🐛 Prevent arc segments > configured size

Fixing MarlinFirmware#22571

Marlin/src/gcode/motion/G2_G3.cpp

@@ -147,8 +147,8 @@ void plan_arc(
 
   const feedRate_t scaled_fr_mm_s = MMS_SCALED(feedrate_mm_s);
 
-  // Start with a nominal segment length
-  float seg_length = (
+  // Determine the segment length based on settings
+  const float seg_length = (
     #ifdef ARC_SEGMENTS_PER_R
       constrain(MM_PER_ARC_SEGMENT * radius, MM_PER_ARC_SEGMENT, ARC_SEGMENTS_PER_R)
     #elif ARC_SEGMENTS_PER_SEC
@@ -157,10 +157,14 @@ void plan_arc(
       MM_PER_ARC_SEGMENT
     #endif
   );
-  // Divide total travel by nominal segment length
+  // Divide total travel by segment length
   uint16_t segments = FLOOR(mm_of_travel / seg_length);
   NOLESS(segments, min_segments);         // At least some segments
-  seg_length = mm_of_travel / segments;
+
+  // Are the segments now too few to reach the destination?
+  const float segmented_length = seg_length * segments;
+  const bool tooshort = segmented_length < mm_of_travel;
+  const float proportion = tooshort ? segmented_length / mm_of_travel : 1.0f;
 
   /**
    * Vector rotation by transformation matrix: r is the original vector, r_T is the rotated vector,
@@ -190,26 +194,29 @@ void plan_arc(
    */
   // Vector rotation matrix values
   xyze_pos_t raw;
-  const float theta_per_segment = angular_travel / segments,
+  const float theta_per_segment = proportion * angular_travel / segments,
               sq_theta_per_segment = sq(theta_per_segment),
               sin_T = theta_per_segment - sq_theta_per_segment * theta_per_segment / 6,
               cos_T = 1 - 0.5f * sq_theta_per_segment; // Small angle approximation
 
   #if HAS_Z_AXIS && DISABLED(AUTO_BED_LEVELING_UBL)
-    const float linear_per_segment = linear_travel / segments;
+    const float linear_per_segment = proportion * linear_travel / segments;
   #endif
   #if HAS_EXTRUDERS
-    const float extruder_per_segment = extruder_travel / segments;
+    const float extruder_per_segment = proportion * extruder_travel / segments;
   #endif
 
+  // For shortened segments, run all but the remainder in the loop
+  if (tooshort) segments++;
+
   // Initialize the linear axis
   TERN_(HAS_Z_AXIS, raw[l_axis] = current_position[l_axis]);
 
   // Initialize the extruder axis
   TERN_(HAS_EXTRUDERS, raw.e = current_position.e);
 
   #if ENABLED(SCARA_FEEDRATE_SCALING)
-    const float inv_duration = scaled_fr_mm_s / seg_length;
+    const float inv_duration = scaled_fr_mm_s * segments / mm_of_travel;
   #endif
 
   millis_t next_idle_ms = millis() + 200UL;
@@ -221,8 +228,9 @@ void plan_arc(
   for (uint16_t i = 1; i < segments; i++) { // Iterate (segments-1) times
 
     thermalManager.manage_heater();
-    if (ELAPSED(millis(), next_idle_ms)) {
-      next_idle_ms = millis() + 200UL;
+    const millis_t ms = millis();
+    if (ELAPSED(ms, next_idle_ms)) {
+      next_idle_ms = ms + 200UL;
       idle();
     }