"""
Schema migration system.

Rules for maintainers
---------------------
- NEVER modify or delete an existing entry in MIGRATIONS.
- ALWAYS append new entries at the end with an incremented version number.
- Use ALTER TABLE to change an existing table, not a new CREATE TABLE.
- Each entry must be a single, complete SQL statement.

How it works
------------
On startup (and after the initial setup form) run_migrations() is called.
It creates a `schema_migrations` table the first time, checks which versions
have already been applied, and runs any that are missing.  A full JSON backup
of all user-data tables is written to the backups/ directory before any
schema changes are made so data can be recovered if something goes wrong.
"""

import json
import os
from datetime import datetime

from util.logger import logger

# ---------------------------------------------------------------------------
# Migration registry — append-only
# ---------------------------------------------------------------------------

MIGRATIONS = [
    (1, "Create users table", """
        CREATE TABLE IF NOT EXISTS users (
            id INT AUTO_INCREMENT PRIMARY KEY,
            username VARCHAR(50) NOT NULL UNIQUE,
            password VARCHAR(255) NOT NULL,
            role VARCHAR(20) NOT NULL
        )
    """),
    (2, "Create websites table", """
        CREATE TABLE IF NOT EXISTS websites (
            id INT AUTO_INCREMENT PRIMARY KEY,
            name VARCHAR(100) NOT NULL,
            url VARCHAR(255) NOT NULL,
            description VARCHAR(255) DEFAULT ''
        )
    """),
    (3, "Create notes table", """
        CREATE TABLE IF NOT EXISTS notes (
            id INT AUTO_INCREMENT PRIMARY KEY,
            user_id INT NOT NULL,
            title VARCHAR(255) NOT NULL,
            content TEXT,
            language VARCHAR(50) DEFAULT 'text',
            created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
            updated_at DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
        )
    """),
    # ── add new migrations below this line ──────────────────────────────────
]

# ---------------------------------------------------------------------------
# Internals
# ---------------------------------------------------------------------------

_BACKUP_DIR = os.path.join(
    os.path.dirname(os.path.abspath(__file__)), "..", "backups"
)


def _ensure_migration_table(conn):
    cur = conn.cursor()
    cur.execute("""
        CREATE TABLE IF NOT EXISTS schema_migrations (
            version     INT          PRIMARY KEY,
            description VARCHAR(255) NOT NULL,
            applied_at  DATETIME     DEFAULT CURRENT_TIMESTAMP
        )
    """)
    conn.commit()
    cur.close()


def _applied_versions(conn):
    cur = conn.cursor()
    cur.execute("SELECT version FROM schema_migrations")
    versions = {row[0] for row in cur.fetchall()}
    cur.close()
    return versions


def backup(conn):
    """
    Dump every user-data table to a timestamped JSON file.
    Returns the path of the written file.
    Datetime values are serialised as ISO-8601 strings.
    """
    os.makedirs(_BACKUP_DIR, exist_ok=True)
    ts = datetime.now().strftime("%Y%m%d_%H%M%S")
    path = os.path.join(_BACKUP_DIR, f"backup_{ts}.json")

    cur = conn.cursor(dictionary=True)
    cur.execute("SHOW TABLES")
    tables = [list(row.values())[0] for row in cur.fetchall()]

    snapshot = {}
    for table in tables:
        if table == "schema_migrations":
            continue
        cur.execute(f"SELECT * FROM `{table}`")
        snapshot[table] = [
            {
                k: v.isoformat() if hasattr(v, "isoformat") else v
                for k, v in row.items()
            }
            for row in cur.fetchall()
        ]
    cur.close()

    with open(path, "w", encoding="utf-8") as f:
        json.dump(snapshot, f, indent=2, ensure_ascii=False)

    logger.info(f"[migrations] Backup written → {path}")
    return path


# ---------------------------------------------------------------------------
# Public API
# ---------------------------------------------------------------------------

def run_migrations(conn):
    """
    Apply every pending migration in version order.
    A backup is created automatically before the first schema change.
    Safe to call on every startup — already-applied migrations are skipped.
    """
    _ensure_migration_table(conn)
    applied = _applied_versions(conn)
    pending = [(v, d, sql) for v, d, sql in MIGRATIONS if v not in applied]

    if not pending:
        logger.info("[migrations] Schema is up to date.")
        return

    backup_path = backup(conn)
    logger.info(
        f"[migrations] {len(pending)} pending migration(s). "
        f"Backup: {backup_path}"
    )

    cur = conn.cursor()
    for version, description, sql in pending:
        logger.info(f"[migrations] Applying v{version}: {description}")
        cur.execute(sql.strip())
        cur.execute(
            "INSERT INTO schema_migrations (version, description) VALUES (%s, %s)",
            (version, description),
        )

    conn.commit()
    cur.close()
    logger.info("[migrations] All migrations applied.")