1 files changed, 76 insertions, 0 deletions
diff --git a/src/mbgl/style/expression/find_zoom_curve.cpp b/src/mbgl/style/expression/find_zoom_curve.cpp
new file mode 100644
index 0000000000..5d39e0791e
--- /dev/null
+++ b/src/mbgl/style/expression/find_zoom_curve.cpp
@@ -0,0 +1,76 @@
+#include <mbgl/style/expression/find_zoom_curve.hpp>
+#include <mbgl/style/expression/compound_expression.hpp>
+#include <mbgl/style/expression/let.hpp>
+#include <mbgl/style/expression/coalesce.hpp>
+
+#include <mbgl/util/variant.hpp>
+#include <mbgl/util/optional.hpp>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+
+optional<variant<const InterpolateBase*, const Step*, ParsingError>> findZoomCurve(const expression::Expression* e) {
+    optional<variant<const InterpolateBase*, const Step*, ParsingError>> result;
+    
+    if (auto let = dynamic_cast<const Let*>(e)) {
+        result = findZoomCurve(let->getResult());
+    } else if (auto coalesce = dynamic_cast<const Coalesce*>(e)) {
+        std::size_t length = coalesce->getLength();
+        for (std::size_t i = 0; i < length; i++) {
+            result = findZoomCurve(coalesce->getChild(i));
+            if (result) {
+                break;
+            }
+        }
+    } else if (auto curve = dynamic_cast<const InterpolateBase*>(e)) {
+        auto z = dynamic_cast<CompoundExpressionBase*>(curve->getInput().get());
+        if (z && z->getName() == "zoom") {
+            result = {curve};
+        }
+    } else if (auto step = dynamic_cast<const Step*>(e)) {
+        auto z = dynamic_cast<CompoundExpressionBase*>(step->getInput().get());
+        if (z && z->getName() == "zoom") {
+            result = {step};
+        }
+    }
+    
+    if (result && result->is<ParsingError>()) {
+        return result;
+    }
+    
+    e->eachChild([&](const Expression& child) {
+        optional<variant<const InterpolateBase*, const Step*, ParsingError>> childResult(findZoomCurve(&child));
+        if (childResult) {
+            if (childResult->is<ParsingError>()) {
+                result = childResult;
+            } else if (!result && childResult) {
+                result = {ParsingError {
+                    R"("zoom" expression may only be used as input to a top-level "step" or "interpolate" expression.)", ""
+                }};
+            } else if (result && childResult && result != childResult) {
+                result = {ParsingError {
+                    R"(Only one zoom-based "step" or "interpolate" subexpression may be used in an expression.)", ""
+                }};
+            }
+        }
+    });
+
+    return result;
+}
+
+variant<const InterpolateBase*, const Step*> findZoomCurveChecked(const expression::Expression* e) {
+    return findZoomCurve(e)->match(
+        [](const ParsingError&) -> variant<const InterpolateBase*, const Step*> {
+            assert(false);
+            return {};
+        },
+        [](auto zoomCurve) -> variant<const InterpolateBase*, const Step*> {
+            return {std::move(zoomCurve)};
+        }
+    );
+}
+
+} // namespace expression
+} // namespace style
+} // namespace mbgl