Fast ISO String Generation

README.md

Results:

fromUnixToISOString x 3,258,470 ops/sec ±1.86% (85 runs sampled)
new Date().toISOString() x 1,408,158 ops/sec ±0.96% (90 runs sampled)

benchmark.ts

import * as Benchmark from 'benchmark';
import { fromUnixToISOString } from './fast-isostring';
import { fromUnixToISOStringWithoutCache } from './fast-isostring-without-cache';

const suite = new Benchmark.Suite();

suite.add('fromUnixToISOString', function () {
  const v = fromUnixToISOString(Date.now());
});

suite.add('fromUnixToISOStringWithoutCache', function () {
  const v = fromUnixToISOStringWithoutCache(Date.now());
});

suite.add('new Date().toISOString()', function () {
  const v = new Date().toISOString();
});

const results = [];

suite
  // add listeners
  .on('cycle', function (event) {
    results.push(String(event.target));
  })
  .on('complete', function () {
    results.forEach(console.log);

    console.log('Fastest is ' + this.filter('fastest').map('name'));
  })
  .run({
    async: true,
  });

fast-isostring-without-cache.ts

const kMsPerMin = 60 * 1000;
const kSecPerDay = 24 * 60 * 60;
const kMsPerDay = kSecPerDay * 1000;
const kMsPerMonth = kMsPerDay * 30;

const kMsPerHour = 60 * 60 * 1000;

const kDaysIn4Years = 4 * 365 + 1;
const kDaysIn100Years = 25 * kDaysIn4Years - 1;
const kDaysIn400Years = 4 * kDaysIn100Years + 1;
const kDays1970to2000 = 30 * 365 + 7;
const kDaysOffset = 1000 * kDaysIn400Years + 5 * kDaysIn400Years - kDays1970to2000;
const kYearsOffset = 400000;
const kDaysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

const pad = n => (n < 10 ? `0${n}` : n);
const padMs = n => (n >= 100 ? n : n >= 10 ? `0${n}` : `00${n}`);

function getYearMonthDayFromDays(days: number): string {
  days += kDaysOffset;
  let year = 400 * Math.floor(days / kDaysIn400Years) - kYearsOffset;
  days %= kDaysIn400Years;

  days--;
  let yd1 = Math.floor(days / kDaysIn100Years);
  days %= kDaysIn100Years;
  year += 100 * yd1;

  days++;
  let yd2 = Math.floor(days / kDaysIn4Years);
  days %= kDaysIn4Years;
  year += 4 * yd2;

  days--;
  let yd3 = Math.floor(days / 365);
  days %= 365;
  year += yd3;

  const is_leap = (!yd1 || yd2) && !yd3;

  if (is_leap) days++;

  let month = 0;
  let day = 0;

  // Check if the date is after February.
  if (days >= 31 + 28 + (is_leap ? 1 : 0)) {
    days -= 31 + 28 + (is_leap ? 1 : 0);
    // Find the date starting from March.
    for (let i = 2; i < 12; i++) {
      if (days < kDaysInMonths[i]) {
        month = i;
        day = days + 1;
        break;
      }
      days -= kDaysInMonths[i];
    }
  } else {
    // Check January and February.
    if (days < 31) {
      month = 0;
      day = days + 1;
    } else {
      month = 1;
      day = days - 31 + 1;
    }
  }

  return `${year}-${pad(month + 1)}-${pad(day)}`;
}

export function fromUnixToISOStringWithoutCache(unix: number): string {
  const days = unix < 0 ? unix - kMsPerDay - 1 : Math.floor(unix / kMsPerDay);
  const timeInDayMs = unix - days * kMsPerDay;

  const hour = Math.floor(timeInDayMs / kMsPerHour);
  const min = Math.floor((timeInDayMs / kMsPerMin) % 60);
  const sec = Math.floor((timeInDayMs / 1000) % 60);
  const ms = Math.floor(timeInDayMs % 1000);

