Consider KeysetScrollPosition direction in WindowIterator.

We now consider the scroll direction in the iterator to properly continue Keyset backward scrolling.

Closes #2851

src/main/java/org/springframework/data/support/WindowIterator.java

@@ -19,6 +19,7 @@ import java.util.Iterator;
 import java.util.NoSuchElementException;
 import java.util.function.Function;
 
+import org.springframework.data.domain.KeysetScrollPosition;
 import org.springframework.data.domain.ScrollPosition;
 import org.springframework.data.domain.Window;
 import org.springframework.lang.Nullable;
@@ -92,7 +93,7 @@ public class WindowIterator<T> implements Iterator<T> {
 
 				if (currentWindow != null && currentWindow.hasNext()) {
 
-					currentPosition = currentWindow.positionAt(currentWindow.size() - 1);
+					currentPosition = getNextPosition(currentPosition, currentWindow);
 					currentIterator = null;
 					currentWindow = null;
 					continue;
@@ -113,6 +114,17 @@ public class WindowIterator<T> implements Iterator<T> {
 		return currentIterator.next();
 	}
 
+	private static ScrollPosition getNextPosition(ScrollPosition currentPosition, Window<?> window) {
+
+		if (currentPosition instanceof KeysetScrollPosition ksp) {
+			if (ksp.scrollsBackward()) {
+				return window.positionAt(0);
+			}
+		}
+
+		return window.positionAt(window.size() - 1);
+	}
+
 	/**
 	 * Builder API to construct a {@link WindowIterator}.
 	 *

src/test/java/org/springframework/data/domain/WindowIteratorUnitTests.java

@@ -22,6 +22,7 @@ import static org.mockito.Mockito.*;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import java.util.NoSuchElementException;
 import java.util.function.Function;
 
@@ -30,7 +31,9 @@ import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.junit.jupiter.MockitoExtension;
 import org.mockito.junit.jupiter.MockitoSettings;
 import org.mockito.quality.Strictness;
+import org.springframework.data.domain.ScrollPosition.Direction;
 import org.springframework.data.support.WindowIterator;
+import org.springframework.data.util.Streamable;
 
 /**
  * Unit tests for {@link WindowIterator}.
@@ -127,4 +130,28 @@ class WindowIteratorUnitTests {
 
 		assertThat(capturedResult).containsExactly("a", "b", "c", "d");
 	}
+
+	@Test // GH-2151
+	void considersBackwardKeysetScrolling() {
+
+		Window<String> initial = Window.from(List.of("c", "d"),
+				value -> KeysetScrollPosition.of(Map.of("k", 10 + value), Direction.BACKWARD), true);
+		Window<String> terminal = Window.from(List.of("a", "b"),
+				value -> KeysetScrollPosition.of(Map.of("k", value), Direction.BACKWARD));
+
+		WindowIterator<String> iterator = WindowIterator.of(it -> {
+
+			if (it instanceof KeysetScrollPosition ksp) {
+				if (Integer.valueOf(10).equals(ksp.getKeys().get("k"))) {
+					return terminal;
+				}
+			}
+
+			return initial;
+
+		}).startingAt(ScrollPosition.keyset().backward());
+
+		List<String> items = Streamable.of(() -> iterator).toList();
+		assertThat(items).containsExactly("c", "d", "a", "b");
+	}
 }