[core] Round-trip linear interpolators

Cherry-picked from c231191dbc6f055341fd25ce97309614dc52802f.

3 files changed, 8 insertions, 2 deletions

diff --git a/mapbox-gl-js b/mapbox-gl-js
-Subproject 3c07b72a59d78bc6240a8e3d77a54bec7619af5
+Subproject 34902f8db3f8a3757a465d62a96b8fa731ee146
diff --git a/src/mbgl/style/expression/interpolate.cpp b/src/mbgl/style/expression/interpolate.cpp
index 30b2cba81b..daad8523f2 100644
--- a/src/mbgl/style/expression/interpolate.cpp
+++ b/src/mbgl/style/expression/interpolate.cpp
@@ -223,7 +223,11 @@ mbgl::Value Interpolate<T>::serialize() const {
     
     interpolator.match(
         [&](const ExponentialInterpolator& exponential) {
-            serialized.emplace_back(std::vector<mbgl::Value>{{ std::string("exponential"), exponential.base }});
+            if (exponential.base == 1) {
+                serialized.emplace_back(std::vector<mbgl::Value>{{ std::string("linear") }});
+            } else {
+                serialized.emplace_back(std::vector<mbgl::Value>{{ std::string("exponential"), exponential.base }});
+            }
         },
         [&](const CubicBezierInterpolator& cubicBezier) {
             static const std::string cubicBezierTag("cubic-bezier");
diff --git a/test/style/conversion/stringify.test.cpp b/test/style/conversion/stringify.test.cpp
index cb3b62dc62..136f276aaf 100644
--- a/test/style/conversion/stringify.test.cpp
+++ b/test/style/conversion/stringify.test.cpp
@@ -80,6 +80,8 @@ TEST(Stringify, Filter) {
 }
 
 TEST(Stringify, CameraFunction) {
+    ASSERT_EQ(stringify(CameraFunction<float>(ExponentialStops<float> { {{0, 1}}, 1 })),
+        "[\"interpolate\",[\"linear\"],[\"zoom\"],0.0,1.0]");
     ASSERT_EQ(stringify(CameraFunction<float>(ExponentialStops<float> { {{0, 1}}, 2 })),
         "[\"interpolate\",[\"exponential\",2.0],[\"zoom\"],0.0,1.0]");
     ASSERT_EQ(stringify(CameraFunction<float>(IntervalStops<float> { {{0, 1}} })),
@@ -109,7 +111,7 @@ TEST(Stringify, CompositeFunction) {
             2
         }, 0.0f)),
         "[\"interpolate\","
-            "[\"exponential\",1.0],"
+            "[\"linear\"],"
             "[\"zoom\"],"
             "0.0,[\"interpolate\",[\"exponential\",2.0],[\"number\",[\"get\",\"property\"]],0.0,1.0],"
             "1.0,[\"interpolate\",[\"exponential\",2.0],[\"number\",[\"get\",\"property\"]],0.0,1.0]]");