"""phase42 — areas.qr_token for per-area public QR landing pages

Adds a unique, unguessable token per area. Each area's QR encodes
/f/area/<qr_token>, a login-free summary scoped to that area plus a
"report a problem" form.

Ported from the single-tenant chain (phase39_area_public_token) and adapted:
the MT column is named `qr_token` (matching `facilities.qr_token` from
phase38_facility_qr), not `public_token`, and is VARCHAR(64) to match the
facility column and MT's `secrets.token_urlsafe(32)` generator.

Unlike the ST original, existing areas are NOT backfilled: MT mints tokens
lazily via `Area.ensure_qr_token()` on first use, exactly as
`Facility.ensure_qr_token()` already does. A backfill would mint tokens for
areas nobody ever prints a code for.

Uses INFORMATION_SCHEMA checks — safe to re-run on every tenant DB.
"""

revision      = 'phase42_area_qr_token'
down_revision = 'phase41_auditor_role'
branch_labels = None
depends_on    = None

from alembic import op
import sqlalchemy as sa


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 _index_exists(conn, table, index):
    return conn.execute(sa.text(
        "SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS "
        "WHERE TABLE_SCHEMA = DATABASE() "
        "AND TABLE_NAME = :t AND INDEX_NAME = :i"
    ), {"t": table, "i": index}).scalar() > 0


def upgrade():
    bind = op.get_bind()

    if not _column_exists(bind, 'areas', 'qr_token'):
        op.execute(sa.text(
            "ALTER TABLE areas ADD COLUMN qr_token VARCHAR(64) NULL"
        ))

    # Unique index tolerates multiple NULLs in MySQL, so it can be created
    # immediately — no backfill needed before enforcing uniqueness.
    if not _index_exists(bind, 'areas', 'uq_area_qr_token'):
        op.execute(sa.text(
            "CREATE UNIQUE INDEX uq_area_qr_token ON areas (qr_token)"
        ))


def downgrade():
    bind = op.get_bind()

    if _index_exists(bind, 'areas', 'uq_area_qr_token'):
        op.execute(sa.text("DROP INDEX uq_area_qr_token ON areas"))
    if _column_exists(bind, 'areas', 'qr_token'):
        op.execute(sa.text("ALTER TABLE areas DROP COLUMN qr_token"))