Code shared from run.dlang.io. Run with '-unittest'

main.d

                auto targetPosition = target.headPosition;

                import std.stdio;
                //writefln!"Writing yaw: %s"(cameraPosition.yawTo(targetPosition));
                //writefln!"Writing pitch: %s"(cameraPosition.pitchTo(targetPosition));

                immutable controlAngles = getControlAngles;
                immutable viewAngles = getViewAngles;
                immutable currentYaw = controlAngles[1];
                immutable currentPitch = controlAngles[0];

                auto spread = 0.0F;
                if (const spreadPointer = localPlayer.weaponName in spreadByWeapon)
                {
                    spread = *spreadPointer;
                    targetPosition.z += 9;
                }

                immutable yawDifference = getDifferenceBetweenAngles(currentYaw, viewAngles[1]);
                immutable pitchDifference = getDifferenceBetweenAngles(currentPitch, viewAngles[0]);
                immutable neededYaw = normalize(cameraPosition.yawTo(targetPosition)
                    - yawDifference * 2 * recoilCompensationScale);
                immutable neededPitch = normalize(cameraPosition.pitchTo(targetPosition)
                    - pitchDifference * 2 * recoilCompensationScale - spread / (180 / PI));
                
                immutable yawDelta = getDifferenceBetweenAngles(currentYaw, neededYaw);
                immutable pitchDelta = getDifferenceBetweenAngles(currentPitch, neededPitch);
                
                if (GetKeyState(aimKey) < 0)
                {
                    immutable angle = atan2(pitchDelta, yawDelta);
                    immutable distance = min(
                        degreesPerSecond.toRadians / 128,
                        sqrt(yawDelta ^^ 2 + pitchDelta ^^ 2));

                    immutable smoothedYawDelta = cos(angle) * distance;
                    
                    immutable smoothedPitchDelta = sin(angle) * distance;
                
                    if (!smoothedYawDelta.isNaN && !smoothedPitchDelta.isNaN)
                    {
                        setControlAngles([
                            currentPitch + smoothedPitchDelta,
                            currentYaw + smoothedYawDelta
                        ]);
                    }
                }