import type { IsoDate } from '@sims/domain'

/**
 * Tax rates are dated configuration rows, never constants (07-DB-AND-CONFIG §1;
 * the Sept-2025 GST 2.0 slab change is the precedent). Rates are basis points:
 * 18% → 1800. Resolution is by the bill's business date, so returns and credit
 * notes against old bills resolve the rate that was law on that day.
 */
export interface TaxClassRow {
  classCode: string
  ratePctBp: number
  cessPctBp: number
  effectiveFrom: IsoDate
  effectiveTo?: IsoDate
}

export function resolveTaxClass(rows: TaxClassRow[], classCode: string, onDate: IsoDate): TaxClassRow {
  let best: TaxClassRow | undefined
  for (const row of rows) {
    if (row.classCode !== classCode) continue
    if (row.effectiveFrom > onDate) continue
    if (row.effectiveTo !== undefined && row.effectiveTo < onDate) continue
    if (best === undefined || row.effectiveFrom > best.effectiveFrom) best = row
  }
  if (best === undefined) {
    // Silent defaults on tax are how wrong GST reaches the portal — fail loudly.
    throw new Error(`No tax rate for class "${classCode}" on ${onDate}`)
  }
  return best
}