Pull scaling field from from #136

Co-authored-by: Henning Scheufler <henning.scheufler@web.de

include/NeoFOAM/fields/scalingField.hpp

@@ -0,0 +1,53 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2023 NeoFOAM authors
+#pragma once
+
+namespace NeoFOAM
+{
+
+/**
+ * @brief A wrapper class that represents either a span of values or a single value.
+ *
+ * This class is used to store either a span of values or a single value of type `ValueType`.
+ * It provides an indexing operator `operator[]` that returns the value at the specified index
+ * or a constant value and is used to scale fields in the DSL.
+ *
+ * @tparam ValueType The type of the values stored in the class.
+ */
+template<typename ValueType>
+struct ScalingField
+{
+    ScalingField() = default;
+    ScalingField(std::span<ValueType> values) : values(values), value(1.0), useSpan(true) {}
+
+    ScalingField(std::span<ValueType> values, NeoFOAM::scalar value)
+        : values(values), value(value), useSpan(true)
+    {}
+
+    ScalingField(NeoFOAM::scalar value) : values(), value(value), useSpan(false) {}
+
+    std::span<ValueType> values;
+    NeoFOAM::scalar value;
+    bool useSpan;
+
+    KOKKOS_INLINE_FUNCTION
+    ValueType operator[](const size_t i) const { return useSpan ? values[i] * value : value; }
+
+    void operator*=(NeoFOAM::scalar scale) { value *= scale; }
+
+    void operator=(NeoFOAM::scalar scale) { value = scale; }
+
+    void operator*=(Field<ValueType> scalingField)
+    {
+        auto scale = scalingField.span();
+        // otherwise we are unable to capture values in the lambda
+        auto selfValue = values;
+        NeoFOAM::parallelFor(
+            scalingField.exec(),
+            {0, scalingField.size()},
+            KOKKOS_LAMBDA(const size_t i) { selfValue[i] *= scale[i]; }
+        );
+    }
+};
+
+} // namespace NeoFOAM