Reverse Applying a Patch to Undo Changes

Reverse Applying a Patch

Reverse apply swaps the meaning of + and - lines in a patch, effectively undoing the changes. This is equivalent to patch -R on the command line.

When to Use Reverse Apply

1. Undo a change — you applied a patch and want to go back
2. Recover original — you have the patched file and the diff, but not the original
3. Test revert — verify that reverting a patch produces the expected original

Example

Patched file (what you have):

function greet(name, greeting = "Hello") {
  console.log(greeting + ", " + name + "!");
  return true;
}

The patch that was applied:

--- a/greet.js
+++ b/greet.js
@@ -1,3 +1,3 @@
-function greet(name) {
-  console.log("Hello, " + name);
+function greet(name, greeting = "Hello") {
+  console.log(greeting + ", " + name + "!");
   return true;
 }

Result of reverse apply (recovered original):

function greet(name) {
  console.log("Hello, " + name);
  return true;
}

How It Works Internally

When reverse is enabled:

• Lines starting with + are treated as lines to remove (they should exist in the current file)
• Lines starting with - are treated as lines to add (they will be inserted)
• Context lines ( prefix) remain unchanged
• Hunk line numbers are swapped: oldStart/oldCount uses the + values and vice versa

Verify Round-Trip

A useful validation: applying a patch and then reverse-applying the same patch should produce the original file. If it does not, the patch may have been edited or the original file was different from what the patch expects.