altos: Perform time comparisons using 16-bit arithmetic to handle wrap

Subtracting two 16-bit unsigned values to perform time comparisons
yields mystic results unless we carefully cast that to int16_t.

Signed-off-by: Keith Packard <keithp@keithp.com>

3 files changed, 11 insertions, 11 deletions

diff --git a/src/kernel/ao_flight.c b/src/kernel/ao_flight.c
index 50f2b68f..b0d911ed 100644
--- a/src/kernel/ao_flight.c
+++ b/src/kernel/ao_flight.c
@@ -269,7 +269,7 @@ ao_flight(void)
 			 * number of seconds.
 			 */
 			if (ao_config.apogee_lockout) {
-				if ((ao_sample_tick - ao_boost_tick) <
+				if ((int16_t) (ao_sample_tick - ao_boost_tick) <
 				    AO_SEC_TO_TICKS(ao_config.apogee_lockout))
 					break;
 			}
diff --git a/src/kernel/ao_kalman.c b/src/kernel/ao_kalman.c
index 69a1b3de..d401a3c8 100644
--- a/src/kernel/ao_kalman.c
+++ b/src/kernel/ao_kalman.c
@@ -55,14 +55,14 @@ static void
 ao_kalman_predict(void)
 {
 #ifdef AO_FLIGHT_TEST
-	if (ao_sample_tick - ao_sample_prev_tick > 50) {
+	if ((int16_t) (ao_sample_tick - ao_sample_prev_tick) > 50) {
 		ao_k_height += ((ao_k_t) ao_speed * AO_K_STEP_1 +
 				(ao_k_t) ao_accel * AO_K_STEP_2_2_1) >> 4;
 		ao_k_speed += (ao_k_t) ao_accel * AO_K_STEP_1;
 
 		return;
 	}
-	if (ao_sample_tick - ao_sample_prev_tick > 5) {
+	if ((int16_t) (ao_sample_tick - ao_sample_prev_tick) > 5) {
 		ao_k_height += ((ao_k_t) ao_speed * AO_K_STEP_10 +
 				(ao_k_t) ao_accel * AO_K_STEP_2_2_10) >> 4;
 		ao_k_speed += (ao_k_t) ao_accel * AO_K_STEP_10;
@@ -141,13 +141,13 @@ ao_kalman_correct_baro(void)
 {
 	ao_kalman_err_height();
 #ifdef AO_FLIGHT_TEST
-	if (ao_sample_tick - ao_sample_prev_tick > 50) {
+	if ((int16_t) (ao_sample_tick - ao_sample_prev_tick) > 50) {
 		ao_k_height += (ao_k_t) AO_BARO_K0_1 * ao_error_h;
 		ao_k_speed  += (ao_k_t) AO_BARO_K1_1 * ao_error_h;
 		ao_k_accel  += (ao_k_t) AO_BARO_K2_1 * ao_error_h;
 		return;
 	}
-	if (ao_sample_tick - ao_sample_prev_tick > 5) {
+	if ((int16_t) (ao_sample_tick - ao_sample_prev_tick) > 5) {
 		ao_k_height += (ao_k_t) AO_BARO_K0_10 * ao_error_h;
 		ao_k_speed  += (ao_k_t) AO_BARO_K1_10 * ao_error_h;
 		ao_k_accel  += (ao_k_t) AO_BARO_K2_10 * ao_error_h;
@@ -180,7 +180,7 @@ ao_kalman_correct_both(void)
 	ao_kalman_err_accel();
 
 #ifdef AO_FLIGHT_TEST
-	if (ao_sample_tick - ao_sample_prev_tick > 50) {
+	if ((int16_t) (ao_sample_tick - ao_sample_prev_tick) > 50) {
 		if (ao_flight_debug) {
 			printf ("correct speed %g + (%g * %g) + (%g * %g) = %g\n",
 				ao_k_speed / (65536.0 * 16.0),
@@ -201,7 +201,7 @@ ao_kalman_correct_both(void)
 			(ao_k_t) AO_BOTH_K21_1 * ao_error_a;
 		return;
 	}
-	if (ao_sample_tick - ao_sample_prev_tick > 5) {
+	if ((int16_t) (ao_sample_tick - ao_sample_prev_tick) > 5) {
 		if (ao_flight_debug) {
 			printf ("correct speed %g + (%g * %g) + (%g * %g) = %g\n",
 				ao_k_speed / (65536.0 * 16.0),
@@ -250,7 +250,7 @@ ao_kalman_correct_accel(void)
 {
 	ao_kalman_err_accel();
 
-	if (ao_sample_tick - ao_sample_prev_tick > 5) {
+	if ((int16_t) (ao_sample_tick - ao_sample_prev_tick) > 5) {
 		ao_k_height +=(ao_k_t) AO_ACCEL_K0_10 * ao_error_a;
 		ao_k_speed  += (ao_k_t) AO_ACCEL_K1_10 * ao_error_a;
 		ao_k_accel  += (ao_k_t) AO_ACCEL_K2_10 * ao_error_a;
@@ -285,9 +285,9 @@ ao_kalman(void)
 		ao_max_height = ao_height;
 	ao_avg_height_scaled = ao_avg_height_scaled - ao_avg_height + ao_sample_height;
 #ifdef AO_FLIGHT_TEST
-	if (ao_sample_tick - ao_sample_prev_tick > 50)
+	if ((int16_t) (ao_sample_tick - ao_sample_prev_tick) > 50)
 		ao_avg_height = (ao_avg_height_scaled + 1) >> 1;
-	else if (ao_sample_tick - ao_sample_prev_tick > 5)
+	else if ((int16_t) (ao_sample_tick - ao_sample_prev_tick) > 5)
 		ao_avg_height = (ao_avg_height_scaled + 7) >> 4;
 	else 
 #endif
diff --git a/src/kernel/ao_sample.c b/src/kernel/ao_sample.c
index 90ea07ad..f0ab0169 100644
--- a/src/kernel/ao_sample.c
+++ b/src/kernel/ao_sample.c
@@ -180,7 +180,7 @@ static void
 ao_sample_rotate(void)
 {
 #ifdef AO_FLIGHT_TEST
-	float	dt = (ao_sample_tick - ao_sample_prev_tick) / TIME_DIV;
+	float	dt = (int16_t) (ao_sample_tick - ao_sample_prev_tick) / TIME_DIV;
 #else
 	static const float dt = 1/TIME_DIV;
 #endif