Use precalculated diff in revision viewer (#2179)

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

src/components/editor-page/document-bar/revisions/invert-unified-patch.ts

@@ -0,0 +1,47 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { Hunk, ParsedDiff } from 'diff'
+
+/**
+ * Inverts a given unified patch.
+ * A patch that e.g. adds a line, will remove it then.
+ *
+ * @param parsedDiff The patch to invert
+ * @return The inverted patch
+ */
+export const invertUnifiedPatch = (parsedDiff: ParsedDiff): ParsedDiff => {
+  const { oldFileName, newFileName, oldHeader, newHeader, hunks, index } = parsedDiff
+
+  const newHunks: Hunk[] = hunks.map((hunk) => {
+    const { oldLines, oldStart, newLines, newStart, lines, linedelimiters } = hunk
+    return {
+      oldLines: newLines,
+      oldStart: newStart,
+      newLines: oldLines,
+      newStart: oldStart,
+      linedelimiters: linedelimiters,
+      lines: lines.map((line) => {
+        if (line.startsWith('-')) {
+          return `+${line.slice(1)}`
+        } else if (line.startsWith('+')) {
+          return `-${line.slice(1)}`
+        } else {
+          return line
+        }
+      })
+    }
+  })
+
+  return {
+    hunks: newHunks,
+    index: index,
+    newFileName: oldFileName,
+    newHeader: oldHeader,
+    oldFileName: newFileName,
+    oldHeader: newHeader
+  }
+}