[core] Implement Expression::serialize()

Issue #10714
- Each expression stores its operator as a string, and default serialization is [operator, serialize(child1), ...]
- Custom implementations of `serialize` for Expression types that don't follow the pattern
- expression::Value -> mbgl::Value converter
- node_expression bindings to expose `serialize`

1 files changed, 12 insertions, 0 deletions

diff --git a/src/mbgl/style/expression/step.cpp b/src/mbgl/style/expression/step.cpp
index 34537d48ae..ddaf9417cb 100644
--- a/src/mbgl/style/expression/step.cpp
+++ b/src/mbgl/style/expression/step.cpp
@@ -168,6 +168,18 @@ ParseResult Step::parse(const mbgl::style::conversion::Convertible& value, Parsi
     return ParseResult(std::make_unique<Step>(*outputType, std::move(*input), std::move(stops)));
 }
 
+mbgl::Value Step::serialize() const {
+    std::vector<mbgl::Value> serialized;
+    serialized.emplace_back(getOperator());
+    serialized.emplace_back(input->serialize());
+    for (auto& entry : stops) {
+        if (entry.first > -std::numeric_limits<double>::infinity()) {
+            serialized.emplace_back(entry.first);
+        }
+        serialized.emplace_back(entry.second->serialize());
+    }
+    return serialized;
+}
 
 } // namespace expression
 } // namespace style