373 lines
12 KiB
Java
373 lines
12 KiB
Java
/*
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* Copyright (c) 1999, 2007, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/*
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* Licensed Materials - Property of IBM
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* RMI-IIOP v1.0
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* Copyright IBM Corp. 1998 1999 All Rights Reserved
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*
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*/
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package sun.rmi.rmic.iiop;
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/**
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* StaticStringsHash takes an array of constant strings and
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* uses several different hash methods to try to find the
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* 'best' one for that set. The set of methods is currently
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* fixed, but with a little work could be made extensible thru
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* subclassing.
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* <p>
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* The current set of methods is:
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* <ol>
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* <li> length() - works well when all strings are different length.</li>
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* <li> charAt(n) - works well when one offset into all strings is different.</li>
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* <li> hashCode() - works well with larger arrays.</li>
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* </ol>
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* After constructing an instance over the set of strings, the
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* <code>getKey(String)</code> method can be used to use the selected hash
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* method to produce a key. The <code>method</code> string will contain
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* "length()", "charAt(n)", or "hashCode()", and is intended for use by
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* code generators.
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* <p>
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* The <code>keys</code> array will contain the full set of unique keys.
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* <p>
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* The <code>buckets</code> array will contain a set of arrays, one for
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* each key in the <code>keys</code>, where <code>buckets[x][y]</code>
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* is an index into the <code>strings</code> array.
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* @author Bryan Atsatt
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*/
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public class StaticStringsHash {
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/** The set of strings upon which the hash info is created */
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public String[] strings = null;
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/** Unique hash keys */
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public int[] keys = null;
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/** Buckets for each key, where buckets[x][y] is an index
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* into the strings[] array. */
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public int[][] buckets = null;
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/** The method to invoke on String to produce the hash key */
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public String method = null;
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/** Get a key for the given string using the
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* selected hash method.
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* @param str the string to return a key for.
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* @return the key.
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*/
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public int getKey(String str) {
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switch (keyKind) {
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case LENGTH: return str.length();
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case CHAR_AT: return str.charAt(charAt);
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case HASH_CODE: return str.hashCode();
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}
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throw new Error("Bad keyKind");
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}
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/** Constructor
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* @param strings the set of strings upon which to
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* find an optimal hash method. Must not contain
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* duplicates.
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*/
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public StaticStringsHash(String[] strings) {
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this.strings = strings;
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length = strings.length;
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tempKeys = new int[length];
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bucketSizes = new int[length];
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setMinStringLength();
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// Decide on the best algorithm based on
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// which one has the smallest maximum
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// bucket depth. First, try length()...
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int currentMaxDepth = getKeys(LENGTH);
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int useCharAt = -1;
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boolean useHashCode = false;
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if (currentMaxDepth > 1) {
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// At least one bucket had more than one
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// entry, so try charAt(i). If there
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// are a lot of strings in the array,
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// and minStringLength is large, limit
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// the search to a smaller number of
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// characters to avoid spending a lot
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// of time here that is most likely to
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// be pointless...
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int minLength = minStringLength;
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if (length > CHAR_AT_MAX_LINES &&
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length * minLength > CHAR_AT_MAX_CHARS) {
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minLength = length/CHAR_AT_MAX_CHARS;
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}
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charAt = 0;
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for (int i = 0; i < minLength; i++) {
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int charAtDepth = getKeys(CHAR_AT);
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if (charAtDepth < currentMaxDepth) {
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currentMaxDepth = charAtDepth;
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useCharAt = i;
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if (currentMaxDepth == 1) {
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break;
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}
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}
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charAt++;
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}
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charAt = useCharAt;
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if (currentMaxDepth > 1) {
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// At least one bucket had more than one
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// entry, try hashCode().
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//
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// Since the cost of computing a full hashCode
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// (for the runtime target string) is much higher
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// than the previous methods, use it only if it is
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// substantially better. The definition of 'substantial'
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// here is not very well founded, and could be improved
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// with some further analysis ;^)
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int hashCodeDepth = getKeys(HASH_CODE);
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if (hashCodeDepth < currentMaxDepth-3) {
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// Using the full hashCode results in at least
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// 3 fewer entries in the worst bucket, so will
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// therefore avoid at least 3 calls to equals()
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// in the worst case.
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//
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// Note that using a number smaller than 3 could
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// result in using a hashCode when there are only
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// 2 strings in the array, and that would surely
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// be a poor performance choice.
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useHashCode = true;
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}
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}
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// Reset keys if needed...
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if (!useHashCode) {
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if (useCharAt >= 0) {
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// Use the charAt(i) method...
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getKeys(CHAR_AT);
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} else {
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// Use length method...
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getKeys(LENGTH);
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}
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}
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}
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// Now allocate and fill our real hashKeys array...
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keys = new int[bucketCount];
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System.arraycopy(tempKeys,0,keys,0,bucketCount);
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// Sort keys and bucketSizes arrays...
