[core] #6071 - extract GeoJSONOptions conversion from GeoJSONSource conversion

* [core] geojson_options - retain original error message

* [core] tests - initial style conversion stub methods

* [core] geojsonoptions conversion - initial unit tests

* [core] tests - fix forward reference issue

* [core] geojsonoptions conversion - unit tests

* [core] geojsonoptions conversion - renamed Holder to Value

2 files changed, 83 insertions, 55 deletions

diff --git a/include/mbgl/style/conversion/geojson_options.hpp b/include/mbgl/style/conversion/geojson_options.hpp
new file mode 100644
index 0000000000..880090b402
--- /dev/null
+++ b/include/mbgl/style/conversion/geojson_options.hpp
@@ -0,0 +1,78 @@
+#pragma once
+
+#include <mbgl/style/conversion.hpp>
+#include <mbgl/style/sources/geojson_source.hpp>
+
+namespace mbgl {
+namespace style {
+namespace conversion {
+
+template <>
+struct Converter<GeoJSONOptions> {
+
+    template <class V>
+    Result<GeoJSONOptions> operator()(const V& value) const {
+        GeoJSONOptions options;
+
+        const auto maxzoomValue = objectMember(value, "maxzoom");
+        if (maxzoomValue) {
+            if (toNumber(*maxzoomValue)) {
+                options.maxzoom = static_cast<uint8_t>(*toNumber(*maxzoomValue));
+            } else {
+                return Error{ "GeoJSON source maxzoom value must be a number" };
+            }
+        }
+
+        const auto bufferValue = objectMember(value, "buffer");
+        if (bufferValue) {
+            if (toNumber(*bufferValue)) {
+                options.buffer = static_cast<uint16_t>(*toNumber(*bufferValue));
+            } else {
+                return Error{ "GeoJSON source buffer value must be a number" };
+            }
+        }
+
+        const auto toleranceValue = objectMember(value, "tolerance");
+        if (toleranceValue) {
+            if (toNumber(*toleranceValue)) {
+                options.tolerance = static_cast<double>(*toNumber(*toleranceValue));
+            } else {
+                return Error{ "GeoJSON source tolerance value must be a number" };
+            }
+        }
+
+        const auto clusterValue = objectMember(value, "cluster");
+        if (clusterValue) {
+            if (toBool(*clusterValue)) {
+                options.cluster = *toBool(*clusterValue);
+            } else {
+                return Error{ "GeoJSON source cluster value must be a boolean" };
+            }
+        }
+
+        const auto clusterMaxZoomValue = objectMember(value, "clusterMaxZoom");
+        if (clusterMaxZoomValue) {
+            if (toNumber(*clusterMaxZoomValue)) {
+                options.clusterMaxZoom = static_cast<uint8_t>(*toNumber(*clusterMaxZoomValue));
+            } else {
+                return Error{ "GeoJSON source clusterMaxZoom value must be a number" };
+            }
+        }
+
+        const auto clusterRadiusValue = objectMember(value, "clusterRadius");
+        if (clusterRadiusValue) {
+            if (toNumber(*clusterRadiusValue)) {
+                options.clusterRadius = static_cast<double>(*toNumber(*clusterRadiusValue));
+            } else {
+                return Error{ "GeoJSON source clusterRadius value must be a number" };
+            }
+        }
+
+        return { options };
+    }
+
+};
+
+} // namespace conversion
+} // namespace style
+} // namespace mbgl
diff --git a/include/mbgl/style/conversion/source.hpp b/include/mbgl/style/conversion/source.hpp
index c4b2fe303f..6e1b4347c3 100644
--- a/include/mbgl/style/conversion/source.hpp
+++ b/include/mbgl/style/conversion/source.hpp
@@ -2,6 +2,7 @@
 
 #include <mbgl/style/conversion.hpp>
 #include <mbgl/style/conversion/geojson.hpp>
+#include <mbgl/style/conversion/geojson_options.hpp>
 #include <mbgl/style/conversion/tileset.hpp>
 #include <mbgl/style/source.hpp>
 #include <mbgl/style/sources/geojson_source.hpp>
@@ -103,63 +104,12 @@ private:
             return Error{ "GeoJSON source must have a data value" };
         }
 
-        GeoJSONOptions options;
-
-        const auto maxzoomValue = objectMember(value, "maxzoom");
-        if (maxzoomValue) {
-            if (toNumber(*maxzoomValue)) {
-                options.maxzoom = static_cast<uint8_t>(*toNumber(*maxzoomValue));
-            } else {
-                return Error{ "GeoJSON source maxzoom value must be a number" };
-            }
-        }
-
-        const auto bufferValue = objectMember(value, "buffer");
-        if (bufferValue) {
-            if (toNumber(*bufferValue)) {
-                options.buffer = static_cast<uint16_t>(*toNumber(*bufferValue));
-            } else {
-                return Error{ "GeoJSON source buffer value must be a number" };
-            }
-        }
-
-        const auto toleranceValue = objectMember(value, "tolerance");
-        if (toleranceValue) {
-            if (toNumber(*toleranceValue)) {
-                options.tolerance = static_cast<double>(*toNumber(*toleranceValue));
-            } else {
-                return Error{ "GeoJSON source tolerance value must be a number" };
-            }
-        }
-
-        const auto clusterValue = objectMember(value, "cluster");
-        if (clusterValue) {
-            if (toBool(*clusterValue)) {
-                options.cluster = *toBool(*clusterValue);
-            } else {
-                return Error{ "GeoJSON source cluster value must be a boolean" };
-            }
-        }
-
-        const auto clusterMaxZoomValue = objectMember(value, "clusterMaxZoom");
-        if (clusterMaxZoomValue) {
-            if (toNumber(*clusterMaxZoomValue)) {
-                options.clusterMaxZoom = static_cast<uint8_t>(*toNumber(*clusterMaxZoomValue));
-            } else {
-                return Error{ "GeoJSON source clusterMaxZoom value must be a number" };
-            }
-        }
-
-        const auto clusterRadiusValue = objectMember(value, "clusterRadius");
-        if (clusterRadiusValue) {
-            if (toNumber(*clusterRadiusValue)) {
-                options.clusterRadius = static_cast<double>(*toNumber(*clusterRadiusValue));
-            } else {
-                return Error{ "GeoJSON source clusterRadius value must be a number" };
-            }
+        Result<GeoJSONOptions> options = convert<GeoJSONOptions>(value);
+        if (!options) {
+            return options.error();
         }
 
-        auto result = std::make_unique<GeoJSONSource>(id, options);
+        auto result = std::make_unique<GeoJSONSource>(id, *options);
 
         if (isObject(*dataValue)) {
             Result<GeoJSON> geoJSON = convertGeoJSON(*dataValue);