/*
* Copyright (c) 1999, 2007, 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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*/
/*
* Licensed Materials - Property of IBM
* RMI-IIOP v1.0
* Copyright IBM Corp. 1998 1999 All Rights Reserved
*
*/
package sun.rmi.rmic.iiop;
/**
* StaticStringsHash takes an array of constant strings and
* uses several different hash methods to try to find the
* 'best' one for that set. The set of methods is currently
* fixed, but with a little work could be made extensible thru
* subclassing.
* <p>
* The current set of methods is:
* <ol>
* <li> length() - works well when all strings are different length.</li>
* <li> charAt(n) - works well when one offset into all strings is different.</li>
* <li> hashCode() - works well with larger arrays.</li>
* </ol>
* After constructing an instance over the set of strings, the
* <code>getKey(String)</code> method can be used to use the selected hash
* method to produce a key. The <code>method</code> string will contain
* "length()", "charAt(n)", or "hashCode()", and is intended for use by
* code generators.
* <p>
* The <code>keys</code> array will contain the full set of unique keys.
* <p>
* The <code>buckets</code> array will contain a set of arrays, one for
* each key in the <code>keys</code>, where <code>buckets[x][y]</code>
* is an index into the <code>strings</code> array.
* @author Bryan Atsatt
*/
public class StaticStringsHash {
/** The set of strings upon which the hash info is created */
public String[] strings = null;
/** Unique hash keys */
public int[] keys = null;
/** Buckets for each key, where buckets[x][y] is an index
* into the strings[] array. */
public int[][] buckets = null;
/** The method to invoke on String to produce the hash key */
public String method = null;
/** Get a key for the given string using the
* selected hash method.
* @param str the string to return a key for.
* @return the key.
*/
public int getKey(String str) {
switch (keyKind) {
case LENGTH: return str.length();
case CHAR_AT: return str.charAt(charAt);
case HASH_CODE: return str.hashCode();
}
throw new Error("Bad keyKind");
}
/** Constructor
* @param strings the set of strings upon which to
* find an optimal hash method. Must not contain
* duplicates.
*/
public StaticStringsHash(String[] strings) {
this.strings = strings;
length = strings.length;
tempKeys = new int[length];
bucketSizes = new int[length];
setMinStringLength();
// Decide on the best algorithm based on
// which one has the smallest maximum
// bucket depth. First, try length()...
int currentMaxDepth = getKeys(LENGTH);
int useCharAt = -1;
boolean useHashCode = false;
if (currentMaxDepth > 1) {
// At least one bucket had more than one
// entry, so try charAt(i). If there
// are a lot of strings in the array,
// and minStringLength is large, limit
// the search to a smaller number of
// characters to avoid spending a lot
// of time here that is most likely to
// be pointless...
int minLength = minStringLength;
if (length > CHAR_AT_MAX_LINES &&
length * minLength > CHAR_AT_MAX_CHARS) {
minLength = length/CHAR_AT_MAX_CHARS;
}
charAt = 0;
for (int i = 0; i < minLength; i++) {
int charAtDepth = getKeys(CHAR_AT);
if (charAtDepth < currentMaxDepth) {
currentMaxDepth = charAtDepth;
useCharAt = i;
if (currentMaxDepth == 1) {
break;
}
}
charAt++;
}
charAt = useCharAt;
if (currentMaxDepth > 1) {
// At least one bucket had more than one
// entry, try hashCode().
//
// Since the cost of computing a full hashCode
// (for the runtime target string) is much higher
// than the previous methods, use it only if it is
// substantially better. The definition of 'substantial'
// here is not very well founded, and could be improved
// with some further analysis ;^)
int hashCodeDepth = getKeys(HASH_CODE);
if (hashCodeDepth < currentMaxDepth-3) {
// Using the full hashCode results in at least
// 3 fewer entries in the worst bucket, so will
// therefore avoid at least 3 calls to equals()
// in the worst case.
