BatchingSpliterator

java-batching-spliterator/pom.xml

@@ -0,0 +1,64 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>com.pivovarit</groupId>
+    <version>1.0</version>
+    <artifactId>java-batching-spliterator</artifactId>
+
+    <name>java-batching-spliterator</name>
+
+    <properties>
+        <jmh.version>1.35</jmh.version>
+    </properties>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>21</source>
+                    <target>21</target>
+                </configuration>
+            </plugin>
+            <plugin>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>2.19.1</version>
+                <dependencies>
+                    <dependency>
+                        <groupId>org.junit.platform</groupId>
+                        <artifactId>junit-platform-surefire-provider</artifactId>
+                        <version>1.1.0</version>
+                    </dependency>
+                </dependencies>
+            </plugin>
+        </plugins>
+    </build>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <version>5.8.2</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>3.22.0</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.openjdk.jmh</groupId>
+            <artifactId>jmh-core</artifactId>
+            <version>${jmh.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.openjdk.jmh</groupId>
+            <artifactId>jmh-generator-annprocess</artifactId>
+            <version>${jmh.version}</version>
+            <scope>provided</scope>
+        </dependency>
+    </dependencies>
+</project>

java-batching-spliterator/src/main/java/com/pivovarit/stream/BatchingSpliterator.java

@@ -0,0 +1,96 @@
+package com.pivovarit.stream;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Spliterator;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+import static java.util.stream.Stream.empty;
+import static java.util.stream.Stream.of;
+import static java.util.stream.StreamSupport.stream;
+
+/**
+ * @author Grzegorz Piwowarek
+ */
+final class BatchingSpliterator<T> implements Spliterator<List<T>> {
+
+    private final List<T> source;
+    private final int maxChunks;
+
+    private int chunks;
+    private int chunkSize;
+    private int consumed;
+
+    private BatchingSpliterator(List<T> list, int batches) {
+        if (batches < 1) {
+            throw new IllegalArgumentException("batches can't be lower than one");
+        }
+        source = list;
+        chunks = batches;
+        maxChunks = Math.min(list.size(), batches);
+        chunkSize = (int) Math.ceil(((double) source.size()) / batches);
+    }
+
+    static <T> Stream<List<T>> partitioned(List<T> list, int numberOfParts) {
+        int size = list.size();
+
+        if (size <= numberOfParts) {
+            return asSingletonListStream(list);
+        } else if (size == 0) {
+            return empty();
+        } else if (numberOfParts == 1) {
+            return of(list);
+        } else {
+            return stream(new BatchingSpliterator<>(list, numberOfParts), false);
+        }
+    }
+
+    private static <T> Stream<List<T>> asSingletonListStream(List<T> list) {
+        Stream.Builder<List<T>> acc = Stream.builder();
+        for (T t : list) {
+            acc.add(Collections.singletonList(t));
+        }
+        return acc.build();
+    }
+
+    static <T, R> Function<List<T>, List<R>> batching(Function<T, R> mapper) {
+        return batch -> {
+            List<R> list = new ArrayList<>(batch.size());
+            for (T t : batch) {
+                list.add(mapper.apply(t));
+            }
+            return list;
+        };
+    }
+
+    @Override
+    public boolean tryAdvance(Consumer<? super List<T>> action) {
+        if (consumed < source.size() && chunks != 0) {
+            List<T> batch = source.subList(consumed, consumed + chunkSize);
+            consumed += chunkSize;
+            chunkSize = (int) Math.ceil(((double) (source.size() - consumed)) / --chunks);
+            action.accept(batch);
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    @Override
+    public Spliterator<List<T>> trySplit() {
+        return null;
+    }
+
+    @Override
+    public long estimateSize() {
+        return maxChunks;
+    }
+
+    @Override
+    public int characteristics() {
+        return ORDERED | SIZED;
+    }
+}

pom.xml

@@ -23,6 +23,7 @@
         <module>java-archunit</module>
         <module>java-sealed-classes</module>
         <module>java-completable-future-timeouts</module>
+        <module>java-batching-spliterator</module>
         <module>java-event-sourcing</module>
         <module>java-completable-future-allof</module>
         <module>java-advanced-groupingby</module>