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The following commit(s) were added to refs/heads/feature_NUMBERS-215 by this
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new dcee12e7 NUMBERS-215: Iterate over k-partitions.
dcee12e7 is described below
commit dcee12e775bae241608103e824c7746dfbe2f091
Author: Gilles Sadowski <[email protected]>
AuthorDate: Sat Sep 19 13:55:00 2026 +0200
NUMBERS-215: Iterate over k-partitions.
Added documentation.
One method changed to "static".
---
.../commons/numbers/combinatorics/Stirling.java | 57 ++++++++++++++++------
.../numbers/combinatorics/StirlingTest.java | 3 +-
2 files changed, 42 insertions(+), 18 deletions(-)
diff --git
a/commons-numbers-combinatorics/src/main/java/org/apache/commons/numbers/combinatorics/Stirling.java
b/commons-numbers-combinatorics/src/main/java/org/apache/commons/numbers/combinatorics/Stirling.java
index df04d8a6..2fc8a4a4 100644
---
a/commons-numbers-combinatorics/src/main/java/org/apache/commons/numbers/combinatorics/Stirling.java
+++
b/commons-numbers-combinatorics/src/main/java/org/apache/commons/numbers/combinatorics/Stirling.java
@@ -305,11 +305,33 @@ public final class Stirling {
/**
* From a collection of {@code n} items, generates all partitions that
contains {@code k} subsets.
- * The number of partitions is {@link #stirlingS2(int,int) stirlingS2(n,
k)}.
+ * For example:
+ * <pre>{@code
+ * Stirling.S2.of(4, 2)
+ * .stream()
+ * .forEach(p -> System.out.println(java.util.Arrays.deepToString(p)));
+ * }</pre>
+ * will output
+ * <pre>
+ * [[0, 1, 2], [3]]
+ * [[0, 1, 3], [2]]
+ * [[0, 1], [2, 3]]
+ * [[0, 2, 3], [1]]
+ * [[0, 2], [1, 3]]
+ * [[0, 3], [1, 2]]
+ * [[0], [1, 2, 3]]
+ * </pre>
+ *
+ * <p>
+ * Method {@link #get() S2.of(n, k).get()} returns the number of
partitions.
+ * </p>
+ *
+ * <p>
* A <a
href="https://mathworld.wolfram.com/RestrictedGrowthString.html";>restrictive
growth string
* (RGS)</a> is used internally. RGS uses integers to represent items:
Position (index) in the
* RGS array is the same as in the original list to be partitioned, value
is the "group" to which
* this element belongs in a given partition.
+ * </p>
*/
public static final class S2 {
/** Number of sublists in every partition (aka "k"). */
@@ -393,23 +415,18 @@ public final class Stirling {
}
/**
- * Iteration wrapped in a stream, where each element is a partition
- * of the given {@code items}.
+ * Factory method for iterating on the partitions of the given list
+ * of {@code items}.
*
+ * @param k Number of sublists in each partition.
* @param items Items to be partitioned.
* @return a stream (without duplicate or "null" elements).
- * @throws IllegalArgumentException if the number of {@code items} does
- * not match the {@link #of(int,int) first argument of the factory
method}.
*
* @param <T> Item type.
*/
- public <T> Stream<List<List<T>>> stream(List<T> items) {
- if (items.size() != numberOfElements) {
- throw new
CombinatoricsException(CombinatoricsException.MISMATCH,
- numberOfElements,
items.size());
- }
-
- return stream().map(o -> mapPartition(o, items));
+ public static <T> Stream<List<List<T>>> stream(List<T> items,
+ int k) {
+ return of(items.size(), k).stream().map(o -> mapPartition(o,
items, k));
}
/**
@@ -423,7 +440,13 @@ public final class Stirling {
* @param <T> Item type.
*/
public <T> Stream<List<List<T>>> stream(T... items) {
- return stream(Arrays.asList(items));
+ if (items.length != numberOfElements) {
+ throw new
CombinatoricsException(CombinatoricsException.MISMATCH,
+ numberOfElements,
items.length);
+ }
+
+ final List<T> list = Arrays.asList(items);
+ return stream().map(o -> mapPartition(o, list, numberOfSubsets));
}
/**
@@ -465,13 +488,15 @@ public final class Stirling {
*
* @param p Partition.
* @param items List of objects.
+ * @param k Number of sublists.
* @return the mapped partition.
*
* @param <T> Item type.
*/
- private <T> List<List<T>> mapPartition(int[][] p,
- List<T> items) {
- final List<List<T>> out = new ArrayList<>(numberOfSubsets);
+ private static <T> List<List<T>> mapPartition(int[][] p,
+ List<T> items,
+ int k) {
+ final List<List<T>> out = new ArrayList<>(k);
for (int[] subset : p) {
final List<T> customSubset = new ArrayList<>(subset.length);
diff --git
a/commons-numbers-combinatorics/src/test/java/org/apache/commons/numbers/combinatorics/StirlingTest.java
b/commons-numbers-combinatorics/src/test/java/org/apache/commons/numbers/combinatorics/StirlingTest.java
index bab5c8a9..e53ae9a3 100644
---
a/commons-numbers-combinatorics/src/test/java/org/apache/commons/numbers/combinatorics/StirlingTest.java
+++
b/commons-numbers-combinatorics/src/test/java/org/apache/commons/numbers/combinatorics/StirlingTest.java
@@ -401,8 +401,7 @@ class StirlingTest {
items.add(a);
items.add(b);
items.add(c);
- final List<List<List<String>>> out = Stirling.S2.of(3, 2)
- .stream(items)
+ final List<List<List<String>>> out = Stirling.S2.stream(items, 2)
.collect(Collectors.toList());
Assertions.assertEquals(3, out.size());
