/**
 * MT202 COV — General Financial Institution Transfer (cover method).
 *
 * Arch §4.3. FIN equivalent of pacs.009 used on SWIFT networks that
 * have not yet migrated to ISO 20022. Generated alongside pacs.009
 * during transitional period — settlement confirmation can arrive on
 * either channel.
 */

import type { Plan, PlanStep } from "../../types/plan";

export interface Mt202Options {
  transactionReference: string;
  relatedReference?: string;
  valueDate: string;           // YYYY-MM-DD
  sendingInstitution: string;  // BIC
  receivingInstitution: string;// BIC
  beneficiaryInstitution: string; // BIC
  orderingInstitution?: string;// BIC
}

export interface Mt202Message {
  sender: string;
  receiver: string;
  fin: string;
  fields: Record<string, string>;
}

function yyMMdd(iso: string): string {
  const m = /^(\d{4})-(\d{2})-(\d{2})$/.exec(iso);
  if (!m) throw new Error(`MT202: valueDate must be YYYY-MM-DD, got '${iso}'`);
  return `${m[1].slice(2)}${m[2]}${m[3]}`;
}

function bicCheck(bic: string, field: string): void {
  if (!/^[A-Z0-9]{8}([A-Z0-9]{3})?$/.test(bic)) {
    throw new Error(`MT202: ${field} must be a valid BIC, got '${bic}'`);
  }
}

function findPayStep(plan: Plan): PlanStep {
  const step = plan.steps.find((s) => s.type === "pay");
  if (!step) throw new Error("MT202: plan must contain a 'pay' step");
  return step;
}

export function generateMt202(plan: Plan, opts: Mt202Options): Mt202Message {
  bicCheck(opts.sendingInstitution, "sendingInstitution");
  bicCheck(opts.receivingInstitution, "receivingInstitution");
  bicCheck(opts.beneficiaryInstitution, "beneficiaryInstitution");
  if (opts.orderingInstitution) bicCheck(opts.orderingInstitution, "orderingInstitution");

  const payStep = findPayStep(plan);
  const ccy = (payStep.asset ?? "USD").toUpperCase();
  const amount = payStep.amount.toFixed(2).replace(".", ",");
  const field32A = `${yyMMdd(opts.valueDate)}${ccy}${amount}`;

  const fields: Record<string, string> = {
    "20": opts.transactionReference,
    "21": opts.relatedReference ?? opts.transactionReference,
    "32A": field32A,
    "52A": opts.orderingInstitution ?? opts.sendingInstitution,
    "57A": opts.receivingInstitution,
    "58A": opts.beneficiaryInstitution,
  };

  const block1 = `{1:F01${opts.sendingInstitution.padEnd(12, "X")}0000000000}`;
  const block2 = `{2:I202${opts.receivingInstitution.padEnd(12, "X")}N}`;
  const block4 = Object.entries(fields).map(([t, v]) => `:${t}:${v}`).join("\n");
  const block4Wrapped = `{4:\n${block4}\n-}`;

  return {
    sender: opts.sendingInstitution,
    receiver: opts.receivingInstitution,
    fin: `${block1}${block2}${block4Wrapped}`,
    fields,
  };
}