[core] Rewrite style parsing logic for reuse in node bindings

1 files changed, 263 insertions, 0 deletions

diff --git a/include/mbgl/style/conversion.hpp b/include/mbgl/style/conversion.hpp
new file mode 100644
index 0000000000..0507d452fd
--- /dev/null
+++ b/include/mbgl/style/conversion.hpp
@@ -0,0 +1,263 @@
+#pragma once
+
+#include <mbgl/style/types.hpp>
+#include <mbgl/style/property_value.hpp>
+#include <mbgl/style/transition_options.hpp>
+
+#include <mbgl/util/variant.hpp>
+#include <mbgl/util/optional.hpp>
+#include <mbgl/util/color.hpp>
+#include <mbgl/util/enum.hpp>
+
+#include <array>
+#include <string>
+#include <vector>
+
+namespace mbgl {
+namespace style {
+namespace conversion {
+
+/*
+   This namespace defines a generic conversion from a templated type `V` representing a JSON object conforming
+   to a style property from the Mapbox Style Specification, to `PropertyValue<T>`:
+
+       template <class T, class V>
+       Result<PropertyValue<T>> convertPropertyValue(const V& value);
+
+   This is used concretely for conversions from RapidJSON types in mbgl core, and from v8 types in
+   the node bindings.
+
+   The requirements are that the following are legal expressions for a value `v` of type `const V&`:
+
+      * `isArray(v)` -- returns a boolean indicating whether `v` represents a JSON array
+      * `arrayLength(v)` -- called only if `isArray(v)`; returns a size_t length
+      * `arrayMember(v)` -- called only if `isArray(v)`; returns `V` or `V&`
+
+      * `isObject(v)` -- returns a boolean indicating whether `v` represents a JSON object
+      * `objectMember(v, name)` -- called only if `isObject(v)`; `name` is `const char *`; return value:
+         * is true when evaluated in a boolean context iff the named member exists
+         * is convertable to a `V` or `V&` when dereferenced
+
+      * `toBool(v)` -- returns `optional<bool>`, absence indicating `v` is not a JSON boolean
+      * `toNumber(v)` -- returns `optional<float>`, absence indicating `v` is not a JSON number
+      * `toString(v)` -- returns `optional<std::string>`, absence indicating `v` is not a JSON string
+
+   If for any reason the conversion fails, the result of `convertPropertyValue` will be the `Error` variant,
+   which includes explanatory text.
+*/
+
+struct Error { const char * message; };
+
+template <class V>
+using Result = variant<V, Error>;
+
+template <class V, class T, class Enable = void>
+struct ConstantConverter {};
+
+template <class V>
+struct ConstantConverter<V, bool> {
+    Result<bool> operator()(const V& value) const {
+        optional<bool> converted = toBool(value);
+        if (!converted) {
+            return Error { "value must be a boolean" };
+        }
+        return *converted;
+    }
+};
+
+template <class V>
+struct ConstantConverter<V, float> {
+    Result<float> operator()(const V& value) const {
+        optional<float> converted = toNumber(value);
+        if (!converted) {
+            return Error { "value must be a number" };
+        }
+        return *converted;
+    }
+};
+
+template <class V>
+struct ConstantConverter<V, std::string> {
+    Result<std::string> operator()(const V& value) const {
+        optional<std::string> converted = toString(value);
+        if (!converted) {
+            return Error { "value must be a string" };
+        }
+        return *converted;
+    }
+};
+
+template <class V, class T>
+struct ConstantConverter<V, T, typename std::enable_if_t<std::is_enum<T>::value>> {
+    Result<T> operator()(const V& value) const {
+        optional<std::string> string = toString(value);
+        if (!string) {
+            return Error { "value must be a string" };
+        }
+
+        const auto result = Enum<T>::toEnum(*string);
+        if (!result) {
+            return Error { "value must be a valid enumeration value" };
+        }
+
+        return *result;
+    }
+};
+
+template <class V>
+struct ConstantConverter<V, Color> {
+    Result<Color> operator()(const V& value) const {
+        optional<std::string> string = toString(value);
+        if (!string) {
+            return Error { "value must be a string" };
+        }
+
+        optional<Color> color = Color::parse(*string);
+        if (!color) {
+            return Error { "value must be a valid color" };
+        }
+
+        return *color;
+    }
+};
+
+template <class V>
+struct ConstantConverter<V, std::array<float, 2>> {
