"""Unit normalization for OpenGoods.

Product parameters arrive in many units (g/kg/ml/L, kcal/kJ, ...). To make
values comparable and searchable we store both the original value and a
normalized value expressed in a canonical unit per dimension.

This module is intentionally dependency-free and pure so it is easy to test.
"""

from __future__ import annotations

from dataclasses import dataclass
from decimal import Decimal

# Conversion factor maps each unit to its canonical unit within a dimension.
# canonical_value = value * factor
_FACTORS: dict[str, tuple[str, str, Decimal]] = {
    # mass -> g
    "mg": ("mass", "g", Decimal("0.001")),
    "g": ("mass", "g", Decimal("1")),
    "kg": ("mass", "g", Decimal("1000")),
    # volume -> ml
    "ml": ("volume", "ml", Decimal("1")),
    "cl": ("volume", "ml", Decimal("10")),
    "l": ("volume", "ml", Decimal("1000")),
    # energy -> kJ
    "kj": ("energy", "kJ", Decimal("1")),
    "kcal": ("energy", "kJ", Decimal("4.184")),
}

# Alias map normalizes common spellings/locales to a canonical unit code.
_ALIASES: dict[str, str] = {
    "kgs": "kg",
    "千克": "kg",
    "公斤": "kg",
    "克": "g",
    "毫升": "ml",
    "升": "l",
    "L": "l",
    "litre": "l",
    "liter": "l",
    "kj": "kj",
    "kJ": "kj",
    "千焦": "kj",
    "千卡": "kcal",
    "大卡": "kcal",
}


class UnitError(ValueError):
    """Raised when a unit cannot be recognized."""


@dataclass(frozen=True)
class Normalized:
    """Result of normalizing a (value, unit) pair to its canonical unit."""

    value: Decimal
    unit: str
    dimension: str
    canonical_value: Decimal
    canonical_unit: str


def canonical_unit_code(unit: str) -> str:
    """Resolve a raw unit string to a known canonical unit code."""
    cleaned = unit.strip()
    cleaned = _ALIASES.get(cleaned, cleaned).lower()
    if cleaned not in _FACTORS:
        raise UnitError(f"unknown unit: {unit!r}")
    return cleaned


def normalize(value: Decimal | float | int | str, unit: str) -> Normalized:
    """Normalize a value+unit to its canonical unit within its dimension."""
    code = canonical_unit_code(unit)
    dimension, canonical, factor = _FACTORS[code]
    dec = value if isinstance(value, Decimal) else Decimal(str(value))
    return Normalized(
        value=dec,
        unit=code,
        dimension=dimension,
        canonical_value=dec * factor,
        canonical_unit=canonical,
    )


def kcal_to_kj(kcal: Decimal | float | int | str) -> Decimal:
    """Convert energy in kcal to kJ (1 kcal = 4.184 kJ)."""
    dec = kcal if isinstance(kcal, Decimal) else Decimal(str(kcal))
    return dec * Decimal("4.184")


def kj_to_kcal(kj: Decimal | float | int | str) -> Decimal:
    """Convert energy in kJ to kcal."""
    dec = kj if isinstance(kj, Decimal) else Decimal(str(kj))
    return dec / Decimal("4.184")