from __future__ import annotations

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

from sqlalchemy import and_, or_, select
from sqlalchemy.ext.asyncio import AsyncSession

from app.core.security import decrypt_field, encrypt_field
from app.db.models.transaction import Transaction
from app.schemas.transaction import TransactionCreate, TransactionFilter, TransactionUpdate
from app.services.account_service import recalculate_balance


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


def _enc(v: str | None) -> bytes | None:
    return encrypt_field(v) if v else None


def _dec(v: bytes | None) -> str | None:
    return decrypt_field(v) if v else None


def _to_response(t: Transaction) -> dict:
    return {
        "id": t.id,
        "account_id": t.account_id,
        "transfer_account_id": t.transfer_account_id,
        "category_id": t.category_id,
        "type": t.type,
        "status": t.status,
        "amount": t.amount,
        "amount_base": t.amount_base,
        "currency": t.currency,
        "base_currency": t.base_currency,
        "exchange_rate": t.exchange_rate,
        "date": t.date,
        "description": _dec(t.description_enc) or "",
        "merchant": _dec(t.merchant_enc),
        "notes": _dec(t.notes_enc),
        "tags": t.tags or [],
        "is_recurring": t.is_recurring,
        "attachment_refs": t.attachment_refs or [],
        "created_at": t.created_at,
        "updated_at": t.updated_at,
    }


async def create_transaction(
    db: AsyncSession,
    user_id: uuid.UUID,
    data: TransactionCreate,
    base_currency: str,
) -> dict:
    now = datetime.now(timezone.utc)
    amount = data.amount

    # For transfers, create mirrored entry on destination account
    txn = Transaction(
        user_id=user_id,
        account_id=data.account_id,
        transfer_account_id=data.transfer_account_id,
        category_id=data.category_id,
        type=data.type,
        status=data.status,
        amount=amount,
        amount_base=amount,  # Phase 3: convert via FX rate
        currency=data.currency,
        base_currency=base_currency,
        exchange_rate=Decimal("1") if data.currency == base_currency else None,
        date=data.date,
        description_enc=encrypt_field(data.description),
        merchant_enc=_enc(data.merchant),
        notes_enc=_enc(data.notes),
        tags=data.tags,
        is_recurring=data.is_recurring,
        recurring_rule=data.recurring_rule,
        created_at=now,
        updated_at=now,
    )
    db.add(txn)
    await db.flush()

    # If transfer, create the counter-entry on the destination account
    if data.type == "transfer" and data.transfer_account_id:
        counter = Transaction(
            user_id=user_id,
            account_id=data.transfer_account_id,
            transfer_account_id=data.account_id,
            category_id=data.category_id,
            type="transfer",
            status=data.status,
            amount=-amount,  # opposite sign
            amount_base=-amount,
            currency=data.currency,
            base_currency=base_currency,
            exchange_rate=Decimal("1") if data.currency == base_currency else None,
            date=data.date,
            description_enc=encrypt_field(data.description),
            merchant_enc=_enc(data.merchant),
            notes_enc=_enc(data.notes),
            tags=data.tags,
            is_recurring=False,
            created_at=now,
            updated_at=now,
        )
        db.add(counter)
        await db.flush()
        await recalculate_balance(db, data.transfer_account_id)

    await recalculate_balance(db, data.account_id)
    return _to_response(txn)


async def list_transactions(
    db: AsyncSession,
    user_id: uuid.UUID,
    filters: TransactionFilter,
) -> dict:
    conditions = [
        Transaction.user_id == user_id,
        Transaction.deleted_at.is_(None),
    ]

    if filters.account_id:
        conditions.append(Transaction.account_id == filters.account_id)
    if filters.category_id:
        conditions.append(Transaction.category_id == filters.category_id)
    if filters.type:
        conditions.append(Transaction.type == filters.type)
    if filters.status:
        conditions.append(Transaction.status == filters.status)
    if filters.date_from:
        conditions.append(Transaction.date >= filters.date_from)
    if filters.date_to:
        conditions.append(Transaction.date <= filters.date_to)
    if filters.min_amount is not None:
        conditions.append(Transaction.amount >= filters.min_amount)
    if filters.max_amount is not None:
        conditions.append(Transaction.amount <= filters.max_amount)
    if filters.is_recurring is not None:
        conditions.append(Transaction.is_recurring == filters.is_recurring)

    query = select(Transaction).where(and_(*conditions)).order_by(Transaction.date.desc(), Transaction.created_at.desc())

