-- Add a human‐readable, sequential task number that is shown in the UI
-- Format: YYYY-MM-##### (resetting each month).

-- 1. Add the column (nullable for now so we can backfill)
ALTER TABLE tasks
ADD COLUMN task_number text;

-- 2. Backfill existing rows using created_at as the timestamp basis.
DO $$
DECLARE
    r RECORD;
    prefix text;
    cnt int;
BEGIN
    FOR r IN
        SELECT id, created_at
        FROM tasks
        WHERE task_number IS NULL
        ORDER BY created_at
    LOOP
        prefix := to_char(r.created_at::timestamp, 'YYYY-MM-');
        cnt := (SELECT count(*)
                FROM tasks t
                WHERE to_char(t.created_at::timestamp, 'YYYY-MM-') = prefix
                  AND t.created_at <= r.created_at);
        UPDATE tasks
        SET task_number = prefix || lpad(cnt::text, 5, '0')
        WHERE id = r.id;
    END LOOP;
END;
$$ LANGUAGE plpgsql;

-- 3. Create trigger function to generate numbers on new inserts
CREATE OR REPLACE FUNCTION tasks_set_task_number()
RETURNS trigger AS $$
DECLARE
    prefix text;
    seq int;
BEGIN
    -- if caller already provided one, do not overwrite
    IF NEW.task_number IS NOT NULL THEN
        RETURN NEW;
    END IF;

    prefix := to_char(now(), 'YYYY-MM-');
    SELECT COALESCE(MAX((substring(task_number FROM 8))::int), 0) + 1
      INTO seq
      FROM tasks
      WHERE task_number LIKE prefix || '%';

    NEW.task_number := prefix || lpad(seq::text, 5, '0');
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER tasks_set_task_number_before_insert
BEFORE INSERT ON tasks
FOR EACH ROW
EXECUTE FUNCTION tasks_set_task_number();

-- 4. Enforce not-null and uniqueness now that every row has a value
ALTER TABLE tasks
    ALTER COLUMN task_number SET NOT NULL;

CREATE UNIQUE INDEX IF NOT EXISTS idx_tasks_task_number
    ON tasks (task_number);