checkout: detect remote stack from current branch (#475)

* detect remote stack from current branch

* clean up phrasing

Author: skarim

README.md

@@ -160,7 +160,7 @@ When a remote stack is referenced, the command fetches the stack on GitHub, pull
 
 When a branch name is provided, the command resolves it against locally tracked stacks only.
 
-When run without arguments in an interactive terminal, opens a searchable picker listing every stack available to you — both the stacks tracked locally and the stacks that exist only on GitHub. Each row shows the stack number, its bottom and top branch, base branch, a status bar summarizing how many of its pull requests are merged, open, closed, or not yet pushed, and whether the stack is available locally or only on the remote. Filter with the All / Local / Remote tabs or type `/` to search; fully merged stacks are omitted. Selecting a remote-only stack clones it locally before switching to it.
+When run without arguments in an interactive terminal, first checks whether the current branch belongs to a stack on remote that is not tracked locally, and offers to check it out. If there is no unique match or you decline, it opens a searchable picker listing every stack available to you — both the stacks tracked locally and the stacks that exist only on GitHub. Each row shows the stack number, its bottom and top branch, base branch, a status bar summarizing how many of its pull requests are merged, open, closed, or not yet pushed, and whether the stack is available locally or only on the remote. Filter with the All / Local / Remote tabs or type `/` to search; fully merged stacks are omitted. Selecting a remote-only stack clones it locally before switching to it.
 
 **Examples:**
 

cmd/checkout.go

@@ -44,10 +44,12 @@ it simply switches to the branch.
 When a branch name is provided, the command resolves it against
 locally tracked stacks only.
 
-When run without arguments, opens an interactive picker listing every
-stack available to you — both the stacks tracked locally and the stacks
-that exist only on GitHub — so you can search, filter, and check one out.
-Fully merged stacks are omitted.`,
+When run without arguments, first checks whether the current branch belongs
+to a stack on remote that is not tracked locally, and offers to check
+it out. Otherwise, it opens an interactive picker listing every stack available
+to you — both the stacks tracked locally and the stacks that exist only on
+GitHub — so you can search, filter, and check one out. Fully merged stacks are
+omitted.`,
 		Example: `  # Check out a stack by its stack number
   $ gh stack checkout 7
 
@@ -643,7 +645,22 @@ func interactiveCheckout(cfg *config.Config, sf *stack.StackFile, gitDir string)
 		return nil, "", fmt.Errorf("no target specified; provide a branch name or PR number, or run interactively to select a stack")
 	}
 
-	rows := gatherCheckoutRows(cfg, sf)
+	rows, remoteStacks := gatherCheckoutRows(cfg, sf)
+	if currentBranch, branchErr := git.CurrentBranch(); branchErr == nil {
+		stackNumber, confirmed, confirmErr := offerRemoteStackForBranch(cfg, sf, remoteStacks, currentBranch)
+		if confirmErr != nil {
+			// The confirmation helper only returns an error for an explicit
+			// interrupt and has already printed the friendly message.
+			return nil, "", ErrSilent
+		}
+		if confirmed {
+			return resolveCheckoutSelection(cfg, sf, gitDir, checkoutview.StackRow{
+				Number: stackNumber,
+				Type:   checkoutview.TypeRemote,
+			})
+		}
+	}
+
 	if len(rows) == 0 {
 		cfg.Infof("No stacks available to check out")
 		cfg.Printf("Create a stack with `%s` or check out a stack by number with `%s`",
@@ -668,14 +685,89 @@ func interactiveCheckout(cfg *config.Config, sf *stack.StackFile, gitDir string)
 // with the local stacks into the picker's rows. Any GitHub failure (stacks not
 // enabled for the repo, no auth, network error) gracefully degrades to a
 // local-only list.
-func gatherCheckoutRows(cfg *config.Config, sf *stack.StackFile) []checkoutview.StackRow {
+func gatherCheckoutRows(cfg *config.Config, sf *stack.StackFile) ([]checkoutview.StackRow, []github.RemoteStack) {
 	var remote []github.RemoteStack
 	if client, err := cfg.GitHubClient(); err == nil {
 		if stacks, err := client.ListStacks(); err == nil {
 			remote = stacks
 		}
 	}
-	return checkoutview.BuildRows(sf.Stacks, remote)
+	return checkoutview.BuildRows(sf.Stacks, remote), remote
+}
+
+// offerRemoteStackForBranch asks to check out the unique active remote stack
+// containing branch when the branch is not already associated with a local
+// stack. Every outcome except confirmation or Ctrl+C falls through to the
+// existing picker.
+func offerRemoteStackForBranch(cfg *config.Config, sf *stack.StackFile, remote []github.RemoteStack, branch string) (int, bool, error) {
+	if branch == "" || len(sf.FindAllStacksForBranch(branch)) > 0 {
+		return 0, false, nil
+	}
+
+	matches := matchingRemoteStacksForBranch(remote, branch)
+	if len(matches) != 1 {
+		return 0, false, nil
+	}
+
+	stackNumber := matches[0].Number
+	prompt := fmt.Sprintf("Found stack #%d that includes branch %q. Check out stack #%d?", stackNumber, branch, stackNumber)
+	confirmed, err := confirmRemoteStackCheckout(cfg, prompt)
+	if err != nil {
+		if errors.Is(err, errInterrupt) {
+			return 0, false, err
+		}
+		return 0, false, nil
+	}
+	if !confirmed {
+		return 0, false, nil
+	}
+	return stackNumber, true, nil
+}
+
+// matchingRemoteStacksForBranch returns picker-eligible remote stacks that
+// contain branch exactly once per stack. Empty and fully merged stacks are not
+// actionable and are omitted, matching the picker.
+func matchingRemoteStacksForBranch(remote []github.RemoteStack, branch string) []*github.RemoteStack {
+	var matches []*github.RemoteStack
+	for i := range remote {
+		rs := &remote[i]
+		if rs.Number <= 0 || len(rs.PRDetails) == 0 {
+			continue
+		}
+
+		containsBranch := false
+		hasUnmergedPR := false
+		for _, pr := range rs.PRDetails {
+			if pr.Head.Ref == branch {
+				containsBranch = true
+			}
+			if !pr.IsMerged() {
+				hasUnmergedPR = true
+			}
+		}
+		if containsBranch && hasUnmergedPR {
+			matches = append(matches, rs)
+		}
+	}
+	return matches
+}
+
+func confirmRemoteStackCheckout(cfg *config.Config, prompt string) (bool, error) {
+	var (
+		confirmed bool
+		err       error
+	)
+	if cfg.ConfirmFn != nil {
+		confirmed, err = cfg.ConfirmFn(prompt, true)
+	} else {
+		p := prompter.New(cfg.In, cfg.Out, cfg.Err)
+		confirmed, err = p.Confirm(prompt, true)
+	}
+	if isInterruptError(err) {
+		printInterrupt(cfg)
+		return false, errInterrupt
+	}
+	return confirmed, err
 }
 
 // resolveCheckoutSelection resolves a picker selection to a local stack and the