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package java.lang.invoke;

import java.io.Serializable;

import java.util.Arrays;

/**

 * <p>Methods to facilitate the creation of simple "function objects" that

 * implement one or more interfaces by delegation to a provided {@link MethodHandle},

 * possibly after type adaptation and partial evaluation of arguments.  These

 * methods are typically used as <em>bootstrap methods</em> for {@code invokedynamic}

 * call sites, to support the <em>lambda expression</em> and <em>method

 * reference expression</em> features of the Java Programming Language.

 *

 * <p>Indirect access to the behavior specified by the provided {@code MethodHandle}

 * proceeds in order through three phases:

 * <ul>

 *     <li><em>Linkage</em> occurs when the methods in this class are invoked.

 *     They take as arguments an interface to be implemented (typically a

 *     <em>functional interface</em>, one with a single abstract method), a

 *     name and signature of a method from that interface to be implemented, a

 *     method handle describing the desired implementation behavior

 *     for that method, and possibly other additional metadata, and produce a

 *     {@link CallSite} whose target can be used to create suitable function

 *     objects.  Linkage may involve dynamically loading a new class that

 *     implements the target interface. The {@code CallSite} can be considered a

 *     "factory" for function objects and so these linkage methods are referred

 *     to as "metafactories".</li>

 *

 *     <li><em>Capture</em> occurs when the {@code CallSite}'s target is

 *     invoked, typically through an {@code invokedynamic} call site,

 *     producing a function object.  This may occur many times for

 *     a single factory {@code CallSite}.  Capture may involve allocation of a

 *     new function object, or may return an existing function object.  The

 *     behavior {@code MethodHandle} may have additional parameters beyond those

 *     of the specified interface method; these are referred to as <em>captured

 *     parameters</em>, which must be provided as arguments to the

 *     {@code CallSite} target, and which may be early-bound to the behavior

 *     {@code MethodHandle}.  The number of captured parameters and their types

 *     are determined during linkage.</li>

 *

 *     <li><em>Invocation</em> occurs when an implemented interface method

 *     is invoked on a function object.  This may occur many times for a single

 *     function object.  The method referenced by the behavior {@code MethodHandle}

 *     is invoked with the captured arguments and any additional arguments

 *     provided on invocation, as if by {@link MethodHandle#invoke(Object...)}.</li>

 * </ul>

 *

 * <p>It is sometimes useful to restrict the set of inputs or results permitted

 * at invocation.  For example, when the generic interface {@code Predicate<T>}

 * is parameterized as {@code Predicate<String>}, the input must be a

 * {@code String}, even though the method to implement allows any {@code Object}.

 * At linkage time, an additional {@link MethodType} parameter describes the

 * "instantiated" method type; on invocation, the arguments and eventual result

 * are checked against this {@code MethodType}.

 *

 * <p>This class provides two forms of linkage methods: a standard version

 * ({@link #metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)})

 * using an optimized protocol, and an alternate version

 * {@link #altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)}).

 * The alternate version is a generalization of the standard version, providing

 * additional control over the behavior of the generated function objects via

 * flags and additional arguments.  The alternate version adds the ability to

 * manage the following attributes of function objects:

 *

 * <ul>

 *     <li><em>Bridging.</em>  It is sometimes useful to implement multiple

 *     variations of the method signature, involving argument or return type

 *     adaptation.  This occurs when multiple distinct VM signatures for a method

 *     are logically considered to be the same method by the language.  The

 *     flag {@code FLAG_BRIDGES} indicates that a list of additional

 *     {@code MethodType}s will be provided, each of which will be implemented

 *     by the resulting function object.  These methods will share the same

 *     name and instantiated type.</li>

 *

 *     <li><em>Multiple interfaces.</em>  If needed, more than one interface

 *     can be implemented by the function object.  (These additional interfaces

 *     are typically marker interfaces with no methods.)  The flag {@code FLAG_MARKERS}

 *     indicates that a list of additional interfaces will be provided, each of

 *     which should be implemented by the resulting function object.</li>

 *

 *     <li><em>Serializability.</em>  The generated function objects do not

 *     generally support serialization.  If desired, {@code FLAG_SERIALIZABLE}

 *     can be used to indicate that the function objects should be serializable.

