mixboard-ios/Sources/Services/BPMDetector.swift

import Accelerate
import AVFoundation
import Foundation

/// BPM detection using spectral flux onset detection and autocorrelation.
struct BPMDetector {

    /// Detect BPM for a track. Analyzes the first 60 seconds.
    static func detectBPM(for track: Track) async throws -> Double {
        let url = track.fileURL
        let file = try AVAudioFile(forReading: url)

        let format = file.processingFormat
        let sampleRate = format.sampleRate
        let maxFrames = AVAudioFrameCount(min(Double(file.length), sampleRate * 60))

        guard let buffer = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: maxFrames) else {
            return 0
        }
        try file.read(into: buffer, frameCount: maxFrames)

        guard let floatData = buffer.floatChannelData else { return 0 }
        let frameCount = Int(buffer.frameLength)

        // Mix to mono
        var mono = [Float](repeating: 0, count: frameCount)
        let channels = Int(format.channelCount)
        for ch in 0..<channels {
            let ptr = floatData[ch]
            for i in 0..<frameCount {
                mono[i] += ptr[i]
            }
        }
        if channels > 1 {
            var div = Float(channels)
            vDSP_vsdiv(mono, 1, &div, &mono, 1, vDSP_Length(frameCount))
        }

        // FFT-based spectral flux
        let fftSize = 2048
        let hopSize = 512
        let log2n = vDSP_Length(log2(Double(fftSize)))
        guard let fftSetup = vDSP_create_fftsetup(log2n, FFTRadix(kFFTRadix2)) else { return 0 }
        defer { vDSP_destroy_fftsetup(fftSetup) }

        let halfN = fftSize / 2
        var window = [Float](repeating: 0, count: fftSize)
        vDSP_hann_window(&window, vDSP_Length(fftSize), Int32(vDSP_HANN_NORM))

        var prevMagnitudes = [Float](repeating: 0, count: halfN)
        var onsetSignal: [Float] = []

        var position = 0
        while position + fftSize <= frameCount {
            var frame = Array(mono[position..<position + fftSize])
            vDSP_vmul(frame, 1, window, 1, &frame, 1, vDSP_Length(fftSize))

            var real = [Float](repeating: 0, count: halfN)
            var imag = [Float](repeating: 0, count: halfN)
            frame.withUnsafeBufferPointer { framePtr in
                real.withUnsafeMutableBufferPointer { realPtr in
                    imag.withUnsafeMutableBufferPointer { imagPtr in
                        var splitComplex = DSPSplitComplex(realp: realPtr.baseAddress!, imagp: imagPtr.baseAddress!)
                        framePtr.baseAddress!.withMemoryRebound(to: DSPComplex.self, capacity: halfN) { complexPtr in
                            vDSP_ctoz(complexPtr, 2, &splitComplex, 1, vDSP_Length(halfN))
                        }
                        vDSP_fft_zrip(fftSetup, &splitComplex, 1, log2n, FFTDirection(kFFTDirection_Forward))
                    }
                }
            }

            // Magnitudes
            var magnitudes = [Float](repeating: 0, count: halfN)
            real.withUnsafeBufferPointer { rPtr in
                imag.withUnsafeBufferPointer { iPtr in
                    var split = DSPSplitComplex(realp: UnsafeMutablePointer(mutating: rPtr.baseAddress!),
                                                imagp: UnsafeMutablePointer(mutating: iPtr.baseAddress!))
                    vDSP_zvabs(&split, 1, &magnitudes, 1, vDSP_Length(halfN))
                }
            }

            // Spectral flux (positive differences only)
            var flux: Float = 0
            for i in 0..<halfN {
                let diff = magnitudes[i] - prevMagnitudes[i]
                if diff > 0 { flux += diff }
            }
            onsetSignal.append(flux)
            prevMagnitudes = magnitudes

            position += hopSize
        }

        guard onsetSignal.count > 2 else { return 0 }

        // Autocorrelation for BPM
        let onsetRate = sampleRate / Double(hopSize)
        let minLag = Int(onsetRate * 60.0 / 200.0)  // 200 BPM
        let maxLag = Int(onsetRate * 60.0 / 60.0)    // 60 BPM

        guard maxLag < onsetSignal.count else { return 0 }

        var bestLag = minLag
        var bestCorrelation: Float = -Float.greatestFiniteMagnitude

        for lag in minLag...maxLag {
            var correlation: Float = 0
            let count = vDSP_Length(onsetSignal.count - lag)
            onsetSignal.withUnsafeBufferPointer { ptr in
                let a = ptr.baseAddress!
                let b = ptr.baseAddress! + lag
                vDSP_dotpr(a, 1, b, 1, &correlation, count)
            }
            if correlation > bestCorrelation {
                bestCorrelation = correlation
                bestLag = lag
            }
        }

        let bpm = (onsetRate * 60.0) / Double(bestLag)

        // Normalize to reasonable range
        var normalizedBPM = bpm
        while normalizedBPM > 200 { normalizedBPM /= 2 }
        while normalizedBPM < 60 { normalizedBPM *= 2 }

        return (normalizedBPM * 10).rounded() / 10
    }
}