Add "new conversation" composer for direct messages

ts/components/leftPane/getConversationInDirection.ts

@@ -0,0 +1,63 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { find as findFirst, findLast, first, last } from 'lodash';
+
+import { PropsData as ConversationListItemPropsType } from '../conversationList/ConversationListItem';
+import { isConversationUnread } from '../../util/isConversationUnread';
+import { FindDirection, ToFindType } from './LeftPaneHelper';
+
+/**
+ * This will look up or down in an array of conversations for the next one to select.
+ * Refer to the tests for the intended behavior.
+ */
+export const getConversationInDirection = (
+  conversations: ReadonlyArray<ConversationListItemPropsType>,
+  toFind: Readonly<ToFindType>,
+  selectedConversationId: undefined | string
+): undefined | { conversationId: string } => {
+  // As an optimization, we don't need to search if no conversation is selected.
+  const selectedConversationIndex = selectedConversationId
+    ? conversations.findIndex(({ id }) => id === selectedConversationId)
+    : -1;
+
+  let conversation: ConversationListItemPropsType | undefined;
+
+  if (selectedConversationIndex < 0) {
+    if (toFind.unreadOnly) {
+      conversation =
+        toFind.direction === FindDirection.Up
+          ? findLast(conversations, isConversationUnread)
+          : findFirst(conversations, isConversationUnread);
+    } else {
+      conversation =
+        toFind.direction === FindDirection.Up
+          ? last(conversations)
+          : first(conversations);
+    }
+  } else if (toFind.unreadOnly) {
+    conversation =
+      toFind.direction === FindDirection.Up
+        ? findLast(
+            conversations.slice(0, selectedConversationIndex),
+            isConversationUnread
+          )
+        : findFirst(
+            conversations.slice(selectedConversationIndex + 1),
+            isConversationUnread
+          );
+  } else {
+    const newIndex =
+      selectedConversationIndex +
+      (toFind.direction === FindDirection.Up ? -1 : 1);
+    if (newIndex < 0) {
+      conversation = last(conversations);
+    } else if (newIndex >= conversations.length) {
+      conversation = first(conversations);
+    } else {
+      conversation = conversations[newIndex];
+    }
+  }
+
+  return conversation ? { conversationId: conversation.id } : undefined;
+};