"""
UK tax service layer for MyMidas.

Pure calculation functions live in tax_calculations.py (no DB imports).
This module provides the DB-backed service layer: rate loading, CRUD, report builder.
"""
from __future__ import annotations

import uuid
from datetime import date, datetime, timezone
from decimal import Decimal
from typing import Any

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

from app.core.security import decrypt_field, encrypt_field
from app.services.tax_calculations import (  # noqa: F401 — re-exported for callers
    calculate_cgt,
    calculate_dividend_tax,
    calculate_income_tax,
    calculate_ni,
    parse_tax_code,
    tax_year_date_range,
    tax_year_for_date,
)


# ---------------------------------------------------------------------------
# DB helpers
# ---------------------------------------------------------------------------

async def load_rates(db: AsyncSession, user_id: uuid.UUID, tax_year: int) -> dict:
    """Load and return rate config dict for a given user/year.

    Returns {"income_tax": {...}, "ni": {...}, "cgt": {...}, "dividend": {...}}
    Raises ValueError if any rate type is missing for the requested year.
    """
    from app.db.models.tax import TaxRateConfig
    result = await db.execute(
        select(TaxRateConfig).where(
            TaxRateConfig.user_id == user_id,
            TaxRateConfig.tax_year == tax_year,
        )
    )
    rows = list(result.scalars())
    if not rows:
        raise ValueError(f"No tax rate config found for year {tax_year}. Please configure rates first.")

    rates: dict = {}
    for row in rows:
        rates[row.rate_type] = row.config

    required = {"income_tax", "ni", "cgt", "dividend"}
    missing = required - rates.keys()
    if missing:
        raise ValueError(f"Incomplete tax rate config for year {tax_year}: missing {missing}")

    return rates


async def seed_default_rates(db: AsyncSession, user_id: uuid.UUID) -> None:
    """Insert default 2025 and 2026 rate configs for a newly registered user."""
    from app.db.models.tax import TaxRateConfig

    now = datetime.now(timezone.utc)

    income_tax_bands = {
        "bands": [
            {"from": 0,      "to": 12570,  "rate": 0.00},
            {"from": 12570,  "to": 50270,  "rate": 0.20},
            {"from": 50270,  "to": 125140, "rate": 0.40},
            {"from": 125140, "to": None,   "rate": 0.45},
        ]
    }
    ni_bands = {
        "bands": [
            {"from": 0,      "to": 12570,  "rate": 0.00},
            {"from": 12570,  "to": 50270,  "rate": 0.08},
            {"from": 50270,  "to": None,   "rate": 0.02},
        ]
    }
    cgt = {"exempt": 3000, "basic_rate": 0.18, "higher_rate": 0.24}
    dividend = {
        "allowance": 500,
        "basic_rate": 0.0875,
        "higher_rate": 0.3375,
        "additional_rate": 0.3935,
    }

    defaults = {
        "income_tax": income_tax_bands,
        "ni": ni_bands,
        "cgt": cgt,
        "dividend": dividend,
    }

    for tax_year in (2025, 2026):
        for rate_type, config in defaults.items():
            existing = await db.execute(
                select(TaxRateConfig).where(
                    TaxRateConfig.user_id == user_id,
                    TaxRateConfig.tax_year == tax_year,
                    TaxRateConfig.rate_type == rate_type,
                )
            )
            if existing.scalar_one_or_none() is None:
                db.add(TaxRateConfig(
                    id=uuid.uuid4(),
                    user_id=user_id,
                    tax_year=tax_year,
                    rate_type=rate_type,
                    config=config,
                    updated_at=now,
                ))


