Largest-Triangle-Three Bucket Downsampling

LTTBD.swift

func downsample(values: [(x:Double, y: Double)], threshold: Int) -> [(x:Double,y: Double)] {

  guard values.count > threshold && values.count > 2 else { return values }
  
  let bucketSize = (values.count - 2) / (threshold - 2)
  
  var A = 0, nextA = 0
  var out = [(x:Double, y: Double)]()
  var maxAreaPoint: (x:Double, y: Double) = (x:0, y: 0)
  out.append(values.first!)
  
  for i in 0..<(threshold - 2) {
    
    var avgRangeStart = (i + 1) * bucketSize + 1
    var avgRangeEnd   = (i + 2) * bucketSize + 1
    
    avgRangeEnd = avgRangeEnd < values.count ? avgRangeEnd : values.count
    
    let avgRangeLength = avgRangeEnd - avgRangeStart
    
    var avgX = 0.0, avgY = 0.0
    
    while avgRangeStart < avgRangeEnd {
      avgX += values[avgRangeStart].x
      avgY += values[avgRangeStart].y
      avgRangeStart += 1;
    }
    
    avgX /= Double(avgRangeLength)
    avgY /= Double(avgRangeLength)
    
    var rangeOffs = (i + 0) * bucketSize + 1
    let rangeTo   = (i + 1) * bucketSize + 1
    
    let pointAx = values[A].x
    let pointAy = values[A].y
    
    var maxArea = -1.0;
    
    while rangeOffs < rangeTo {
      
      let x = (pointAx - avgX) * ( values[rangeOffs].y - pointAy)
      let y = (pointAx - values[rangeOffs].x ) * (avgY - pointAy)
      let area = abs ( x - y ) * 0.5;
      
      if area > maxArea {
        maxArea = area;
        maxAreaPoint = values[rangeOffs]
        nextA = rangeOffs
      }
      rangeOffs += 1
    }
    out.append( maxAreaPoint  )
    A = nextA
  }
  out.append (values.last!)
  return out
}