AP_Declination: save some more memory by putting the declination keys in progmem

there was a comment saying this was too slow, but it actually costs
about 30 usec extra, which is trivial given the full auto declination
call costs 680 usec and its only called once per boot

libraries/AP_Declination/AP_Declination.cpp

@@ -7,11 +7,11 @@
  *	Scott Ferguson
  *	scottfromscott@gmail.com
  *
-
- This library is free software; you can redistribute it and/or
- modify it under the terms of the GNU Lesser General Public License
- as published by the Free Software Foundation; either version 2.1
- of the License, or (at your option) any later version.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public License
+ * as published by the Free Software Foundation; either version 2.1
+ * of the License, or (at your option) any later version.
 */
 
 #include <FastSerial.h>
@@ -65,9 +65,7 @@ static const uint8_t exception_signs[10][10] PROGMEM = \
 };
 
 // 76 bytes
-// I decided NOT to store this in PROGMEM because it is small and the expense of pulling
-// the value out of the PROGMEM is too high
-static const uint8_t declination_keys[2][37] = \
+static const uint8_t declination_keys[2][37] PROGMEM = \
 { \
 // Row start values
 {36,30,25,21,18,16,14,12,11,10,9,9,9,8,8,8,7,6,6,5,4,4,4,3,4,4,4}, \
@@ -107,6 +105,8 @@ static const row_value declination_values[] PROGMEM = \
 {0,0,0},{3,0,5},{2,0,1},{1,0,0},{0,0,0},{1,1,0},{2,1,0},{5,1,0},{8,1,0},{12,1,0},{14,1,0},{13,1,0},{9,1,0},{6,1,0},{3,1,0},{1,1,0},{0,0,0},{2,0,0},{1,0,0},{3,0,0},{2,0,0},{3,0,0},{4,0,0},{3,0,1},{4,0,0},{3,0,0},{4,0,1},{3,0,0},{4,0,0},{3,0,2},{4,0,0},{3,0,1},{4,0,0},{3,0,0},{2,0,0},{3,0,0},{2,0,2},{0,0,1},{1,1,0},{2,1,0},{4,1,0},{5,1,0},{7,1,0},{8,1,0},{6,1,1},{5,1,0},{3,1,0},{1,1,1},{1,0,1},{2,0,0},{3,0,0},{2,0,0},{3,0,1},{2,0,0},{3,0,0}, \
 };
 
+#define PGM_UINT8(p) (uint8_t)pgm_read_byte_far(p)
+
 float
 AP_Declination::get_declination(float lat, float lon)
 {
@@ -149,10 +149,10 @@ AP_Declination::get_lookup_value(uint8_t x, uint8_t y)
 		if(x >= 34) x -= 27;
 
 		// Read the unsigned value from the array
-		val = (uint8_t)pgm_read_byte_far(&exceptions[x][y]);
+		val = PGM_UINT8(&exceptions[x][y]);
 
 		// Read the 8 bit compressed sign values
-		uint8_t sign = (uint8_t)pgm_read_byte_far(&exception_signs[x][y/8]);
+		uint8_t sign = PGM_UINT8(&exception_signs[x][y/8]);
 
 		// Check the sign bit for this index
 		if(sign & (0x80 >> y%8))
@@ -169,7 +169,7 @@ AP_Declination::get_lookup_value(uint8_t x, uint8_t y)
 
 	// If we are looking for the first value we can just use the
 	// row start value from declination_keys
-	if(y == 0) return declination_keys[0][x];
+	if(y == 0) return PGM_UINT8(&declination_keys[0][x]);
 
 	// Init vars
 	row_value stval;
@@ -182,16 +182,16 @@ AP_Declination::get_lookup_value(uint8_t x, uint8_t y)
 	uint16_t start_index = 0, i;
 
 	// Init value to row start
-	val = declination_keys[0][x];
+	val = PGM_UINT8(&declination_keys[0][x]);
 
 	// Find the first element in the 1D array
 	// that corresponds with the target row
 	for(i = 0; i < x; i++){
-		start_index += declination_keys[1][i];
+		start_index += PGM_UINT8(&declination_keys[1][i]);
 	}
 
 	// Traverse the row until we find our value
-	for(i = start_index; i < (start_index + declination_keys[1][x]) && current_virtual_index <= y; i++){
+	for(i = start_index; i < (start_index + PGM_UINT8(&declination_keys[1][x])) && current_virtual_index <= y; i++){
 
 		// Pull out the row_value struct
 		memcpy_P((void*) &stval, (const prog_char *)&declination_values[i], sizeof(struct row_value));