"""Financial-health formulas — regime-agnostic, useful for any small business.

These are the numbers a good accountant brings up unprompted: am I liquid, am I
profitable, when do I break even, how long does my cash last. All deterministic.
"""

from __future__ import annotations

from decimal import Decimal

from .money import D, money
from .result import CalcResult


def _ratio(numerator: Decimal, denominator: Decimal) -> Decimal:
    if denominator == 0:
        return D(0)
    return numerator / denominator


def current_ratio(current_assets, current_liabilities) -> CalcResult:
    """Liquidity: can short-term assets cover short-term debts? (>1 is healthy.)"""
    ca, cl = D(current_assets), D(current_liabilities)
    r = _ratio(ca, cl)
    notes = []
    if cl == 0:
        notes.append("No current liabilities — ratio undefined, reported as 0.")
    elif r < 1:
        notes.append("Below 1.0 — short-term obligations exceed liquid assets.")
    return CalcResult(
        amount=r.quantize(Decimal("0.01")),
        label="Razón circulante (current ratio)",
        breakdown=[("Activo circulante", money(ca)), ("Pasivo circulante", money(cl))],
        notes=notes,
    )


def profit_margin(net_profit, revenue) -> CalcResult:
    """Net margin as a fraction of revenue."""
    np_, rev = D(net_profit), D(revenue)
    r = _ratio(np_, rev)
    return CalcResult(
        amount=r.quantize(Decimal("0.0001")),
        label="Margen de utilidad neta",
        breakdown=[("Utilidad neta", money(np_)), ("Ingresos", money(rev))],
        notes=[f"≈ {(r * 100).quantize(Decimal('0.01'))}% of revenue is profit."],
    )


def break_even_units(fixed_costs, price_per_unit, variable_cost_per_unit) -> CalcResult:
    """Units to sell to cover all costs: FC / (price − variable cost)."""
    fc = D(fixed_costs)
    contribution = D(price_per_unit) - D(variable_cost_per_unit)
    if contribution <= 0:
        return CalcResult(
            amount=D(0),
            label="Punto de equilibrio (unidades)",
            breakdown=[
                ("Costos fijos", money(fc)),
                ("Margen de contribución por unidad", money(contribution)),
            ],
            notes=["Contribution margin ≤ 0 — break-even impossible at this price."],
        )
    units = fc / contribution
    return CalcResult(
        amount=units.quantize(Decimal("0.01")),
        label="Punto de equilibrio (unidades)",
        breakdown=[
            ("Costos fijos", money(fc)),
            ("Margen de contribución por unidad", money(contribution)),
        ],
    )


def cash_runway_months(cash_on_hand, monthly_net_burn) -> CalcResult:
    """How many months the cash lasts at the current net burn rate."""
    cash = D(cash_on_hand)
    burn = D(monthly_net_burn)
    if burn <= 0:
        return CalcResult(
            amount=D(0),
            label="Meses de pista (runway)",
            breakdown=[("Efectivo disponible", money(cash)), ("Quema neta mensual", money(burn))],
            notes=["Non-positive burn — cash-flow positive, runway effectively infinite."],
        )
    months = cash / burn
    return CalcResult(
        amount=months.quantize(Decimal("0.1")),
        label="Meses de pista (runway)",
        breakdown=[("Efectivo disponible", money(cash)), ("Quema neta mensual", money(burn))],
    )