/**
 * Module that defines an `Optional` type, which is a simplified version of
 * Phobos' Nullable, that also supports mapping the underlying data.
 */
module handy_http_primitives.optional;

/**
 * A simple wrapper around a value to make it optionally present.
 */
struct Optional(T) {
    /// The internal value of this optional.
    T value;

    /// Whether this optional is empty.
    bool isNull = true;

    /**
     * Constructs an optional value using a given value.
     * Params:
     *   value = The value to use.
     * Returns: An optional that contains the given value.
     */
    static Optional!T of(T value) {
        return Optional!T(value, false);
    }

    /**
     * Constructs an optional that's empty.
     * Returns: An optional that is empty.
     */
    static Optional!T empty() {
        return Optional!T(T.init, true);
    }

    /**
     * Gets the value of this optional if it exists, otherwise uses a given
     * default value.
     * Params:
     *   defaultValue = The value to return if no default value exists.
     * Returns: The value of the optional, or the default value if this
     * optional is empty.
     */
    T orElse(T defaultValue) {
        if (this.isNull) return defaultValue;
        return this.value;
    }

    /**
     * Gets the value of this optional if it exists, or throws an exception.
     * Params:
     *   msg = A message to put in the exception.
     * Returns: The value of this optional.
     */
    T orElseThrow(string msg = "Optional value is null.") {
        if (this.isNull) throw new Exception(msg);
        return this.value;
    }

    /**
     * Gets the value of this optional if it exists, or throws an exception as
     * produced by the given delegate.
     * Params:
     *   exceptionSupplier = A delegate that returns an exception to throw if
     *                       this optional is null.
     * Returns: The value of this optional.
     */
    T orElseThrow(Exception delegate() exceptionSupplier) {
        if (this.isNull) throw exceptionSupplier();
        return this.value;
    }

    /**
     * Provides a mechanism to allow usage in boolean expressions.
     * Returns: true if non-null, false if null
     * ---
     * auto optInt = Optional!int.empty();
     * assert(!optInt);
     * auto optStr = Optional!string.of("Hello");
     * assert(optStr);
     * ---
     */
    bool opCast(B : bool)() const {
        return !this.isNull;
    }

    // Optional support for ASDF serialization and deserialization.
    // See here: https://code.dlang.org/packages/asdf
    version (Have_asdf) {
        import asdf;

        static if (
            __traits(compiles, deserialize!T(Asdf.init))
        ) {
            /**
            * Deserializes an optional value from raw ASDF data. This function
            * is defined to allow for automatic deserialization of Optionals when
            * parsing JSON or other data types.
            * Params:
            *   data = The data representing an optional value.
            * Returns: An exception if one is thrown.
            */
            SerdeException deserializeFromAsdf(Asdf data) {
                static if (!__traits(compiles, {T t; t = T.init;})) {
                    return new SerdeException(
                        "Cannot deserialize Optional that contains immutable or otherwise unassignable value. " ~
                        "Ensure that the following code compiles: T t; t = T.init; for the given type T."
                    );
                } else {
                    if (data == null) {
                        this.isNull = true;
                        this.value = T.init;
                    } else {
                        this.isNull = false;
                        this.value = deserialize!T(data);
                    }
                    return null;
                }
            }

            /**
            * Serializes an optional value to allow for the automatic serialization
            * of Optionals when writing JSON or other data types.
            * Params:
            *   serializer = The serializer to use (provided by ASDF).
            */
            void serialize(S)(ref S serializer) const {
                if (this.isNull) {
                    serializer.putValue(null);
                } else {
                    serializeValue(serializer, this.value);
                }
            }
        }
    }
}

/**
 * Maps the value of a given optional to another type using a given function.
 * Params:
 *   opt = The optional to map.
 * Returns: An optional whose type is the return-type of the given `fn`
 * template argument function.
 */
auto mapIfPresent(alias fn, T)(Optional!T opt) {
    alias U = typeof(fn(T.init));
    if (opt.isNull) return Optional!U.empty();
    return Optional!U.of(fn(opt.value));
}

unittest {
    Optional!string s = Optional!string.of("hello");
    assert(!s.isNull);
    assert(s.value == "hello");
    assert(s); // test boolean conversion
    Optional!int mapped = s.mapIfPresent!(str => 1);
    assert(!mapped.isNull);
    assert(mapped.value == 1);

    // Additional tests if the ASDF library is present:
    version (Have_asdf) {
        import asdf;

        assert(deserialize!(Optional!string)(`null`).isNull);
        assert(!deserialize!(Optional!bool)(`true`).isNull);
        assert(deserialize!(Optional!bool)(`true`).value == true);

        struct Sd {
            int x;
            int y;
        }

        struct S {
            Optional!bool a;
            Optional!string b;
            Optional!float c;
            Optional!Sd d;
        }

        string json = `{
            "a": true,
            "b": "hello world!",
            "c": "3.14",
            "d": {
                "x": 42,
                "y": 67
            }
        }`;
        S sa = deserialize!S(json);
        assert(sa.a && sa.a.value == true);
        assert(sa.b && sa.b.value == "hello world!");
        assert(sa.c && sa.c.value == 3.14f);
        assert(sa.d && sa.d.value == Sd(42, 67));

        // Check that optionals containing immutable data aren't supported for serialization & deserialization:
        struct Invalid {
            immutable int x;
        }
        Optional!Invalid opt = Optional!(Invalid).of(Invalid(42));
        static assert(__traits(compiles, serializeToJson(opt)));
        try {
            deserialize!(Optional!(Invalid))(`{"x": 123}`);
            assert(
                false,
                "Failed to ensure that an exception is thrown when attempting to deserialize an incompatible Optional."
            );
        } catch (SerdeException exc) {
            // pass.
        }
    }
}