How to use Apple's Accelerate FFT in swift

fft-ios.swift

//
// MIT LICENSE: Copy past as much as you want :)
//

// Your signal, array of length 1024
let signal: [Float] = (0 ... 1024)

// --- INITIALIZATION
// The length of the input
length = vDSP_Length(signal.count)
// The power of two of two times the length of the input.
// Do not forget this factor 2.
log2n = vDSP_Length(ceil(log2(Float(length * 2))))
// Create the instance of the FFT class which allow computing FFT of complex vector with length
// up to `length`.
fftSetup = vDSP.FFT(log2n: log2n, radix: .radix2, ofType: DSPSplitComplex.self)!


// --- Input / Output arrays
var forwardInputReal = [Float](signal) // Copy the signal here
var forwardInputImag = [Float](repeating: 0, count: Int(length))
var forwardOutputReal = [Float](repeating: 0, count: Int(length))
var forwardOutputImag = [Float](repeating: 0, count: Int(length))
var magnitudes = [Float](repeating: 0, count: Int(length))

/// --- Compute FFT
forwardInputReal.withUnsafeMutableBufferPointer { forwardInputRealPtr in
  forwardInputImag.withUnsafeMutableBufferPointer { forwardInputImagPtr in
    forwardOutputReal.withUnsafeMutableBufferPointer { forwardOutputRealPtr in
      fforwardOutputImag.withUnsafeMutableBufferPointer { forwardOutputImagPtr in
        // Input
        let forwardInput = DSPSplitComplex(realp: forwardInputRealPtr.baseAddress!, imagp: forwardInputImagPtr.baseAddress!)
        // Output
        var forwardOutput = DSPSplitComplex(realp: forwardOutputRealPtr.baseAddress!, imagp: forwardOutputImagPtr.baseAddress!)
   
        fftSetup.forward(input: forwardInput, output: &forwardOutput)
        vDSP.absolute(forwardOutput, result: &magnitudes)
      }
    }
  }
}