Fix edge case in composite function interpolation (#8613)

This fixes a bug where, for a zoom value greater than that of the highest zoom
stop, composite function interpolation would return nan. (Blocking a render
test over in #8593)

* Add failing tests for composite function edge case

The failing cases here are:
 - Should interpolate before the first stop
 - Should interpolate past the last stop

* Fix edge case in composite function interpolation

* Hold functions constant outside stop-defined domain

1 files changed, 46 insertions, 0 deletions

diff --git a/test/style/function/composite_function.test.cpp b/test/style/function/composite_function.test.cpp
new file mode 100644
index 0000000000..e0804d4b27
--- /dev/null
+++ b/test/style/function/composite_function.test.cpp
@@ -0,0 +1,46 @@
+#include <mbgl/test/util.hpp>
+#include <mbgl/test/stub_geometry_tile_feature.hpp>
+
+#include <mbgl/style/function/composite_function.hpp>
+
+using namespace mbgl;
+using namespace mbgl::style;
+
+using namespace std::string_literals;
+
+static StubGeometryTileFeature oneInteger {
+    PropertyMap {{ "property", uint64_t(1) }}
+};
+
+TEST(CompositeFunction, ZoomInterpolation) {
+    EXPECT_EQ(40.0f, CompositeFunction<float>("property", CompositeExponentialStops<float>({
+        {0.0f, {{uint64_t(1), 24.0f}}},
+        {1.5f, {{uint64_t(1), 36.0f}}},
+        {3.0f, {{uint64_t(1), 48.0f}}}
+    }), 0.0f)
+    .evaluate(2.0f, oneInteger, -1.0f)) << "Should interpolate between stops";
+    
+    EXPECT_EQ(33.0, CompositeFunction<float>("property", CompositeExponentialStops<float>({
+        {5.0f, {{uint64_t(1), 33.0f}}},
+        {10.0f, {{uint64_t(1), 66.0f}}}
+    }), 0.0f)
+    .evaluate(0.0f, oneInteger, -1.0f)) << "Use first stop output for input values from -inf to first stop";
+    
+    EXPECT_EQ(66.0, CompositeFunction<float>("property", CompositeExponentialStops<float>({
+        {0.0f, {{uint64_t(1), 33.0f}}},
+        {10.0f, {{uint64_t(1), 66.0f}}}
+    }), 0.0f)
+    .evaluate(20.0f, oneInteger, -1.0f)) << "Use last stop output for input values from last stop to +inf";
+
+    EXPECT_EQ(66.0f, CompositeFunction<float>("property", CompositeExponentialStops<float>({
+        {0.0f, {{uint64_t(1), 33.0f}}},
+        {10.0f, {{uint64_t(1), 66.0f}}}
+    }), 0.0f)
+    .evaluate(10.0f, oneInteger, -1.0f)) << "Should interpolate TO the last stop.";
+    
+    EXPECT_EQ(33.0f, CompositeFunction<float>("property", CompositeExponentialStops<float>({
+        {0.0f, {{uint64_t(1), 33.0f}}},
+        {10.0f, {{uint64_t(1), 66.0f}}}
+    }), 0.0f)
+    .evaluate(0.0f, oneInteger, -1.0f)) << "Should interpolate TO the first stop";
+}