"""Payment-history risk scorer.

Pure-Python: takes payment history (list of payment events) + average days-late
and returns a risk score 0-100. Higher = more risky."""

from dataclasses import dataclass


@dataclass
class PartnerRiskScore:
    score: int
    band: str
    drivers: list[str]


def score_partner(*, total_invoices: int = 0, paid_late_count: int = 0,
                    avg_days_late: float = 0.0,
                    longest_overdue_days: int = 0,
                    open_overdue_amount: float = 0.0,
                    average_invoice_amount: float = 1000.0) -> PartnerRiskScore:
    """Compute a 0-100 risk score from payment-history primitives.

    Heuristic weights:
    - 30% : late-payment ratio (paid_late_count / total_invoices)
    - 25% : avg days late (capped at 60 days)
    - 25% : longest current overdue (capped at 120 days)
    - 20% : open overdue amount as multiple of average invoice
    """
    drivers: list[str] = []
    score = 0.0

    if total_invoices > 0:
        late_ratio = paid_late_count / total_invoices
        score += min(late_ratio * 100, 100) * 0.30
        if late_ratio > 0.5:
            drivers.append(f"{paid_late_count}/{total_invoices} invoices paid late")

    score += min(avg_days_late / 60, 1) * 100 * 0.25
    if avg_days_late > 14:
        drivers.append(f"Avg {avg_days_late:.1f} days late on payment")

    score += min(longest_overdue_days / 120, 1) * 100 * 0.25
    if longest_overdue_days > 30:
        drivers.append(f"Longest currently overdue: {longest_overdue_days} days")

    if average_invoice_amount > 0:
        ratio = open_overdue_amount / average_invoice_amount
        score += min(ratio / 5, 1) * 100 * 0.20
        if ratio > 1.5:
            drivers.append(f"Open overdue ${open_overdue_amount:,.2f} ({ratio:.1f}x avg invoice)")

    final = int(round(score))
    if final >= 80:
        band = 'critical'
    elif final >= 60:
        band = 'high'
    elif final >= 30:
        band = 'medium'
    else:
        band = 'low'

    return PartnerRiskScore(score=final, band=band, drivers=drivers)