/**
 * The handoff an exit leaves behind: what the branch holds, and the command that
 * finishes the task.
 *
 * `AGENTS.md` ends a worktree task with a merge into the main checkout, or a pull
 * request. The exit already decides the branch's fate and keeps one that holds
 * commits; this says how many, whether the default branch already contains them,
 * and the exact commands — so the next step is read, not remembered.
 *
 * Pure: the caller has the facts, this shapes them. The tool never commits, merges,
 * pushes, or opens a pull request, because each of those acts on a checkout the
 * session is no longer isolated in.
 */

/** What a leaving worktree hands back to the main checkout. */
export interface Handoff {
	/** The worktree branch, when the session had one. */
	branch: string | null;
	/** The branch a merge would target, when it could be resolved. */
	base: string | null;
	/** Commits on the branch that the base does not contain. */
	ahead: number;
	/** Whether the base already contains every commit on the branch. */
	merged: boolean;
}

/** Pluralized commit count, e.g. `1 commit` / `3 commits`. */
function commits(count: number): string {
	return `${count} commit${count === 1 ? "" : "s"}`;
}

/**
 * The `Handoff:` block: one headline and, only when there is something to do, the
 * two commands. A merged or empty branch prints no command — inviting a merge of
 * work that is already in the base is how a branch gets merged twice.
 */
export function handoffLines(handoff: Handoff): string[] {
	if (!handoff.branch) return ["Handoff: the worktree had no branch to hand off."];

	if (handoff.base === null) {
		return [
			`Handoff: branch ${handoff.branch} holds ${commits(handoff.ahead)}; its base branch could not be resolved.`,
		];
	}
	if (handoff.merged || handoff.ahead === 0) {
		const state = handoff.merged ? `is merged into ${handoff.base}` : `has no commits beyond ${handoff.base}`;
		return [`Handoff: ${handoff.branch} ${state} — nothing to merge.`];
	}

	return [
		`Handoff: ${handoff.branch} is ${commits(handoff.ahead)} ahead of ${handoff.base}.`,
		`  merge:  git switch ${handoff.base} && git merge --no-ff ${handoff.branch}`,
		`  or PR:  gh pr create --head ${handoff.branch} --fill`,
	];
}