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Nashorn is an Open Source JavaScript (ECMAScript 5.1 and some 6 features) engine for the JVM.
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/*
 * Copyright (c) 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.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * 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 org.openjdk.nashorn.internal.runtime;

import static org.openjdk.nashorn.internal.lookup.Lookup.MH;

import java.io.Serializable;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.ref.WeakReference;
import org.openjdk.nashorn.internal.codegen.Compiler;
import org.openjdk.nashorn.internal.codegen.CompilerConstants;
import org.openjdk.nashorn.internal.codegen.ObjectClassGenerator;

/**
 * Encapsulates the allocation strategy for a function when used as a constructor.
 */
final public class AllocationStrategy implements Serializable {
    private static final long serialVersionUID = 1L;

    private static final MethodHandles.Lookup LOOKUP = MethodHandles.lookup();

    /** Number of fields in the allocated object */
    private final int fieldCount;

    /** Whether to use dual field representation */
    private final boolean dualFields;

    /** Name of class where allocator function resides */
    private transient String allocatorClassName;

    /** lazily generated allocator */
    private transient MethodHandle allocator;

    /** Last used allocator map */
    private transient AllocatorMap lastMap;

    /**
     * Construct an allocation strategy with the given map and class name.
     * @param fieldCount number of fields in the allocated object
     * @param dualFields whether to use dual field representation
     */
    public AllocationStrategy(final int fieldCount, final boolean dualFields) {
        this.fieldCount = fieldCount;
        this.dualFields = dualFields;
    }

    private String getAllocatorClassName() {
        if (allocatorClassName == null) {
            // These classes get loaded, so an interned variant of their name is most likely around anyway.
            allocatorClassName = Compiler.binaryName(ObjectClassGenerator.getClassName(fieldCount, dualFields)).intern();
        }
        return allocatorClassName;
    }

    /**
     * Get the property map for the allocated object.
     * @param prototype the prototype object
     * @return the property map
     */
    synchronized PropertyMap getAllocatorMap(final ScriptObject prototype) {
        assert prototype != null;
        final PropertyMap protoMap = prototype.getMap();

        if (lastMap != null) {
            if (!lastMap.hasSharedProtoMap()) {
                if (lastMap.hasSamePrototype(prototype)) {
                    return lastMap.allocatorMap;
                }
                if (lastMap.hasSameProtoMap(protoMap) && lastMap.hasUnchangedProtoMap()) {
                    // Convert to shared prototype map. Allocated objects will use the same property map
                    // that can be used as long as none of the prototypes modify the shared proto map.
                    final PropertyMap allocatorMap = PropertyMap.newMap(null, getAllocatorClassName(), 0, fieldCount, 0);
                    final SharedPropertyMap sharedProtoMap = new SharedPropertyMap(protoMap);
                    allocatorMap.setSharedProtoMap(sharedProtoMap);
                    prototype.setMap(sharedProtoMap);
                    lastMap = new AllocatorMap(prototype, protoMap, allocatorMap);
                    return allocatorMap;
                }
            }

            if (lastMap.hasValidSharedProtoMap() && lastMap.hasSameProtoMap(protoMap)) {
                prototype.setMap(lastMap.getSharedProtoMap());
                return lastMap.allocatorMap;
            }
        }

        final PropertyMap allocatorMap = PropertyMap.newMap(null, getAllocatorClassName(), 0, fieldCount, 0);
        lastMap = new AllocatorMap(prototype, protoMap, allocatorMap);

        return allocatorMap;
    }

    /**
     * Allocate an object with the given property map
     * @param map the property map
     * @return the allocated object
     */
    ScriptObject allocate(final PropertyMap map) {
        try {
            if (allocator == null) {
                allocator = MH.findStatic(LOOKUP, Context.forStructureClass(getAllocatorClassName()),
                        CompilerConstants.ALLOCATE.symbolName(), MH.type(ScriptObject.class, PropertyMap.class));
            }
            return (ScriptObject)allocator.invokeExact(map);
        } catch (final RuntimeException | Error e) {
            throw e;
        } catch (final Throwable t) {
            throw new RuntimeException(t);
        }
    }

    @Override
    public String toString() {
        return "AllocationStrategy[fieldCount=" + fieldCount + "]";
    }

    static class AllocatorMap {
        final private WeakReference prototype;
        final private WeakReference prototypeMap;

        private final PropertyMap allocatorMap;

        AllocatorMap(final ScriptObject prototype, final PropertyMap protoMap, final PropertyMap allocMap) {
            this.prototype = new WeakReference<>(prototype);
            this.prototypeMap = new WeakReference<>(protoMap);
            this.allocatorMap = allocMap;
        }

        boolean hasSamePrototype(final ScriptObject proto) {
            return prototype.get() == proto;
        }

        boolean hasSameProtoMap(final PropertyMap protoMap) {
            return prototypeMap.get() == protoMap || allocatorMap.getSharedProtoMap() == protoMap;
        }

        boolean hasUnchangedProtoMap() {
            final ScriptObject proto = prototype.get();
            return proto != null && proto.getMap() == prototypeMap.get();
        }

        boolean hasSharedProtoMap() {
            return getSharedProtoMap() != null;
        }

        boolean hasValidSharedProtoMap() {
            return hasSharedProtoMap() && getSharedProtoMap().isValidSharedProtoMap();
        }

        PropertyMap getSharedProtoMap() {
            return allocatorMap.getSharedProtoMap();
        }

    }
}