  const datePart = getYearMonthDayFromDays(days);

  return `${datePart}T${pad(hour)}:${pad(min)}:${pad(sec)}.${padMs(ms)}Z`;
}

fast-isostring.ts

const kMsPerMin = 60 * 1000;
const kSecPerDay = 24 * 60 * 60;
const kMsPerDay = kSecPerDay * 1000;
const kMsPerMonth = kMsPerDay * 30;

const kMsPerHour = 60 * 60 * 1000;

const kDaysIn4Years = 4 * 365 + 1;
const kDaysIn100Years = 25 * kDaysIn4Years - 1;
const kDaysIn400Years = 4 * kDaysIn100Years + 1;
const kDays1970to2000 = 30 * 365 + 7;
const kDaysOffset = 1000 * kDaysIn400Years + 5 * kDaysIn400Years - kDays1970to2000;
const kYearsOffset = 400000;
const kDaysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

const pad = n => (n < 10 ? `0${n}` : n);
const padMs = n => (n >= 100 ? n : n >= 10 ? `0${n}` : `00${n}`);

let cachedValid: boolean = false;
let cachedDays: number | undefined;
let cachedYear: number | undefined;
let cachedMonth: number | undefined;
let cachedDay: number | undefined;

function getYearMonthDayFromDays(days: number): string {
  if (cachedValid) {
    // Check conservatively if the given 'days' has
    // the same year and month as the cached 'days'.
    const new_day = cachedDay + (days - cachedDays);

    if (new_day >= 1 && new_day <= 28) {
      cachedDay = new_day;
      cachedDays = days;

      return `${cachedYear}-${pad(cachedMonth)}-${pad(new_day)}`;
    }
  }

  days += kDaysOffset;
  let year = 400 * Math.floor(days / kDaysIn400Years) - kYearsOffset;
  days %= kDaysIn400Years;

  days--;
  let yd1 = Math.floor(days / kDaysIn100Years);
  days %= kDaysIn100Years;
  year += 100 * yd1;

  days++;
  let yd2 = Math.floor(days / kDaysIn4Years);
  days %= kDaysIn4Years;
  year += 4 * yd2;

  days--;
  let yd3 = Math.floor(days / 365);
  days %= 365;
  year += yd3;

  const is_leap = (!yd1 || yd2) && !yd3;

  if (is_leap) days++;

  let month = 0;
  let day = 0;

  // Check if the date is after February.
  if (days >= 31 + 28 + (is_leap ? 1 : 0)) {
    days -= 31 + 28 + (is_leap ? 1 : 0);
    // Find the date starting from March.
    for (let i = 2; i < 12; i++) {
      if (days < kDaysInMonths[i]) {
        month = i;
        day = days + 1;
        break;
      }
      days -= kDaysInMonths[i];
    }
  } else {
    // Check January and February.
    if (days < 31) {
      month = 0;
      day = days + 1;
    } else {
      month = 1;
      day = days - 31 + 1;
    }
  }

  cachedValid = true;
  cachedYear = year;
  cachedMonth = month + 1;
  cachedDay = day;
  cachedDays = days;

  return `${cachedYear}-${pad(cachedMonth)}-${pad(cachedDay)}`;
}

export function fromUnixToISOString(unix: number): string {
  const days = unix < 0 ? unix - kMsPerDay - 1 : Math.floor(unix / kMsPerDay);
  const timeInDayMs = unix - days * kMsPerDay;

  const hour = Math.floor(timeInDayMs / kMsPerHour);
  const min = Math.floor((timeInDayMs / kMsPerMin) % 60);
  const sec = Math.floor((timeInDayMs / 1000) % 60);
  const ms = Math.floor(timeInDayMs % 1000);

  const datePart = getYearMonthDayFromDays(days);

  return `${datePart}T${pad(hour)}:${pad(min)}:${pad(sec)}.${padMs(ms)}Z`;
}

export function fastISOString(): string {
  return fromUnixToISOString(Date.now());
}