#include <mbgl/style/conversion/function.hpp>
#include <mbgl/style/conversion/position.hpp>
#include <mbgl/style/conversion_impl.hpp>
#include <mbgl/style/expression/dsl.hpp>
#include <mbgl/style/expression/dsl_impl.hpp>
#include <mbgl/style/expression/step.hpp>
#include <mbgl/style/expression/interpolate.hpp>
#include <mbgl/style/expression/match.hpp>
#include <mbgl/style/expression/case.hpp>
#include <mbgl/style/expression/format_expression.hpp>
#include <mbgl/util/string.hpp>

#include <cassert>

namespace mbgl {
namespace style {
namespace conversion {

using namespace expression;
using namespace expression::dsl;

const static std::string tokenReservedChars = "{}";

bool hasTokens(const std::string& source) {
    auto pos = source.begin();
    const auto end = source.end();

    while (pos != end) {
        auto brace = std::find(pos, end, '{');
        if (brace == end)
            return false;
        for (brace++; brace != end && tokenReservedChars.find(*brace) == std::string::npos; brace++);
        if (brace != end && *brace == '}') {
            return true;
        }
        pos = brace;
    }

    return false;
}
    
std::unique_ptr<Expression> convertTokenStringToFormatExpression(const std::string& source) {
    auto textExpression = convertTokenStringToExpression(source);
    std::vector<FormatExpressionSection> sections;
    sections.emplace_back(std::move(textExpression), nullopt, nullopt, nullopt);
    return std::make_unique<FormatExpression>(sections);
}

std::unique_ptr<Expression> convertTokenStringToExpression(const std::string& source) {
    std::vector<std::unique_ptr<Expression>> inputs;

    auto pos = source.begin();
    const auto end = source.end();

    while (pos != end) {
        auto brace = std::find(pos, end, '{');
        if (pos != brace) {
            inputs.push_back(literal(std::string(pos, brace)));
        }
        pos = brace;
        if (pos != end) {
            for (brace++; brace != end && tokenReservedChars.find(*brace) == std::string::npos; brace++);
            if (brace != end && *brace == '}') {
                inputs.push_back(get(literal(std::string(pos + 1, brace))));
                pos = brace + 1;
            } else {
                inputs.push_back(literal(std::string(pos, brace)));
                pos = brace;
            }
        }
    }

    switch (inputs.size()) {
    case 0:
        return literal(source);
    case 1:
        return toString(std::move(inputs[0]));
    default:
        return concat(std::move(inputs));
    }
}

template <class T>
optional<PropertyExpression<T>> convertFunctionToExpression(const Convertible& value, Error& error, bool convertTokens) {
    auto expression = convertFunctionToExpression(expression::valueTypeToExpressionType<T>(), value, error, convertTokens);
    if (!expression) {
        return nullopt;
    }

    optional<T> defaultValue{};

    auto defaultValueValue = objectMember(value, "default");
    if (defaultValueValue) {
        defaultValue = convert<T>(*defaultValueValue, error);
        if (!defaultValue) {
            error.message = R"(wrong type for "default": )" + error.message;
            return nullopt;
        }
    }

