#! /usr/bin/env python3
#
def c_is_left_of_ab_test ( ):

  import matplotlib.pyplot as plt
  import numpy as np

  print ( '' )
  print ( 'c_is_left_of_ab():' )
  print ( '  True if point C is to the left of line A-B.' )

  a = np.array ( [ 0.0, 0.0 ] )
  b = np.array ( [ 1.0, 0.5 ] )

  plt.clf ( )

  plt.plot ( [ a[0], b[0] ], [ a[1], b[1] ], 'k-' )

  for i in range ( 0, 50 ):
    c = np.random.rand ( 2 )
    value = c_is_left_of_ab ( a, b, c )
    if ( value ):
      plt.plot ( c[0], c[1], 'bo' )
    else:
      plt.plot ( c[0], c[1], 'ro' )

  plt.title ( 'Blue points left, red on right.' )
  plt.axis ( 'equal' )
  plt.grid ( True )
  filename = 'c_is_left_of_ab.png'
  plt.savefig ( filename )
  print ( '  Graphics saved as "' + filename + '"' )
  plt.show ( )

  return

def c_is_left_of_ab ( a, b, c ):

  t = ( c[0] - a[0] ) * ( b[1] - a[1] ) \
    - ( c[1] - a[1] ) * ( b[0] - a[0] )

  value = ( t < 0.0 )
  return value

if ( __name__ == "__main__" ):
  c_is_left_of_ab_test ( )