mintMaker exo with Interface guard for prepareIssuerKit

makeMintExo.js

import { Nat } from '@endo/nat';
import { M } from '@agoric/store';

// reproducing purseShape found in ERTP
// https://github.com/Agoric/agoric-sdk/blob/master/packages/ERTP/src/typeGuards.js#L8
const purseShape = M.remotable('Purse');

const IssuerI = M.interface('Issuer', {
  makeEmptyPurse: M.call().returns(purseShape),
});

const MintI = M.interface('Mint', {
  makePurse: M.call(M.bigint()).returns(purseShape),
});

/**
 * Mint maker, transformed to an exo class kit.
 *
 * @param {import('@agoric/zone').Zone} zone
 */
export const prepareIssuerKit = zone => {
  const makePurse = zone.exoClass(
    'Purse',
    undefined,
    (issuer, ledger) => ({ issuer, ledger }),
    {
      getIssuer() {
        return this.state.issuer;
      },
      getBalance() {
        const { self } = this;
        return this.state.ledger.get(self);
      },
      /**
       * @param {bigint} amount
       * @param {unknown} src
       */
      deposit(amount, src) {
        const { self } = this;
        const { ledger } = this.state;
        Nat(ledger.get(self) + Nat(amount));
        ledger.set(src, Nat(ledger.get(src) - amount));
        ledger.set(self, ledger.get(self) + amount);
      },
      withdraw(amount) {
        const { self } = this;
        const { issuer } = this.state;
        const newPurse = issuer.makeEmptyPurse();
        newPurse.deposit(amount, self);
        return newPurse;
      },
    },
  );

  const makeIssuerKit = zone.exoClassKit(
    'Mint',
    { issuer: IssuerI, mint: MintI },
    () => ({
      /** @type {WeakMapStore<unknown, bigint>} */
      ledger: zone.weakMapStore('ledger'),
    }),
    {
      issuer: {
        makeEmptyPurse() {
          const { mint } = this.facets;
          return mint.makePurse(0n);
        },
      },
      mint: {
        /** @param {bigint} initialBalance */
        makePurse(initialBalance) {
          const { issuer } = this.facets;
          const { ledger } = this.state;
          const purse = makePurse(issuer, ledger);
          ledger.init(purse, initialBalance);
          return purse;
        },
      },
    },
  );

  return makeIssuerKit;
};