C++ code:

class Ray {
  float3 o;   // origin
  float3 dir; // direction
};

// Minimum threshold: any point closer to the surface than this is counted as part of it.
const float kMinThreshold = 1e-5;

// Maximum threshold: if we are ever this far away from the surface then assume we will never
// intersect. In practice you'll probably clamp the ray to some bounding volume instead.
const float kMaxThreshold = 1e2;

bool march_ray(Ray& r, SDF f) {
  float distance;
  while(true) {
    // Compute the distance to the surface. We can move this far without intersecting.
    distance = f(r.o);

    // Specifically, we can move this far in the ray direction without intersecting.
    r.o = r.o + distance * r.dir;

    // If we're sufficiently close to the surface, count as an intersection.
    if (distance < kMinThreshold) {
      return true;
    }

    // If we're sufficiently far away from the surface, count as escaping.
    if (kMaxThreshold < distance) {
      return false;
    }
  }
}