    return PropertyExpression<T>(std::move(*expression), defaultValue);
}

template optional<PropertyExpression<AlignmentType>>
    convertFunctionToExpression<AlignmentType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<bool>>
    convertFunctionToExpression<bool>(const Convertible&, Error&, bool);
template optional<PropertyExpression<CirclePitchScaleType>>
    convertFunctionToExpression<CirclePitchScaleType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<float>>
    convertFunctionToExpression<float>(const Convertible&, Error&, bool);
template optional<PropertyExpression<HillshadeIlluminationAnchorType>>
    convertFunctionToExpression<HillshadeIlluminationAnchorType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<IconTextFitType>>
    convertFunctionToExpression<IconTextFitType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<LightAnchorType>>
    convertFunctionToExpression<LightAnchorType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<LineCapType>>
    convertFunctionToExpression<LineCapType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<LineJoinType>>
    convertFunctionToExpression<LineJoinType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<Color>>
    convertFunctionToExpression<Color>(const Convertible&, Error&, bool);
template optional<PropertyExpression<Position>>
    convertFunctionToExpression<Position>(const Convertible&, Error&, bool);
template optional<PropertyExpression<RasterResamplingType>>
    convertFunctionToExpression<RasterResamplingType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<std::array<float, 2>>>
    convertFunctionToExpression<std::array<float, 2>>(const Convertible&, Error&, bool);
template optional<PropertyExpression<std::array<float, 4>>>
    convertFunctionToExpression<std::array<float, 4>>(const Convertible&, Error&, bool);
template optional<PropertyExpression<std::string>>
    convertFunctionToExpression<std::string>(const Convertible&, Error&, bool);
template optional<PropertyExpression<std::vector<float>>>
    convertFunctionToExpression<std::vector<float>>(const Convertible&, Error&, bool);
template optional<PropertyExpression<std::vector<std::string>>>
    convertFunctionToExpression<std::vector<std::string>>(const Convertible&, Error&, bool);
template optional<PropertyExpression<SymbolAnchorType>>
    convertFunctionToExpression<SymbolAnchorType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<std::vector<TextVariableAnchorType>>>
    convertFunctionToExpression<std::vector<TextVariableAnchorType>>(const Convertible&, Error&, bool);
template optional<PropertyExpression<SymbolPlacementType>>
    convertFunctionToExpression<SymbolPlacementType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<SymbolZOrderType>>
    convertFunctionToExpression<SymbolZOrderType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<TextJustifyType>>
    convertFunctionToExpression<TextJustifyType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<TextTransformType>>
    convertFunctionToExpression<TextTransformType>(const Convertible&, Error&, bool);
template optional<PropertyExpression<TranslateAnchorType>>
    convertFunctionToExpression<TranslateAnchorType>(const Convertible&, Error&, bool);
    
template optional<PropertyExpression<Formatted>>
    convertFunctionToExpression<Formatted>(const Convertible&, Error&, bool);

template optional<PropertyExpression<RadialOffsetType>>
    convertFunctionToExpression<RadialOffsetType>(const Convertible&, Error&, bool);


// Ad-hoc Converters for double and int64_t. We should replace float with double wholesale,
// and promote the int64_t Converter to general use (and it should check that the input is
// an integer).
template <>
struct Converter<double> {
    optional<double> operator()(const Convertible& value, Error& error) const {
        auto converted = convert<float>(value, error);
        if (!converted) {
            return nullopt;
        }
        return *converted;
    }
};

template <>
struct Converter<int64_t> {
    optional<int64_t> operator()(const Convertible& value, Error& error) const {
        auto converted = convert<float>(value, error);
        if (!converted) {
            return nullopt;
        }
        return *converted;
    }
};

enum class FunctionType {
    Interval,
    Exponential,
    Categorical,
    Identity,
    Invalid
};

static bool interpolatable(type::Type type) {
    return type.match(
        [&] (const type::NumberType&) {
            return true;
        },
        [&] (const type::ColorType&) {
            return true;
        },
        [&] (const type::Array& array) {
            return array.N && array.itemType == type::Number;
        },
        [&] (const auto&) {
            return false;
        }
    );
}

