code: func (p *Point) Quadkey(level int) int64 {
  	x, y := scalarMercatorProject(p.Lng(), p.Lat(), uint64(level))
  
  	var i uint
  	var result uint64
  	for i = 0; i < uint(level); i++ {
  		result |= (x & (1 << i)) << i
  		result |= (y & (1 << i)) << (i + 1)
  	}
  
  	return int64(result)
  }
docs: // Quadkey returns the quad key for the given point at the provided level.
  // See http://msdn.microsoft.com/en-us/library/bb259689.aspx for more information
  // about this coordinate system.