wallet-core/src/util/amounts.ts

/*
 This file is part of GNU Taler
 (C) 2019 Taler Systems S.A.

 GNU Taler is free software; you can redistribute it and/or modify it under the
 terms of the GNU General Public License as published by the Free Software
 Foundation; either version 3, or (at your option) any later version.

 GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
 WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
 A PARTICULAR PURPOSE.  See the GNU General Public License for more details.

 You should have received a copy of the GNU General Public License along with
 GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
 */

/**
 * Types and helper functions for dealing with Taler amounts.
 */

/**
 * Imports.
 */
import {
  typecheckedCodec,
  makeCodecForObject,
  codecForString,
  codecForNumber,
} from "./codec";

/**
 * Number of fractional units that one value unit represents.
 */
export const fractionalBase = 1e8;

/**
 * How many digits behind the comma are required to represent the
 * fractional value in human readable decimal format?  Must match
 * lg(fractionalBase)
 */
export const fractionalLength = 8;

/**
 * Maximum allowed value field of an amount.
 */
export const maxAmountValue = 2 ** 52;

/**
 * Non-negative financial amount.  Fractional values are expressed as multiples
 * of 1e-8.
 */
export interface AmountJson {
  /**
   * Value, must be an integer.
   */
  readonly value: number;

  /**
   * Fraction, must be an integer.  Represent 1/1e8 of a unit.
   */
  readonly fraction: number;

  /**
   * Currency of the amount.
   */
  readonly currency: string;
}

export const codecForAmountJson = () =>
  typecheckedCodec<AmountJson>(
    makeCodecForObject<AmountJson>()
      .property("currency", codecForString)
      .property("value", codecForNumber)
      .property("fraction", codecForNumber)
      .build("AmountJson"),
  );

/**
 * Result of a possibly overflowing operation.
 */
export interface Result {
  /**
   * Resulting, possibly saturated amount.
   */
  amount: AmountJson;
  /**
   * Was there an over-/underflow?
   */
  saturated: boolean;
}

/**
 * Get an amount that represents zero units of a currency.
 */
export function getZero(currency: string): AmountJson {
  return {
    currency,
    fraction: 0,
    value: 0,
  };
}

export function sum(amounts: AmountJson[]) {
  if (amounts.length <= 0) {
    throw Error("can't sum zero amounts");
  }
  return add(amounts[0], ...amounts.slice(1));
}

/**
 * Add two amounts.  Return the result and whether
 * the addition overflowed.  The overflow is always handled
 * by saturating and never by wrapping.
 *
 * Throws when currencies don't match.
 */
export function add(first: AmountJson, ...rest: AmountJson[]): Result {
  const currency = first.currency;
  let value = first.value + Math.floor(first.fraction / fractionalBase);
  if (value > maxAmountValue) {
    return {
      amount: { currency, value: maxAmountValue, fraction: fractionalBase - 1 },
      saturated: true,
    };
  }
  let fraction = first.fraction % fractionalBase;
  for (const x of rest) {
    if (x.currency !== currency) {
      throw Error(`Mismatched currency: ${x.currency} and ${currency}`);
    }

    value =
      value + x.value + Math.floor((fraction + x.fraction) / fractionalBase);
    fraction = Math.floor((fraction + x.fraction) % fractionalBase);
    if (value > maxAmountValue) {
      return {
        amount: {
          currency,
          value: maxAmountValue,
          fraction: fractionalBase - 1,
        },
        saturated: true,
      };
    }
  }
  return { amount: { currency, value, fraction }, saturated: false };
}

/**
 * Subtract two amounts.  Return the result and whether
 * the subtraction overflowed.  The overflow is always handled
 * by saturating and never by wrapping.
 *
 * Throws when currencies don't match.
 */
export function sub(a: AmountJson, ...rest: AmountJson[]): Result {
  const currency = a.currency;
  let value = a.value;
  let fraction = a.fraction;

  for (const b of rest) {
    if (b.currency !== currency) {
      throw Error(`Mismatched currency: ${b.currency} and ${currency}`);
    }
    if (fraction < b.fraction) {
      if (value < 1) {
        return { amount: { currency, value: 0, fraction: 0 }, saturated: true };
      }
      value--;
      fraction += fractionalBase;
    }
    console.assert(fraction >= b.fraction);
    fraction -= b.fraction;
    if (value < b.value) {
      return { amount: { currency, value: 0, fraction: 0 }, saturated: true };
    }
    value -= b.value;
  }