    # Count total
    from sqlalchemy import func
    count_result = await db.execute(select(func.count()).select_from(query.subquery()))
    total = count_result.scalar_one()

    # Paginate
    offset = (filters.page - 1) * filters.page_size
    query = query.offset(offset).limit(filters.page_size)
    result = await db.execute(query)
    items = [_to_response(t) for t in result.scalars()]

    # Filter by search (post-decrypt — Phase 3 will add FTS)
    if filters.search:
        term = filters.search.lower()
        items = [
            t for t in items
            if term in t["description"].lower()
            or (t["merchant"] and term in t["merchant"].lower())
        ]

    return {
        "items": items,
        "total": total,
        "page": filters.page,
        "page_size": filters.page_size,
        "pages": max(1, -(-total // filters.page_size)),
    }


async def get_transaction(db: AsyncSession, txn_id: uuid.UUID, user_id: uuid.UUID) -> Transaction:
    result = await db.execute(
        select(Transaction).where(
            Transaction.id == txn_id,
            Transaction.user_id == user_id,
            Transaction.deleted_at.is_(None),
        )
    )
    txn = result.scalar_one_or_none()
    if not txn:
        raise TransactionError("Transaction not found", status_code=404)
    return txn


async def update_transaction(
    db: AsyncSession,
    txn_id: uuid.UUID,
    user_id: uuid.UUID,
    data: TransactionUpdate,
    base_currency: str,
) -> dict:
    txn = await get_transaction(db, txn_id, user_id)
    now = datetime.now(timezone.utc)
    old_account_id = txn.account_id

    if data.category_id is not None:
        txn.category_id = data.category_id
    if data.status is not None:
        txn.status = data.status
    if data.amount is not None:
        txn.amount = data.amount
        txn.amount_base = data.amount
    if data.date is not None:
        txn.date = data.date
    if data.description is not None:
        txn.description_enc = encrypt_field(data.description)
    if data.merchant is not None:
        txn.merchant_enc = _enc(data.merchant)
    if data.notes is not None:
        txn.notes_enc = _enc(data.notes)
    if data.tags is not None:
        txn.tags = data.tags

    txn.updated_at = now
    await db.flush()
    await recalculate_balance(db, old_account_id)
    return _to_response(txn)


async def delete_transaction(db: AsyncSession, txn_id: uuid.UUID, user_id: uuid.UUID) -> None:
    txn = await get_transaction(db, txn_id, user_id)
    account_id = txn.account_id
    txn.deleted_at = datetime.now(timezone.utc)
    txn.updated_at = datetime.now(timezone.utc)
    await db.flush()
    await recalculate_balance(db, account_id)


async def import_csv(
    db: AsyncSession,
    user_id: uuid.UUID,
    account_id: uuid.UUID,
    rows: list[dict],
    base_currency: str,
) -> dict:
    """
    Import transactions from parsed CSV rows.
    Each row must have: date, description, amount
    Optional: merchant, notes, category_name
    Returns counts of imported vs skipped (duplicates).
    """
    imported = 0
    skipped = 0
    now = datetime.now(timezone.utc)

    for row in rows:
        # Build dedup hash from date + description + amount
        raw = f"{row['date']}|{row['description']}|{row['amount']}"
        import_hash = hashlib.sha256(raw.encode()).hexdigest()

        # Check duplicate
        exists = await db.scalar(
            select(Transaction.id).where(
                Transaction.user_id == user_id,
                Transaction.import_hash == import_hash,
            )
        )
        if exists:
            skipped += 1
            continue

        try:
            amount = Decimal(str(row["amount"]))
            from datetime import date as date_type
            import dateutil.parser
            txn_date = dateutil.parser.parse(str(row["date"])).date()
        except Exception:
            skipped += 1
            continue

        txn_type = "income" if amount > 0 else "expense"

        txn = Transaction(
            user_id=user_id,
            account_id=account_id,
            type=txn_type,
            status="cleared",
            amount=amount,
            amount_base=amount,
            currency=row.get("currency", base_currency),
            base_currency=base_currency,
            exchange_rate=Decimal("1"),
            date=txn_date,
            description_enc=encrypt_field(str(row.get("description", ""))),
            merchant_enc=_enc(row.get("merchant")),
            notes_enc=_enc(row.get("notes")),
            tags=[],
            is_recurring=False,
            import_hash=import_hash,
            created_at=now,
            updated_at=now,
        )
        db.add(txn)
        imported += 1

    await db.flush()
    if imported > 0:
        await recalculate_balance(db, account_id)

    return {"imported": imported, "skipped": skipped}