"""Database abstraction layer with MySQL and file-based backends.

Usage:
    from refcurator.db import get_backend

    db = get_backend()
    rows = db.fetch_all("SELECT * FROM documents WHERE crawl_status = %s", ("pending",))
    doc_id = db.insert_returning_id("INSERT INTO documents (...) VALUES (...)", params)
"""

from __future__ import annotations

import json
import logging
from contextlib import contextmanager
from pathlib import Path
from typing import Any, Optional, Protocol, Sequence

from refcurator.config import get_db_config, get_state_backend, get_state_dir

logger = logging.getLogger("refcurator.db")


class DatabaseBackend(Protocol):
    """Protocol for database backends."""

    def execute(self, sql: str, params: Sequence = ()) -> int:
        """Execute a statement, return affected row count."""
        ...

    def fetch_one(self, sql: str, params: Sequence = ()) -> Optional[dict]:
        """Fetch a single row as dict."""
        ...

    def fetch_all(self, sql: str, params: Sequence = ()) -> list[dict]:
        """Fetch all rows as list of dicts."""
        ...

    def insert_returning_id(self, sql: str, params: Sequence = ()) -> int:
        """Insert a row and return the auto-generated ID."""
        ...

    def close(self) -> None:
        """Close the connection."""
        ...


class MySQLBackend:
    """MySQL backend using PyMySQL."""

    def __init__(self, config: dict | None = None):
        import pymysql
        import pymysql.cursors

        if config is None:
            config = get_db_config().get("mysql", {})

        self._conn = pymysql.connect(
            host=config.get("host", "localhost"),
            port=int(config.get("port", 3306)),
            user=config.get("user", "root"),
            password=config.get("password", ""),
            database=config.get("database", "reference_library"),
            charset="utf8mb4",
            cursorclass=pymysql.cursors.DictCursor,
            autocommit=True,
        )

    def execute(self, sql: str, params: Sequence = ()) -> int:
        with self._conn.cursor() as cur:
            return cur.execute(sql, params)

    def fetch_one(self, sql: str, params: Sequence = ()) -> Optional[dict]:
        with self._conn.cursor() as cur:
            cur.execute(sql, params)
            return cur.fetchone()

    def fetch_all(self, sql: str, params: Sequence = ()) -> list[dict]:
        with self._conn.cursor() as cur:
            cur.execute(sql, params)
            return cur.fetchall()

    def insert_returning_id(self, sql: str, params: Sequence = ()) -> int:
        with self._conn.cursor() as cur:
            cur.execute(sql, params)
            return cur.lastrowid

    def close(self) -> None:
        self._conn.close()


class FileBackend:
    """JSON file-based backend for use without MySQL.

    Stores data as JSON arrays in the state directory.
    Supports basic CRUD but not complex queries or JOINs.
    """

    def __init__(self, state_dir: Path | None = None):
        self._dir = state_dir or get_state_dir()
        self._dir.mkdir(parents=True, exist_ok=True)
        self._cache: dict[str, list[dict]] = {}
        self._counters: dict[str, int] = {}
        self._load_counters()

    def _table_path(self, table: str) -> Path:
        return self._dir / f"{table}.json"

    def _load_table(self, table: str) -> list[dict]:
        if table not in self._cache:
            path = self._table_path(table)
            if path.is_file():
                self._cache[table] = json.loads(path.read_text())
            else:
                self._cache[table] = []
        return self._cache[table]

    def _save_table(self, table: str) -> None:
        path = self._table_path(table)
        path.write_text(json.dumps(self._cache.get(table, []), indent=2, default=str))

    def _load_counters(self) -> None:
        counter_path = self._dir / "_counters.json"
        if counter_path.is_file():
            self._counters = json.loads(counter_path.read_text())

    def _save_counters(self) -> None:
        counter_path = self._dir / "_counters.json"
        counter_path.write_text(json.dumps(self._counters, indent=2))

    def _next_id(self, table: str) -> int:
        current = self._counters.get(table, 0)
        self._counters[table] = current + 1
        self._save_counters()
        return current + 1

    # --- Protocol methods ---
    # These provide basic support for the most common operations.
    # Complex SQL is not supported; use MySQL for full functionality.

