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

3 files changed, 55 insertions, 1 deletions

diff --git a/cmake/test-files.cmake b/cmake/test-files.cmake
index 951514e38a..dab5b48745 100644
--- a/cmake/test-files.cmake
+++ b/cmake/test-files.cmake
@@ -87,6 +87,7 @@ set(MBGL_TEST_FILES
 
     # style/function
     test/style/function/camera_function.test.cpp
+    test/style/function/composite_function.test.cpp
     test/style/function/source_function.test.cpp
 
     # style
diff --git a/include/mbgl/style/function/composite_function.hpp b/include/mbgl/style/function/composite_function.hpp
index b31329b75e..2b4ae504ca 100644
--- a/include/mbgl/style/function/composite_function.hpp
+++ b/include/mbgl/style/function/composite_function.hpp
@@ -56,9 +56,13 @@ public:
                 assert(!s.stops.empty());
                 auto minIt = s.stops.lower_bound(zoom);
                 auto maxIt = s.stops.upper_bound(zoom);
-                if (minIt != s.stops.begin()) {
+                
+                // lower_bound yields first element >= zoom, but we want the *last*
+                // element <= zoom, so if we found a stop > zoom, back up by one.
+                if (minIt != s.stops.begin() && minIt->first > zoom) {
                     minIt--;
                 }
+                
                 return std::make_tuple(
                     Range<float> {
                         minIt == s.stops.end() ? s.stops.rbegin()->first : minIt->first,
@@ -96,6 +100,9 @@ public:
     T evaluate(float zoom, const GeometryTileFeature& feature, T finalDefaultValue) const {
         std::tuple<Range<float>, Range<InnerStops>> ranges = coveringRanges(zoom);
         Range<T> resultRange = evaluate(std::get<1>(ranges), feature, finalDefaultValue);
+        // If the covering stop range is constant, just return the output value directly.
+        if (resultRange.min == resultRange.max) return resultRange.min;
+        // Otherwise, interpolate between the two stops.
         return util::interpolate(
             resultRange.min,
             resultRange.max,
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";
+}