 *     Serializable function objects will use, as their serialized form,

 *     instances of the class {@code SerializedLambda}, which requires additional

 *     assistance from the capturing class (the class described by the

 *     {@link MethodHandles.Lookup} parameter {@code caller}); see

 *     {@link SerializedLambda} for details.</li>

 * </ul>

 *

 * <p>Assume the linkage arguments are as follows:

 * <ul>

 *      <li>{@code invokedType} (describing the {@code CallSite} signature) has

 *      K parameters of types (D1..Dk) and return type Rd;</li>

 *      <li>{@code samMethodType} (describing the implemented method type) has N

 *      parameters, of types (U1..Un) and return type Ru;</li>

 *      <li>{@code implMethod} (the {@code MethodHandle} providing the

 *      implementation has M parameters, of types (A1..Am) and return type Ra

 *      (if the method describes an instance method, the method type of this

 *      method handle already includes an extra first argument corresponding to

 *      the receiver);</li>

 *      <li>{@code instantiatedMethodType} (allowing restrictions on invocation)

 *      has N parameters, of types (T1..Tn) and return type Rt.</li>

 * </ul>

 *

 * <p>Then the following linkage invariants must hold:

 * <ul>

 *     <li>Rd is an interface</li>

 *     <li>{@code implMethod} is a <em>direct method handle</em></li>

 *     <li>{@code samMethodType} and {@code instantiatedMethodType} have the same

 *     arity N, and for i=1..N, Ti and Ui are the same type, or Ti and Ui are

 *     both reference types and Ti is a subtype of Ui</li>

 *     <li>Either Rt and Ru are the same type, or both are reference types and

 *     Rt is a subtype of Ru</li>

 *     <li>K + N = M</li>

 *     <li>For i=1..K, Di = Ai</li>

 *     <li>For i=1..N, Ti is adaptable to Aj, where j=i+k</li>

 *     <li>The return type Rt is void, or the return type Ra is not void and is

 *     adaptable to Rt</li>

 * </ul>

 *

 * <p>Further, at capture time, if {@code implMethod} corresponds to an instance

 * method, and there are any capture arguments ({@code K > 0}), then the first

 * capture argument (corresponding to the receiver) must be non-null.

 *

 * <p>A type Q is considered adaptable to S as follows:

 * <table summary="adaptable types">

 *     <tr><th>Q</th><th>S</th><th>Link-time checks</th><th>Invocation-time checks</th></tr>

 *     <tr>

 *         <td>Primitive</td><td>Primitive</td>

 *         <td>Q can be converted to S via a primitive widening conversion</td>

 *         <td>None</td>

 *     </tr>

 *     <tr>

 *         <td>Primitive</td><td>Reference</td>

 *         <td>S is a supertype of the Wrapper(Q)</td>

 *         <td>Cast from Wrapper(Q) to S</td>

 *     </tr>

 *     <tr>

 *         <td>Reference</td><td>Primitive</td>

 *         <td>for parameter types: Q is a primitive wrapper and Primitive(Q)

 *         can be widened to S

 *         <br>for return types: If Q is a primitive wrapper, check that

 *         Primitive(Q) can be widened to S</td>

 *         <td>If Q is not a primitive wrapper, cast Q to the base Wrapper(S);

 *         for example Number for numeric types</td>

 *     </tr>

 *     <tr>

 *         <td>Reference</td><td>Reference</td>

 *         <td>for parameter types: S is a supertype of Q

 *         <br>for return types: none</td>

 *         <td>Cast from Q to S</td>

 *     </tr>

 * </table>

 *

 * @apiNote These linkage methods are designed to support the evaluation

 * of <em>lambda expressions</em> and <em>method references</em> in the Java

 * Language.  For every lambda expressions or method reference in the source code,

 * there is a target type which is a functional interface.  Evaluating a lambda

 * expression produces an object of its target type. The recommended mechanism

 * for evaluating lambda expressions is to desugar the lambda body to a method,

 * invoke an invokedynamic call site whose static argument list describes the

 * sole method of the functional interface and the desugared implementation

 * method, and returns an object (the lambda object) that implements the target

 * type. (For method references, the implementation method is simply the

 * referenced method; no desugaring is needed.)

 *

 * <p>The argument list of the implementation method and the argument list of

 * the interface method(s) may differ in several ways.  The implementation

 * methods may have additional arguments to accommodate arguments captured by

 * the lambda expression; there may also be differences resulting from permitted

 * adaptations of arguments, such as casting, boxing, unboxing, and primitive

 * widening. (Varargs adaptations are not handled by the metafactories; these are

 * expected to be handled by the caller.)

