downsample.swift

/// Creates an array of values to display
    ///
    /// Averages values and performs in batches in the background
    func resetDrawValues() {
        guard values.count > 0 else {
            print("load data first")
            return
        }
        
        drawValues = []
        drawPoints = []
        
        //let logValues = self.values
        
        let binCount = Int(self.bounds.size.width / resolutionWidth)
        let binSize = values.count / binCount
        
        if self.shapeLayer == nil {
            setupLayer()
        }
        
        DispatchQueue.global().async {
            let threshhold: Float = -50
            var logValues: [Float] = self.values.map {
                let value = 10 * log10f( $0 )
                return value < threshhold ? threshhold : value
            }
            
            var drawValuesBuffer = [Float]()
            var drawPointsBuffer = [NSPoint]()
            var drawValuesCount = 0
            
            var nextThreshhold = logValues.count / 5
            for i in stride(from: 0, to: logValues.count, by: binSize) {
                
                let range = i..<(i+binSize >= logValues.count ? logValues.count - 1 : i+binSize)
                let bin = logValues[range]
                let value = bin.reduce(0,+) / Float(bin.count)
                
                let point = NSPoint(x: CGFloat(drawValuesCount) / CGFloat(binCount),
                                    y: CGFloat(value - threshhold))
                
                drawValuesBuffer.append(value - threshhold)
                drawPointsBuffer.append(point)
                drawValuesCount += 1
                
                if i > nextThreshhold {
                    DispatchQueue.main.async {
                        self.drawValues.append(contentsOf: drawValuesBuffer)
                        self.drawPoints.append(contentsOf: drawPointsBuffer)
                        self.refreshLayer()
                    }
                    
                    drawValuesBuffer.removeAll(keepingCapacity: true)
                    drawPointsBuffer.removeAll(keepingCapacity: true)
                    nextThreshhold += logValues.count / 5
                }
            }
            

            DispatchQueue.main.async {
                self.drawValues.append(contentsOf: drawValuesBuffer)
                self.drawPoints.append(contentsOf: drawPointsBuffer)
                self.refreshLayer()
            }
        
        }
        
    }