[core, ios, macos, android, node] Heatmap layer (#11046)

Co-Authored-By: Konstantin Käfer <mail@kkaefer.com>
Co-Authored-By: Anand Thakker <anandthakker@users.noreply.github.com>
Co-Authored-By: Minh Nguyễn <1ec5@users.noreply.github.com>

1 files changed, 49 insertions, 0 deletions

diff --git a/include/mbgl/style/heatmap_color_property_value.hpp b/include/mbgl/style/heatmap_color_property_value.hpp
new file mode 100644
index 0000000000..130639c6e2
--- /dev/null
+++ b/include/mbgl/style/heatmap_color_property_value.hpp
@@ -0,0 +1,49 @@
+#pragma once
+
+#include <mbgl/util/variant.hpp>
+#include <mbgl/style/undefined.hpp>
+#include <mbgl/style/function/camera_function.hpp>
+
+namespace mbgl {
+namespace style {
+
+/*
+ * Special-case implementation of (a subset of) the PropertyValue<T> interface
+ * used for building the HeatmapColor paint property traits class.
+ */
+class HeatmapColorPropertyValue {
+private:
+    std::shared_ptr<expression::Expression> value;
+
+    friend bool operator==(const HeatmapColorPropertyValue& lhs, const HeatmapColorPropertyValue& rhs) {
+        return (lhs.isUndefined() && rhs.isUndefined()) || (lhs.value && rhs.value && *(lhs.value) == *(rhs.value));
+    }
+
+    friend bool operator!=(const HeatmapColorPropertyValue& lhs, const HeatmapColorPropertyValue& rhs) {
+        return !(lhs == rhs);
+    }
+
+public:
+    HeatmapColorPropertyValue() : value(nullptr) {}
+    HeatmapColorPropertyValue(std::shared_ptr<expression::Expression> value_) : value(std::move(value_)) {}
+
+    bool isUndefined() const { return value.get() == nullptr; }
+
+    // noop, needed for batch evaluation of paint property values to compile
+    template <typename Evaluator>
+    Color evaluate(const Evaluator&, TimePoint = {}) const { return {}; }
+
+    Color evaluate(double heatmapDensity) const {
+        const auto result = value->evaluate(expression::EvaluationContext({}, nullptr, {heatmapDensity}));
+        return *expression::fromExpressionValue<Color>(*result);
+    }
+
+    bool isDataDriven()     const { return false; }
+    bool hasDataDrivenPropertyDifference(const HeatmapColorPropertyValue&) const { return false; }
+    
+    const expression::Expression& getExpression() const { return *value; }
+};
+
+
+} // namespace style
+} // namespace mbgl