292 lines
6.9 KiB
Java
292 lines
6.9 KiB
Java
/* ObjectIdentityMapToInt.java -- Helper class for faster serialization
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Copyright (C) 2006 Free Software Foundation, Inc.
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This file is part of GNU Classpath.
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GNU Classpath is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GNU Classpath is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GNU Classpath; see the file COPYING. If not, write to the
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Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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02110-1301 USA.
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Linking this library statically or dynamically with other modules is
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making a combined work based on this library. Thus, the terms and
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conditions of the GNU General Public License cover the whole
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combination.
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As a special exception, the copyright holders of this library give you
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permission to link this library with independent modules to produce an
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executable, regardless of the license terms of these independent
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modules, and to copy and distribute the resulting executable under
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terms of your choice, provided that you also meet, for each linked
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independent module, the terms and conditions of the license of that
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module. An independent module is a module which is not derived from
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or based on this library. If you modify this library, you may extend
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this exception to your version of the library, but you are not
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obligated to do so. If you do not wish to do so, delete this
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exception statement from your version. */
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package gnu.java.io;
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/**
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* This class provides a map from Object to non-negative int values.
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* Objects are considered equal only if their references are equal.
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*
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* This can be used to equip objects with an integer id. This class
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* is implemented to use as little memory as possible, particularly
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* not to create hashtable buckets and Integer instances for each
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* mapping.
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*
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* @author Fridtjof Siebert (siebert@aicas.com)
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*/
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public class ObjectIdentityMap2Int
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{
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/**
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* Prime numbers used as size of array. We need the size to be a
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* prime number since the delta used for conflict resulution must
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* not have any common divisors with the length.
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*/
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private static final int[] PRIMES = {
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0x1f,
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0x3d,
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0x7f,
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0xfb,
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0x1fd,
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0x3fd,
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0x7f7,
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0xffd,
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0x1fff,
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0x3ffd,
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0x7fed,
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0xfff1,
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0x1ffff,
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0x3fffb,
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0x7ffff,
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0xffffd,
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0x1ffff7,
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0x3ffffd,
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0x7ffff1,
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0xfffffd,
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0x1ffffd9,
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0x3fffffb,
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0x7ffffd9,
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0xfffffc7,
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0x1ffffffd,
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0x3fffffdd,
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0x7fffffff};
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/**
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* Object to be used instead of "null"
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*/
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private static final Object NIL = new Object();
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/**
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* The objects in this map:
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*
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* invariant
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* objectTable.size == PRIMES[cap]
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*/
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private Object[] objectTable;
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/**
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* The corresponding integer ids.
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*
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* invariant
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* intTable.size == PRIMES[cap]
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*/
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private int[] intTable;
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/**
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* The number of entries in this map.
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*
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* invariant
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* size < limit
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*/
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private int size = 0;
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/**
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* The index in primes of the size of the tables.
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*/
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private int cap = 0;
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/**
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* The limit for size at which the table size is increased.
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*
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* invariant
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* limit = PRIMES[cap] / 4 * 3;
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*/
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private int limit = 0;
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/**
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* Constructs an empty <code>ObjectIdentityMap2Int</code>.
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*/
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public ObjectIdentityMap2Int()
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{
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alloc(0);
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}
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/**
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* Helper function to alloc the object and int array for the given
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* capacity. Set limit, reset size to 0.
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*
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* No elements will be stored in the newly allocated arrays.
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*
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* @param c the capacity: this is an index in PRIMES, PRIMES[c]
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* gives the size of the arrays.
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*
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* @throws InternalError if c >= PRIMES.length (in this case, a
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* normal Hashtable would throw an OutOfMemoryError or a
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* NegativeArraySizeException since the array size exceeds the range
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* of positive integers).
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*/
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private void alloc(int c)
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{
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if (c >= PRIMES.length)
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throw new InternalError("Hash table size overflow");
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cap = c;
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int len = PRIMES[c];
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objectTable = new Object[len];
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intTable = new int[len];
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limit = len / 4 * 3;
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size = 0;
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}
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/**
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* Add a mapping to this Map.
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*
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* ensures
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* (get(o) == i);
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*
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* @param o object reference or null that is to be mapped.
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*
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* @param i the integer id to be associated with o
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*
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* @throws IllegalArgumentException if i<0
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*
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* @throws InternalError if hash tables has grown to more then
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* 0x7fffffff entries (ie., size >= 0x7fffffff*3/4).
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*/
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public void put(Object o, int i)
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{
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if (i < 0)
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throw new IllegalArgumentException("int argument must be postive: "+i);
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o = (o == null) ? NIL : o;
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int s = slot(o);
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Object[] ot = objectTable;
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intTable[s] = i;
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if (objectTable[s] == null)
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{
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objectTable[s] = o;
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size++;
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if (size >= limit)
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{
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rehash();
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}
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}
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}
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/**
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* Helper function to find the index of a free or existing slot for
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* object o
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*
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* ensure
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* ((objectTable[result] != null) IMPLIES (objectTable[result] == o));
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*
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* @param o an object, must not be null.
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*
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* @return an index of o
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*/
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private int slot(Object o)
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{
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Object[] ot = objectTable;
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int hc = System.identityHashCode(o);
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int len = ot.length;
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int result = hc % len;
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result = result < 0 ? -result : result;
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int delta = 16 - (hc & 15);
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Object existing = ot[result];
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while ((existing != null) && (existing != o))
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{
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result += delta;
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if (result >= len)
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result -= len;
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existing = ot[result];
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}
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return result;
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}
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/**
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* Helper function for put() to increaes the capacity of this table
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* to the next size (approx. double the size). Keep the mapping and
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* the size unchanged.
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*
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* ensure
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* (cap == \old cap+1);
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*/
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private void rehash()
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{
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Object[] ot = objectTable;
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int [] it = intTable;
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alloc(cap + 1);
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for (int i = 0; i < ot.length; i++)
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put(ot[i], it[i]);
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}
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/**
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* Obtain an element from this map
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*
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* @param o an object or null
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*
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* @return the corresponding integer id for o or -1 if o has not
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* been put into this map.
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*/
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public int get(Object o)
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{
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o = (o == null) ? NIL : o;
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int s = slot(o);
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return objectTable[s] == null ? -1 : intTable[s];
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}
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/**
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* Clear this map
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*
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* ensures
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* ((size == 0) && \forall Object o: get(o) == -1)
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*/
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public void clear()
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{
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Object[] ot = objectTable;
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size = 0;
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for (int i = 0; i < ot.length; i++)
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ot[i] = null;
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}
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}
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