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boolean didSwap;
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do {
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didSwap = false;
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for (int i = 0; i < bucketCount - 1; i++) {
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if (keys[i] > keys[i+1]) {
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int temp = keys[i];
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keys[i] = keys[i+1];
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keys[i+1] = temp;
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temp = bucketSizes[i];
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bucketSizes[i] = bucketSizes[i+1];
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bucketSizes[i+1] = temp;
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didSwap = true;
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}
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}
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}
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while (didSwap == true);
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// Allocate our buckets array. Fill the string
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// index slot with an unused key so we can
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// determine which are free...
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int unused = findUnusedKey();
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buckets = new int[bucketCount][];
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for (int i = 0; i < bucketCount; i++) {
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buckets[i] = new int[bucketSizes[i]];
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for (int j = 0; j < bucketSizes[i]; j++) {
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buckets[i][j] = unused;
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}
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}
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// And fill it in...
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for(int i = 0; i < strings.length; i++) {
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int key = getKey(strings[i]);
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for (int j = 0; j < bucketCount; j++) {
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if (keys[j] == key) {
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int k = 0;
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while (buckets[j][k] != unused) {
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k++;
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}
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buckets[j][k] = i;
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break;
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}
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}
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}
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}
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/** Print an optimized 'contains' method for the
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* argument strings
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*/
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public static void main (String[] args) {
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StaticStringsHash hash = new StaticStringsHash(args);
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System.out.println();
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System.out.println(" public boolean contains(String key) {");
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System.out.println(" switch (key."+hash.method+") {");
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for (int i = 0; i < hash.buckets.length; i++) {
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System.out.println(" case "+hash.keys[i]+": ");
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for (int j = 0; j < hash.buckets[i].length; j++) {
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if (j > 0) {
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System.out.print(" } else ");
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} else {
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System.out.print(" ");
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}
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System.out.println("if (key.equals(\""+ hash.strings[hash.buckets[i][j]] +"\")) {");
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System.out.println(" return true;");
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}
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System.out.println(" }");
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}
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System.out.println(" }");
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System.out.println(" return false;");
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System.out.println(" }");
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}
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private int length;
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private int[] tempKeys;
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private int[] bucketSizes;
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private int bucketCount;
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private int maxDepth;
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private int minStringLength = Integer.MAX_VALUE;
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private int keyKind;
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private int charAt;
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private static final int LENGTH = 0;
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private static final int CHAR_AT = 1;
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private static final int HASH_CODE = 2;
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/* Determines the maximum number of charAt(i)
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* tests that will be done. The search is
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* limited because if the number of characters
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* is large enough, the likelyhood of finding
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* a good hash key based on this method is
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* low. The CHAR_AT_MAX_CHARS limit only
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* applies f there are more strings than
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* CHAR_AT_MAX_LINES.
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*/
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private static final int CHAR_AT_MAX_LINES = 50;
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private static final int CHAR_AT_MAX_CHARS = 1000;
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private void resetKeys(int keyKind) {
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this.keyKind = keyKind;
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switch (keyKind) {
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case LENGTH: method = "length()"; break;
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case CHAR_AT: method = "charAt("+charAt+")"; break;
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case HASH_CODE: method = "hashCode()"; break;
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}
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maxDepth = 1;
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bucketCount = 0;
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for (int i = 0; i < length; i++) {
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tempKeys[i] = 0;
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bucketSizes[i] = 0;
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}
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}
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private void setMinStringLength() {
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for (int i = 0; i < length; i++) {
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if (strings[i].length() < minStringLength) {
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minStringLength = strings[i].length();
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}
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}
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}
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private int findUnusedKey() {
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int unused = 0;
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int keysLength = keys.length;
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// Note that we just assume that resource
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// exhaustion will occur rather than an
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// infinite loop here if the set of keys
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// is very large.
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while (true) {
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boolean match = false;
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for (int i = 0; i < keysLength; i++) {
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if (keys[i] == unused) {
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match = true;
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break;
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}
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}
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if (match) {
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unused--;
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} else {
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break;
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}
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}
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return unused;
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}
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private int getKeys(int methodKind) {
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resetKeys(methodKind);
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for(int i = 0; i < strings.length; i++) {
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addKey(getKey(strings[i]));
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}
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return maxDepth;
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}
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private void addKey(int key) {
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// Have we seen this one before?
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boolean addIt = true;
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for (int j = 0; j < bucketCount; j++) {
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if (tempKeys[j] == key) {
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addIt = false;
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bucketSizes[j]++;
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if (bucketSizes[j] > maxDepth) {
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maxDepth = bucketSizes[j];
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}
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break;
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}
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}
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if (addIt) {
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tempKeys[bucketCount] = key;
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bucketSizes[bucketCount] = 1;
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bucketCount++;
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}
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}
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}
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