R/is.clockwise.R

In eggs: Distances and areas on the surface of an egg

is.clockwise = function(p) {
  # Uses the shoelace formula to determine whether or not the
  # vertices of a polygon are arranged clockwise
  area = 0
  for(i in 1:nrow(p)) {
    j = i %% nrow(p) + 1
    area = area + p[i,1]*p[j,2]
    area = area - p[j,1]*p[i,2]
  }
  return (area < 0)
}