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/* Copyright (c) 2006 Google Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package sample.gbase.recipe;
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import com.google.api.gbase.client.AttributeHistogram;
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import com.google.api.gbase.client.FeedURLFactory;
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import com.google.api.gbase.client.GoogleBaseEntry;
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import com.google.api.gbase.client.GoogleBaseFeed;
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import com.google.api.gbase.client.GoogleBaseQuery;
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import com.google.api.gbase.client.GoogleBaseService;
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import com.google.api.gbase.client.MetadataEntryExtension;
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import com.google.api.gbase.client.AttributeHistogram.UniqueValue;
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import com.google.gdata.util.ServiceException;
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import java.io.IOException;
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import java.net.URL;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.Collection;
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import java.util.Hashtable;
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import java.util.Iterator;
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import java.util.List;
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import java.util.Timer;
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import java.util.TimerTask;
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import javax.servlet.ServletContext;
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/**
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* Holds the most used values of attributes in Google Base and
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* periodically refreshes them.
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*
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* A MostUsedValues object is focused on a GoogleBaseService and
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* a FeedURLFactory. One object can be used to analyse only the items that
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* match a specific query.
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*
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* The class currently works for TEXT attributes only.
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*
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* The cache() and clear() calls are synchronized,
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* so that the different Maps and Collections remain in sync.
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*
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* Typically, right after you create a MostUsedValues object, you call the
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* {@link #cache(long, int, ServletContext, String[])} method to set it up
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* to cache the most used values of your attributes. This creates
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* {@link java.util.TimerTask} objects that periodically refresh the cached
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* values. After this, you use {@link #getMostUsedValuesForAttribute(String)}
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* to get the most used values for the attributes you previously specified.
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* In the end, when you no longer need the cache, you can call
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* {@link #clear()} to stop the TimerTask objects and clear the cache.
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*/
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public class MostUsedValues {
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protected static final String TEXT_TYPE = "(text)";
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/**
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* The initial value of the max values limit when making a query;
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* the step used to increase that limit if some of the attributes
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* are not found.
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*/
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protected static final int STEP_MAXRESULTS = 25;
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/**
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* The max of the max values limit when making a query.
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*/
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protected static final int MAX_MAXRESULTS = 200;
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/**
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* The cached most used values.
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* This is a synchronized map.
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*/
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private java.util.Map<String, String[]> mostUsedValues;
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/**
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* The timers that periodically refresh the cache.
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* The access to this object has to be synchronized.
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*/
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private Collection<Timer> timers;
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private GoogleBaseService service;
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private FeedURLFactory urlFactory;
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private String queryString;
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/**
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* Creates an empty MostUsedValues.
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*
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* @param service any GoogleBaseService used to retrieve attribute histograms
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* @param urlFactory a FeedURLFactory used to create the URLs of
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* the attribute histograms
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* @param queryString the query string used to filter the analyzed items,
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* for example one that focuses only on the items that
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* have a specific item type.
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*/
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public MostUsedValues(GoogleBaseService service,
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FeedURLFactory urlFactory,
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String queryString) {
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this.service = service;
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this.urlFactory = urlFactory;
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this.queryString = queryString;
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mostUsedValues = new Hashtable<String, String[]>();
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timers = new ArrayList<Timer>();
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}
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/**
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* Gets the cached most used values of an attribute.
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*
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* @param attrName the name of the attribute
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* @return a cached list of the most used values
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*/
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public String[] getMostUsedValuesForAttribute(String attrName) {
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return mostUsedValues.get(attrName);
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}
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/**
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* Sets up the object to cache a limited number of the most used values of
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* each of the specified attributes; the cache is refreshed periodically.
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*
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* This method does not check if the specified attributes are already cached.
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* You have to make sure an attribute is specified only once in your calls.
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*
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* @param interval the cache refresh period, in millis
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* @param maxValues the maximum number of values to cache
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* @param servletContext a ServletContext to be used for logging
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* @param attrNames the names of the attributes to be cached
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*/
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synchronized public void cache(long interval,
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final int maxValues,
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ServletContext servletContext,
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final String... attrNames) {
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if (attrNames.length == 0) {
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return;
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}
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Timer timer = new Timer(true);
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TimerTask task = createRefresher(maxValues, servletContext, attrNames);
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task.run();
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timer.schedule(task, interval, interval);
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timers.add(timer);
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}
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/**
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* Creates a TimerTask that refreshes the cached most used values of
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* the specified attributes.
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*
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* @param maxValues how many values to cache for each attribute
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* @param servletContext servlet context for logging error messages
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* @param attrNames the names of the attributes
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* @return a TimerTask that refreshes the cache
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*/
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private TimerTask createRefresher(final int maxValues,
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final ServletContext servletContext,
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final String... attrNames) {
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TimerTask task = new TimerTask() {
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/**
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* Tells the MostUsedValues object that created this TimerTask to
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* refresh the cached most used values of some of the attributes.