# ---------------------------------------------------------------------------
# Tax profile CRUD
# ---------------------------------------------------------------------------

async def get_tax_profile(
    db: AsyncSession, user_id: uuid.UUID, tax_year: int
):
    from app.db.models.tax import TaxProfile
    result = await db.execute(
        select(TaxProfile).where(
            TaxProfile.user_id == user_id,
            TaxProfile.tax_year == tax_year,
        )
    )
    return result.scalar_one_or_none()


async def upsert_tax_profile(
    db: AsyncSession,
    user_id: uuid.UUID,
    tax_year: int,
    tax_code: str,
    employer_name: str | None,
    is_cumulative: bool,
):
    from app.db.models.tax import TaxProfile
    from app.core.audit import write_audit

    now = datetime.now(timezone.utc)
    profile = await get_tax_profile(db, user_id, tax_year)

    employer_enc = encrypt_field(employer_name) if employer_name else None

    if profile is None:
        profile = TaxProfile(
            id=uuid.uuid4(),
            user_id=user_id,
            tax_year=tax_year,
            tax_code=tax_code,
            employer_name_enc=employer_enc,
            is_cumulative=is_cumulative,
            created_at=now,
            updated_at=now,
        )
        db.add(profile)
        action = "tax_profile_create"
    else:
        profile.tax_code = tax_code
        profile.employer_name_enc = employer_enc
        profile.is_cumulative = is_cumulative
        profile.updated_at = now
        action = "tax_profile_update"

    await db.flush()
    await write_audit(db, user_id=user_id, action=action,
                          resource_type="tax_profile", resource_id=profile.id)
    return profile


def _profile_to_response(profile) -> dict:
    from app.db.models.tax import TaxProfile
    employer = None
    if profile.employer_name_enc:
        try:
            employer = decrypt_field(profile.employer_name_enc)
        except Exception:
            employer = None
    return {
        "id": str(profile.id),
        "tax_year": profile.tax_year,
        "tax_code": profile.tax_code,
        "employer_name": employer,
        "is_cumulative": profile.is_cumulative,
        "created_at": profile.created_at.isoformat(),
        "updated_at": profile.updated_at.isoformat(),
    }


# ---------------------------------------------------------------------------
# Payslip CRUD
# ---------------------------------------------------------------------------

async def list_payslips(db: AsyncSession, user_id: uuid.UUID, tax_year: int) -> list:
    from app.db.models.tax import Payslip, TaxProfile
    result = await db.execute(
        select(Payslip)
        .join(TaxProfile, Payslip.tax_profile_id == TaxProfile.id)
        .where(
            Payslip.user_id == user_id,
            TaxProfile.tax_year == tax_year,
        )
        .order_by(Payslip.period_year, Payslip.period_month.nulls_last())
    )
    return list(result.scalars())


async def create_payslip(
    db: AsyncSession,
    user_id: uuid.UUID,
    tax_year: int,
    period_month: int | None,
    period_year: int,
    gross_pay: Decimal,
    income_tax_withheld: Decimal,
    ni_withheld: Decimal,
    net_pay: Decimal,
    is_p60: bool = False,
    notes: str | None = None,
):
    from app.db.models.tax import Payslip
    from app.core.audit import write_audit

    profile = await get_tax_profile(db, user_id, tax_year)
    if profile is None:
        raise ValueError(f"No tax profile for year {tax_year}. Create a profile first.")

    now = datetime.now(timezone.utc)
    notes_enc = encrypt_field(notes) if notes else None

    payslip = Payslip(
        id=uuid.uuid4(),
        user_id=user_id,
        tax_profile_id=profile.id,
        period_month=period_month,
        period_year=period_year,
        gross_pay=gross_pay,
        income_tax_withheld=income_tax_withheld,
        ni_withheld=ni_withheld,
        net_pay=net_pay,
        is_p60=is_p60,
        notes_enc=notes_enc,
        created_at=now,
    )
    db.add(payslip)
    await db.flush()
    await write_audit(db, user_id=user_id, action="payslip_create",
                          resource_type="payslip", resource_id=payslip.id)
    return payslip


async def update_payslip(
    db: AsyncSession,
    user_id: uuid.UUID,
    payslip_id: uuid.UUID,
    **kwargs,
):
    from app.db.models.tax import Payslip
    from app.core.audit import write_audit

    result = await db.execute(
        select(Payslip).where(Payslip.id == payslip_id, Payslip.user_id == user_id)
    )
    payslip = result.scalar_one_or_none()
    if payslip is None:
        raise ValueError("Payslip not found")