    def execute(self, sql: str, params: Sequence = ()) -> int:
        """Basic INSERT/UPDATE/DELETE support via SQL pattern matching."""
        sql_lower = sql.strip().lower()
        table = _extract_table_name(sql)

        if sql_lower.startswith("insert"):
            return self._handle_insert(table, sql, params)
        elif sql_lower.startswith("update"):
            return self._handle_update(table, sql, params)
        elif sql_lower.startswith("delete"):
            return self._handle_delete(table, sql, params)

        logger.warning("FileBackend: unsupported SQL operation: %s", sql[:60])
        return 0

    def fetch_one(self, sql: str, params: Sequence = ()) -> Optional[dict]:
        rows = self.fetch_all(sql, params)
        return rows[0] if rows else None

    def fetch_all(self, sql: str, params: Sequence = ()) -> list[dict]:
        table = _extract_table_name(sql)
        if not table:
            logger.warning("FileBackend: cannot extract table from: %s", sql[:60])
            return []

        # Hard-fail on SQL patterns that FileBackend cannot handle correctly
        _reject_unsupported_sql(sql)

        rows = self._load_table(table)

        # Basic WHERE clause filtering
        conditions = _extract_where_conditions(sql, params)
        if conditions:
            rows = [r for r in rows if _matches_conditions(r, conditions)]

        # Basic ORDER BY
        order_col = _extract_order_by(sql)
        if order_col:
            desc = "desc" in sql.lower().split("order by")[-1].lower()
            rows = sorted(rows, key=lambda r: r.get(order_col, ""), reverse=desc)

        # Basic LIMIT
        limit = _extract_limit(sql)
        if limit is not None:
            rows = rows[:limit]

        return rows

    def insert_returning_id(self, sql: str, params: Sequence = ()) -> int:
        table = _extract_table_name(sql)
        self._handle_insert(table, sql, params)
        return self._counters.get(table, 0)

    def close(self) -> None:
        pass  # No connection to close

    # --- Internal handlers ---

    def _handle_insert(self, table: str, sql: str, params: Sequence) -> int:
        columns = _extract_insert_columns(sql)
        if not columns or len(columns) != len(params):
            logger.warning("FileBackend: column/param mismatch for INSERT into %s", table)
            return 0

        row = dict(zip(columns, params))
        pk = _primary_key_for(table)
        if pk and pk not in row:
            row[pk] = self._next_id(table)

        rows = self._load_table(table)
        rows.append(row)
        self._cache[table] = rows
        self._save_table(table)
        return 1

    def _handle_update(self, table: str, sql: str, params: Sequence) -> int:
        rows = self._load_table(table)
        set_cols = _extract_set_columns(sql)
        conditions = _extract_where_conditions(sql, params[len(set_cols):])
        set_values = list(params[:len(set_cols)])

        count = 0
        for row in rows:
            if _matches_conditions(row, conditions):
                for col, val in zip(set_cols, set_values):
                    row[col] = val
                count += 1

        if count > 0:
            self._save_table(table)
        return count

    def _handle_delete(self, table: str, sql: str, params: Sequence) -> int:
        rows = self._load_table(table)
        conditions = _extract_where_conditions(sql, params)
        before = len(rows)
        self._cache[table] = [r for r in rows if not _matches_conditions(r, conditions)]
        self._save_table(table)
        return before - len(self._cache[table])


class UnsupportedQueryError(Exception):
    """Raised when FileBackend encounters SQL it cannot handle correctly."""
    pass


def _reject_unsupported_sql(sql: str) -> None:
    """Raise UnsupportedQueryError if the SQL uses patterns FileBackend cannot handle.