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*/
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@Override public void run() {
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try {
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MostUsedValues.this.retrieveMostUsedValues(maxValues, attrNames);
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} catch (IOException e) {
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servletContext.log(e.getMessage(), e);
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} catch (ServiceException e) {
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servletContext.log(e.getMessage() + " " +
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e.getHttpErrorCodeOverride() + " " +
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e.getResponseContentType() + ": " +
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e.getResponseBody(), e);
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}
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}
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};
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return task;
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}
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/**
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* Retrieves the most used values for some attributes
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* for the items that match the query string
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* and stores a limited number of those values for each attribute.
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*
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* @param numValue maximum number of values to store
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* @param attrNames the names of the attributes
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*/
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protected void retrieveMostUsedValues(int numValue, final String... attrNames)
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throws ServiceException, IOException {
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URL url = urlFactory.getAttributesFeedURL();
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GoogleBaseQuery query = new GoogleBaseQuery(url);
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StringBuffer queryString = createQueryString(attrNames);
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query.setGoogleBaseQuery(queryString.toString());
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query.setMaxValues(numValue);
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int numResults = 0;
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int lastNumResults = 0;
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Collection<String> attrToRetrieve =
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new ArrayList<String>(Arrays.asList(attrNames));
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do {
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// Get the feed
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numResults += STEP_MAXRESULTS;
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query.setMaxResults(numResults);
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GoogleBaseFeed feed = service.query(query);
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if (lastNumResults == feed.getTotalResults()) {
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// No new entries to process
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break;
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}
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lastNumResults = feed.getTotalResults();
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// Extract the values from the entries
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Iterator<String> attrIter = attrToRetrieve.iterator();
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while (attrIter.hasNext()) {
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String attrName = attrIter.next();
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// Searching for the entry with the following name
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String entryTitle = attrName + TEXT_TYPE;
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for (GoogleBaseEntry entry : feed.getEntries()) {
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if (entryTitle.equals(entry.getTitle().getPlainText())) {
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extractValuesFromEntry(numValue, attrName, entry);
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attrIter.remove();
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}
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}
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}
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} while (!attrToRetrieve.isEmpty() && numResults <= MAX_MAXRESULTS);
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if (!attrToRetrieve.isEmpty()) {
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throw new ServiceException("The retrieved histograms do not contain" +
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"some of the attributes. The most used values of these attributes " +
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"have not been refreshed.");
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}
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}
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/**
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* Returns the query string extended so that it filters out the items
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* that do not have at least one of the specified attributes of type TEXT.
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*
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* @param attrNames the attributes we are interested in
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* @return an extended query string
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*/
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protected StringBuffer createQueryString(final String... attrNames) {
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StringBuffer queryString = new StringBuffer(this.queryString);
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queryString.append("(");
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queryString.append("[").append(attrNames[0]).append(TEXT_TYPE).append("]");
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for (int i = 1; i < attrNames.length; i++) {
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String attrName = attrNames[i];
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queryString.append("|[").append(attrName).append(TEXT_TYPE).append("]");
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}
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queryString.append(")");
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return queryString;
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}
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/**
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* Caches a limited number of the values of a GoogleBaseEntry.
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*
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* @param numValue maximum number of values to cache
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* @param attrName the name of the attribute that has the values
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* @param entry an entry with a MetadataEntryExtension
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*/
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private void extractValuesFromEntry(int numValue,
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String attrName,
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GoogleBaseEntry entry) {
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MetadataEntryExtension metadata = entry.getGoogleBaseMetadata();
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AttributeHistogram attributeHistogram = metadata.getAttributeHistogram();
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List<? extends UniqueValue> values = attributeHistogram.getValues();
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int valuesCount = Math.min(numValue, values.size());
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String[] usedValues = new String[valuesCount];
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for (int i = 0; i < valuesCount; i++) {
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usedValues[i] = values.get(i).getValueAsString();
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}
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updateMostUsedValue(attrName, usedValues);
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}
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/**
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* Cancels all refresh timers and clears the cache.
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*/
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synchronized public void clear() {
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for (Timer timer : timers) {
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timer.cancel();
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}
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timers.clear();
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mostUsedValues.clear();
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}
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/**
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* Returns the number of cached attributes.
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*/
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public int size() {
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return mostUsedValues.size();
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}
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/**
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* Returns true if no attribute is cached.
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*/
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public boolean isEmpty() {
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return mostUsedValues.isEmpty();
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}
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public String getQueryString() {
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return queryString;
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}
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/**
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* Caches the most used values of an attribute.
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* @param attrName
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* @param stringValues
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*/
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protected void updateMostUsedValue(String attrName, String[] stringValues) {
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mostUsedValues.put(attrName, stringValues);
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}
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}
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