    for field, value in kwargs.items():
        if field == "notes":
            payslip.notes_enc = encrypt_field(value) if value else None
        else:
            setattr(payslip, field, value)

    await db.flush()
    await write_audit(db, user_id=user_id, action="payslip_update",
                          resource_type="payslip", resource_id=payslip.id)
    return payslip


async def delete_payslip(db: AsyncSession, user_id: uuid.UUID, payslip_id: uuid.UUID) -> None:
    from app.db.models.tax import Payslip
    from app.core.audit import write_audit

    result = await db.execute(
        select(Payslip).where(Payslip.id == payslip_id, Payslip.user_id == user_id)
    )
    payslip = result.scalar_one_or_none()
    if payslip is None:
        raise ValueError("Payslip not found")

    await write_audit(db, user_id=user_id, action="payslip_delete",
                          resource_type="payslip", resource_id=payslip_id)
    await db.delete(payslip)
    await db.flush()


async def replace_with_p60(
    db: AsyncSession,
    user_id: uuid.UUID,
    tax_year: int,
    gross_pay: Decimal,
    income_tax_withheld: Decimal,
    ni_withheld: Decimal,
    net_pay: Decimal,
) -> None:
    """Delete all existing payslips for the tax year and replace with a single P60."""
    from app.db.models.tax import Payslip, TaxProfile
    from app.core.audit import write_audit

    profile = await get_tax_profile(db, user_id, tax_year)
    if profile is None:
        raise ValueError(f"No tax profile for year {tax_year}. Create a profile first.")

    await db.execute(
        delete(Payslip).where(
            Payslip.user_id == user_id,
            Payslip.tax_profile_id == profile.id,
        )
    )

    now = datetime.now(timezone.utc)
    p60 = Payslip(
        id=uuid.uuid4(),
        user_id=user_id,
        tax_profile_id=profile.id,
        period_month=None,
        period_year=tax_year - 1,  # P60 covers year ending 5 Apr tax_year; the employer year starts the prior calendar year
        gross_pay=gross_pay,
        income_tax_withheld=income_tax_withheld,
        ni_withheld=ni_withheld,
        net_pay=net_pay,
        is_p60=True,
        notes_enc=None,
        created_at=now,
    )
    db.add(p60)
    await db.flush()
    await write_audit(db, user_id=user_id, action="payslip_p60_replace",
                          resource_type="payslip", resource_id=p60.id)


def _payslip_to_response(payslip) -> dict:
    notes = None
    if payslip.notes_enc:
        try:
            notes = decrypt_field(payslip.notes_enc)
        except Exception:
            notes = None
    return {
        "id": str(payslip.id),
        "tax_profile_id": str(payslip.tax_profile_id),
        "period_month": payslip.period_month,
        "period_year": payslip.period_year,
        "gross_pay": str(payslip.gross_pay),
        "income_tax_withheld": str(payslip.income_tax_withheld),
        "ni_withheld": str(payslip.ni_withheld),
        "net_pay": str(payslip.net_pay),
        "is_p60": payslip.is_p60,
        "notes": notes,
        "created_at": payslip.created_at.isoformat(),
    }


# ---------------------------------------------------------------------------
# Manual CGT disposal CRUD
# ---------------------------------------------------------------------------

async def list_manual_disposals(
    db: AsyncSession, user_id: uuid.UUID, tax_year: int
) -> list:
    from app.db.models.tax import ManualCGTDisposal
    result = await db.execute(
        select(ManualCGTDisposal).where(
            ManualCGTDisposal.user_id == user_id,
            ManualCGTDisposal.tax_year == tax_year,
        ).order_by(ManualCGTDisposal.disposal_date)
    )
    return list(result.scalars())


async def create_manual_disposal(
    db: AsyncSession,
    user_id: uuid.UUID,
    tax_year: int,
    disposal_date: date,
    asset_description: str,
    proceeds: Decimal,
    cost_basis: Decimal,
    notes: str | None = None,
):
    from app.db.models.tax import ManualCGTDisposal
    from app.core.audit import write_audit

