Author: srowen
Date: Wed Dec 7 13:49:29 2011
New Revision: 1211439
URL: http://svn.apache.org/viewvc?rev=1211439&view=rev
Log:
MAHOUT-910 merge ideas from MAHOUT-914, better docs, new no-limit arg,
different defaults from Sebastian
Modified:
mahout/trunk/core/src/main/java/org/apache/mahout/cf/taste/impl/recommender/SamplingCandidateItemsStrategy.java
Modified:
mahout/trunk/core/src/main/java/org/apache/mahout/cf/taste/impl/recommender/SamplingCandidateItemsStrategy.java
URL:
http://svn.apache.org/viewvc/mahout/trunk/core/src/main/java/org/apache/mahout/cf/taste/impl/recommender/SamplingCandidateItemsStrategy.java?rev=1211439&r1=1211438&r2=1211439&view=diff
==============================================================================
---
mahout/trunk/core/src/main/java/org/apache/mahout/cf/taste/impl/recommender/SamplingCandidateItemsStrategy.java
(original)
+++
mahout/trunk/core/src/main/java/org/apache/mahout/cf/taste/impl/recommender/SamplingCandidateItemsStrategy.java
Wed Dec 7 13:49:29 2011
@@ -38,29 +38,56 @@ import java.util.Iterator;
* things, noted above:</p>
*
* <ol>
- * <li>The items</li>
+ * <li>The items that the user has preferred</li>
+ * <li>The users who also prefer each of those items</li>
+ * <li>The items those users also prefer</li>
* </ol>
*
- * <p><pre>
- * max(defaultMaxPrefsPerItemConsidered, userItemCountFactor *
log(max(N_users, N_items)))
- * </pre></p>
+ * <p>There is a maximum associated with each of these three things; if the
number of items or users exceeds
+ * that max, it is sampled so that the expected number of items or users
actually used in that part of the
+ * computation is equal to the max.</p>
*
- * <p>This limit is applied in two ways. First, for each item that the current
user prefers, it limits
- * the number of other users who preferred that item that are then considered.
Then for each of those users,
- * it limits the number of their preferred items that are added to the list of
candidates.</p>
+ * <p>Three arguments control these three maxima. Each is a "factor" f, which
establishes the max at
+ * f * log2(n), where n is the number of users or items in the data. For
example if factor #2 is 5,
+ * which controls the number of users sampled per item, then 5 * log2(# users)
is the maximum for this
+ * part of the computation.</p>
+ *
+ * <p>Each can be set to not do any limiting with value {@link
#NO_LIMIT_FACTOR}.</p>
*/
public class SamplingCandidateItemsStrategy extends
AbstractCandidateItemsStrategy {
- private static final int DEFAULT_FACTOR = 5;
+ /**
+ * Default factor used if not otherwise specified, for all limits. (30).
+ */
+ public static final int DEFAULT_FACTOR = 30;
+ /**
+ * Specify this value as a factor to mean no limit.
+ */
+ public static final int NO_LIMIT_FACTOR = Integer.MAX_VALUE;
+ private static final int MAX_LIMIT = Integer.MAX_VALUE;
+ private static final double LOG2 = Math.log(2.0);
private final int maxItems;
private final int maxUsersPerItem;
private final int maxItemsPerUser;
+ /**
+ * Defaults to using no limit ({@link #NO_LIMIT_FACTOR}) for all factors,
except
+ * {@code candidatesPerUserFactor} which defaults to {@link #DEFAULT_FACTOR}.
+ *
+ * @see #SamplingCandidateItemsStrategy(int, int, int, int, int)
+ */
public SamplingCandidateItemsStrategy(int numUsers, int numItems) {
this(DEFAULT_FACTOR, DEFAULT_FACTOR, DEFAULT_FACTOR, numUsers, numItems);
}
+ /**
+ * @param itemsFactor factor controlling max items considered for a user
+ * @param usersPerItemFactor factor controlling max users considered for
each of those items
+ * @param candidatesPerUserFactor factor controlling max candidate items
considered from each of those users
+ * @param numUsers number of users currently in the data
+ * @param numItems number of items in the data
+ */
public SamplingCandidateItemsStrategy(int itemsFactor,
int usersPerItemFactor,
int candidatesPerUserFactor,
@@ -71,9 +98,17 @@ public class SamplingCandidateItemsStrat
Preconditions.checkArgument(candidatesPerUserFactor > 0);
Preconditions.checkArgument(numUsers > 0);
Preconditions.checkArgument(numItems > 0);
- maxItems = (int) (itemsFactor * (1.0 + Math.log(numItems)));
- maxUsersPerItem = (int) (itemsFactor * (1.0 + Math.log(numUsers)));
- maxItemsPerUser = (int) (itemsFactor *(1.0 + Math.log(numItems)));
+ maxItems = computeMaxFrom(itemsFactor, numItems);
+ maxUsersPerItem = computeMaxFrom(usersPerItemFactor, numUsers);
+ maxItemsPerUser = computeMaxFrom(candidatesPerUserFactor, numItems);
+ }
+
+ private static int computeMaxFrom(int factor, int numThings) {
+ if (factor == NO_LIMIT_FACTOR) {
+ return MAX_LIMIT;
+ }
+ long max = (long) (factor * (1.0 + Math.log(numThings) / LOG2));
+ return max > MAX_LIMIT ? MAX_LIMIT : (int) max;
}
@Override
