Add QuantityType.toUnitRelative

bundles/org.openhab.core.thing/src/main/java/org/openhab/core/thing/internal/profiles/SystemOffsetProfile.java

@@ -21,8 +21,6 @@
 import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.library.types.DecimalType;
 import org.openhab.core.library.types.QuantityType;
-import org.openhab.core.library.unit.ImperialUnits;
-import org.openhab.core.library.unit.SIUnits;
 import org.openhab.core.library.unit.Units;
 import org.openhab.core.thing.profiles.ProfileCallback;
 import org.openhab.core.thing.profiles.ProfileContext;
@@ -44,10 +42,6 @@
 @NonNullByDefault
 public class SystemOffsetProfile implements StateProfile {
 
-    private static final @Nullable QuantityType<Temperature> ZERO_CELSIUS_IN_KELVIN = new QuantityType<>(0,
-            SIUnits.CELSIUS).toUnit(Units.KELVIN);
-    private static final @Nullable QuantityType<Temperature> ZERO_FAHRENHEIT_IN_KELVIN = new QuantityType<>(0,
-            ImperialUnits.FAHRENHEIT).toUnit(Units.KELVIN);
     static final String OFFSET_PARAM = "offset";
 
     private final Logger logger = LoggerFactory.getLogger(SystemOffsetProfile.class);
@@ -152,20 +146,11 @@ private Type applyOffset(Type state, boolean towardsItem) {
             QuantityType<Temperature> offset) {
         // do the math in Kelvin and afterwards convert it back to the unit of the state
         final QuantityType<Temperature> kelvinState = qtState.toUnit(Units.KELVIN);
-        final QuantityType<Temperature> kelvinOffset = offset.toUnit(Units.KELVIN);
+        final QuantityType<Temperature> kelvinOffset = offset.toUnitRelative(Units.KELVIN);
         if (kelvinState == null || kelvinOffset == null) {
             return null;
         }
 
-        final QuantityType<Temperature> finalOffset;
-        if (SIUnits.CELSIUS.equals(offset.getUnit())) {
-            finalOffset = kelvinOffset.add(ZERO_CELSIUS_IN_KELVIN.negate());
-        } else if (ImperialUnits.FAHRENHEIT.equals(offset.getUnit())) {
-            finalOffset = kelvinOffset.add(ZERO_FAHRENHEIT_IN_KELVIN.negate());
-        } else {
-            // offset is already in Kelvin
-            finalOffset = offset;
-        }
-        return kelvinState.add(finalOffset).toUnit(qtState.getUnit());
+        return kelvinState.add(kelvinOffset).toUnit(qtState.getUnit());
     }
 }

bundles/org.openhab.core/src/main/java/org/openhab/core/library/types/QuantityType.java

@@ -314,6 +314,31 @@ public Dimension getDimension() {
         return null;
     }
 
+    /**
+     * Convert this QuantityType to a new {@link QuantityType} using the given target unit.
+     *
+     * Similar to {@link toUnit}, except that it treats the values as relative instead of absolute.
+     * This means that any offsets in the conversion of absolute values are ignored.
+     * This is useful when your quantity represents a delta, and not necessarily a measured
+     * value itself. For example, 32 °F, when converted with toUnit to Celsius, it will become 0 °C.
+     * But when converted with toUnitRelative, it will become 17.8 °C.
+     *
+     * @param targetUnit the unit to which this {@link QuantityType} will be converted to.
+     * @return the new {@link QuantityType} in the given {@link Unit} or {@code null} in case of an error.
+     */
+    @SuppressWarnings("unchecked")
+    public @Nullable QuantityType<T> toUnitRelative(Unit<T> targetUnit) {
+        if (targetUnit.equals(getUnit())) {
+            return this;
+        }
+        if (!quantity.getUnit().isCompatible(targetUnit)) {
+            return null;
+        }
+        Quantity<?> result = quantity.to(targetUnit);
+
+        return new QuantityType<T>(result.getValue(), (Unit<T>) targetUnit);
+    }
+
     public BigDecimal toBigDecimal() {
         return new BigDecimal(quantity.getValue().toString());
     }

bundles/org.openhab.core/src/test/java/org/openhab/core/library/types/QuantityTypeTest.java

@@ -40,6 +40,7 @@
 import org.openhab.core.library.dimension.Density;
 import org.openhab.core.library.dimension.Intensity;
 import org.openhab.core.library.unit.BinaryPrefix;
+import org.openhab.core.library.unit.ImperialUnits;
 import org.openhab.core.library.unit.MetricPrefix;
 import org.openhab.core.library.unit.SIUnits;
 import org.openhab.core.library.unit.Units;
@@ -483,4 +484,11 @@ public void testMireds() {
         QuantityType<?> andBack = mireds.toInvertibleUnit(Units.KELVIN);
         assertEquals(2700, andBack.intValue());
     }
+
+    @Test
+    public void testRelativeConversion() {
+        QuantityType<Temperature> c = new QuantityType("1 °C");
+        QuantityType<Temperature> f = c.toUnitRelative(ImperialUnits.FAHRENHEIT);
+        assertEquals(1.8, f.doubleValue());
+    }
 }