consolidate functions to only use stops

refs mapbox/mapbox-gl-style-spec#96

2 files changed, 58 insertions, 136 deletions

diff --git a/src/style/function_properties.cpp b/src/style/function_properties.cpp
index 2b378fb6b9..879de84f85 100644
--- a/src/style/function_properties.cpp
+++ b/src/style/function_properties.cpp
@@ -6,82 +6,36 @@
 namespace mbgl {
 
 
-template <>
-bool LinearFunction<bool>::evaluate(float z) const {
-    return z < z_base ? slope >= 0 : z > z_base ? slope >= 0 : value;
-}
-
-template <>
-float LinearFunction<float>::evaluate(float z) const {
-    return std::fmin(std::fmax(min, value + (z - z_base) * slope), max);
-}
-
-template <>
-Color LinearFunction<Color>::evaluate(float z) const {
-    return {{
-        std::fmin(std::fmax(min[0], value[0] + (z - z_base) * slope), max[0]),
-        std::fmin(std::fmax(min[1], value[1] + (z - z_base) * slope), max[1]),
-        std::fmin(std::fmax(min[2], value[2] + (z - z_base) * slope), max[2]),
-        std::fmin(std::fmax(min[3], value[3] + (z - z_base) * slope), max[3])
-    }};
-}
-
-
-template <>
-bool ExponentialFunction<bool>::evaluate(float z) const {
-    return z < z_base ? slope >= 0 : z > z_base ? slope >= 0 : value;
-}
-
-template <>
-float ExponentialFunction<float>::evaluate(float z) const {
-    return std::fmin(std::fmax(min, value + std::pow(exp_base, (z - z_base)) * slope), max);
-}
-
-template <>
-Color ExponentialFunction<Color>::evaluate(float z) const {
-    return {{
-        std::fmin(std::fmax(min[0], value[0] + float(std::pow(exp_base, (z - z_base))) * slope), max[0]),
-        std::fmin(std::fmax(min[1], value[1] + float(std::pow(exp_base, (z - z_base))) * slope), max[1]),
-        std::fmin(std::fmax(min[2], value[2] + float(std::pow(exp_base, (z - z_base))) * slope), max[2]),
-        std::fmin(std::fmax(min[3], value[3] + float(std::pow(exp_base, (z - z_base))) * slope), max[3])
-    }};
-}
-
 template <typename T>
-inline T exponentialInterpolate(T smaller, T larger, const float factor);
+inline T interpolate(T smaller, T larger, const float factor);
 
 template <>
-inline float exponentialInterpolate(const float smaller, const float larger, const float factor) {
-    // Linear interpolation if base is 0
-    if (smaller == 0.0f) {
-        return factor * larger;
-    }
-    // Exponential interpolation between the values
-    return smaller * std::pow(larger / smaller, factor);
+inline float interpolate(const float smaller, const float larger, const float factor) {
+    return (smaller * (1 - factor)) + (larger * factor);
 }
 
 template <>
-inline bool exponentialInterpolate(const bool smaller, const bool larger, const float factor) {
-    return exponentialInterpolate(float(smaller), float(larger), factor);
+inline bool interpolate(const bool smaller, const bool larger, const float factor) {
+    return interpolate(float(smaller), float(larger), factor);
 }
 
 template <>
-inline Color exponentialInterpolate(const Color smaller, const Color larger, const float factor) {
+inline Color interpolate(const Color smaller, const Color larger, const float factor) {
     return {{
-        exponentialInterpolate(smaller[0], larger[0], factor),
-        exponentialInterpolate(smaller[1], larger[1], factor),
-        exponentialInterpolate(smaller[2], larger[2], factor),
-        exponentialInterpolate(smaller[3], larger[3], factor)
+        interpolate(smaller[0], larger[0], factor),
+        interpolate(smaller[1], larger[1], factor),
+        interpolate(smaller[2], larger[2], factor),
+        interpolate(smaller[3], larger[3], factor)
     }};
 }
 
 
 template <typename T>
-T exponentialDefault();
+inline T defaultStopsValue();
 
-template <> bool exponentialDefault() { return true; }
-template <> float exponentialDefault() { return 1.0f; }
-template <> Color exponentialDefault() { return {{ 0, 0, 0, 1 }}; }
+template <> inline bool defaultStopsValue() { return true; }
+template <> inline float defaultStopsValue() { return 1.0f; }
+template <> inline Color defaultStopsValue() { return {{ 0, 0, 0, 1 }}; }
 
 
 template <typename T>
@@ -112,15 +66,22 @@ T StopsFunction<T>::evaluate(float z) const {
         if (larger_z == smaller_z || larger_val == smaller_val) {
             return smaller_val;
         }
-        float factor = (z - smaller_z) / (larger_z - smaller_z);
-        return exponentialInterpolate<T>(smaller_val, larger_val, factor);
+        const float zoomDiff = larger_z - smaller_z;
+        const float zoomProgress = z - smaller_z;
+        if (base == 1.0f) {
+            const float t = zoomProgress / zoomDiff;
+            return interpolate<T>(smaller_val, larger_val, t);
+        } else {
+            const float t = (std::pow(base, zoomProgress) - 1) / (std::pow(base, zoomDiff) - 1);
+            return interpolate<T>(smaller_val, larger_val, t);
+        }
     } else if (larger) {
         return larger_val;
     } else if (smaller) {
         return smaller_val;
     } else {
         // No stop defined.
-        return exponentialDefault<T>();
+        return defaultStopsValue<T>();
     }
 }
 
diff --git a/src/style/style_parser.cpp b/src/style/style_parser.cpp
index faa8caeb98..3c79ee3c92 100644
--- a/src/style/style_parser.cpp
+++ b/src/style/style_parser.cpp
@@ -230,95 +230,56 @@ Color StyleParser::parseFunctionArgument(JSVal value) {
     return parseColor(rvalue);
 }
 
