import AVFoundation
import Foundation

// The maximum number of audio buffers in flight. Setting to two allows one
// buffer to be played while the next is being written.
private let kInFlightAudioBuffers: Int = 2

// The number of audio samples per buffer. A lower value reduces latency for
// changes but requires more processing but increases the risk of being unable
// to fill the buffers in time. A setting of 1024 represents about 23ms of
// samples.
private let kSamplesPerBuffer: AVAudioFrameCount = 1024


public class FMSynthesizer {

    // The audio engine manages the sound system.
    private let engine: AVAudioEngine = AVAudioEngine()

    // The player node schedules the playback of the audio buffers.
    private let playerNode: AVAudioPlayerNode = AVAudioPlayerNode()

    // Use standard non-interleaved PCM audio.
    let audioFormat = AVAudioFormat(standardFormatWithSampleRate: 44100.0, channels: 2)

    // A circular queue of audio buffers.
    private var audioBuffers: [AVAudioPCMBuffer] = [AVAudioPCMBuffer]()

    // The index of the next buffer to fill.
    private var bufferIndex: Int = 0

    // The dispatch queue to render audio samples.
    private let audioQueue = DispatchQueue(label: "FMSynthesizerQueue")

    // A semaphore to gate the number of buffers processed.
    private let audioSemaphore = DispatchSemaphore(value: kInFlightAudioBuffers)

    public class func sharedSynth() -> FMSynthesizer {
        return FMSynthesizer()
    }

    init() {
        // Create a pool of audio buffers.
        for _ in 0 ..< kInFlightAudioBuffers {
            let audioBuffer = AVAudioPCMBuffer(pcmFormat: audioFormat, frameCapacity: kSamplesPerBuffer)
            audioBuffers.append(audioBuffer)
        }

        // Attach and connect the player node.
        engine.attach(playerNode)
        engine.connect(playerNode, to: engine.mainMixerNode, format: audioFormat)

        do {
            try engine.start()
        } catch {
            print(error.localizedDescription)
        }

        NotificationCenter.default.addObserver(self, selector: Selector(("audioEngineConfigurationChange:")), name: NSNotification.Name.AVAudioEngineConfigurationChange, object: engine)
    }

    public func play(carrierFrequency: Float32, modulatorFrequency: Float32, modulatorAmplitude: Float32) {
        let unitVelocity = Float32(2.0 * M_PI / audioFormat.sampleRate)
        let carrierVelocity = carrierFrequency * unitVelocity
        let modulatorVelocity = modulatorFrequency * unitVelocity
        audioQueue.async {
            var sampleTime: Float32 = 0
            while true {
                // Wait for a buffer to become available.
                self.audioSemaphore.wait(timeout: DispatchTime.distantFuture)

                // Fill the buffer with new samples.
                let audioBuffer = self.audioBuffers[self.bufferIndex]
                let leftChannel = audioBuffer.floatChannelData?[0]
                let rightChannel = audioBuffer.floatChannelData?[1]
                for sampleIndex in 0 ..< Int(kSamplesPerBuffer) {
                    let sample = sin(carrierVelocity * sampleTime + modulatorAmplitude * sin(modulatorVelocity * sampleTime))
                    leftChannel?[sampleIndex] = sample
                    rightChannel?[sampleIndex] = sample
                    sampleTime += 1
                }
                audioBuffer.frameLength = kSamplesPerBuffer

                // Schedule the buffer for playback and release it for reuse after
                // playback has finished.
                self.playerNode.scheduleBuffer(audioBuffer) {
                    self.audioSemaphore.signal()
                    return
                }

                self.bufferIndex = (self.bufferIndex + 1) % self.audioBuffers.count
            }
        }

        playerNode.pan = 0.8
        playerNode.play()
    }

    @objc private func audioEngineConfigurationChange(notification: NSNotification) -> Void {
        NSLog("Audio engine configuration change: \(notification)")
    }

}


// The single FM synthesizer instance.
private let gFMSynthesizer: FMSynthesizer = FMSynthesizer()



// Play a bell sound:
FMSynthesizer.sharedSynth().play(carrierFrequency: 440.0, modulatorFrequency: 679.0, modulatorAmplitude: 0.8)