const GSTIN_SHAPE = /^[0-9]{2}[A-Z]{5}[0-9]{4}[A-Z][1-9A-Z]Z[0-9A-Z]$/
const B36 = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'

/** Standard GSTIN check digit: alternating 1/2 factors over base-36 values. */
export function gstinCheckDigit(first14: string): string {
  let sum = 0
  for (let i = 0; i < 14; i++) {
    const v = B36.indexOf(first14[i]!)
    const product = v * (i % 2 === 0 ? 1 : 2)
    sum += Math.trunc(product / 36) + (product % 36)
  }
  return B36[(36 - (sum % 36)) % 36]!
}

export function validateGstin(gstin: string): { ok: boolean; stateCode?: string; reason?: string } {
  const g = gstin.toUpperCase().trim()
  if (!GSTIN_SHAPE.test(g)) return { ok: false, reason: 'shape' }
  if (gstinCheckDigit(g.slice(0, 14)) !== g[14]) return { ok: false, reason: 'checksum' }
  return { ok: true, stateCode: g.slice(0, 2) }
}

/** Minimal buyer identity a place-of-supply decision needs. */
export interface SupplyCustomer {
  gstin?: string
  /** 2-digit GST state code; when absent it is read from the GSTIN's first two digits. */
  stateCode?: string
}

export interface SupplyDecision {
  /** True when the buyer carries a GSTIN — a registered B2B sale. */
  b2b: boolean
  /** GST state code that is the place of supply (buyer's for B2B, the store's for B2C). */
  placeOfSupplyStateCode: string
  /** Place of supply differs from the store ⇒ the IGST path; else CGST+SGST. */
  interState: boolean
  /** Buyer GSTIN snapshot (upper-cased) when this is a B2B sale. */
  buyerGstin?: string
}

/**
 * Place-of-supply decision at the counter — the single rule the POS and the
 * store server must agree on so their engine runs match to the paisa (R5).
 * A buyer with a GSTIN is B2B: place of supply is the buyer's state (an explicit
 * stateCode, else the GSTIN's first two digits). Everyone else is a B2C
 * over-the-counter sale — place of supply is the store. Inter-state ⇒ IGST.
 */
export function deriveSupply(customer: SupplyCustomer | undefined, storeStateCode: string): SupplyDecision {
  const gstin = customer?.gstin?.toUpperCase().trim()
  const b2b = gstin !== undefined && gstin !== ''
  const buyerState = customer?.stateCode !== undefined && customer.stateCode !== ''
    ? customer.stateCode
    : b2b ? gstin.slice(0, 2) : undefined
  const placeOfSupplyStateCode = b2b && buyerState !== undefined ? buyerState : storeStateCode
  return {
    b2b,
    placeOfSupplyStateCode,
    interState: placeOfSupplyStateCode !== storeStateCode,
    ...(b2b ? { buyerGstin: gstin } : {}),
  }
}