-- B05分支只保存起点；正文内容仍归既有document/version表。
CREATE TABLE public.muse_document_branch (
    document_id uuid PRIMARY KEY REFERENCES public.muse_document(document_id),
    source_document_id uuid NOT NULL,
    source_revision bigint NOT NULL,
    source_hash text NOT NULL,
    author_id text NOT NULL,
    created_at timestamptz NOT NULL DEFAULT clock_timestamp(),
    CHECK(document_id <> source_document_id),
    FOREIGN KEY(source_document_id,source_revision)
        REFERENCES public.muse_document_version(document_id,revision)
);
CREATE TABLE public.muse_branch_merge_source (
    candidate_id uuid NOT NULL,
    candidate_revision bigint NOT NULL,
    source_document_id uuid NOT NULL REFERENCES public.muse_document_branch(document_id),
    source_revision bigint NOT NULL,
    source_hash text NOT NULL,
    merge_basis jsonb NOT NULL,
    PRIMARY KEY(candidate_id,candidate_revision),
    FOREIGN KEY(candidate_id,candidate_revision)
        REFERENCES public.muse_writing_candidate_version(candidate_id,revision),
    FOREIGN KEY(source_document_id,source_revision)
        REFERENCES public.muse_document_version(document_id,revision)
);
CREATE FUNCTION public.muse_branch_source_immutable() RETURNS trigger LANGUAGE plpgsql AS $$
BEGIN
    RAISE EXCEPTION 'branch and merge origins are immutable';
END;
$$;
CREATE TRIGGER document_branch_immutable BEFORE UPDATE OR DELETE ON public.muse_document_branch
    FOR EACH ROW EXECUTE FUNCTION public.muse_branch_source_immutable();
CREATE TRIGGER branch_merge_source_immutable BEFORE UPDATE OR DELETE ON public.muse_branch_merge_source
    FOR EACH ROW EXECUTE FUNCTION public.muse_branch_source_immutable();