Module: RGeo::Cartesian::Analysis

Defined in:
lib/rgeo/cartesian/analysis.rb

Overview

This provides includes some spatial analysis algorithms supporting Cartesian data.

Class Method Summary collapse

Class Method Details

.ring_direction(ring_) ⇒ Object

Given a LineString, which must be a ring, determine whether the ring proceeds clockwise or counterclockwise. Returns 1 for counterclockwise, or -1 for clockwise.

Returns 0 if the ring is empty. The return value is undefined if the object is not a ring, or is not in a Cartesian coordinate system.



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# File 'lib/rgeo/cartesian/analysis.rb', line 58

def ring_direction(ring_)
  size_ = ring_.num_points - 1
  return 0 if size_ == 0
  
  # Extract unit-length segments from the ring.
  segs_ = []
  size_.times do |i_|
    p0_ = ring_.point_n(i_)
    p1_ = ring_.point_n(i_+1)
    x_ = p1_.x - p0_.x
    y_ = p1_.y - p0_.y
    r_ = ::Math.sqrt(x_*x_ + y_*y_)
    if r_ > 0.0
      segs_ << x_/r_ << y_/r_
    end
  end
  segs_ << segs_[0] << segs_[1]
  
  # Extract angles from the segments by subtracting the segments.
  # Note angles are represented as cos/sin pairs so we don't
  # have to calculate any trig functions.
  angs_ = []
  size_.times do |i_|
    x0_, y0_, x1_, y1_ = segs_[i_*2,4]
    angs_ << x0_*x1_ + y0_*y1_ << x0_*y1_ - x1_*y0_
  end
  
  # Now add the angles and count revolutions.
  # Again, our running sum is represented as a cos/sin pair.
  revolutions_ = 0
  sin_ = 0.0
  cos_ = 1.0
  angs_.each_slice(2) do |(x_, y_)|
    ready_ = y_ > 0.0 && sin_ > 0.0 || y_ < 0.0 && sin_ < 0.0
    if y_ != 0.0
      s_ = sin_*x_ + cos_*y_
      c_ = cos_*x_ - sin_*y_
      r_ = ::Math.sqrt(s_*s_ + c_*c_)
      sin_ = s_ / r_
      cos_ = c_ / r_
    end
    if ready_
      if y_ > 0.0 && sin_ <= 0.0
        revolutions_ += 1
      elsif y_ < 0.0 && sin_ >= 0.0
        revolutions_ -= 1
      end
    end
  end
  revolutions_
end