c++ - Determining whether a circular number is nearly equal to another -

i'm working radians within range of [-pi, pi]. values outside range converted valid values following function

double radian::convert(double input) {     if (input > rad_upper_bound) return rad_lower_bound + fmod(input, rad_upper_bound);     if (input < rad_lower_bound) return rad_upper_bound + fmod(input, rad_upper_bound);                                  return input; } 

my question is: how can implement logic check whether radian within x in either direction of radian b. want specifiy x when calling function.

assuming = 3.0; b = -2.5; x = 1; function should return true because , b less x apart.

i'm assuming there standard way of handling type of problem.

the main trick here realize values can't more half total span away each other.

bool iswithindistance(double a,double b,double within_distance) {     assert(a >= rad_lower_bound);     assert(a <= rad_upper_bound);     assert(b >= rad_lower_bound);     assert(b <= rad_upper_bound);     assert(within_distance >= 0);      double span = radian_upper_bound-radian_lower_bound;     double distance = fabs(a-b);      if (distance > span/2) {         distance = span-distance;     }      return distance<within_distance; } 

in example:

a = 3.0;  b = -2.5;  within_distance = 1; span = 2*pi; distance = fabs(a-b) = 5.5; distance > pi,     distance = 2*pi - 5.5 ~= 0.7832; result = (distance < 1) = true;