fix: Move content into to frontend directory

Doing this BEFORE the merge prevents a lot of merge conflicts.

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

frontend/src/redux/history/reducers.ts

@@ -0,0 +1,30 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { Reducer } from 'redux'
+import type { HistoryActions } from './types'
+import { HistoryActionType } from './types'
+import type { HistoryEntryWithOrigin } from '../../api/history/types'
+
+// Q: Why is the reducer initialized with an empty array instead of the actual history entries like in the config reducer?
+// A: The history reducer will be created without entries because of async entry retrieval.
+//    Entries will be added after reducer initialization.
+
+export const HistoryReducer: Reducer<HistoryEntryWithOrigin[], HistoryActions> = (
+  state: HistoryEntryWithOrigin[] = [],
+  action: HistoryActions
+) => {
+  switch (action.type) {
+    case HistoryActionType.SET_ENTRIES:
+      return action.entries
+    case HistoryActionType.UPDATE_ENTRY:
+      return [...state.filter((entry) => entry.identifier !== action.noteId), action.newEntry]
+    case HistoryActionType.REMOVE_ENTRY:
+      return state.filter((entry) => entry.identifier !== action.noteId)
+    default:
+      return state
+  }
+}