Add support for pagination of list entries

fixes #29

plugins/src/main/java/org/smartregister/fhir/gateway/plugins/BaseEndpoint.java

@@ -28,7 +28,9 @@ public Bundle createEmptyBundle(String requestURL) {
         Bundle responseBundle = new Bundle();
         responseBundle.setId(UUID.randomUUID().toString());
         Bundle.BundleLinkComponent linkComponent = new Bundle.BundleLinkComponent();
+        linkComponent.setRelation(Bundle.LINK_PREV);
         linkComponent.setRelation(Bundle.LINK_SELF);
+        linkComponent.setRelation(Bundle.LINK_NEXT);
         linkComponent.setUrl(requestURL);
         responseBundle.setLink(Collections.singletonList(linkComponent));
         responseBundle.setType(Bundle.BundleType.SEARCHSET);

plugins/src/main/java/org/smartregister/fhir/gateway/plugins/SyncAccessDecision.java

@@ -168,7 +168,7 @@ public String postProcess(RequestDetailsReader request, HttpResponse response)
 
             switch (gatewayMode) {
                 case SyncAccessDecisionConstants.LIST_ENTRIES:
-                    resultContentBundle = postProcessModeListEntries(responseResource);
+                    resultContentBundle = postProcessModeListEntries(responseResource, request);
                     break;
 
                 default:
@@ -216,11 +216,29 @@ private static OperationOutcome createOperationOutcome(String exception) {
 
     @NotNull
     private static Bundle processListEntriesGatewayModeByListResource(
-            ListResource responseListResource) {
+            ListResource responseListResource, RequestDetailsReader request) {
         Bundle requestBundle = new Bundle();
         requestBundle.setType(Bundle.BundleType.BATCH);
 
-        for (ListResource.ListEntryComponent listEntryComponent : responseListResource.getEntry()) {
+        String[] pageSize = request.getParameters().get("_count");
+        String[] pageNumber = request.getParameters().get("_page");
+
+        int count =
+                pageSize != null && pageSize.length > 0
+                        ? Integer.parseInt(pageSize[0])
+                        : 20; // Default page size to 20 if not provided
+        int page =
+                pageNumber != null && pageNumber.length > 0
+                        ? Integer.parseInt(pageNumber[0])
+                        : 1; // Default page number to 1 if not provided
+
+        int start = Math.max(0, (page - 1)) * count;
+        int end = start + count;
+
+        List<ListResource.ListEntryComponent> entries = responseListResource.getEntry();
+
+        for (int i = start; i < Math.min(end, entries.size()); i++) {
+            ListResource.ListEntryComponent listEntryComponent = entries.get(i);
             requestBundle.addEntry(
                     createBundleEntryComponent(
                             Bundle.HTTPVerb.GET,
@@ -271,18 +289,20 @@ static Bundle.BundleEntryComponent createBundleEntryComponent(
      * Generates a Bundle result from making a batch search request with the contained entries in
      * the List as parameters
      *
-     * @param responseResource FHIR Resource result returned byt the HTTPResponse
+     * @param responseResource FHIR Resource result returned by the HTTPResponse
      * @return String content of the result Bundle
      */
-    private Bundle postProcessModeListEntries(IBaseResource responseResource) {
+    private Bundle postProcessModeListEntries(
+            IBaseResource responseResource, RequestDetailsReader request) {
 
         Bundle requestBundle = null;
 
         if (responseResource instanceof ListResource
                 && ((ListResource) responseResource).hasEntry()) {
 
             requestBundle =
-                    processListEntriesGatewayModeByListResource((ListResource) responseResource);
+                    processListEntriesGatewayModeByListResource(
+                            (ListResource) responseResource, request);
 
         } else if (responseResource instanceof Bundle) {
 

plugins/src/test/java/org/smartregister/fhir/gateway/plugins/SyncAccessDecisionTest.java

@@ -494,6 +494,80 @@ public void testPostProcessWithListModeHeaderSearchByTagShouldFetchListEntriesBu
                 resultContent);
     }
 
