@agoric/ertp/src/mathHelpers/natMathHelpers.js

Electronic Rights Transfer Protocol (ERTP). A smart contract framework for exchanging electronic rights

// @ts-check

import { Nat, isNat } from '@agoric/nat';

import '../types.js';

const { details: X } = assert;
const empty = 0n;

/**
 * Fungible digital assets use the natMathHelpers to manage balances -
 * the operations are merely arithmetic on natural, non-negative
 * numbers.
 *
 * Natural numbers are used for fungible erights such as money because
 * rounding issues make floats problematic. All operations should be
 * done with the smallest whole unit such that the NatMathHelpers never
 * deals with fractional parts.
 *
 * @type {NatMathHelpers}
 */
export const natMathHelpers = harden({
  doCoerce: nat => {
    // TODO: tighten the definition of Nat in @agoric/nat to throw on `number`
    assert.typeof(nat, 'bigint');
    assert(isNat(nat), X`value ${nat} must be a natural number`);
    return Nat(nat);
  },
  doMakeEmpty: () => empty,
  doIsEmpty: nat => nat === empty,
  doIsGTE: (left, right) => left >= right,
  doIsEqual: (left, right) => left === right,
  // BigInts don't observably overflow
  doAdd: (left, right) => left + right,
  doSubtract: (left, right) => Nat(left - right),
});