反時計回り | 計算幾何学 | Aizu Online Judge http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=CGL_1_C&lang=jp

CGL_1_C.rb

# coding: utf-8
require 'matrix'
xp0, yp0, xp1, yp1 = gets.split.map(&:to_f)
p01 = Vector[xp1-xp0, yp1-yp0]
gets.to_i.times {
  xp2, yp2 = gets.split.map(&:to_f)
  p02 = Vector[xp2-xp0, yp2-yp0]
  cross =  p01[0]*p02[1]-p01[1]*p02[0] # 外積
#  cross = p01.cross(p02)        # wrong number of arguments (1 for 0) (ArgumentError)
  if cross > 0
    puts "COUNTER_CLOCKWISE"
  elsif cross < 0
    puts "CLOCKWISE"
  else
    dot = p01.dot(p02)
    if dot < 0
      puts "ONLINE_BACK"
    else
      if p01.r >= p02.r
        puts "ON_SEGMENT"
      else
        puts "ONLINE_FRONT"
      end
    end
  end
}