static optional<std::unique_ptr<Expression>> convertLiteral(type::Type type, const Convertible& value, Error& error, bool convertTokens = false) {
    return type.match(
        [&] (const type::NumberType&) -> optional<std::unique_ptr<Expression>> {
            auto result = convert<float>(value, error);
            if (!result) {
                return nullopt;
            }
            return literal(double(*result));
        },
        [&] (const type::BooleanType&) -> optional<std::unique_ptr<Expression>> {
            auto result = convert<bool>(value, error);
            if (!result) {
                return nullopt;
            }
            return literal(*result);
        },
        [&] (const type::StringType&) -> optional<std::unique_ptr<Expression>> {
            auto result = convert<std::string>(value, error);
            if (!result) {
                return nullopt;
            }
            return convertTokens ? convertTokenStringToExpression(*result) : literal(*result);
        },
        [&] (const type::ColorType&) -> optional<std::unique_ptr<Expression>> {
            auto result = convert<Color>(value, error);
            if (!result) {
                return nullopt;
            }
            return literal(*result);
        },
        [&] (const type::Array& array) -> optional<std::unique_ptr<Expression>> {
            if (!isArray(value)) {
                error.message = "value must be an array";
                return nullopt;
            }
            if (array.N && arrayLength(value) != *array.N) {
                error.message = "value must be an array of length " + util::toString(*array.N);
                return nullopt;
            }
            return array.itemType.match(
                [&] (const type::NumberType&) -> optional<std::unique_ptr<Expression>> {
                    std::vector<expression::Value> result;
                    result.reserve(arrayLength(value));
                    for (std::size_t i = 0; i < arrayLength(value); ++i) {
                        optional<float> number = toNumber(arrayMember(value, i));
                        if (!number) {
                            error.message = "value must be an array of numbers";
                            return nullopt;
                        }
                        result.emplace_back(double(*number));
                    }
                    return literal(result);
                },
                [&] (const type::StringType&) -> optional<std::unique_ptr<Expression>> {
                    std::vector<expression::Value> result;
                    result.reserve(arrayLength(value));
                    for (std::size_t i = 0; i < arrayLength(value); ++i) {
                        optional<std::string> string = toString(arrayMember(value, i));
                        if (!string) {
                            error.message = "value must be an array of strings";
                            return nullopt;
                        }
                        result.emplace_back(*string);
                    }
                    return literal(result);
                },
                [&] (const auto&) -> optional<std::unique_ptr<Expression>> {
                    assert(false); // No properties use this type.
                    return nullopt;
                }
            );
        },
        [&] (const type::NullType&) -> optional<std::unique_ptr<Expression>> {
            assert(false); // No properties use this type.
            return nullopt;
        },
        [&] (const type::ObjectType&) -> optional<std::unique_ptr<Expression>> {
            assert(false); // No properties use this type.
            return nullopt;
        },
        [&] (const type::ErrorType&) -> optional<std::unique_ptr<Expression>> {
            assert(false); // No properties use this type.
            return nullopt;
        },
        [&] (const type::ValueType&) -> optional<std::unique_ptr<Expression>> {
            assert(false); // No properties use this type.
            return nullopt;
        },
        [&] (const type::CollatorType&) -> optional<std::unique_ptr<Expression>> {
            assert(false); // No properties use this type.
            return nullopt;
        },
        [&] (const type::FormattedType&) -> optional<std::unique_ptr<Expression>> {
            auto result = convert<std::string>(value, error);
            if (!result) {
                return nullopt;
            }
            return convertTokens ?
                convertTokenStringToFormatExpression(*result) :
                literal(Formatted(result->c_str()));
        }
    );
}

static optional<std::map<double, std::unique_ptr<Expression>>> convertStops(type::Type type,
                                                                            const Convertible& value,
                                                                            Error& error,
                                                                            bool convertTokens) {
    auto stopsValue = objectMember(value, "stops");
    if (!stopsValue) {
        error.message = "function value must specify stops";
        return nullopt;
    }

    if (!isArray(*stopsValue)) {
        error.message = "function stops must be an array";
        return nullopt;
    }

    if (arrayLength(*stopsValue) == 0) {
        error.message = "function must have at least one stop";
        return nullopt;
    }

    std::map<double, std::unique_ptr<Expression>> stops;
    for (std::size_t i = 0; i < arrayLength(*stopsValue); ++i) {
        const auto& stopValue = arrayMember(*stopsValue, i);

        if (!isArray(stopValue)) {
            error.message = "function stop must be an array";
            return nullopt;
        }

        if (arrayLength(stopValue) != 2) {
            error.message = "function stop must have two elements";
            return nullopt;
        }

        optional<float> t = convert<float>(arrayMember(stopValue, 0), error);
        if (!t) {
            return nullopt;
        }

        optional<std::unique_ptr<Expression>> e = convertLiteral(type, arrayMember(stopValue, 1), error, convertTokens);
        if (!e) {
            return nullopt;
        }

