Fix computed pagination bugs

inventory-framework-core/src/main/java/me/devnatan/inventoryframework/component/PaginationImpl.java

@@ -155,7 +155,10 @@ private CompletableFuture<List<?>> loadSourceForTheCurrentPage() {
             isLoading = false;
             simulateStateUpdate();
 
-            if (isLazy())
+            // Computed pagination always recomputes the full source (never reuses it, see the
+            // condition above), so it must be sliced down to the current page just like lazy
+            // pagination is, otherwise every page would render the same leading slice of items.
+            if (isLazy() || isComputed())
                 return Pagination.splitSourceForPage(currentPageIndex(), getPageSize(), getPagesCount(), result);
             else return result;
         });
@@ -550,8 +553,10 @@ public void updated(@NotNull IFSlotRenderContext context) {
                 "[Pagination] #updated(IFSlotRenderContext) called (forceUpdated = %b, pageWasChanged = %b)",
                 forceUpdated, pageWasChanged);
 
+        // Computed pagination must be recomputed on every update, not just on page change or a
+        // forced update, since its whole premise is that the source is never reused/cached.
         // If page was changed all components will be removed, so don't trigger update on them
-        if (forceUpdated || pageWasChanged) {
+        if (forceUpdated || pageWasChanged || isComputed()) {
             clear(renderContext);
             components = new ArrayList<>();
             loadCurrentPage(renderContext).thenRun(() -> {
@@ -675,7 +680,6 @@ public boolean isLastPage() {
 
     @Override
     public boolean hasPage(int pageIndex) {
-        if (isComputed()) return true;
         if (pageIndex < 0) return false;
         return pageIndex < getPagesCount();
     }

inventory-framework-core/src/test/java/me/devnatan/inventoryframework/component/PaginationImplTest.java

@@ -1,5 +1,7 @@
 package me.devnatan.inventoryframework.component;
 
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.atLeastOnce;
 import static org.mockito.Mockito.doCallRealMethod;
@@ -9,13 +11,84 @@
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
+import java.lang.reflect.Field;
 import java.util.Collections;
+import java.util.List;
+import java.util.function.Supplier;
+import me.devnatan.inventoryframework.context.IFContext;
 import me.devnatan.inventoryframework.context.IFSlotClickContext;
+import me.devnatan.inventoryframework.state.State;
 import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 public class PaginationImplTest {
 
+    @SuppressWarnings("unchecked")
+    private static PaginationImpl newComputedPagination() {
+        final Supplier<List<Integer>> sourceProvider = () -> List.of(1, 2, 3);
+        return new PaginationImpl(
+                mock(State.class),
+                mock(IFContext.class),
+                'X',
+                sourceProvider,
+                mock(PaginationElementFactory.class),
+                null,
+                /* isAsync = */ false,
+                /* isComputed = */ true,
+                Pagination.Orientation.HORIZONTAL);
+    }
+
+    private static void setPagesCount(PaginationImpl pagination, int pagesCount) throws ReflectiveOperationException {
+        final Field field = PaginationImpl.class.getDeclaredField("pagesCount");
+        field.setAccessible(true);
+        field.set(pagination, pagesCount);
+    }
+
+    private static void setCurrentPageIndex(PaginationImpl pagination, int pageIndex)
+            throws ReflectiveOperationException {
+        final Field field = PaginationImpl.class.getDeclaredField("currPageIndex");
+        field.setAccessible(true);
+        field.set(pagination, pageIndex);
+    }
+
+    @Test
+    void computedPaginationCannotAdvanceOrBackWithoutMorePages() throws ReflectiveOperationException {
+        final PaginationImpl pagination = newComputedPagination();
+        setPagesCount(pagination, 1);
+        setCurrentPageIndex(pagination, 0);
+
+        // A single-page computed pagination must not report being able to advance or go back,
+        // regression test for https://github.com/DevNatan/inventory-framework/issues/682
+        assertFalse(pagination.canAdvance());
+        assertFalse(pagination.canBack());
+    }
+
+    @Test
+    void computedPaginationCanAdvanceAndBackWithinBounds() throws ReflectiveOperationException {
+        final PaginationImpl pagination = newComputedPagination();
+        setPagesCount(pagination, 3);
+
+        setCurrentPageIndex(pagination, 0);
+        assertTrue(pagination.canAdvance());
+        assertFalse(pagination.canBack());
+
+        setCurrentPageIndex(pagination, 1);
+        assertTrue(pagination.canAdvance());
+        assertTrue(pagination.canBack());
+
+        setCurrentPageIndex(pagination, 2);
+        assertFalse(pagination.canAdvance());
+        assertTrue(pagination.canBack());
+    }
+
+    @Test
+    void hasPageRejectsNegativeIndexEvenWhenComputed() throws ReflectiveOperationException {
+        final PaginationImpl pagination = newComputedPagination();
+        setPagesCount(pagination, 5);
+
+        assertFalse(pagination.hasPage(-1));
+    }
+
     @Test
     @Disabled("This test is not working as expected, because it is passed to parent")
     void callAnyChildInteractionHandlerOnClickInteraction() {