 *

 * <p>Invokedynamic call sites have two argument lists: a static argument list

 * and a dynamic argument list.  The static argument list is stored in the

 * constant pool; the dynamic argument is pushed on the operand stack at capture

 * time.  The bootstrap method has access to the entire static argument list

 * (which in this case, includes information describing the implementation method,

 * the target interface, and the target interface method(s)), as well as a

 * method signature describing the number and static types (but not the values)

 * of the dynamic arguments and the static return type of the invokedynamic site.

 *

 * @implNote The implementation method is described with a method handle. In

 * theory, any method handle could be used. Currently supported are direct method

 * handles representing invocation of virtual, interface, constructor and static

 * methods.

 */

public class LambdaMetafactory {

    /** Flag for alternate metafactories indicating the lambda object

     * must be serializable */

    public static final int FLAG_SERIALIZABLE = 1 << 0;

    /**

     * Flag for alternate metafactories indicating the lambda object implements

     * other marker interfaces

     * besides Serializable

     */

    public static final int FLAG_MARKERS = 1 << 1;

    /**

     * Flag for alternate metafactories indicating the lambda object requires

     * additional bridge methods

     */

    public static final int FLAG_BRIDGES = 1 << 2;

    private static final Class<?>[] EMPTY_CLASS_ARRAY = new Class<?>[0];

    private static final MethodType[] EMPTY_MT_ARRAY = new MethodType[0];

    /**

     * Facilitates the creation of simple "function objects" that implement one

     * or more interfaces by delegation to a provided {@link MethodHandle},

     * after appropriate type adaptation and partial evaluation of arguments.

     * Typically used as a <em>bootstrap method</em> for {@code invokedynamic}

     * call sites, to support the <em>lambda expression</em> and <em>method

     * reference expression</em> features of the Java Programming Language.

     *

     * <p>This is the standard, streamlined metafactory; additional flexibility

     * is provided by {@link #altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)}.

     * A general description of the behavior of this method is provided

     * {@link LambdaMetafactory above}.

     *

     * <p>When the target of the {@code CallSite} returned from this method is

     * invoked, the resulting function objects are instances of a class which

     * implements the interface named by the return type of {@code invokedType},

     * declares a method with the name given by {@code invokedName} and the

     * signature given by {@code samMethodType}.  It may also override additional

     * methods from {@code Object}.

     *

     * @param caller Represents a lookup context with the accessibility

     *               privileges of the caller.  When used with {@code invokedynamic},

     *               this is stacked automatically by the VM.

     * @param invokedName The name of the method to implement.  When used with

     *                    {@code invokedynamic}, this is provided by the

     *                    {@code NameAndType} of the {@code InvokeDynamic}

     *                    structure and is stacked automatically by the VM.

     * @param invokedType The expected signature of the {@code CallSite}.  The

     *                    parameter types represent the types of capture variables;

     *                    the return type is the interface to implement.   When

     *                    used with {@code invokedynamic}, this is provided by

     *                    the {@code NameAndType} of the {@code InvokeDynamic}

     *                    structure and is stacked automatically by the VM.

     *                    In the event that the implementation method is an

     *                    instance method and this signature has any parameters,

     *                    the first parameter in the invocation signature must

     *                    correspond to the receiver.

     * @param samMethodType Signature and return type of method to be implemented

     *                      by the function object.

     * @param implMethod A direct method handle describing the implementation

     *                   method which should be called (with suitable adaptation

     *                   of argument types, return types, and with captured

     *                   arguments prepended to the invocation arguments) at

     *                   invocation time.

     * @param instantiatedMethodType The signature and return type that should

     *                               be enforced dynamically at invocation time.

     *                               This may be the same as {@code samMethodType},

     *                               or may be a specialization of it.