        stops.emplace(*t, std::move(*e));
    }

    return { std::move(stops) };
}

static void omitFirstStop(std::map<double, std::unique_ptr<Expression>>& stops) {
    double min = std::numeric_limits<double>::max();
    for (auto& s : stops) {
        if (s.first < min) {
            min = s.first;
        }
    }
    stops.emplace(-std::numeric_limits<double>::infinity(), std::move(stops[min]));
    stops.erase(min);
}

template <class T>
optional<std::map<T, std::unique_ptr<Expression>>> convertBranches(type::Type type,
                                                                   const Convertible& value,
                                                                   Error& error) {
    auto stopsValue = objectMember(value, "stops");
    if (!stopsValue) {
        error.message = "function value must specify stops";
        return nullopt;
    }

    if (!isArray(*stopsValue)) {
        error.message = "function stops must be an array";
        return nullopt;
    }

    if (arrayLength(*stopsValue) == 0) {
        error.message = "function must have at least one stop";
        return nullopt;
    }

    std::map<T, std::unique_ptr<Expression>> stops;
    for (std::size_t i = 0; i < arrayLength(*stopsValue); ++i) {
        const auto& stopValue = arrayMember(*stopsValue, i);

        if (!isArray(stopValue)) {
            error.message = "function stop must be an array";
            return nullopt;
        }

        if (arrayLength(stopValue) != 2) {
            error.message = "function stop must have two elements";
            return nullopt;
        }

        optional<T> t = convert<T>(arrayMember(stopValue, 0), error);
        if (!t) {
            return nullopt;
        }

        optional<std::unique_ptr<Expression>> e = convertLiteral(type, arrayMember(stopValue, 1), error);
        if (!e) {
            return nullopt;
        }

        stops.emplace(*t, std::move(*e));
    }

    return { std::move(stops) };
}

static optional<double> convertBase(const Convertible& value, Error& error) {
    auto baseValue = objectMember(value, "base");

    if (!baseValue) {
        return 1.0;
    }

    auto base = toNumber(*baseValue);
    if (!base) {
        error.message = "function base must be a number";
        return nullopt;
    }

    return *base;
}

static std::unique_ptr<Expression> step(type::Type type, std::unique_ptr<Expression> input, std::map<double, std::unique_ptr<Expression>> stops) {
    return std::make_unique<Step>(type, std::move(input), std::move(stops));
}

static std::unique_ptr<Expression> interpolate(type::Type type, Interpolator interpolator, std::unique_ptr<Expression> input, std::map<double, std::unique_ptr<Expression>> stops) {
    ParsingContext ctx;
    auto result = createInterpolate(type, std::move(interpolator), std::move(input), std::move(stops), ctx);
    if (!result) {
        assert(false);
        return {};
    }
    return std::move(*result);
}

template <class T>
std::unique_ptr<Expression> categorical(type::Type type,
                                        const std::string& property,
                                        std::map<T, std::unique_ptr<Expression>> branches,
                                        std::unique_ptr<Expression> def) {
    std::unordered_map<T, std::shared_ptr<Expression>> convertedBranches;
    for (auto& b : branches) {
        convertedBranches[b.first] = std::move(b.second);
    }
    return std::make_unique<Match<T>>(type,
                                      get(literal(property)),
                                      std::move(convertedBranches),
                                      def ? std::move(def) : error("replaced with default"));
}

template <>
std::unique_ptr<Expression> categorical<bool>(type::Type type,
                                              const std::string& property,
                                              std::map<bool, std::unique_ptr<Expression>> branches,
                                              std::unique_ptr<Expression> def) {
    auto it = branches.find(true);
    std::unique_ptr<Expression> trueCase = it == branches.end() ?
        error("replaced with default") :
        std::move(it->second);

    it = branches.find(false);
    std::unique_ptr<Expression> falseCase = it == branches.end() ?
        error("replaced with default") :
        std::move(it->second);

    std::vector<typename Case::Branch> convertedBranches;
    convertedBranches.emplace_back(eq(get(literal(property)), literal(Value(true))), std::move(trueCase));
    convertedBranches.emplace_back(eq(get(literal(property)), literal(Value(false))), std::move(falseCase));

    return std::make_unique<Case>(type, std::move(convertedBranches),
                                  def ? std::move(def) : error("replaced with default"));
}

