[branch-bender] Fix verify logic to allow retries (#220)

# Exercise Review

## Exercise Discussion

Fixes #216 

## Checklist

- [ ] If you require a new remote repository on the `Git-Mastery`
organization, have you [created a
request](https://github.com/git-mastery/exercises/issues/new?template=request_exercise_repository.md)
for it?
- [X] Have you written unit tests using
[`repo-smith`](https://github.com/git-mastery/repo-smith) to validate
the exercise grading scheme?
- [X] Have you tested your changes using the instructions posted?
- [X] Have you verified that this exercise does not already exist or is
not currently in review?
- [ ] Did you introduce a new grading mechanism that should belong to
[`git-autograder`](https://github.com/git-mastery/git-autograder)?
- [ ] Did you introduce a new dependency that should belong to
[`app`](https://github.com/git-mastery/app)?

Author: jovnc

branch_bender/test_verify.py

@@ -59,6 +59,18 @@ def test_base():
         assert_output(output, GitAutograderStatus.SUCCESSFUL)
 
 
+def test_merge_undo_succeeds():
+    with base_setup() as (test, rs):
+        rs.git.merge("feature/login", no_ff=False)
+        rs.git.reset("HEAD~1", hard=True)
+        rs.git.merge("feature/login", no_ff=True)
+        rs.git.merge("feature/dashboard", no_ff=True)
+        rs.git.merge("feature/payments", no_ff=True)
+
+        output = test.run()
+        assert_output(output, GitAutograderStatus.SUCCESSFUL)
+
+
 def test_ff_fails():
     with base_setup() as (test, rs):
         rs.git.merge("feature/login")

branch_bender/verify.py

@@ -41,26 +41,28 @@ def verify(exercise: GitAutograderExercise) -> GitAutograderOutput:
     if len(merge_order) < 3:
         raise exercise.wrong_answer([MISSING_MERGES])
 
-    if merge_order[0] != "feature/login":
+    # Use negative indexing to check the last 3 merges (most recent)
+    # This allows users to undo mistakes (e.g., reset --hard after accidental ff) and redo properly
+    if merge_order[-3] != "feature/login":
         raise exercise.wrong_answer([FEATURE_LOGIN_MERGE_MISSING, RESET_MESSAGE])
 
-    if messages[0] == "Fast-forward":
+    if messages[-3] == "Fast-forward":
         raise exercise.wrong_answer(
             [NO_FAST_FORWARDING.format(branch_name="feature/login"), RESET_MESSAGE]
         )
 
-    if merge_order[1] != "feature/dashboard":
+    if merge_order[-2] != "feature/dashboard":
         raise exercise.wrong_answer([FEATURE_DASHBOARD_MERGE_MISSING, RESET_MESSAGE])
 
-    if messages[1] == "Fast-forward":
+    if messages[-2] == "Fast-forward":
         raise exercise.wrong_answer(
             [NO_FAST_FORWARDING.format(branch_name="feature/dashboard"), RESET_MESSAGE]
         )
 
-    if merge_order[2] != "feature/payments":
+    if merge_order[-1] != "feature/payments":
         raise exercise.wrong_answer([FEATURE_PAYMENTS_MERGE_MISSING, RESET_MESSAGE])
 
-    if messages[2] == "Fast-forward":
+    if messages[-1] == "Fast-forward":
         raise exercise.wrong_answer(
             [NO_FAST_FORWARDING.format(branch_name="feature/payments"), RESET_MESSAGE]
         )