     * @return a CallSite whose target can be used to perform capture, generating

     *         instances of the interface named by {@code invokedType}

     * @throws LambdaConversionException If any of the linkage invariants

     *                                   described {@link LambdaMetafactory above}

     *                                   are violated

     */

    public static CallSite metafactory(MethodHandles.Lookup caller,

                                       String invokedName,

                                       MethodType invokedType,

                                       MethodType samMethodType,

                                       MethodHandle implMethod,

                                       MethodType instantiatedMethodType)

            throws LambdaConversionException {

        AbstractValidatingLambdaMetafactory mf;

        mf = new InnerClassLambdaMetafactory(caller, invokedType,

                                             invokedName, samMethodType,

                                             implMethod, instantiatedMethodType,

                                             false, EMPTY_CLASS_ARRAY, EMPTY_MT_ARRAY);

        mf.validateMetafactoryArgs();

        return mf.buildCallSite();

    }

    /**

     * Facilitates the creation of simple "function objects" that implement one

     * or more interfaces by delegation to a provided {@link MethodHandle},

     * after appropriate type adaptation and partial evaluation of arguments.

     * Typically used as a <em>bootstrap method</em> for {@code invokedynamic}

     * call sites, to support the <em>lambda expression</em> and <em>method

     * reference expression</em> features of the Java Programming Language.

     *

     * <p>This is the general, more flexible metafactory; a streamlined version

     * is provided by {@link #metafactory(java.lang.invoke.MethodHandles.Lookup,

     * String, MethodType, MethodType, MethodHandle, MethodType)}.

     * A general description of the behavior of this method is provided

     * {@link LambdaMetafactory above}.

     *

     * <p>The argument list for this method includes three fixed parameters,

     * corresponding to the parameters automatically stacked by the VM for the

     * bootstrap method in an {@code invokedynamic} invocation, and an {@code Object[]}

     * parameter that contains additional parameters.  The declared argument

     * list for this method is:

     *

     * <pre>{@code

     *  CallSite altMetafactory(MethodHandles.Lookup caller,

     *                          String invokedName,

     *                          MethodType invokedType,

     *                          Object... args)

     * }</pre>

     *

     * <p>but it behaves as if the argument list is as follows:

     *

     * <pre>{@code

     *  CallSite altMetafactory(MethodHandles.Lookup caller,

     *                          String invokedName,

     *                          MethodType invokedType,

     *                          MethodType samMethodType,

     *                          MethodHandle implMethod,

     *                          MethodType instantiatedMethodType,

     *                          int flags,

     *                          int markerInterfaceCount,  // IF flags has MARKERS set

     *                          Class... markerInterfaces, // IF flags has MARKERS set

     *                          int bridgeCount,           // IF flags has BRIDGES set

     *                          MethodType... bridges      // IF flags has BRIDGES set

     *                          )

     * }</pre>

     *

     * <p>Arguments that appear in the argument list for

     * {@link #metafactory(MethodHandles.Lookup, String, MethodType, MethodType, MethodHandle, MethodType)}

     * have the same specification as in that method.  The additional arguments

     * are interpreted as follows:

     * <ul>

     *     <li>{@code flags} indicates additional options; this is a bitwise

     *     OR of desired flags.  Defined flags are {@link #FLAG_BRIDGES},

     *     {@link #FLAG_MARKERS}, and {@link #FLAG_SERIALIZABLE}.</li>

     *     <li>{@code markerInterfaceCount} is the number of additional interfaces

     *     the function object should implement, and is present if and only if the

     *     {@code FLAG_MARKERS} flag is set.</li>

     *     <li>{@code markerInterfaces} is a variable-length list of additional

     *     interfaces to implement, whose length equals {@code markerInterfaceCount},

     *     and is present if and only if the {@code FLAG_MARKERS} flag is set.</li>

     *     <li>{@code bridgeCount} is the number of additional method signatures

     *     the function object should implement, and is present if and only if

     *     the {@code FLAG_BRIDGES} flag is set.</li>

     *     <li>{@code bridges} is a variable-length list of additional

     *     methods signatures to implement, whose length equals {@code bridgeCount},

     *     and is present if and only if the {@code FLAG_BRIDGES} flag is set.</li>

     * </ul>

     *

     * <p>Each class named by {@code markerInterfaces} is subject to the same

     * restrictions as {@code Rd}, the return type of {@code invokedType},

     * as described {@link LambdaMetafactory above}.  Each {@code MethodType}

     * named by {@code bridges} is subject to the same restrictions as

     * {@code samMethodType}, as described {@link LambdaMetafactory above}.

     *

     * <p>When FLAG_SERIALIZABLE is set in {@code flags}, the function objects

     * will implement {@code Serializable}, and will have a {@code writeReplace}

     * method that returns an appropriate {@link SerializedLambda}.  The

     * {@code caller} class must have an appropriate {@code $deserializeLambda$}

     * method, as described in {@link SerializedLambda}.

