/**
 * Single source of truth for PO money math (issue #133).
 *
 * A PO's value is built up as:
 *
 *     taxable   = Σ qty · unitPrice                 (ex-GST)
 *     gst       = Σ qty · unitPrice · gstRate
 *     inclGst   = taxable + gst                      (the line-items total)
 *     netPayable = inclGst + tcs − discount          (PO-level charges, below GST)
 *
 * `netPayable` is what is stored in `PurchaseOrder.totalAmount`, so payments,
 * reports, and the advance-payment slider all operate on the true amount due.
 *
 * TCS and Discount are **absolute** rupee amounts (the UI's % control is only a
 * convenience that writes back the rupee value). Discount is applied **post-GST**.
 * The percentage shown for either is taken against `inclGst` (the GST-inclusive
 * line-items total) — see `amountToPercent` / `percentToAmount`.
 */

export interface MoneyItem {
  quantity: number;
  unitPrice: number;
  gstRate?: number | null;
}

export const DEFAULT_GST_RATE = 0.18;

export interface PoMoney {
  taxable: number;
  gst: number;
  inclGst: number;
  tcs: number;
  discount: number;
  netPayable: number;
}

export function computePoMoney(
  items: MoneyItem[],
  tcs = 0,
  discount = 0
): PoMoney {
  const taxable = items.reduce((s, i) => s + i.quantity * i.unitPrice, 0);
  const gst = items.reduce(
    (s, i) => s + i.quantity * i.unitPrice * (i.gstRate ?? DEFAULT_GST_RATE),
    0
  );
  const inclGst = taxable + gst;
  const t = tcs || 0;
  const d = discount || 0;
  return { taxable, gst, inclGst, tcs: t, discount: d, netPayable: inclGst + t - d };
}

/** Net payable (PO totalAmount) for a set of line items plus PO-level charges. */
export function poNetPayable(items: MoneyItem[], tcs = 0, discount = 0): number {
  return computePoMoney(items, tcs, discount).netPayable;
}

/** Convert an absolute charge to its percentage of a base (0 when base is 0). */
export function amountToPercent(amount: number, base: number): number {
  if (!base) return 0;
  return (amount / base) * 100;
}

/** Convert a percentage of a base back to an absolute charge. */
export function percentToAmount(percent: number, base: number): number {
  return (percent / 100) * base;
}