lazy streams with pager spliterator (#295)

Author: mariuszs

src/main/java/org/gitlab4j/api/Pager.java

@@ -7,6 +7,7 @@
 import java.util.List;
 import java.util.NoSuchElementException;
 import java.util.stream.Stream;
+import java.util.stream.StreamSupport;
 
 import javax.ws.rs.core.MultivaluedMap;
 import javax.ws.rs.core.Response;
@@ -333,4 +334,12 @@ public Stream<T> stream() throws GitLabApiException {
 
     	return (streamBuilder.build());
     }
+
+    public Stream<T> lazyStream() {
+        // Make sure that current page is 0, this will ensure the whole list is streamed
+        // regardless of what page the instance is currently on.
+        currentPage = 0;
+
+        return StreamSupport.stream(new PagerSpliterator<T>(this), false);
+    }
 }

src/main/java/org/gitlab4j/api/PagerSpliterator.java

@@ -0,0 +1,47 @@
+package org.gitlab4j.api;
+
+import java.util.Iterator;
+import java.util.Spliterator;
+import java.util.function.Consumer;
+
+class PagerSpliterator<T> implements Spliterator<T> {
+
+	private Pager<T> pager;
+
+	private Iterator<T> elements;
+
+	PagerSpliterator(Pager<T> pager) {
+		this.pager = pager;
+		if (pager.hasNext()) {
+			elements = this.pager.next().iterator();
+		}
+	}
+
+	@Override
+	public boolean tryAdvance(Consumer<? super T> action) {
+		if (elements.hasNext()) {
+			action.accept(elements.next());
+			return true;
+		} else if (pager.hasNext()) {
+			elements = pager.next().iterator();
+			action.accept(elements.next());
+			return true;
+		}
+		return false;
+	}
+
+	@Override
+	public Spliterator<T> trySplit() {
+		return null;
+	}
+
+	@Override
+	public long estimateSize() {
+		return pager.getTotalItems();
+	}
+
+	@Override
+	public int characteristics() {
+		return 0;
+	}
+}