+template <typename T> inline float defaultBaseValue() { return 1.75; }
+template <> inline float defaultBaseValue<Color>() { return 1.0; }
+
 template <typename T>
 std::tuple<bool, Function<T>> StyleParser::parseFunction(JSVal value) {
-    if (!value.HasMember("fn")) {
-        fprintf(stderr, "[WARNING] function must specify a function name\n");
-        return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-    }
-
-    JSVal value_fn = value["fn"];
-    if (!value_fn.IsString()) {
-        fprintf(stderr, "[WARNING] function must specify a function type\n");
+    if (!value.HasMember("stops")) {
+        fprintf(stderr, "[WARNING] stops function must specify a stops array\n");
         return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
     }
 
-    const std::string type { value_fn.GetString(), value_fn.GetStringLength() };
+    float base = defaultBaseValue<T>();
 
-    if (type == "linear") {
-        if (!value.HasMember("z")) {
-            fprintf(stderr, "[WARNING] linear function must specify a base zoom level\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
-        if (!value.HasMember("val")) {
-            fprintf(stderr, "[WARNING] linear function must specify a base value\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+    if (value.HasMember("base")) {
+        JSVal value_base = value["base"];
+        if (value_base.IsNumber()) {
+            base = value_base.GetDouble();
+        } else {
+            fprintf(stderr, "[WARNING] base must be numeric\n");
         }
-        const float z_base = parseFunctionArgument<float>(value["z"]);
-        const T val = parseFunctionArgument<T>(value["val"]);
-        const float slope = value.HasMember("slope") ? parseFunctionArgument<float>(value["slope"]) : 1;
-        const T min = value.HasMember("min") ? parseFunctionArgument<T>(value["min"]) : T();
-        const T max = value.HasMember("max") ? parseFunctionArgument<T>(value["max"]) : T();
-        return std::tuple<bool, Function<T>> { true, LinearFunction<T>(val, z_base, slope, min, max) };
     }
-    else if (type == "exponential") {
-        if (!value.HasMember("z")) {
-            fprintf(stderr, "[WARNING] exponential function must specify a base zoom level\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
-        if (!value.HasMember("val")) {
-            fprintf(stderr, "[WARNING] exponential function must specify a base value\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
-        const float z_base = parseFunctionArgument<float>(value["z"]);
-        const float exp_base =  value.HasMember("base") ? parseFunctionArgument<float>(value["base"]) : 1.75;
-        const T val = parseFunctionArgument<T>(value["val"]);
-        const float slope = value.HasMember("slope") ? parseFunctionArgument<float>(value["slope"]) : 1;
-        const T min = value.HasMember("min") ? parseFunctionArgument<T>(value["min"]) : T();
-        const T max = value.HasMember("max") ? parseFunctionArgument<T>(value["max"]) : T();
-        return std::tuple<bool, Function<T>> { true, ExponentialFunction<T>(val, z_base, exp_base, slope, min, max) };
-    }
-    else if (type == "stops") {
-        if (!value.HasMember("stops")) {
-            fprintf(stderr, "[WARNING] stops function must specify a stops array\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
 
-        JSVal value_stops = value["stops"];
-        if (!value_stops.IsArray()) {
-            fprintf(stderr, "[WARNING] stops function must specify a stops array\n");
-            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-        }
-
-        std::vector<std::pair<float, T>> stops;
-        for (rapidjson::SizeType i = 0; i < value_stops.Size(); ++i) {
-            JSVal stop = value_stops[i];
-            if (stop.IsArray()) {
-                if (stop.Size() != 2) {
-                    fprintf(stderr, "[WARNING] stop must have zoom level and value specification\n");
-                    return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-                }
+    JSVal value_stops = value["stops"];
+    if (!value_stops.IsArray()) {
+        fprintf(stderr, "[WARNING] stops function must specify a stops array\n");
+        return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+    }
 
-                JSVal z = stop[rapidjson::SizeType(0)];
-                if (!z.IsNumber()) {
-                    fprintf(stderr, "[WARNING] zoom level in stop must be a number\n");
-                    return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
-                }
+    std::vector<std::pair<float, T>> stops;
+    for (rapidjson::SizeType i = 0; i < value_stops.Size(); ++i) {
+        JSVal stop = value_stops[i];
+        if (stop.IsArray()) {
+            if (stop.Size() != 2) {
+                fprintf(stderr, "[WARNING] stop must have zoom level and value specification\n");
+                return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+            }
 
-                stops.emplace_back(z.GetDouble(), parseFunctionArgument<T>(stop[rapidjson::SizeType(1)]));
-            } else {
-                fprintf(stderr, "[WARNING] function argument must be a numeric value\n");
+            JSVal z = stop[rapidjson::SizeType(0)];
+            if (!z.IsNumber()) {
+                fprintf(stderr, "[WARNING] zoom level in stop must be a number\n");
                 return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
             }
-        }
 
-        return std::tuple<bool, Function<T>> { true, StopsFunction<T>(stops) };
-    }
-    else {
-        fprintf(stderr, "[WARNING] function type '%s' is unknown\n", type.c_str());
+            stops.emplace_back(z.GetDouble(), parseFunctionArgument<T>(stop[rapidjson::SizeType(1)]));
+        } else {
+            fprintf(stderr, "[WARNING] function argument must be a numeric value\n");
+            return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+        }
     }
 
-    return std::tuple<bool, Function<T>> { false, ConstantFunction<T>(T()) };
+    return std::tuple<bool, Function<T>> { true, StopsFunction<T>(stops, base) };
 }