"""phase43 — scheduled inspection day-of-week / day-of-month recurrence

Adds the recurrence-detail columns to `scheduled_inspections` so a weekly
schedule can name the weekdays it runs on (Mon/Wed/Fri) and a monthly schedule
can name either a day of the month ("the 15th") or an nth weekday ("the 2nd
Tuesday"):

    weekdays     VARCHAR(20)  -- CSV of Python weekday ints, Mon=0, e.g. '0,2,4'
    month_mode   VARCHAR(20)  -- 'day_of_month' | 'nth_weekday'
    day_of_month SMALLINT     -- 1–31, clamped to the month's last day
    nth_week     SMALLINT     -- 1–5, or -1 for "last"
    nth_weekday  SMALLINT     -- 0–6, Mon=0

All nullable with no backfill: existing phase36 rows keep NULLs and fall back to
the legacy "+7 days" / "same day next month" behaviour in
ScheduledInspection._add_interval(), so no schedule changes cadence on deploy.

`month_mode` is VARCHAR rather than ENUM so adding a recurrence style later
needs no 3-step MySQL ENUM migration (CLAUDE.md rule 3).

Uses INFORMATION_SCHEMA column-existence checks — safe to re-run.
"""

revision      = 'phase43_sched_recurrence'
down_revision = 'phase42_internal_contact'
branch_labels = None
depends_on    = None

from alembic import op
import sqlalchemy as sa


_COLUMNS = (
    ('weekdays',     'VARCHAR(20) NULL'),
    ('month_mode',   'VARCHAR(20) NULL'),
    ('day_of_month', 'SMALLINT NULL'),
    ('nth_week',     'SMALLINT NULL'),
    ('nth_weekday',  'SMALLINT NULL'),
)


def _column_exists(conn, table, column):
    result = conn.execute(sa.text(
        "SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS "
        "WHERE TABLE_SCHEMA = DATABASE() "
        "AND TABLE_NAME = :t AND COLUMN_NAME = :c"
    ), {"t": table, "c": column})
    return result.scalar() > 0


def upgrade():
    bind = op.get_bind()
    for name, ddl in _COLUMNS:
        if not _column_exists(bind, 'scheduled_inspections', name):
            op.execute(sa.text(
                f"ALTER TABLE scheduled_inspections ADD COLUMN {name} {ddl}"
            ))


def downgrade():
    bind = op.get_bind()
    for name, _ in reversed(_COLUMNS):
        if _column_exists(bind, 'scheduled_inspections', name):
            op.execute(sa.text(
                f"ALTER TABLE scheduled_inspections DROP COLUMN {name}"
            ))