    now = datetime.now(timezone.utc)
    disposal = ManualCGTDisposal(
        id=uuid.uuid4(),
        user_id=user_id,
        tax_year=tax_year,
        disposal_date=disposal_date,
        asset_description_enc=encrypt_field(asset_description),
        proceeds=proceeds,
        cost_basis=cost_basis,
        notes_enc=encrypt_field(notes) if notes else None,
        created_at=now,
    )
    db.add(disposal)
    await db.flush()
    await write_audit(db, user_id=user_id, action="cgt_disposal_create",
                          resource_type="manual_cgt_disposal", resource_id=disposal.id)
    return disposal


async def update_manual_disposal(
    db: AsyncSession,
    user_id: uuid.UUID,
    disposal_id: uuid.UUID,
    disposal_date: date,
    asset_description: str,
    proceeds: Decimal,
    cost_basis: Decimal,
    notes: str | None = None,
):
    from app.db.models.tax import ManualCGTDisposal
    from app.core.audit import write_audit

    result = await db.execute(
        select(ManualCGTDisposal).where(
            ManualCGTDisposal.id == disposal_id,
            ManualCGTDisposal.user_id == user_id,
        )
    )
    disposal = result.scalar_one_or_none()
    if disposal is None:
        raise ValueError("Disposal not found")

    disposal.disposal_date = disposal_date
    disposal.asset_description_enc = encrypt_field(asset_description)
    disposal.proceeds = proceeds
    disposal.cost_basis = cost_basis
    disposal.notes_enc = encrypt_field(notes) if notes else None
    await db.flush()
    await write_audit(db, user_id=user_id, action="cgt_disposal_update",
                          resource_type="manual_cgt_disposal", resource_id=disposal.id)
    return disposal


async def delete_manual_disposal(
    db: AsyncSession, user_id: uuid.UUID, disposal_id: uuid.UUID
) -> None:
    from app.db.models.tax import ManualCGTDisposal
    from app.core.audit import write_audit

    result = await db.execute(
        select(ManualCGTDisposal).where(
            ManualCGTDisposal.id == disposal_id,
            ManualCGTDisposal.user_id == user_id,
        )
    )
    disposal = result.scalar_one_or_none()
    if disposal is None:
        raise ValueError("Disposal not found")

    await write_audit(db, user_id=user_id, action="cgt_disposal_delete",
                          resource_type="manual_cgt_disposal", resource_id=disposal_id)
    await db.delete(disposal)
    await db.flush()


def _disposal_to_response(disposal) -> dict:
    asset_desc = ""
    try:
        asset_desc = decrypt_field(disposal.asset_description_enc)
    except Exception:
        pass
    notes = None
    if disposal.notes_enc:
        try:
            notes = decrypt_field(disposal.notes_enc)
        except Exception:
            pass
    gain_loss = disposal.proceeds - disposal.cost_basis
    return {
        "id": str(disposal.id),
        "tax_year": disposal.tax_year,
        "disposal_date": disposal.disposal_date.isoformat(),
        "asset_description": asset_desc,
        "proceeds": str(disposal.proceeds),
        "cost_basis": str(disposal.cost_basis),
        "gain_loss": str(gain_loss),
        "notes": notes,
        "created_at": disposal.created_at.isoformat(),
    }


# ---------------------------------------------------------------------------
# Tax rate config CRUD
# ---------------------------------------------------------------------------

async def list_configured_years(db: AsyncSession, user_id: uuid.UUID) -> list[int]:
    from app.db.models.tax import TaxRateConfig
    result = await db.execute(
        select(TaxRateConfig.tax_year)
        .where(TaxRateConfig.user_id == user_id)
        .distinct()
        .order_by(TaxRateConfig.tax_year)
    )
    return [row[0] for row in result]


