from decimal import Decimal, ROUND_HALF_UP

from pydantic import BaseModel, Field, field_validator

CURRENCY_SYMBOLS = {
    "RUB": "₽",
    "USD": "$",
    "EUR": "€",
    "GBP": "£",
    "CNY": "¥",
    "JPY": "¥",
    "KZT": "₸",
    "UAH": "₴",
}

# ISO 4217 minor units per currency (https://en.wikipedia.org/wiki/ISO_4217).
# Keys are uppercase 3-letter codes, values are the number of decimal digits used
# when DISPLAYING the currency. Internal arithmetic keeps full Decimal precision
# (do not modify — see T8 decision `internal_precision`).
#
# - 0 decimals: JPY (Japanese yen)
# - 2 decimals: most currencies (RUB, USD, EUR, GBP, CNY, KZT, UAH, ...)
# - 3 decimals: BHD (Bahraini dinar), KWD (Kuwaiti dinar), OMR (Omani rial)
#
# Unknown currencies fall back to `DEFAULT_CURRENCY_DECIMALS` (2).
CURRENCY_DECIMALS: dict[str, int] = {
    "JPY": 0,
    "RUB": 2,
    "USD": 2,
    "EUR": 2,
    "GBP": 2,
    "CNY": 2,
    "KZT": 2,
    "UAH": 2,
    "BHD": 3,
    "KWD": 3,
    "OMR": 3,
}

DEFAULT_CURRENCY_DECIMALS = 2


class ExchangeRate(BaseModel):
    from_currency: str = "USD"
    to_currency: str = "RUB"
    rate: Decimal = Field(default=Decimal("80"))

    @field_validator("rate")
    @classmethod
    def _rate_positive(cls, v: Decimal) -> Decimal:
        if v <= 0:
            raise ValueError("rate must be positive")
        return v


DEFAULT_RATES: list[ExchangeRate] = [
    ExchangeRate(from_currency="USD", to_currency="RUB", rate=Decimal("80")),
]


class CurrencyError(Exception):
    pass


class CurrencyConverter:
    def __init__(self, rates: list[ExchangeRate] | None = None):
        self._rates: dict[tuple[str, str], Decimal] = {}
        if rates:
            for r in rates:
                self.set_rate(r.from_currency, r.to_currency, r.rate)

    def set_rate(
        self,
        from_currency: str,
        to_currency: str,
        rate: Decimal | float | int | str,
    ) -> None:
        d = Decimal(str(rate)) if not isinstance(rate, Decimal) else rate
        if d <= 0:
            raise CurrencyError(f"Rate must be positive: {d}")
        self._rates[(from_currency, to_currency)] = d
        self._rates[(to_currency, from_currency)] = Decimal("1") / d

    def get_rate(self, from_currency: str, to_currency: str) -> Decimal:
        if from_currency == to_currency:
            return Decimal("1")
        try:
            return self._rates[(from_currency, to_currency)]
        except KeyError:
            raise CurrencyError(f"No exchange rate: {from_currency} → {to_currency}")

    def convert(
        self,
        amount: Decimal | float | int | str,
        from_currency: str,
        to_currency: str,
    ) -> Decimal:
        a = Decimal(str(amount)) if not isinstance(amount, Decimal) else amount
        if from_currency == to_currency:
            return a
        rate = self.get_rate(from_currency, to_currency)
        return (a * rate).quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)

    def get_symbol(self, currency: str) -> str:
        return CURRENCY_SYMBOLS.get(currency, currency)

    @staticmethod
    def format_money(
        amount: Decimal | float | int | str,
        currency: str,
    ) -> str:
        """Format `amount` for display using ISO 4217 decimal digits for `currency`.

        Rounding: ROUND_HALF_UP (0.5 -> 1) before display — internal arithmetic in
        Decimal is unchanged. Known symbols are rendered as a tight prefix
        (e.g. "$1,234.57"); unknown codes are rendered as a spaced prefix
        ("BHD 1.234") so the layout is consistent regardless of symbol availability.
        Negative values render with the sign before the symbol ("-₽1,200.00").
        """
        cur = currency.upper()
        decimals = CURRENCY_DECIMALS.get(cur, DEFAULT_CURRENCY_DECIMALS)
        a = Decimal(str(amount)) if not isinstance(amount, Decimal) else amount
        quantizer = Decimal("1") if decimals == 0 else Decimal(1).scaleb(-decimals)
        quantized = a.quantize(quantizer, rounding=ROUND_HALF_UP)
        sign = "-" if quantized < 0 else ""
        abs_quantized = -quantized if quantized < 0 else quantized
        num_str = f"{abs_quantized:,.{decimals}f}"
        if cur in CURRENCY_SYMBOLS:
            return f"{sign}{CURRENCY_SYMBOLS[cur]}{num_str}"
        return f"{sign}{cur} {num_str}"

    @classmethod
    def with_defaults(cls) -> "CurrencyConverter":
        return cls(DEFAULT_RATES)