From 16f8adb4b3fa8b2649dee347c1702c99722f4144 Mon Sep 17 00:00:00 2001
From: Daniel Agar <daniel@agar.ca>
Date: Tue, 27 Sep 2022 22:07:31 -0400
Subject: [PATCH] hysteresis: avoid subtracting unsigned integers

 - avoid potential errors with timestamp passed in from different sources
---
 src/lib/hysteresis/hysteresis.cpp | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/lib/hysteresis/hysteresis.cpp b/src/lib/hysteresis/hysteresis.cpp
index 07f14e4b2e..22593b2d6e 100644
--- a/src/lib/hysteresis/hysteresis.cpp
+++ b/src/lib/hysteresis/hysteresis.cpp
@@ -74,17 +74,15 @@ Hysteresis::update(const hrt_abstime &now_us)
 {
 	if (_requested_state != _state) {
 
-		const hrt_abstime elapsed = now_us - _last_time_to_change_state;
-
 		if (_state && !_requested_state) {
 			// true -> false
-			if (elapsed >= _time_from_true_us) {
+			if (now_us >= _last_time_to_change_state + _time_from_true_us) {
 				_state = false;
 			}
 
 		} else if (!_state && _requested_state) {
 			// false -> true
-			if (elapsed >= _time_from_false_us) {
+			if (now_us >= _last_time_to_change_state + _time_from_false_us) {
 				_state = true;
 			}
 		}