From 8bf5f08dbcfa4c5870a907c04ef2fdc5393da97d Mon Sep 17 00:00:00 2001
From: Mark Amery <markrobertamery@gmail.com>
Date: Fri, 29 Dec 2023 12:11:57 +0000
Subject: [PATCH] Add function to reverse a patch

---
 src/patch/reverse.js | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)
 create mode 100644 src/patch/reverse.js

diff --git a/src/patch/reverse.js b/src/patch/reverse.js
new file mode 100644
index 00000000..76f072ec
--- /dev/null
+++ b/src/patch/reverse.js
@@ -0,0 +1,25 @@
+export function reversePatch(structuredPatch) {
+  const result = {...structuredPatch, hunks: structuredPatch.hunks.slice()};
+  const { oldFileName, newFileName, oldHeader, newHeader, hunks } = result;
+
+  result.oldFileName = newFileName;
+  result.oldHeader = newHeader;
+  result.newFileName = oldFileName;
+  result.newHeader = oldHeader;
+
+  for (const hunk of hunks) {
+    const { oldLines, oldStart, newLines, newStart, lines } = hunk;
+    hunk.oldLines = newLines.slice();
+    hunk.oldStart = newStart;
+    hunk.newLines = oldLines.slice();
+    hunk.newStart = oldStart;
+
+    hunk.lines = lines.map(l => {
+      if (l.startsWith('-')) return `+${l.slice(1)}`;
+      if (l.startsWith('+')) return `-${l.slice(1)}`;
+      return l;
+    });
+  }
+
+  return structuredPatch;
+}