1 files changed, 64 insertions, 0 deletions
diff --git a/src/mbgl/style/expression/at.cpp b/src/mbgl/style/expression/at.cpp
new file mode 100644
index 0000000000..e447d33bc7
--- /dev/null
+++ b/src/mbgl/style/expression/at.cpp
@@ -0,0 +1,64 @@
+#include <mbgl/style/expression/at.hpp>
+#include <mbgl/util/string.hpp>
+
+
+namespace mbgl {
+namespace style {
+namespace expression {
+
+EvaluationResult At::evaluate(const EvaluationContext& params) const {
+    const EvaluationResult evaluatedIndex = index->evaluate(params);
+    const EvaluationResult evaluatedInput = input->evaluate(params);
+    if (!evaluatedIndex) {
+        return evaluatedIndex.error();
+    }
+    if (!evaluatedInput) {
+        return evaluatedInput.error();
+    }
+    
+    const auto i = evaluatedIndex->get<double>();
+    const auto inputArray = evaluatedInput->get<std::vector<Value>>();
+    
+    if (i < 0 || i >= inputArray.size()) {
+        return EvaluationError {
+            "Array index out of bounds: " + stringify(i) +
+            " > " + util::toString(inputArray.size()) + "."
+        };
+    }
+    if (i != std::floor(i)) {
+        return EvaluationError {
+            "Array index must be an integer, but found " + stringify(i) + " instead."
+        };
+    }
+    return inputArray[static_cast<std::size_t>(i)];
+}
+
+void At::eachChild(const std::function<void(const Expression&)>& visit) const {
+    visit(*index);
+    visit(*input);
+}
+
+using namespace mbgl::style::conversion;
+ParseResult At::parse(const Convertible& value, ParsingContext& ctx) {
+    assert(isArray(value));
+
+    std::size_t length = arrayLength(value);
+    if (length != 3) {
+        ctx.error("Expected 2 arguments, but found " + util::toString(length - 1) + " instead.");
+        return ParseResult();
+    }
+
+    ParseResult index = ctx.parse(arrayMember(value, 1), 1, {type::Number});
+    
+    type::Type inputType = type::Array(ctx.getExpected() ? *ctx.getExpected() : type::Value);
+    ParseResult input = ctx.parse(arrayMember(value, 2), 2, {inputType});
+
+    if (!index || !input) return ParseResult();
+
+    return ParseResult(std::make_unique<At>(std::move(*index), std::move(*input)));
+
+}
+
+} // namespace expression
+} // namespace style
+} // namespace mbgl