+    Result<std::array<float, 2>> operator()(const V& value) const {
+        if (!isArray(value) || arrayLength(value) != 2) {
+            return Error { "value must be an array of two numbers" };
+        }
+
+        optional<float> first = toNumber(arrayMember(value, 0));
+        optional<float> second = toNumber(arrayMember(value, 1));
+        if (!first || !second) {
+            return Error { "value must be an array of two numbers" };
+        }
+
+        return std::array<float, 2> {{ *first, *second }};
+    }
+};
+
+template <class V>
+struct ConstantConverter<V, std::array<float, 4>> {
+    Result<std::array<float, 4>> operator()(const V& value) const {
+        if (!isArray(value) || arrayLength(value) != 4) {
+            return Error { "value must be an array of four numbers" };
+        }
+
+        optional<float> first = toNumber(arrayMember(value, 0));
+        optional<float> second = toNumber(arrayMember(value, 1));
+        optional<float> third = toNumber(arrayMember(value, 2));
+        optional<float> fourth = toNumber(arrayMember(value, 3));
+        if (!first || !second) {
+            return Error { "value must be an array of four numbers" };
+        }
+
+        return std::array<float, 4> {{ *first, *second, *third, *fourth }};
+    }
+};
+
+template <class V>
+struct ConstantConverter<V, std::vector<float>> {
+    Result<std::vector<float>> operator()(const V& value) const {
+        if (!isArray(value)) {
+            return Error { "value must be an array" };
+        }
+
+        std::vector<float> result;
+        result.reserve(arrayLength(value));
+
+        for (std::size_t i = 0; i < arrayLength(value); ++i) {
+            optional<float> number = toNumber(arrayMember(value, i));
+            if (!number) {
+                return Error { "value must be an array of numbers" };
+            }
+            result.push_back(*number);
+        }
+
+        return result;
+    }
+};
+
+template <class V>
+struct ConstantConverter<V, std::vector<std::string>> {
+    Result<std::vector<std::string>> operator()(const V& value) const {
+        if (!isArray(value)) {
+            return Error { "value must be an array" };
+        }
+
+        std::vector<std::string> result;
+        result.reserve(arrayLength(value));
+
+        for (std::size_t i = 0; i < arrayLength(value); ++i) {
+            optional<std::string> string = toString(arrayMember(value, i));
+            if (!string) {
+                return Error { "value must be an array of strings" };
+            }
+            result.push_back(*string);
+        }
+
+        return result;
+    }
+};
+
+template <class T, class V>
+Result<PropertyValue<T>> convertPropertyValue(const V& value) {
+    if (!isObject(value)) {
+        Result<T> constant = ConstantConverter<V, T>()(value);
+        if (constant.template is<Error>()) {
+            return constant.template get<Error>();
+        }
+        return constant.template get<T>();
+    }
+
+    auto stopsValue = objectMember(value, "stops");
+    if (!stopsValue) {
+        return Error { "function value must specify stops" };
+    }
+
+    if (!isArray(*stopsValue)) {
+        return Error { "function stops must be an array" };
+    }
+
+    std::vector<std::pair<float, T>> stops;
+    for (std::size_t i = 0; i < arrayLength(*stopsValue); ++i) {
+        const auto& stopValue = arrayMember(*stopsValue, i);
+
+        if (!isArray(stopValue)) {
+            return Error { "function stop must be an array" };
+        }
+
+        if (arrayLength(stopValue) != 2) {
+            return Error { "function stop must have two elements" };
+        }
+
+        optional<float> z = toNumber(arrayMember(stopValue, 0));
+        if (!z) {
+            return Error { "function stop zoom level must be a number" };
+        }
+
+        Result<T> v = ConstantConverter<V, T>()(arrayMember(stopValue, 1));
+        if (v.template is<Error>()) {
+            return v.template get<Error>();
+        }
+
+        stops.emplace_back(*z, v.template get<T>());
+    }
+
+    auto baseValue = objectMember(value, "base");
+    if (!baseValue) {
+        return Function<T>(stops, 1.0f);
+    }
+
+    optional<float> base = toNumber(*baseValue);
+    if (!base) {
+        return Error { "function base must be a number"};
+    }
+
+    return Function<T>(stops, *base);
+}
+
+} // namespace conversion
+} // namespace style
+} // namespace mbgl