From dc5cd8ddf716fd3de480b9d2d2290d1f8a15ebf1 Mon Sep 17 00:00:00 2001
From: Ben Bachem <10088265+bezbac@users.noreply.github.com>
Date: Wed, 27 Mar 2024 23:20:55 +0100
Subject: [PATCH 1/2] improve `magit-branch-spinoff` implementation

---
 src/commands/branchingCommands.ts | 46 +++++++++++++++++++++++++++----
 1 file changed, 41 insertions(+), 5 deletions(-)

diff --git a/src/commands/branchingCommands.ts b/src/commands/branchingCommands.ts
index 0d05dda..772818c 100644
--- a/src/commands/branchingCommands.ts
+++ b/src/commands/branchingCommands.ts
@@ -207,13 +207,49 @@ async function createNewSpinoff({ repository }: MenuState) {
     prompt: 'Name for new branch',
   });
 
-  if (newBranchName && newBranchName.length > 0) {
-    const args = ['checkout', '-B', newBranchName];
-    return gitRun(repository.gitRepository, args);
-  } else {
-    window.setStatusBarMessage(
+  if (!newBranchName || newBranchName.length < 1) {
+    return window.setStatusBarMessage(
       'No name given for new branch',
       Constants.StatusMessageDisplayTimeout
     );
   }
+
+  if (repository.branches.find(b => b.name === newBranchName)) {
+    return window.setStatusBarMessage(
+      `Cannot spin off ${newBranchName}. It already exists`,
+      Constants.StatusMessageDisplayTimeout
+    );
+  }
+
+  const base = repository.HEAD?.name;
+
+  if (!base) {
+    return window.setStatusBarMessage(
+      'No branch checked out',
+      Constants.StatusMessageDisplayTimeout
+    );
+  }
+
+  // Save current commit hash of base branch for later reference
+  const baseCommit = repository.HEAD?.commit;
+
+  // Get upsteam branch commit hash of base branch
+  const upstreamBranchCommit = repository.HEAD.upstreamRemote?.commit?.hash;
+
+  // Checkout new branch
+  await gitRun(repository.gitRepository, ['checkout', '-b', newBranchName]);
+
+  if (upstreamBranchCommit && baseCommit !== upstreamBranchCommit) {
+    await gitRun(repository.gitRepository, ['branch', '-f', base, upstreamBranchCommit]);
+
+    window.setStatusBarMessage(
+      `Branch ${base} was reset to upstream`,
+      Constants.StatusMessageDisplayTimeout
+    );
+  } else {
+    window.setStatusBarMessage(
+      `Branch ${base} not changed`,
+      Constants.StatusMessageDisplayTimeout
+    );
+  }
 }

From e0faf63d850d765bca2bd84d5bc316a164152d21 Mon Sep 17 00:00:00 2001
From: Ben Bachem <10088265+bezbac@users.noreply.github.com>
Date: Fri, 29 Mar 2024 15:04:23 +0100
Subject: [PATCH 2/2] address pr comments

---
 src/commands/branchingCommands.ts | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/src/commands/branchingCommands.ts b/src/commands/branchingCommands.ts
index 772818c..f76abeb 100644
--- a/src/commands/branchingCommands.ts
+++ b/src/commands/branchingCommands.ts
@@ -239,11 +239,21 @@ async function createNewSpinoff({ repository }: MenuState) {
   // Checkout new branch
   await gitRun(repository.gitRepository, ['checkout', '-b', newBranchName]);
 
-  if (upstreamBranchCommit && baseCommit !== upstreamBranchCommit) {
-    await gitRun(repository.gitRepository, ['branch', '-f', base, upstreamBranchCommit]);
+  if (baseCommit && upstreamBranchCommit && baseCommit !== upstreamBranchCommit) {
+    // Find common ancestor of base branch and upstream branch
+    const mergeBase = await repository.gitRepository.getMergeBase(baseCommit, upstreamBranchCommit);
+
+    // Reset the original branch to the common ancestor
+    await gitRun(repository.gitRepository, [
+      'update-ref',
+      '-m',
+      `"reset: moving to ${mergeBase}"`,
+      `refs/heads/${base}`,
+      mergeBase
+    ]);
 
     window.setStatusBarMessage(
-      `Branch ${base} was reset to upstream`,
+      `Branch ${base} was reset to ${mergeBase}`,
       Constants.StatusMessageDisplayTimeout
     );
   } else {