static std::unique_ptr<Expression> numberOrDefault(type::Type type,
                                                   std::unique_ptr<Expression> get,
                                                   std::unique_ptr<Expression> expr,
                                                   std::unique_ptr<Expression> def) {
    if (!def) {
        return expr;
    }

    std::vector<Case::Branch> branches;
    branches.emplace_back(eq(compound("typeof", std::move(get)), literal("number")),
                          std::move(expr));
    return std::make_unique<Case>(type, std::move(branches), std::move(def));
}

static optional<std::unique_ptr<Expression>> convertIntervalFunction(type::Type type,
                                                                     const Convertible& value,
                                                                     Error& error,
                                                                     std::function<std::unique_ptr<Expression> (bool)> makeInput,
                                                                     std::unique_ptr<Expression> def,
                                                                     bool convertTokens = false) {
    auto stops = convertStops(type, value, error, convertTokens);
    if (!stops) {
        return nullopt;
    }
    omitFirstStop(*stops);

    return numberOrDefault(type,
                           makeInput(false),
                           step(type, makeInput(true), std::move(*stops)),
                           std::move(def));
}

static optional<std::unique_ptr<Expression>> convertExponentialFunction(type::Type type,
                                                                        const Convertible& value,
                                                                        Error& error,
                                                                        std::function<std::unique_ptr<Expression> (bool)> makeInput,
                                                                        std::unique_ptr<Expression> def,
                                                                        bool convertTokens = false) {
    auto stops = convertStops(type, value, error, convertTokens);
    if (!stops) {
        return nullopt;
    }
    auto base = convertBase(value, error);
    if (!base) {
        return nullopt;
    }

    return numberOrDefault(type,
                           makeInput(false),
                           interpolate(type, exponential(*base), makeInput(true), std::move(*stops)),
                           std::move(def));
}

static optional<std::unique_ptr<Expression>> convertCategoricalFunction(type::Type type,
                                                                        const Convertible& value,
                                                                        Error& err,
                                                                        const std::string& property,
                                                                        std::unique_ptr<Expression> def) {
    auto stopsValue = objectMember(value, "stops");
    if (!stopsValue) {
        err.message = "function value must specify stops";
        return nullopt;
    }

    if (!isArray(*stopsValue)) {
        err.message = "function stops must be an array";
        return nullopt;
    }

    if (arrayLength(*stopsValue) == 0) {
        err.message = "function must have at least one stop";
        return nullopt;
    }

    const auto& first = arrayMember(*stopsValue, 0);

    if (!isArray(first)) {
        err.message = "function stop must be an array";
        return nullopt;
    }

    if (arrayLength(first) != 2) {
        err.message = "function stop must have two elements";
        return nullopt;
    }

    if (toBool(arrayMember(first, 0))) {
        auto branches = convertBranches<bool>(type, value, err);
        if (!branches) {
            return nullopt;
        }
        return categorical(type, property, std::move(*branches), std::move(def));
    }

    if (toNumber(arrayMember(first, 0))) {
        auto branches = convertBranches<int64_t>(type, value, err);
        if (!branches) {
            return nullopt;
        }
        return categorical(type, property, std::move(*branches), std::move(def));
    }

    if (toString(arrayMember(first, 0))) {
        auto branches = convertBranches<std::string>(type, value, err);
        if (!branches) {
            return nullopt;
        }
        return categorical(type, property, std::move(*branches), std::move(def));
    }

    err.message = "stop domain value must be a number, string, or boolean";
    return nullopt;
}

template <class T, class Fn>
optional<std::unique_ptr<Expression>> composite(type::Type type,
                                                const Convertible& value,
                                                Error& error,
                                                const Fn& makeInnerExpression) {
    auto base = convertBase(value, error);
    if (!base) {
        return nullopt;
    }

    auto stopsValue = objectMember(value, "stops");

