accounted/lib/invoices/line-amounts.ts

import { roundOre } from '@/lib/money'

/**
 * Shared per-line amount math for invoice items with an optional percentage
 * discount (rabatt i procent per artikelrad).
 *
 * The formula set is deliberately exact in öre so every surface (editor
 * preview, build-invoice-write, staged-operation commit, PDF, Peppol BG-27
 * line allowance) agrees to the öre:
 *
 *   gross    = roundOre(quantity * unit_price)
 *   discount = roundOre(gross * discount_percent / 100)
 *   net      = roundOre(gross - discount)
 *
 * `net` is what is stored as invoice_items.line_total and what VAT is
 * computed on (the discount reduces the beskattningsunderlag, ML 8 kap 13 §).
 * Because `discount` is rounded before the subtraction, gross - discount is
 * exact 2-decimal arithmetic and the UBL line check
 * LineExtensionAmount = base - allowance holds without a tolerance.
 *
 * A line with no discount keeps the legacy unrounded `quantity * unit_price`
 * as its net so existing invoices, stored line_totals and the Peppol
 * LINE_TOTAL_MISMATCH check stay byte-identical.
 */
export interface LineAmounts {
  /** Line amount before discount (rounded to öre when a discount applies). */
  gross: number
  /** Discount amount in invoice currency (0 when no discount). */
  discount: number
  /** Line amount after discount: what line_total stores and VAT applies to. */
  net: number
}

/** True when the value is a discount that actually changes the line. */
export function hasLineDiscount(discountPercent: number | null | undefined): boolean {
  return typeof discountPercent === 'number' && discountPercent > 0
}

export function computeLineAmounts(
  quantity: number,
  unitPrice: number,
  discountPercent?: number | null,
): LineAmounts {
  const raw = (quantity || 0) * (unitPrice || 0)
  if (!hasLineDiscount(discountPercent)) {
    return { gross: raw, discount: 0, net: raw }
  }
  const gross = roundOre(raw)
  const discount = roundOre((gross * (discountPercent as number)) / 100)
  return { gross, discount, net: roundOre(gross - discount) }
}

/** Convenience: the net line total (what invoice_items.line_total stores). */
export function computeLineNet(
  quantity: number,
  unitPrice: number,
  discountPercent?: number | null,
): number {
  return computeLineAmounts(quantity, unitPrice, discountPercent).net
}