const PAN_RE = /^[A-Z]{5}[0-9]{4}[A-Z]$/;
const IFSC_RE = /^[A-Z]{4}0[A-Z0-9]{6}$/;
const GSTIN_RE = /^[0-9]{2}[A-Z]{5}[0-9]{4}[A-Z][1-9A-Z]Z[0-9A-Z]$/;
const EMAIL_RE = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
const PHONE_RE = /^[0-9+\-()\s]{6,16}$/;

const norm = (v: string) => v.trim().toUpperCase();

export function panError(value: string): string | undefined {
  if (!value.trim()) return undefined;
  return PAN_RE.test(norm(value)) ? undefined : "Enter a valid PAN, e.g. CPOPP7388K";
}

export function ifscError(value: string): string | undefined {
  if (!value.trim()) return undefined;
  return IFSC_RE.test(norm(value)) ? undefined : "Enter a valid IFSC, e.g. SBIN0017060";
}

export function gstinError(value: string): string | undefined {
  const v = norm(value);
  if (!v || v === "NA") return undefined;
  return GSTIN_RE.test(v) ? undefined : "Enter a valid 15-character GSTIN";
}

const GSTIN_CHARS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

/** Standard GSTIN mod-36 check digit. Returns an error message, or undefined when valid. */
export function gstinChecksumError(value: string): string | undefined {
  const v = norm(value);
  if (!GSTIN_RE.test(v)) return "Enter a valid 15-character GSTIN";
  let sum = 0;
  for (let i = 0; i < 14; i++) {
    const product = GSTIN_CHARS.indexOf(v[i]) * (i % 2 === 0 ? 1 : 2);
    sum += Math.floor(product / 36) + (product % 36);
  }
  const expected = GSTIN_CHARS[(36 - (sum % 36)) % 36];
  return v[14] === expected ? undefined : `GSTIN check digit looks wrong (expected ${expected})`;
}

/**
 * Full GSTIN check: shape, check digit and, when a PAN is given, that the GSTIN
 * contains that PAN. Empty (or "NA") is valid; the field is optional.
 */
export function gstinFullError(value: string, pan = ""): string | undefined {
  const v = norm(value);
  if (!v || v === "NA") return undefined;
  const base = gstinError(v) ?? gstinChecksumError(v);
  if (base) return base;
  if (pan.trim() && v.slice(2, 12) !== norm(pan)) return "GSTIN does not contain the PAN";
  return undefined;
}

/** Supplier GSTIN rules for a registered business; an unregistered one needs none. */
export function vendorGstinError(
  registration: string,
  gstin: string,
  pan: string,
  stateCode: string,
): string | undefined {
  if (registration === "unregistered") return undefined;
  const g = norm(gstin);
  if (!g) return "A registered business needs a GSTIN";
  const base = gstinFullError(g, pan);
  if (base) return base;
  if (g.slice(0, 2) !== stateCode) return `GSTIN starts with ${g.slice(0, 2)}, but your state code is ${stateCode}`;
  return undefined;
}

export function emailError(value: string): string | undefined {
  if (!value.trim()) return undefined;
  return EMAIL_RE.test(value.trim()) ? undefined : "Enter a valid email address";
}

export function phoneError(value: string): string | undefined {
  if (!value.trim()) return undefined;
  return PHONE_RE.test(value.trim()) ? undefined : "Enter a valid phone number";
}

export function accountError(value: string): string | undefined {
  if (!value.trim()) return undefined;
  return /^[0-9]{6,20}$/.test(value.trim()) ? undefined : "Account number should be 6–20 digits";
}

export const isPan = (v: string) => PAN_RE.test(norm(v));
export const isIfsc = (v: string) => IFSC_RE.test(norm(v));
export const isGstin = (v: string) => GSTIN_RE.test(norm(v));