async def get_rate_config(
    db: AsyncSession, user_id: uuid.UUID, tax_year: int
) -> dict:
    from app.db.models.tax import TaxRateConfig
    result = await db.execute(
        select(TaxRateConfig).where(
            TaxRateConfig.user_id == user_id,
            TaxRateConfig.tax_year == tax_year,
        )
    )
    rows = list(result.scalars())
    if not rows:
        raise ValueError(f"No rate config for year {tax_year}")
    return {
        "tax_year": tax_year,
        "rates": {row.rate_type: row.config for row in rows},
        "updated_at": max(row.updated_at for row in rows).isoformat(),
    }


async def upsert_rate_config(
    db: AsyncSession,
    user_id: uuid.UUID,
    tax_year: int,
    rates: dict,
) -> dict:
    from app.db.models.tax import TaxRateConfig
    from app.core.audit import write_audit

    now = datetime.now(timezone.utc)
    for rate_type, config in rates.items():
        result = await db.execute(
            select(TaxRateConfig).where(
                TaxRateConfig.user_id == user_id,
                TaxRateConfig.tax_year == tax_year,
                TaxRateConfig.rate_type == rate_type,
            )
        )
        row = result.scalar_one_or_none()
        if row is None:
            db.add(TaxRateConfig(
                id=uuid.uuid4(),
                user_id=user_id,
                tax_year=tax_year,
                rate_type=rate_type,
                config=config,
                updated_at=now,
            ))
        else:
            row.config = config
            row.updated_at = now

    await db.flush()
    await write_audit(db, user_id=user_id, action="tax_rate_config_update",
                          resource_type="tax_rate_config",
                          resource_id=None,
                          metadata={"tax_year": tax_year})
    return await get_rate_config(db, user_id, tax_year)


# ---------------------------------------------------------------------------
# Report builder
# ---------------------------------------------------------------------------

async def build_tax_report(
    db: AsyncSession, user_id: uuid.UUID, tax_year: int
) -> dict[str, Any]:
    """Build the full tax report for a given year.

    Steps:
    1. Load rates
    2. Load profile + payslip totals
    3. Load investment sell disposals within the tax year
    4. Load investment dividend transactions within the tax year
    5. Load manual CGT disposals
    6. Run calculations: income tax → NI → CGT → dividend tax
    7. Return full report dict
    """
    from app.db.models.tax import ManualCGTDisposal, Payslip, TaxProfile
    from app.db.models.investment_transaction import InvestmentTransaction
    from app.db.models.investment_holding import InvestmentHolding
    from app.db.models.asset import Asset

    rates = await load_rates(db, user_id, tax_year)
    start_date, end_date = tax_year_date_range(tax_year)

    # ---- Profile ----
    profile = await get_tax_profile(db, user_id, tax_year)
    tax_code = profile.tax_code if profile else "1257L"
    profile_data = _profile_to_response(profile) if profile else None

    # ---- Payslip totals ----
    payslips = await list_payslips(db, user_id, tax_year)
    gross_income = sum((Decimal(str(p.gross_pay)) for p in payslips), Decimal("0"))
    income_tax_withheld = sum((Decimal(str(p.income_tax_withheld)) for p in payslips), Decimal("0"))
    ni_withheld = sum((Decimal(str(p.ni_withheld)) for p in payslips), Decimal("0"))
    payslip_rows = [_payslip_to_response(p) for p in payslips]

    # ---- Investment sell disposals ----
    inv_disposals_result = await db.execute(
        select(InvestmentTransaction, InvestmentHolding, Asset)
        .join(InvestmentHolding, InvestmentTransaction.holding_id == InvestmentHolding.id)
        .join(Asset, InvestmentHolding.asset_id == Asset.id)
        .where(
            InvestmentTransaction.user_id == user_id,
            InvestmentTransaction.type == "sell",
            InvestmentTransaction.date >= start_date,
            InvestmentTransaction.date <= end_date,
        )
        .order_by(InvestmentTransaction.date)
    )
    inv_disposal_rows = []
    total_inv_gain = Decimal("0")
    for inv_txn, holding, asset in inv_disposals_result:
        proceeds = Decimal(str(inv_txn.total_amount))
        cost = Decimal(str(inv_txn.quantity)) * Decimal(str(holding.avg_cost_basis))
        gain = proceeds - cost - Decimal(str(inv_txn.fees))
        total_inv_gain += gain
        inv_disposal_rows.append({
            "date": inv_txn.date.isoformat(),
            "asset": asset.name,
            "symbol": asset.symbol,
            "quantity": str(inv_txn.quantity),
            "proceeds": str(proceeds),
            "cost_basis": str(cost),
            "fees": str(inv_txn.fees),
            "gain_loss": str(gain),
        })

