"""phase46 — inspection schedule day-of-week / day-of-month recurrence

Ports single-tenant phase43 onto MT's `inspection_schedules` table (MT's name
for the same thing — no rename, rule 7). Adds the recurrence-detail columns so a
weekly schedule can name the weekdays it runs on (Mon/Wed/Fri) and a monthly (or
quarterly / bi-annual / annual) 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 phase34/phase43 rows keep NULLs and fall
back to the plain-interval behaviour in InspectionSchedule._add_interval(), so
no schedule changes cadence on deploy.

`next_run_at` is NOT touched — it keeps its name, its DATETIME type and its
index. It remains the due datetime (ST's `next_due_date` by another name); the
recurrence maths runs on its DATE part and preserves its TIME part.

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

Uses INFORMATION_SCHEMA column-existence checks — safe to re-run on every
tenant DB. Additive only: nothing is renamed, retyped or dropped.
"""

revision      = 'phase46_schedule_recurrence'
down_revision = 'phase45_schedule_frequency_enum'
branch_labels = None
depends_on    = None

from alembic import op
import sqlalchemy as sa


_TABLE = 'inspection_schedules'

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


def _table_exists(conn, table):
    return conn.execute(sa.text(
        "SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLES "
        "WHERE TABLE_SCHEMA = DATABASE() AND TABLE_NAME = :t"
    ), {"t": table}).scalar() > 0


def _column_exists(conn, table, column):
    return 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}).scalar() > 0


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


def downgrade():
    bind = op.get_bind()
    if not _table_exists(bind, _TABLE):
        return
    for name, _ddl in reversed(_COLUMNS):
        if _column_exists(bind, _TABLE, name):
            op.execute(sa.text(
                f"ALTER TABLE {_TABLE} DROP COLUMN {name}"
            ))