/*
 * Copyright (c) 2014, 2014, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */
package com.oracle.truffle.sl.runtime;

import com.oracle.truffle.api.*;
import com.oracle.truffle.api.interop.*;
import com.oracle.truffle.api.utilities.*;

/**
 * Represents a SL function. On the Truffle level, a callable element is represented by a
 * {@link RootCallTarget call target}. This class encapsulates a call target, and adds version
 * support: functions in SL can be redefined, i.e. changed at run time. When a function is
 * redefined, the call target managed by this function object is changed (and {@link #callTarget} is
 * therefore not a final field).
 * <p>
 * Function redefinition is expected to be rare, therefore optimized call nodes want to speculate
 * that the call target is stable. This is possible with the help of a Truffle {@link Assumption}: a
 * call node can keep the call target returned by {@link #getCallTarget()} cached until the
 * assumption returned by {@link #getCallTargetStable()} is valid.
 * <p>
 * The {@link #callTarget} can be {@code null}. To ensure that only one {@link SLFunction} instance
 * per name exists, the {@link SLFunctionRegistry} creates an instance also when performing name
 * lookup. A function that has been looked up, i.e., used, but not defined, has no call target.
 */
public final class SLFunction implements TruffleObject {

    /** The name of the function. */
    private final String name;

    /** The current implementation of this function. */
    private RootCallTarget callTarget;

    /**
     * Manages the assumption that the {@link #callTarget} is stable. We use the utility class
     * {@link CyclicAssumption}, which automatically creates a new {@link Assumption} when the old
     * one gets invalidated.
     */
    private final CyclicAssumption callTargetStable;

    protected SLFunction(String name) {
        this.name = name;
        this.callTargetStable = new CyclicAssumption(name);
    }

    public String getName() {
        return name;
    }

    protected void setCallTarget(RootCallTarget callTarget) {
        this.callTarget = callTarget;
        /*
         * We have a new call target. Invalidate all code that speculated that the old call target
         * was stable.
         */
        callTargetStable.invalidate();
    }

    public RootCallTarget getCallTarget() {
        return callTarget;
    }

    public Assumption getCallTargetStable() {
        return callTargetStable.getAssumption();
    }

    /**
     * This method is, e.g., called when using a function literal in a string concatenation. So
     * changing it has an effect on SL programs.
     */
    @Override
    public String toString() {
        return name;
    }

    /**
     * In case you want some of your objects to co-operate with other languages, you need to make
     * them implement {@link TruffleObject} and provide additional {@link SLFunctionForeignAccess
     * foreign access implementation}.
     */
    @Override
    public ForeignAccessFactory getForeignAccessFactory() {
        return SLFunctionForeignAccess.INSTANCE;
    }
}