// Previously duplicated as groupByTaxRate() independently inside
// invoicePdf.tsx and correctionInvoicePdf.tsx (and, on the Payload backend,
// their own copies) — pulled out so the storefront's own MwSt. breakdowns
// (checkout summary, order confirmation page/email, account order pages)
// can share the exact same math instead of a fourth hand-rolled version
// drifting out of sync with what the actual invoices say.

export type TaxBreakdownLine = { quantity: number; unitPrice: number; taxRatePercent: number };
export type TaxBreakdownGroup = { rate: number; net: number; tax: number; gross: number };

// Groups line items by their effective VAT rate, then scales each group's
// gross total by however much shipping/discount moved the grand total away
// from the raw item subtotal — proportional to that group's own share of
// the subtotal, not a flat split. Passing discountAmount=0 and
// shippingCost=0 (e.g. a Gutschrift, which excludes both) collapses `scale`
// to 1, i.e. no adjustment at all.
export function computeTaxBreakdown(
  items: TaxBreakdownLine[],
  subtotal: number,
  discountAmount: number,
  shippingCost: number,
): TaxBreakdownGroup[] {
  const groups = new Map<number, number>();
  for (const item of items) {
    const lineGross = item.quantity * item.unitPrice;
    groups.set(item.taxRatePercent, (groups.get(item.taxRatePercent) ?? 0) + lineGross);
  }
  const scale = subtotal > 0 ? (subtotal - discountAmount + shippingCost) / subtotal : 1;
  return Array.from(groups.entries())
    .map(([rate, lineGross]) => {
      const gross = lineGross * scale;
      const net = gross / (1 + rate / 100);
      return { rate, net, tax: gross - net, gross };
    })
    .sort((a, b) => b.rate - a.rate);
}