    // Checked by caller.
    assert(stopsValue);
    assert(isArray(*stopsValue));

    std::map<float, std::map<T, std::unique_ptr<Expression>>> map;

    for (std::size_t i = 0; i < arrayLength(*stopsValue); ++i) {
        const auto& stopValue = arrayMember(*stopsValue, i);

        if (!isArray(stopValue)) {
            error.message = "function stop must be an array";
            return nullopt;
        }

        if (arrayLength(stopValue) != 2) {
            error.message = "function stop must have two elements";
            return nullopt;
        }

        const auto& stopInput = arrayMember(stopValue, 0);

        if (!isObject(stopInput)) {
            error.message = "stop input must be an object";
            return nullopt;
        }

        auto zoomValue = objectMember(stopInput, "zoom");
        if (!zoomValue) {
            error.message = "stop input must specify zoom";
            return nullopt;
        }

        auto sourceValue = objectMember(stopInput, "value");
        if (!sourceValue) {
            error.message = "stop input must specify value";
            return nullopt;
        }

        optional<float> z = convert<float>(*zoomValue, error);
        if (!z) {
            return nullopt;
        }

        optional<T> d = convert<T>(*sourceValue, error);
        if (!d) {
            return nullopt;
        }

        optional<std::unique_ptr<Expression>> r = convertLiteral(type, arrayMember(stopValue, 1), error);
        if (!r) {
            return nullopt;
        }

        map[*z].emplace(*d, std::move(*r));
    }

    std::map<double, std::unique_ptr<Expression>> stops;

    for (auto& e : map) {
        stops.emplace(e.first, makeInnerExpression(type, *base, std::move(e.second)));
    }

    if (interpolatable(type)) {
        return interpolate(type, linear(), zoom(), std::move(stops));
    } else {
        return step(type, zoom(), std::move(stops));
    }
}

optional<std::unique_ptr<Expression>> convertFunctionToExpression(type::Type type,
                                                                  const Convertible& value,
                                                                  Error& err,
                                                                  bool convertTokens) {
    if (!isObject(value)) {
        err.message = "function must be an object";
        return nullopt;
    }

    FunctionType functionType = FunctionType::Invalid;

    auto typeValue = objectMember(value, "type");
    if (!typeValue) {
        functionType = interpolatable(type) ? FunctionType::Exponential : FunctionType::Interval;
    } else {
        optional<std::string> string = toString(*typeValue);
        if (string) {
            if (*string == "interval")
                functionType = FunctionType::Interval;
            if (*string == "exponential" && interpolatable(type))
                functionType = FunctionType::Exponential;
            if (*string == "categorical")
                functionType = FunctionType::Categorical;
            if (*string == "identity")
                functionType = FunctionType::Identity;
        }
    }

    auto defaultExpr = [&]() -> std::unique_ptr<Expression> {
        auto member = objectMember(value, "default");
        if (member) {
            auto literal = convertLiteral(type, *member, err);
            if (literal) {
                return std::move(*literal);
            }
        }
        return nullptr;
    };

    if (!objectMember(value, "property")) {
        // Camera function.
        switch (functionType) {
        case FunctionType::Interval:
            return convertIntervalFunction(type, value, err, [](bool) { return zoom(); }, defaultExpr(), convertTokens);
        case FunctionType::Exponential:
            return convertExponentialFunction(type, value, err, [](bool) { return zoom(); }, defaultExpr(), convertTokens);
        default:
            err.message = "unsupported function type";
            return nullopt;
        }
    }

    auto propertyValue = objectMember(value, "property");
    if (!propertyValue) {
        err.message = "function must specify property";
        return nullopt;
    }

    auto property = toString(*propertyValue);
    if (!property) {
        err.message = "function property must be a string";
        return nullopt;
    }

    if (functionType == FunctionType::Identity) {
        return type.match(
            [&] (const type::StringType&) -> optional<std::unique_ptr<Expression>> {
                return string(get(literal(*property)), defaultExpr());
            },
            [&] (const type::NumberType&) -> optional<std::unique_ptr<Expression>> {
                return number(get(literal(*property)), defaultExpr());
            },
            [&] (const type::BooleanType&) -> optional<std::unique_ptr<Expression>> {
                return boolean(get(literal(*property)), defaultExpr());
            },
            [&] (const type::ColorType&) -> optional<std::unique_ptr<Expression>> {
                return toColor(get(literal(*property)), defaultExpr());
            },
            [&] (const type::Array& array) -> optional<std::unique_ptr<Expression>> {
                return assertion(array, get(literal(*property)), defaultExpr());
            },
            [&] (const type::FormattedType&) -> optional<std::unique_ptr<Expression>> {
                return toFormatted(get(literal(*property)), defaultExpr());
            },
            [&] (const auto&) -> optional<std::unique_ptr<Expression>>  {
                assert(false); // No properties use this type.
                return nullopt;
            }
        );
    }

