Fix G2/G3 arcs > 180° (MarlinFirmware#20292)

Marlin/src/gcode/motion/G2_G3.cpp

@@ -77,22 +77,29 @@ void plan_arc(
               rt_Y = cart[q_axis] - center_Q,
               start_L = current_position[l_axis];
 
-  // CCW angle of rotation between position and target from the circle center. Only one atan2() trig computation required.
+  // Angle of rotation between position and target from the circle center.
   float angular_travel = ATAN2(rvec.a * rt_Y - rvec.b * rt_X, rvec.a * rt_X + rvec.b * rt_Y);
-  if (angular_travel < 0) angular_travel += RADIANS(360);
+
   #ifdef MIN_ARC_SEGMENTS
-    uint16_t min_segments = CEIL((MIN_ARC_SEGMENTS) * (angular_travel / RADIANS(360)));
-    NOLESS(min_segments, 1U);
+    uint16_t min_segments = MIN_ARC_SEGMENTS;
   #else
     constexpr uint16_t min_segments = 1;
   #endif
-  if (clockwise) angular_travel -= RADIANS(360);
 
-  // Make a circle if the angular rotation is 0 and the target is current position
-  if (NEAR_ZERO(angular_travel) && NEAR(current_position[p_axis], cart[p_axis]) && NEAR(current_position[q_axis], cart[q_axis])) {
-    angular_travel = RADIANS(360);
+  // Do a full circle if angular rotation is near 0 and the target is current position
+  if (!angular_travel || (NEAR_ZERO(angular_travel) && NEAR(current_position[p_axis], cart[p_axis]) && NEAR(current_position[q_axis], cart[q_axis]))) {
+    // Preserve direction for circles
+    angular_travel = clockwise ? -RADIANS(360) : RADIANS(360);
+  }
+  else {
+    // Make sure angular travel over 180 degrees goes the other way around.
+    switch (((angular_travel < 0) << 1) | clockwise) {
+      case 1: angular_travel -= RADIANS(360); break; // Positive but CW? Reverse direction.
+      case 2: angular_travel += RADIANS(360); break; // Negative but CCW? Reverse direction.
+    }
     #ifdef MIN_ARC_SEGMENTS
-      min_segments = MIN_ARC_SEGMENTS;
+      min_segments = CEIL(min_segments * ABS(angular_travel) / RADIANS(360));
+      NOLESS(min_segments, 1U);
     #endif
   }