question: private static boolean isNonLeft ( int i0 , int i1 , int i2 , int i3 , double [ ] pts ) { double l1 , l2 , l4 , l5 , l6 , angle1 , angle2 , angle ; l1 = Math . sqrt ( Math . pow ( pts [ i2 + 1 ] - pts [ i1 + 1 ] , 2 ) + Math . pow ( pts [ i2 ] - pts [ i1 ] , 2 ) ) ; l2 = Math . sqrt ( Math . pow ( pts [ i3 + 1 ] - pts [ i2 + 1 ] , 2 ) + Math . pow ( pts [ i3 ] - pts [ i2 ] , 2 ) ) ; l4 = Math . sqrt ( Math . pow ( pts [ i3 + 1 ] - pts [ i0 + 1 ] , 2 ) + Math . pow ( pts [ i3 ] - pts [ i0 ] , 2 ) ) ; l5 = Math . sqrt ( Math . pow ( pts [ i1 + 1 ] - pts [ i0 + 1 ] , 2 ) + Math . pow ( pts [ i1 ] - pts [ i0 ] , 2 ) ) ; l6 = Math . sqrt ( Math . pow ( pts [ i2 + 1 ] - pts [ i0 + 1 ] , 2 ) + Math . pow ( pts [ i2 ] - pts [ i0 ] , 2 ) ) ; angle1 = Math . acos ( ( ( l2 * l2 ) + ( l6 * l6 ) - ( l4 * l4 ) ) / ( 2 * l2 * l6 ) ) ; angle2 = Math . acos ( ( ( l6 * l6 ) + ( l1 * l1 ) - ( l5 * l5 ) ) / ( 2 * l6 * l1 ) ) ; angle = ( Math . PI - angle1 ) - angle2 ; if ( angle <= 0.0 ) { return ( true ) ; } else { return ( false ) ; } }
target:  Convex hull helper method for detecting a non left turn about 3 points