+    @Test
+    public void testPostProcessWithListModePaginateEntriesBundle() throws IOException {
+        locationIds.add("Location-1");
+        testInstance = Mockito.spy(createSyncAccessDecisionTestInstance(Constants.LOCATION));
+
+        FhirContext fhirR4Context = mock(FhirContext.class);
+        IGenericClient iGenericClient = mock(IGenericClient.class);
+        ITransaction iTransaction = mock(ITransaction.class);
+        ITransactionTyped<Bundle> iClientExecutable = mock(ITransactionTyped.class);
+        testInstance.setFhirR4Client(iGenericClient);
+        testInstance.setFhirR4Context(fhirR4Context);
+
+        Mockito.when(iGenericClient.transaction()).thenReturn(iTransaction);
+        Mockito.when(iTransaction.withBundle(any(Bundle.class))).thenReturn(iClientExecutable);
+
+        Bundle resultBundle = new Bundle();
+        resultBundle.setType(Bundle.BundleType.BATCHRESPONSE);
+        resultBundle.setId("bundle-result-id");
+
+        Mockito.when(iClientExecutable.execute()).thenReturn(resultBundle);
+
+        ArgumentCaptor<Bundle> bundleArgumentCaptor = ArgumentCaptor.forClass(Bundle.class);
+
+        testInstance.setFhirR4Context(fhirR4Context);
+
+        RequestDetailsReader requestDetailsSpy = Mockito.mock(RequestDetailsReader.class);
+
+        Mockito.when(
+                        requestDetailsSpy.getHeader(
+                                SyncAccessDecision.SyncAccessDecisionConstants.FHIR_GATEWAY_MODE))
+                .thenReturn(SyncAccessDecision.SyncAccessDecisionConstants.LIST_ENTRIES);
+
+        Map<String, String[]> params = new HashMap<>();
+        params.put("_count", new String[] {"1"});
+        params.put("_page", new String[] {"1"});
+
+        Mockito.when(requestDetailsSpy.getParameters()).thenReturn(params);
+
+        URL listUrl = Resources.getResource("test_list_resource.json");
+        String testListJson = Resources.toString(listUrl, StandardCharsets.UTF_8);
+
+        HttpResponse fhirResponseMock =
+                Mockito.mock(HttpResponse.class, Answers.RETURNS_DEEP_STUBS);
+
+        TestUtil.setUpFhirResponseMock(fhirResponseMock, testListJson);
+
+        String resultContent = testInstance.postProcess(requestDetailsSpy, fhirResponseMock);
+
+        Mockito.verify(iTransaction).withBundle(bundleArgumentCaptor.capture());
+        Bundle requestBundle = bundleArgumentCaptor.getValue();
+
+        // Verify modified request to the server
+        Assert.assertNotNull(requestBundle);
+        Assert.assertEquals(Bundle.BundleType.BATCH, requestBundle.getType());
+        List<Bundle.BundleEntryComponent> requestBundleEntries = requestBundle.getEntry();
+
+        // Only one returned one _page = 1 and _count = 1
+        Assert.assertEquals(1, requestBundleEntries.size());
+
+        Assert.assertEquals(
+                Bundle.HTTPVerb.GET, requestBundleEntries.get(0).getRequest().getMethod());
+        Assert.assertEquals(
+                "Group/proxy-list-entry-id-1", requestBundleEntries.get(0).getRequest().getUrl());
+
+        Assert.assertEquals(
+                Bundle.HTTPVerb.GET, requestBundleEntries.get(0).getRequest().getMethod());
+
+        // Verify returned result content from the server request
+        Assert.assertNotNull(resultContent);
+        Assert.assertEquals(
+                "{\"resourceType\":\"Bundle\",\"id\":\"bundle-result-id\",\"type\":\"batch-response\"}",
+                resultContent);
+    }
+
     @After
     public void cleanUp() {
         locationIds.clear();