refactor: extract paging helpers

feature/messageshistory/src/main/kotlin/dev/meloda/fast/messageshistory/MessagesHistoryViewModelImpl.kt

@@ -31,6 +31,10 @@ import dev.meloda.fast.common.extensions.orDots
 import dev.meloda.fast.common.extensions.removeIfCompat
 import dev.meloda.fast.common.extensions.setValue
 import dev.meloda.fast.common.provider.ResourceProvider
+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.UserConfig
 import dev.meloda.fast.data.VkUtils
@@ -915,11 +919,7 @@ class MessagesHistoryViewModelImpl(
                 error = ::handleError,
                 success = { response ->
                     val messages = response.messages
-                    val fullMessages = if (offset == 0) {
-                        messages
-                    } else {
-                        this.messages.value.plus(messages)
-                    }.sorted()
+                    val fullMessages = mergePage(this.messages.value, messages, offset).sorted()
 
                     val convos = response.convos
 
@@ -930,10 +930,12 @@ class MessagesHistoryViewModelImpl(
                     messagesUseCase.storeMessages(messages)
                     convoUseCase.storeConvos(convos)
 
-                    val itemsCountSufficient = messages.size == MESSAGES_LOAD_COUNT
-
-                    val paginationExhausted = !itemsCountSufficient &&
-                            this.messages.value.isNotEmpty()
+                    val itemsCountSufficient = canPaginatePage(MESSAGES_LOAD_COUNT, messages.size)
+                    val paginationExhausted = isPaginationExhaustedPage(
+                        pageSize = MESSAGES_LOAD_COUNT,
+                        loadedCount = messages.size,
+                        hasExistingItems = this.messages.value.isNotEmpty()
+                    )
                     screenState.setValue { old ->
                         old.copy(isPaginationExhausted = paginationExhausted)
                     }
@@ -944,10 +946,11 @@ class MessagesHistoryViewModelImpl(
                 }
             )
 
+            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
                 )
             }
         }