//
// Note that using a number smaller than 3 could
// result in using a hashCode when there are only
// 2 strings in the array, and that would surely
// be a poor performance choice.
useHashCode = true;
}
}
// Reset keys if needed...
if (!useHashCode) {
if (useCharAt >= 0) {
// Use the charAt(i) method...
getKeys(CHAR_AT);
} else {
// Use length method...
getKeys(LENGTH);
}
}
}
// Now allocate and fill our real hashKeys array...
keys = new int[bucketCount];
System.arraycopy(tempKeys,0,keys,0,bucketCount);
// Sort keys and bucketSizes arrays...
boolean didSwap;
do {
didSwap = false;
for (int i = 0; i < bucketCount - 1; i++) {
if (keys[i] > keys[i+1]) {
int temp = keys[i];
keys[i] = keys[i+1];
keys[i+1] = temp;
temp = bucketSizes[i];
bucketSizes[i] = bucketSizes[i+1];
bucketSizes[i+1] = temp;
didSwap = true;
}
}
}
while (didSwap == true);
// Allocate our buckets array. Fill the string
// index slot with an unused key so we can
// determine which are free...
int unused = findUnusedKey();
buckets = new int[bucketCount][];
for (int i = 0; i < bucketCount; i++) {
buckets[i] = new int[bucketSizes[i]];
for (int j = 0; j < bucketSizes[i]; j++) {
buckets[i][j] = unused;
}
}
// And fill it in...
for(int i = 0; i < strings.length; i++) {
int key = getKey(strings[i]);
for (int j = 0; j < bucketCount; j++) {
if (keys[j] == key) {
int k = 0;
while (buckets[j][k] != unused) {
k++;
}
buckets[j][k] = i;
break;
}
}
}
}
/** Print an optimized 'contains' method for the
* argument strings
*/
public static void main (String[] args) {
StaticStringsHash hash = new StaticStringsHash(args);
System.out.println();
System.out.println(" public boolean contains(String key) {");
System.out.println(" switch (key."+hash.method+") {");
for (int i = 0; i < hash.buckets.length; i++) {
System.out.println(" case "+hash.keys[i]+": ");
for (int j = 0; j < hash.buckets[i].length; j++) {
if (j > 0) {
System.out.print(" } else ");
} else {
System.out.print(" ");
}
System.out.println("if (key.equals(\""+ hash.strings[hash.buckets[i][j]] +"\")) {");
System.out.println(" return true;");
}
System.out.println(" }");
}
System.out.println(" }");
System.out.println(" return false;");
System.out.println(" }");
}
private int length;
private int[] tempKeys;
private int[] bucketSizes;
private int bucketCount;
private int maxDepth;
private int minStringLength = Integer.MAX_VALUE;
private int keyKind;
private int charAt;
private static final int LENGTH = 0;
private static final int CHAR_AT = 1;
private static final int HASH_CODE = 2;
/* Determines the maximum number of charAt(i)
* tests that will be done. The search is
* limited because if the number of characters
* is large enough, the likelyhood of finding
* a good hash key based on this method is
* low. The CHAR_AT_MAX_CHARS limit only
* applies f there are more strings than
* CHAR_AT_MAX_LINES.
*/
private static final int CHAR_AT_MAX_LINES = 50;
private static final int CHAR_AT_MAX_CHARS = 1000;
private void resetKeys(int keyKind) {
this.keyKind = keyKind;
switch (keyKind) {
case LENGTH: method = "length()"; break;
case CHAR_AT: method = "charAt("+charAt+")"; break;
case HASH_CODE: method = "hashCode()"; break;
}
maxDepth = 1;
bucketCount = 0;
for (int i = 0; i < length; i++) {
tempKeys[i] = 0;
bucketSizes[i] = 0;
}
}
private void setMinStringLength() {
for (int i = 0; i < length; i++) {
if (strings[i].length() < minStringLength) {
minStringLength = strings[i].length();
}
}
}
private int findUnusedKey() {
int unused = 0;
int keysLength = keys.length;
// Note that we just assume that resource
// exhaustion will occur rather than an
// infinite loop here if the set of keys
// is very large.
while (true) {
boolean match = false;
for (int i = 0; i < keysLength; i++) {
if (keys[i] == unused) {
match = true;
break;
}
}
if (match) {
unused--;
} else {
break;
}
}
return unused;
}
private int getKeys(int methodKind) {
resetKeys(methodKind);
for(int i = 0; i < strings.length; i++) {
addKey(getKey(strings[i]));
}
return maxDepth;
}
private void addKey(int key) {
// Have we seen this one before?
boolean addIt = true;
for (int j = 0; j < bucketCount; j++) {
if (tempKeys[j] == key) {
addIt = false;
bucketSizes[j]++;
if (bucketSizes[j] > maxDepth) {
maxDepth = bucketSizes[j];
}
break;
}
}
if (addIt) {
tempKeys[bucketCount] = key;
bucketSizes[bucketCount] = 1;
bucketCount++;
}
}
}