refactor: extract paging helpers

feature/chatmaterials/src/main/kotlin/dev/meloda/fast/chatmaterials/ChatMaterialsViewModel.kt

@@ -9,6 +9,10 @@ import dev.meloda.fast.chatmaterials.navigation.ChatMaterials
 import dev.meloda.fast.chatmaterials.util.asPresentation
 import dev.meloda.fast.common.extensions.listenValue
 import dev.meloda.fast.common.extensions.setValue
+import dev.meloda.fast.common.paging.canPaginate as canPaginatePage
+import dev.meloda.fast.common.paging.isPaginationExhausted as isPaginationExhaustedPage
+import dev.meloda.fast.common.paging.loadingFlags
+import dev.meloda.fast.common.paging.mergePage
 import dev.meloda.fast.data.State
 import dev.meloda.fast.data.VkUtils
 import dev.meloda.fast.data.processState
@@ -89,11 +93,14 @@ class ChatMaterialsViewModelImpl(
             state.processState(
                 error = ::handleError,
                 success = { response ->
-                    val itemsCountSufficient = response.size == LOAD_COUNT
+                    val itemsCountSufficient = canPaginatePage(LOAD_COUNT, response.size)
                     canPaginate.setValue { itemsCountSufficient }
 
-                    val paginationExhausted = !itemsCountSufficient
-                            && screenState.value.materials.isNotEmpty()
+                    val paginationExhausted = isPaginationExhaustedPage(
+                        pageSize = LOAD_COUNT,
+                        loadedCount = response.size,
+                        hasExistingItems = screenState.value.materials.isNotEmpty()
+                    )
 
                     val loadedMaterials = response.mapNotNull(VkAttachmentHistoryMessage::asPresentation)
 
@@ -107,24 +114,19 @@ class ChatMaterialsViewModelImpl(
                         }
                     )
 
-                    if (offset == 0) {
-                        screenState.setValue {
-                            newState.copy(materials = loadedMaterials)
-                        }
-                    } else {
-                        screenState.setValue {
-                            newState.copy(
-                                materials = newState.materials.plus(loadedMaterials)
-                            )
-                        }
+                    screenState.setValue {
+                        newState.copy(
+                            materials = mergePage(newState.materials, loadedMaterials, offset)
+                        )
                     }
                 }
             )
 
+            val flags = loadingFlags(offset, state.isLoading())
             screenState.setValue { old ->
                 old.copy(
-                    isLoading = offset == 0 && state.isLoading(),
-                    isPaginating = offset > 0 && state.isLoading()
+                    isLoading = flags.isLoading,
+                    isPaginating = flags.isPaginating
                 )
             }
         }