    auto stopsValue = objectMember(value, "stops");
    if (!stopsValue) {
        err.message = "function value must specify stops";
        return nullopt;
    }

    if (!isArray(*stopsValue)) {
        err.message = "function stops must be an array";
        return nullopt;
    }

    if (arrayLength(*stopsValue) == 0) {
        err.message = "function must have at least one stop";
        return nullopt;
    }

    const auto& first = arrayMember(*stopsValue, 0);

    if (!isArray(first)) {
        err.message = "function stop must be an array";
        return nullopt;
    }

    if (arrayLength(first) != 2) {
        err.message = "function stop must have two elements";
        return nullopt;
    }

    const auto& stop = arrayMember(first, 0);

    const auto getProperty = [&](bool coerce) {
        if (coerce) {
            return number(get(literal(*property)));
        } else {
            return get(literal(*property));
        }
    };

    if (!isObject(stop)) {
        // Source function.
        switch (functionType) {
        case FunctionType::Interval:
            return convertIntervalFunction(type, value, err, getProperty, defaultExpr());
        case FunctionType::Exponential:
            return convertExponentialFunction(type, value, err, getProperty, defaultExpr());
        case FunctionType::Categorical:
            return convertCategoricalFunction(type, value, err, *property, defaultExpr());
        default:
            err.message = "unsupported function type";
            return nullopt;
        }
    } else {
        // Composite function.
        auto sourceValue = objectMember(stop, "value");
        if (!sourceValue) {
            err.message = "stop must specify value";
            return nullopt;
        }

        if (toBool(*sourceValue)) {
            switch (functionType) {
            case FunctionType::Categorical:
                return composite<bool>(type, value, err, [&] (type::Type type_, double, std::map<bool, std::unique_ptr<Expression>> stops) {
                    return categorical<bool>(type_, *property, std::move(stops), defaultExpr());
                });
            default:
                err.message = "unsupported function type";
                return nullopt;
            }
        }

        if (toNumber(*sourceValue)) {
            switch (functionType) {
            case FunctionType::Interval:
                return composite<double>(type, value, err, [&] (type::Type type_, double, std::map<double, std::unique_ptr<Expression>> stops) {
                    omitFirstStop(stops);
                    return numberOrDefault(type,
                                           getProperty(false),
                                           step(type_, getProperty(true), std::move(stops)),
                                           defaultExpr());
                });
            case FunctionType::Exponential:
                return composite<double>(type, value, err, [&] (type::Type type_, double base, std::map<double, std::unique_ptr<Expression>> stops) {
                    return numberOrDefault(type,
                                           getProperty(false),
                                           interpolate(type_, exponential(base), getProperty(true), std::move(stops)),
                                           defaultExpr());
                });
            case FunctionType::Categorical:
                return composite<int64_t>(type, value, err, [&] (type::Type type_, double, std::map<int64_t, std::unique_ptr<Expression>> stops) {
                    return categorical<int64_t>(type_, *property, std::move(stops), defaultExpr());
                });
            default:
                err.message = "unsupported function type";
                return nullopt;
            }
        }

        if (toString(*sourceValue)) {
            switch (functionType) {
            case FunctionType::Categorical:
                return composite<std::string>(type, value, err, [&] (type::Type type_, double, std::map<std::string, std::unique_ptr<Expression>> stops) {
                    return categorical<std::string>(type_, *property, std::move(stops), defaultExpr());
                });
            default:
                err.message = "unsupported function type";
                return nullopt;
            }
        }

        err.message = "stop domain value must be a number, string, or boolean";
        return nullopt;
    }
}

} // namespace conversion
} // namespace style
} // namespace mbgl