from __future__ import annotations

import uuid
from datetime import datetime, timezone
from decimal import Decimal

from sqlalchemy import select, func
from sqlalchemy.ext.asyncio import AsyncSession

from app.core.security import encrypt_field, decrypt_field
from app.db.models.account import Account
from app.db.models.currency import ExchangeRate
from app.db.models.transaction import Transaction
from app.schemas.account import AccountCreate, AccountUpdate

# Account types that are liabilities (balance is negative contribution to net worth)
LIABILITY_TYPES = {"credit_card", "loan", "mortgage"}


class AccountError(Exception):
    def __init__(self, detail: str, status_code: int = 400):
        self.detail = detail
        self.status_code = status_code


def _encrypt(value: str | None) -> bytes | None:
    if value is None:
        return None
    return encrypt_field(value)


def _decrypt(data: bytes | None) -> str | None:
    if not data:
        return None
    return decrypt_field(data)


def _to_response(account: Account) -> dict:
    return {
        "id": account.id,
        "name": _decrypt(account.name_enc) or "",
        "institution": _decrypt(account.institution_enc),
        "type": account.type,
        "currency": account.currency,
        "current_balance": account.current_balance,
        "credit_limit": account.credit_limit,
        "interest_rate": account.interest_rate,
        "is_active": account.is_active,
        "include_in_net_worth": account.include_in_net_worth,
        "color": account.color,
        "icon": account.icon,
        "notes": _decrypt(account.notes_enc),
        "created_at": account.created_at,
        "updated_at": account.updated_at,
    }


async def create_account(
    db: AsyncSession,
    user_id: uuid.UUID,
    data: AccountCreate,
) -> dict:
    now = datetime.now(timezone.utc)
    account = Account(
        user_id=user_id,
        name_enc=encrypt_field(data.name),
        institution_enc=_encrypt(data.institution),
        type=data.type,
        currency=data.currency,
        current_balance=data.opening_balance,
        credit_limit=data.credit_limit,
        interest_rate=data.interest_rate,
        include_in_net_worth=data.include_in_net_worth,
        color=data.color,
        icon=data.icon,
        notes_enc=_encrypt(data.notes),
        created_at=now,
        updated_at=now,
    )
    db.add(account)
    await db.flush()
    return _to_response(account)


async def list_accounts(db: AsyncSession, user_id: uuid.UUID) -> list[dict]:
    result = await db.execute(
        select(Account).where(
            Account.user_id == user_id,
            Account.deleted_at.is_(None),
        ).order_by(Account.created_at)
    )
    return [_to_response(a) for a in result.scalars()]


async def get_account(db: AsyncSession, account_id: uuid.UUID, user_id: uuid.UUID) -> Account:
    result = await db.execute(
        select(Account).where(
            Account.id == account_id,
            Account.user_id == user_id,
            Account.deleted_at.is_(None),
        )
    )
    account = result.scalar_one_or_none()
    if not account:
        raise AccountError("Account not found", status_code=404)
    return account


async def update_account(
    db: AsyncSession,
    account_id: uuid.UUID,
    user_id: uuid.UUID,
    data: AccountUpdate,
) -> dict:
    account = await get_account(db, account_id, user_id)
    now = datetime.now(timezone.utc)

    if data.name is not None:
        account.name_enc = encrypt_field(data.name)
    if data.institution is not None:
        account.institution_enc = _encrypt(data.institution)
    if data.opening_balance is not None:
        account.current_balance = data.opening_balance
    if data.credit_limit is not None:
        account.credit_limit = data.credit_limit
    if data.interest_rate is not None:
        account.interest_rate = data.interest_rate
    if data.include_in_net_worth is not None:
        account.include_in_net_worth = data.include_in_net_worth
    if data.is_active is not None:
        account.is_active = data.is_active
    if data.color is not None:
        account.color = data.color
    if data.icon is not None:
        account.icon = data.icon
    if data.notes is not None:
        account.notes_enc = _encrypt(data.notes)

    account.updated_at = now
    await db.flush()
    return _to_response(account)


async def delete_account(
    db: AsyncSession,
    account_id: uuid.UUID,
    user_id: uuid.UUID,
) -> None:
    account = await get_account(db, account_id, user_id)
    account.deleted_at = datetime.now(timezone.utc)
    account.updated_at = datetime.now(timezone.utc)
    await db.flush()


async def recalculate_balance(db: AsyncSession, account_id: uuid.UUID) -> None:
    """Recompute current_balance from all non-deleted transactions."""
    result = await db.execute(
        select(func.sum(Transaction.amount)).where(
            Transaction.account_id == account_id,
            Transaction.deleted_at.is_(None),
        )
    )
    total = result.scalar_one_or_none() or Decimal("0")

    account = await db.get(Account, account_id)
    if account:
        account.current_balance = total
        account.updated_at = datetime.now(timezone.utc)
    await db.flush()


async def _fx_rate(db: AsyncSession, from_currency: str, to_currency: str) -> Decimal:
    if from_currency == to_currency:
        return Decimal("1")
    result = await db.execute(
        select(ExchangeRate)
        .where(ExchangeRate.base_currency == from_currency, ExchangeRate.quote_currency == to_currency)
        .order_by(ExchangeRate.fetched_at.desc())
        .limit(1)
    )
    er = result.scalar_one_or_none()
    return er.rate if er else Decimal("1")


async def get_net_worth(db: AsyncSession, user_id: uuid.UUID, base_currency: str) -> dict:
    accounts = await db.execute(
        select(Account).where(
            Account.user_id == user_id,
            Account.include_in_net_worth == True,
            Account.deleted_at.is_(None),
        )
    )
    total_assets = Decimal("0")
    total_liabilities = Decimal("0")

    for account in accounts.scalars():
        bal = account.current_balance or Decimal("0")
        acct_currency = account.currency or base_currency
        if acct_currency != base_currency:
            rate = await _fx_rate(db, acct_currency, base_currency)
            bal = bal * rate
        if account.type in LIABILITY_TYPES:
            total_liabilities += abs(bal)
        else:
            total_assets += bal

    return {
        "total_assets": total_assets,
        "total_liabilities": total_liabilities,
        "net_worth": total_assets - total_liabilities,
        "base_currency": base_currency,
    }