git-smart-fixup

name	git-smart-fixup
description	Split staged changes or HEAD commit into `fixup!` commits that target the appropriate previous commits in the current branch.

Git Smart Fixup

Trigger

User invokes /smart-fixup or asks to split changes into fixup commits.

Arguments

• --from-head: Use the HEAD commit instead of staged changes (will reset HEAD and recreate as fixups)
• --dry-run: Show what fixup commits would be created without actually creating them
• --base <ref>: Specify the base branch (default: auto-detect master/main)

Instructions

Step 1: Determine the Base Branch and Commit Range

1. Find the merge base between current branch and main/master:

   git merge-base HEAD master
   

2. Get the list of commits from merge base to HEAD (these are candidates for fixup targets):

   git log --oneline --reverse <merge-base>..HEAD
   

3. Store commit hashes and their messages for later matching.

Step 2: Get the Changes to Split

If using staged changes (default):

git diff --cached --unified=0

If using --from-head:

# Get the diff from HEAD commit
git show HEAD --unified=0 --format=""

# Store HEAD message for potential fallback
git log -1 --format="%s" HEAD

# Soft reset to unstage HEAD's changes
git reset --soft HEAD~1

Step 3: Analyze Each Hunk and Find Target Commits

For each changed file and hunk:

1. Parse the diff to extract file paths and line ranges.

2. For each modified/deleted line range, use git blame to find which commit last touched those lines:

   # For the version before our changes, blame the lines being modified
   git blame -l -L <start>,<end> <merge-base>..HEAD~1 -- <file> 2>/dev/null
   

   If blaming the range fails (new lines), check the surrounding context:

   git blame -l -L <start-3>,<start> HEAD~1 -- <file> 2>/dev/null
   

3. For each hunk, determine the target commit:

   • Extract commit hashes from blame output
   • Check if any blamed commit is within our branch's commits (between merge-base and HEAD)
   • If multiple commits touched the lines, prefer the most recent one in our branch
   • If no branch commit is found, this hunk will go into a "new" commit

Step 4: Group Changes by Target Commit

Create a mapping of target commits to their associated hunks:

{
  "<commit-hash-1>": [hunk1, hunk2, ...],
  "<commit-hash-2>": [hunk3, ...],
  "NEW": [hunk4, hunk5, ...]  // Changes not attributable to any branch commit
}

Step 5: Create Fixup Commits

For each target commit with associated hunks:

1. Stage only the relevant hunks using git add -p or by creating patch files:

   # Create a patch for this group of hunks
   # Apply with: git apply --cached <patch-file>
   

2. Create the fixup commit:

   git commit --fixup=<target-commit-hash>
   

3. For changes marked as "NEW" (not attributable to branch commits):

   • If --from-head was used, use the original HEAD commit message
   • Otherwise, prompt the user for a commit message or create with a descriptive message

Step 6: Report Results

Display a summary:

Original commit: <original-head>

Created X fixup commits:
  fixup! <original-commit-message-1> (N files, M lines)
  fixup! <original-commit-message-2> (N files, M lines)

Remaining changes (not matched to branch commits):
  <new-commit-message> (N files, M lines)

To confirm that these commits match the original state, run:
  git diff <original-head>..<new-head>

To squash these fixups, run:
  git rebase -i --autosquash <merge-base>

Edge Cases

1. No staged changes and no --from-head: Error with helpful message.

2. New files: These typically become a new commit unless the user added them alongside modifications to existing code.

3. Binary files: Skip blame analysis, treat as new changes.

4. Renamed files: Use git log --follow to track through renames.

Implementation Notes

• Use git diff --cached -U0 for minimal context when parsing hunks
• Parse unified diff format: @@ -start,count +start,count @@
• Handle the case where blame returns commits outside the branch range
• Consider using git log -S or git log -G as fallback for finding related commits
• Preserve the order of fixup commits to match the original commit order for cleaner rebases