weather: Fix incorrect rounding for negative temperatures

src/components/ble/SimpleWeatherService.h

@@ -34,6 +34,7 @@
 #include "components/datetime/DateTimeController.h"
 #include <lvgl/lvgl.h>
 #include "displayapp/InfiniTimeTheme.h"
+#include "utility/Math.h"
 
 int WeatherCallback(uint16_t connHandle, uint16_t attrHandle, struct ble_gatt_access_ctxt* ctxt, void* arg);
 
@@ -77,11 +78,11 @@ namespace Pinetime {
         }
 
         [[nodiscard]] int16_t Celsius() const {
-          return (PreciseCelsius() + 50) / 100;
+          return Utility::RoundedDiv(PreciseCelsius(), static_cast<int16_t>(100));
         }
 
         [[nodiscard]] int16_t Fahrenheit() const {
-          return (PreciseFahrenheit() + 50) / 100;
+          return Utility::RoundedDiv(PreciseFahrenheit(), static_cast<int16_t>(100));
         }
 
         [[nodiscard]] lv_color_t Color() const {

src/utility/Math.h

@@ -1,10 +1,33 @@
 #pragma once
 
 #include <cstdint>
+#include <concepts>
 
 namespace Pinetime {
   namespace Utility {
     // returns the arcsin of `arg`. asin(-32767) = -90, asin(32767) = 90
     int16_t Asin(int16_t arg);
+
+    // Round half away from zero integer division
+    // If T signed, divisor cannot be std::numeric_limits<T>::min()
+    // Adapted from https://github.com/lucianpls/rounding_integer_division
+    // Under the MIT license
+    template <std::integral T>
+    constexpr T RoundedDiv(T dividend, T divisor) {
+      bool neg = divisor < 0;
+      if (neg) {
+        // overflows if divisor is minimum value for T
+        divisor = -divisor;
+      }
+
+      T m = dividend % divisor;
+      T h = divisor / 2 + divisor % 2;
+      T res = (dividend / divisor) + (!(dividend < 0) & (m >= h)) - ((dividend < 0) & ((m + h) <= 0));
+
+      if (neg) {
+        res = -res;
+      }
+      return res;
+    }
   }
 }