     *

     * <p>When the target of the {@code CallSite} returned from this method is

     * invoked, the resulting function objects are instances of a class with

     * the following properties:

     * <ul>

     *     <li>The class implements the interface named by the return type

     *     of {@code invokedType} and any interfaces named by {@code markerInterfaces}</li>

     *     <li>The class declares methods with the name given by {@code invokedName},

     *     and the signature given by {@code samMethodType} and additional signatures

     *     given by {@code bridges}</li>

     *     <li>The class may override methods from {@code Object}, and may

     *     implement methods related to serialization.</li>

     * </ul>

     *

     * @param caller Represents a lookup context with the accessibility

     *               privileges of the caller.  When used with {@code invokedynamic},

     *               this is stacked automatically by the VM.

     * @param invokedName The name of the method to implement.  When used with

     *                    {@code invokedynamic}, this is provided by the

     *                    {@code NameAndType} of the {@code InvokeDynamic}

     *                    structure and is stacked automatically by the VM.

     * @param invokedType The expected signature of the {@code CallSite}.  The

     *                    parameter types represent the types of capture variables;

     *                    the return type is the interface to implement.   When

     *                    used with {@code invokedynamic}, this is provided by

     *                    the {@code NameAndType} of the {@code InvokeDynamic}

     *                    structure and is stacked automatically by the VM.

     *                    In the event that the implementation method is an

     *                    instance method and this signature has any parameters,

     *                    the first parameter in the invocation signature must

     *                    correspond to the receiver.

     * @param  args       An {@code Object[]} array containing the required

     *                    arguments {@code samMethodType}, {@code implMethod},

     *                    {@code instantiatedMethodType}, {@code flags}, and any

     *                    optional arguments, as described

     *                    {@link #altMetafactory(MethodHandles.Lookup, String, MethodType, Object...)} above}

     * @return a CallSite whose target can be used to perform capture, generating

     *         instances of the interface named by {@code invokedType}

     * @throws LambdaConversionException If any of the linkage invariants

     *                                   described {@link LambdaMetafactory above}

     *                                   are violated

     */

    public static CallSite altMetafactory(MethodHandles.Lookup caller,

                                          String invokedName,

                                          MethodType invokedType,

                                          Object... args)

            throws LambdaConversionException {

        MethodType samMethodType = (MethodType)args[0];

        MethodHandle implMethod = (MethodHandle)args[1];

        MethodType instantiatedMethodType = (MethodType)args[2];

        int flags = (Integer) args[3];

        Class<?>[] markerInterfaces;

        MethodType[] bridges;

        int argIndex = 4;

        if ((flags & FLAG_MARKERS) != 0) {

            int markerCount = (Integer) args[argIndex++];

            markerInterfaces = new Class<?>[markerCount];

            System.arraycopy(args, argIndex, markerInterfaces, 0, markerCount);

            argIndex += markerCount;

        }

        else

            markerInterfaces = EMPTY_CLASS_ARRAY;

        if ((flags & FLAG_BRIDGES) != 0) {

            int bridgeCount = (Integer) args[argIndex++];

            bridges = new MethodType[bridgeCount];

            System.arraycopy(args, argIndex, bridges, 0, bridgeCount);

            argIndex += bridgeCount;

        }

        else

            bridges = EMPTY_MT_ARRAY;

        boolean isSerializable = ((flags & FLAG_SERIALIZABLE) != 0);

        if (isSerializable) {

            boolean foundSerializableSupertype = Serializable.class.isAssignableFrom(invokedType.returnType());

            for (Class<?> c : markerInterfaces)

                foundSerializableSupertype |= Serializable.class.isAssignableFrom(c);

            if (!foundSerializableSupertype) {

                markerInterfaces = Arrays.copyOf(markerInterfaces, markerInterfaces.length + 1);

                markerInterfaces[markerInterfaces.length-1] = Serializable.class;

            }

        }

        AbstractValidatingLambdaMetafactory mf

                = new InnerClassLambdaMetafactory(caller, invokedType,

                                                  invokedName, samMethodType,

                                                  implMethod,

                                                  instantiatedMethodType,

                                                  isSerializable,

                                                  markerInterfaces, bridges);

        mf.validateMetafactoryArgs();

        return mf.buildCallSite();

    }

}