
	SampleRate int         `json:"sample_rate"`
	FreqBins   int         `json:"freq_bins"`
	Duration   float64     `json:"duration"`
	MaxFreq    float64     `json:"max_freq"`
}

// TimeSlice represents spectrum data at a point in time
type TimeSlice struct {
	Time       float64   `json:"time"`
	Magnitudes []float64 `json:"magnitudes"`
}

// AnalyzeSpectrum decodes FLAC file and performs FFT analysis
func AnalyzeSpectrum(filepath string) (*SpectrumData, error) {
	// Open FLAC file
	stream, err := flac.ParseFile(filepath)
	if err != nil {
		return nil, fmt.Errorf("failed to parse FLAC: %w", err)
	}
	defer stream.Close()

	info := stream.Info
	sampleRate := int(info.SampleRate)
	channels := int(info.NChannels)

	// Read audio samples
	samples, err := readSamples(stream, channels)
	if err != nil {
		return nil, fmt.Errorf("failed to read samples: %w", err)
	}

	if len(samples) == 0 {
		return nil, fmt.Errorf("no audio samples found")
	}

	// Calculate spectrum
	return calculateSpectrum(samples, sampleRate), nil
}

// readSamples reads and decodes audio samples from FLAC stream
func readSamples(stream *flac.Stream, channels int) ([]float64, error) {
	var allSamples []float64
	maxSamples := 10 * 1024 * 1024 // Limit to ~10 million samples to avoid memory issues

	// Decode frames
	for {
		frame, err := stream.ParseNext()
		if err != nil {
			// End of stream
			break
		}

		// Convert samples to float64 and mix channels to mono
		for i := 0; i < frame.Subframes[0].NSamples; i++ {
			var sample float64

			// Mix all channels to mono by averaging
			for ch := 0; ch < channels; ch++ {
				sample += float64(frame.Subframes[ch].Samples[i])
			}
			sample /= float64(channels)

			allSamples = append(allSamples, sample)

			// Limit sample count
			if len(allSamples) >= maxSamples {
				return allSamples, nil
			}
		}
	}

	return allSamples, nil
}

// calculateSpectrum performs FFT analysis on audio samples
func calculateSpectrum(samples []float64, sampleRate int) *SpectrumData {
	fftSize := 8192
	numTimeSlices := 300

	duration := float64(len(samples)) / float64(sampleRate)

	samplesPerSlice := len(samples) / numTimeSlices
	if samplesPerSlice < fftSize {
		samplesPerSlice = fftSize
		numTimeSlices = len(samples) / fftSize
	}

	timeSlices := make([]TimeSlice, 0, numTimeSlices)
	freqBins := fftSize / 2
	maxFreq := float64(sampleRate) / 2.0

	for i := 0; i < numTimeSlices; i++ {
		startIdx := i * samplesPerSlice
		if startIdx+fftSize > len(samples) {
			break
		}

		window := samples[startIdx : startIdx+fftSize]

		windowedSamples := applyHannWindow(window)

		spectrum := fft(windowedSamples)

		magnitudes := make([]float64, freqBins)
		for j := 0; j < freqBins; j++ {
			magnitude := cmplx.Abs(spectrum[j])

			if magnitude < 1e-10 {
				magnitude = 1e-10
			}
			magnitudes[j] = 20 * math.Log10(magnitude)
		}

		timeSlice := TimeSlice{
			Time:       float64(startIdx) / float64(sampleRate),
			Magnitudes: magnitudes,
		}
		timeSlices = append(timeSlices, timeSlice)
	}

	return &SpectrumData{
		TimeSlices: timeSlices,
		SampleRate: sampleRate,
		FreqBins:   freqBins,
		Duration:   duration,
		MaxFreq:    maxFreq,
	}
}

// applyHannWindow applies Hann window to reduce spectral leakage
func applyHannWindow(samples []float64) []float64 {
	n := len(samples)
	windowed := make([]float64, n)

	for i := 0; i < n; i++ {
		window := 0.5 * (1.0 - math.Cos(2.0*math.Pi*float64(i)/float64(n-1)))
		windowed[i] = samples[i] * window
	}

	return windowed
}

// fft performs Fast Fourier Transform using Cooley-Tukey algorithm
func fft(samples []float64) []complex128 {
	n := len(samples)

	x := make([]complex128, n)
	for i := 0; i < n; i++ {
		x[i] = complex(samples[i], 0)
	}

	return fftRecursive(x)
}

// fftRecursive performs recursive FFT
func fftRecursive(x []complex128) []complex128 {
	n := len(x)

	if n <= 1 {
		return x
	}

	even := make([]complex128, n/2)
	odd := make([]complex128, n/2)

	for i := 0; i < n/2; i++ {
		even[i] = x[2*i]
		odd[i] = x[2*i+1]
	}

	evenFFT := fftRecursive(even)
	oddFFT := fftRecursive(odd)

	result := make([]complex128, n)
	for k := 0; k < n/2; k++ {
		t := cmplx.Exp(complex(0, -2*math.Pi*float64(k)/float64(n))) * oddFFT[k]
		result[k] = evenFFT[k] + t
		result[k+n/2] = evenFFT[k] - t
	}

	return result
}