  return { amount: { currency, value, fraction }, saturated: false };
}

/**
 * Compare two amounts.  Returns 0 when equal, -1 when a < b
 * and +1 when a > b.  Throws when currencies don't match.
 */
export function cmp(a: AmountJson, b: AmountJson): -1 | 0 | 1 {
  if (a.currency !== b.currency) {
    throw Error(`Mismatched currency: ${a.currency} and ${b.currency}`);
  }
  const av = a.value + Math.floor(a.fraction / fractionalBase);
  const af = a.fraction % fractionalBase;
  const bv = b.value + Math.floor(b.fraction / fractionalBase);
  const bf = b.fraction % fractionalBase;
  switch (true) {
    case av < bv:
      return -1;
    case av > bv:
      return 1;
    case af < bf:
      return -1;
    case af > bf:
      return 1;
    case af === bf:
      return 0;
    default:
      throw Error("assertion failed");
  }
}

/**
 * Create a copy of an amount.
 */
export function copy(a: AmountJson): AmountJson {
  return {
    currency: a.currency,
    fraction: a.fraction,
    value: a.value,
  };
}

/**
 * Divide an amount.  Throws on division by zero.
 */
export function divide(a: AmountJson, n: number): AmountJson {
  if (n === 0) {
    throw Error(`Division by 0`);
  }
  if (n === 1) {
    return { value: a.value, fraction: a.fraction, currency: a.currency };
  }
  const r = a.value % n;
  return {
    currency: a.currency,
    fraction: Math.floor((r * fractionalBase + a.fraction) / n),
    value: Math.floor(a.value / n),
  };
}

/**
 * Check if an amount is non-zero.
 */
export function isNonZero(a: AmountJson): boolean {
  return a.value > 0 || a.fraction > 0;
}

export function isZero(a: AmountJson): boolean {
  return a.value === 0 && a.fraction === 0;
}

/**
 * Parse an amount like 'EUR:20.5' for 20 Euros and 50 ct.
 */
export function parse(s: string): AmountJson | undefined {
  const res = s.match(/^([a-zA-Z0-9_*-]+):([0-9]+)([.][0-9]+)?$/);
  if (!res) {
    return undefined;
  }
  const tail = res[3] || ".0";
  if (tail.length > fractionalLength + 1) {
    return undefined;
  }
  const value = Number.parseInt(res[2]);
  if (value > maxAmountValue) {
    return undefined;
  }
  return {
    currency: res[1],
    fraction: Math.round(fractionalBase * Number.parseFloat(tail)),
    value,
  };
}

/**
 * Parse amount in standard string form (like 'EUR:20.5'),
 * throw if the input is not a valid amount.
 */
export function parseOrThrow(s: string): AmountJson {
  const res = parse(s);
  if (!res) {
    throw Error(`Can't parse amount: "${s}"`);
  }
  return res;
}

/**
 * Convert a float to a Taler amount.
 * Loss of precision possible.
 */
export function fromFloat(floatVal: number, currency: string) {
  return {
    currency,
    fraction: Math.floor((floatVal - Math.floor(floatVal)) * fractionalBase),
    value: Math.floor(floatVal),
  };
}

/**
 * Convert to standard human-readable string representation that's
 * also used in JSON formats.
 */
export function stringify(a: AmountJson): string {
  const av = a.value + Math.floor(a.fraction / fractionalBase);
  const af = a.fraction % fractionalBase;
  let s = av.toString();

  if (af) {
    s = s + ".";
    let n = af;
    for (let i = 0; i < fractionalLength; i++) {
      if (!n) {
        break;
      }
      s = s + Math.floor((n / fractionalBase) * 10).toString();
      n = (n * 10) % fractionalBase;
    }
  }

  return `${a.currency}:${s}`;
}

/**
 * Check if the argument is a valid amount in string form.
 */
function check(a: any): boolean {
  if (typeof a !== "string") {
    return false;
  }
  try {
    const parsedAmount = parse(a);
    return !!parsedAmount;
  } catch {
    return false;
  }
}

// Export all amount-related functions here for better IDE experience.
export const Amounts = {
  stringify: stringify,
  parse: parse,
  parseOrThrow: parseOrThrow,
  cmp: cmp,
  add: add,
  sum: sum,
  sub: sub,
  check: check,
  getZero: getZero,
  isZero: isZero,
  maxAmountValue: maxAmountValue,
  fromFloat: fromFloat,
};