    FileBackend only supports single-table SELECT with simple WHERE col = %s.
    JOINs, subqueries, aggregates, and GROUP BY would return wrong results silently.
    """
    import re
    sql_upper = sql.upper()

    unsupported = []
    if re.search(r"\bJOIN\b", sql_upper):
        unsupported.append("JOIN")
    if re.search(r"\bGROUP\s+BY\b", sql_upper):
        unsupported.append("GROUP BY")
    if re.search(r"\b(MAX|MIN|SUM|AVG|COUNT)\s*\(", sql_upper):
        unsupported.append("aggregate functions")
    if re.search(r"\bLEFT\s+JOIN\b", sql_upper):
        unsupported.append("LEFT JOIN")
    if re.search(r"\(\s*SELECT\b", sql_upper):
        unsupported.append("subquery")

    if unsupported:
        raise UnsupportedQueryError(
            f"FileBackend cannot execute queries with {', '.join(unsupported)}. "
            f"Configure MySQL or use 'backend: mysql' in pipeline_config.yaml. "
            f"Query: {sql[:80]}..."
        )


# --- SQL parsing helpers (minimal, covers common patterns) ---

def _extract_table_name(sql: str) -> str:
    """Extract the primary table name from a SQL statement."""
    import re
    sql_clean = sql.strip()

    # INSERT INTO table
    m = re.search(r"(?:insert\s+into|update|delete\s+from|from)\s+(\w+)", sql_clean, re.I)
    if m:
        return m.group(1)
    return ""


def _extract_insert_columns(sql: str) -> list[str]:
    """Extract column names from INSERT INTO table (col1, col2, ...)."""
    import re
    m = re.search(r"\(([^)]+)\)\s*VALUES", sql, re.I)
    if m:
        return [c.strip() for c in m.group(1).split(",")]
    return []


def _extract_set_columns(sql: str) -> list[str]:
    """Extract column names from UPDATE ... SET col1 = %s, col2 = %s."""
    import re
    m = re.search(r"SET\s+(.+?)(?:\s+WHERE|$)", sql, re.I | re.S)
    if m:
        return [c.strip().split("=")[0].strip() for c in m.group(1).split(",")]
    return []


def _extract_where_conditions(sql: str, params: Sequence) -> list[tuple[str, Any]]:
    """Extract simple col = %s conditions from WHERE clause."""
    import re
    m = re.search(r"WHERE\s+(.+?)(?:\s+ORDER|\s+LIMIT|$)", sql, re.I | re.S)
    if not m:
        return []

    where_clause = m.group(1)
    cols = re.findall(r"(\w+)\s*=\s*%s", where_clause)
    # Map to params (taking from the end of params for UPDATE, all for SELECT)
    return list(zip(cols, params[-len(cols):] if cols else []))


def _extract_order_by(sql: str) -> str:
    """Extract the first ORDER BY column."""
    import re
    m = re.search(r"ORDER\s+BY\s+(\w+)", sql, re.I)
    return m.group(1) if m else ""


def _extract_limit(sql: str) -> int | None:
    """Extract LIMIT value."""
    import re
    m = re.search(r"LIMIT\s+(\d+)", sql, re.I)
    return int(m.group(1)) if m else None


def _matches_conditions(row: dict, conditions: list[tuple[str, Any]]) -> bool:
    """Check if a row matches all WHERE conditions."""
    return all(str(row.get(col)) == str(val) for col, val in conditions)


def _primary_key_for(table: str) -> str:
    """Return the auto-increment primary key name for a table."""
    pk_map = {
        "sources": "source_id",
        "documents": "doc_id",
        "distilled_content": "distill_id",
        "review_logs": "review_id",
        "topics": "topic_id",
        "export_jobs": "export_id",
        "pipeline_runs": "run_id",
        "pipeline_iteration_tracker": "tracker_id",
        "crawl_schedule": "schedule_id",
        "change_detection": "change_id",
    }
    return pk_map.get(table, "")


# --- Factory ---

def get_backend(backend_type: str | None = None) -> DatabaseBackend:
    """Create and return the appropriate database backend.

    Args:
        backend_type: 'mysql' or 'file'. If None, reads from pipeline config.

    Returns:
        A DatabaseBackend instance.
    """
    if backend_type is None:
        backend_type = get_state_backend()

    if backend_type == "mysql":
        return MySQLBackend()

    return FileBackend()


@contextmanager
def db_session(backend_type: str | None = None):
    """Context manager for database sessions.

    Usage:
        with db_session() as db:
            rows = db.fetch_all("SELECT * FROM documents")
    """
    db = get_backend(backend_type)
    try:
        yield db
    finally:
        db.close()