    # ---- Manual CGT disposals ----
    manual_disposals = await list_manual_disposals(db, user_id, tax_year)
    manual_disposal_rows = [_disposal_to_response(d) for d in manual_disposals]
    total_manual_gain = sum(
        (Decimal(str(d.proceeds)) - Decimal(str(d.cost_basis)) for d in manual_disposals),
        Decimal("0"),
    )
    total_cgt_gain = total_inv_gain + total_manual_gain

    # ---- Investment dividends ----
    div_result = await db.execute(
        select(InvestmentTransaction, InvestmentHolding, Asset)
        .join(InvestmentHolding, InvestmentTransaction.holding_id == InvestmentHolding.id)
        .join(Asset, InvestmentHolding.asset_id == Asset.id)
        .where(
            InvestmentTransaction.user_id == user_id,
            InvestmentTransaction.type == "dividend",
            InvestmentTransaction.date >= start_date,
            InvestmentTransaction.date <= end_date,
        )
        .order_by(InvestmentTransaction.date)
    )
    dividend_rows = []
    total_dividends = Decimal("0")
    for inv_txn, holding, asset in div_result:
        amount = Decimal(str(inv_txn.total_amount))
        total_dividends += amount
        dividend_rows.append({
            "date": inv_txn.date.isoformat(),
            "asset": asset.name,
            "symbol": asset.symbol,
            "amount": str(amount),
        })

    # ---- Calculations ----
    income_tax_result = calculate_income_tax(gross_income, tax_code, rates)
    ni_result = calculate_ni(gross_income, rates)
    cgt_result = calculate_cgt(
        total_cgt_gain,
        income_tax_result["remaining_basic_rate_band"],
        rates,
    )
    dividend_result = calculate_dividend_tax(
        total_dividends,
        income_tax_result["remaining_basic_rate_band"],
        rates,
    )

    # ---- Totals ----
    total_liability = (
        income_tax_result["liability"]
        + ni_result["liability"]
        + cgt_result["liability"]
        + dividend_result["liability"]
    )
    total_withheld = income_tax_withheld + ni_withheld
    net_owed = total_liability - total_withheld

    return {
        "tax_year": tax_year,
        "tax_year_display": f"{tax_year - 1}/{str(tax_year)[2:]}",
        "profile": profile_data,
        "income": {
            "gross_income": str(gross_income),
            "income_tax_withheld": str(income_tax_withheld),
            "ni_withheld": str(ni_withheld),
            "payslips": payslip_rows,
        },
        "income_tax": {
            **{k: str(v) if isinstance(v, Decimal) else v
               for k, v in income_tax_result.items()
               if k != "remaining_basic_rate_band"},
            "withheld": str(income_tax_withheld),
            "owed": str(income_tax_result["liability"] - income_tax_withheld),
        },
        "ni": {
            **{k: str(v) if isinstance(v, Decimal) else v
               for k, v in ni_result.items()},
            "withheld": str(ni_withheld),
            "owed": str(ni_result["liability"] - ni_withheld),
        },
        "cgt": {
            **{k: str(v) if isinstance(v, Decimal) else v
               for k, v in cgt_result.items()},
            "investment_disposals": inv_disposal_rows,
            "manual_disposals": manual_disposal_rows,
            "total_gain": str(total_cgt_gain),
        },
        "dividends": {
            **{k: str(v) if isinstance(v, Decimal) else v
               for k, v in dividend_result.items()},
            "dividend_transactions": dividend_rows,
        },
        "summary": {
            "total_liability": str(total_liability),
            "total_withheld": str(total_withheld),
            "net_owed": str(net_owed),
            "overpaid": net_owed < 0,
        },
    }