More '%' modulo opertor removals and some housecleaning.

- Serial functions contained quite a few modulo operations that would
be executed with high frequency when streaming. AVR processors are very
slow when operating these. In one test on the Arduino forums, it showed
about a 15x slow down compared to a simple if-then statement. -
Clarified some variable names and types and comments.

gcode.c

@@ -328,7 +328,7 @@ uint8_t gc_execute_line(char *line) {
       }
 
       // Set clockwise/counter-clockwise sign for mc_arc computations
-      int8_t isclockwise = false;
+      uint8_t isclockwise = false;
       if (gc.motion_mode == MOTION_MODE_CW_ARC) { isclockwise = true; }
 
       // Trace the arc

motion_control.c

@@ -48,7 +48,7 @@ void mc_dwell(uint32_t milliseconds)
 // The arc is approximated by generating a huge number of tiny, linear segments. The length of each 
 // segment is configured in settings.mm_per_arc_segment.  
 void mc_arc(double *position, double *target, double *offset, uint8_t axis_0, uint8_t axis_1, 
-  uint8_t axis_linear, double feed_rate, uint8_t invert_feed_rate, double radius, int8_t isclockwise)
+  uint8_t axis_linear, double feed_rate, uint8_t invert_feed_rate, double radius, uint8_t isclockwise)
 {      
 //   int acceleration_manager_was_enabled = plan_is_acceleration_manager_enabled();
 //   plan_set_acceleration_manager_enabled(false); // disable acceleration management for the duration of the arc
@@ -106,15 +106,15 @@ void mc_arc(double *position, double *target, double *offset, uint8_t axis_0, ui
   double cos_T = 1-0.5*theta_per_segment*theta_per_segment; // Small angle approximation
   double sin_T = theta_per_segment;
 
-  double trajectory[3];
+  double arc_target[3];
   double sin_Ti;
   double cos_Ti;
   double r_axisi;
   uint16_t i;
   int8_t count = 0;
 
   // Initialize the linear axis
-  trajectory[axis_linear] = position[axis_linear];
+  arc_target[axis_linear] = position[axis_linear];
 
   for (i = 1; i<segments; i++) { // Increment (segments-1)
 
@@ -134,11 +134,11 @@ void mc_arc(double *position, double *target, double *offset, uint8_t axis_0, ui
       count = 0;
     }
 
-    // Update trajectory location
-    trajectory[axis_0] = center_axis0 + r_axis0;
-    trajectory[axis_1] = center_axis1 + r_axis1;
-    trajectory[axis_linear] += linear_per_segment;
-    plan_buffer_line(trajectory[X_AXIS], trajectory[Y_AXIS], trajectory[Z_AXIS], feed_rate, invert_feed_rate);
+    // Update arc_target location
+    arc_target[axis_0] = center_axis0 + r_axis0;
+    arc_target[axis_1] = center_axis1 + r_axis1;
+    arc_target[axis_linear] += linear_per_segment;
+    plan_buffer_line(arc_target[X_AXIS], arc_target[Y_AXIS], arc_target[Z_AXIS], feed_rate, invert_feed_rate);
 
   }
   // Ensure last segment arrives at target location.

motion_control.h

@@ -38,10 +38,10 @@
 #ifdef __AVR_ATmega328P__
 // Execute an arc in offset mode format. position == current xyz, target == target xyz, 
 // offset == offset from current xyz, axis_XXX defines circle plane in tool space, axis_linear is
-// the direction of helical travel, radius == circle radius, clockwise_sign == -1 or 1. Used
+// the direction of helical travel, radius == circle radius, isclockwise boolean. Used
 // for vector transformation direction.
 void mc_arc(double *position, double *target, double *offset, uint8_t axis_0, uint8_t axis_1,
-  uint8_t axis_linear, double feed_rate, uint8_t invert_feed_rate, double radius, int8_t isclockwise);
+  uint8_t axis_linear, double feed_rate, uint8_t invert_feed_rate, double radius, uint8_t isclockwise);
 #endif
 
 // Dwell for a couple of time units

serial.c

@@ -67,7 +67,8 @@ void serial_init(long baud)
 
 void serial_write(uint8_t data) {
   // Calculate next head
-  uint8_t next_head = (tx_buffer_head + 1) % TX_BUFFER_SIZE;
+  uint8_t next_head = tx_buffer_head + 1;
+  if (next_head == TX_BUFFER_SIZE) { next_head = 0; }
 
   // Wait until there's a space in the buffer
   while (next_head == tx_buffer_tail) { sleep_mode(); };
@@ -90,7 +91,7 @@ SIGNAL(USART_UDRE_vect) {
 
   // Update tail position
   tail ++;
-  tail %= TX_BUFFER_SIZE;
+  if (tail == TX_BUFFER_SIZE) { tail = 0; }
 
   // Turn off Data Register Empty Interrupt to stop tx-streaming if this concludes the transfer
   if (tail == tx_buffer_head) { UCSR0B &= ~(1 << UDRIE0); }
@@ -104,15 +105,17 @@ uint8_t serial_read()
 		return SERIAL_NO_DATA;
 	} else {
 		uint8_t data = rx_buffer[rx_buffer_tail];
-		rx_buffer_tail = (rx_buffer_tail + 1) % RX_BUFFER_SIZE;
+		rx_buffer_tail++;
+		if (rx_buffer_tail == RX_BUFFER_SIZE) { rx_buffer_tail = 0; }
 		return data;
 	}
 }
 
 SIGNAL(USART_RX_vect)
 {
 	uint8_t data = UDR0;
-	uint8_t next_head = (rx_buffer_head + 1) % RX_BUFFER_SIZE;
+	uint8_t next_head = rx_buffer_head + 1;
+	if (next_head == RX_BUFFER_SIZE) { next_head = 0; }
 
   // Write data to buffer unless it is full.
 	if (next_head != rx_buffer_tail) {

settings.c

@@ -3,7 +3,7 @@
   Part of Grbl
 
   Copyright (c) 2009-2011 Simen Svale Skogsrud
-  Copyright (c) 2011 Sungeun Jeon  
+  Copyright (c) 2011 Sungeun K. Jeon  
 
   Grbl is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by