Doomsday method trainer

doomsday.mjs

import * as readline from "readline";

const rl = readline.createInterface({ input: process.stdin, output: process.stdout });
const lower = new Date(process.argv[2] || "0000-01-01");
const upper = new Date(process.argv[3] || "9999-12-31");
const diff = upper - lower;

function randomDate() {
  return new Date(lower.getTime() + Math.random() * diff);
}

const centuryLookup = {
  0: 2,
  1: 0,
  2: 5,
  3: 3,
};

const dayNames = {
  0: "Sunday",
  1: "Monday",
  2: "Tuesday",
  3: "Wednesday",
  4: "Thursday",
  5: "Friday",
  6: "Saturday",
}

function getDoomsday(month, day, isLeapYear) {
  return {
    1: isLeapYear ? 4 : 3,
    2: 7 * Math.floor(day / 7) + (isLeapYear ? 1 : 0),
    3: 7 * Math.floor(day / 7),
    4: 4,
    5: 9,
    6: 6,
    7: day <= 4 ? 4 : 11,
    8: 8,
    9: 5,
    10: 10,
    11: 7,
    12: day >= 26 ? 26 : 12,
  }[month];
}

function printDetails(date) {
  // console.log(date.toISOString().slice(0, 10));
  const result = [];
  const year = date.getUTCFullYear();
  const firstTwo = Math.floor(year / 100);
  const centuryNum = centuryLookup[firstTwo % 4];
  result.push(`Century: ${firstTwo} % 4 = ${firstTwo % 4} -> ${centuryNum} <---`);
  const lastTwo = year - firstTwo * 100;
  let centuryResult;
  if (lastTwo === 0) {
    result.push(`Year 00 – use century: ${centuryNum} <---`);
    centuryResult = centuryNum;
  } else {
    const lastTwo11 =
      lastTwo % 2 === 0
        ? (result.push(`Even: ${lastTwo}`), lastTwo)
        : (result.push(`Odd: ${lastTwo} + 11 = ${lastTwo + 11}`), lastTwo + 11);
    const half = lastTwo11 / 2;
    result.push(`Half: ${lastTwo11} / 2 = ${half}`);
    const half11 =
      half % 2 === 0
        ? (result.push(`Even: ${half}`), half)
        : (result.push(`Odd: ${half} + 11 = ${half + 11}`), half + 11);
    const mod7 = half11 % 7;
    const diff7 = 7 - mod7;
    result.push(`Mod7: ${half11} % 7 = ${mod7} -> 7 - ${mod7} = ${diff7}`);
    const centurySum = centuryNum + diff7;
    centuryResult = centurySum % 7;
    if (centurySum < 7) {
      result.push(`Century result: ${centuryNum} + ${diff7} = ${centurySum} <---`);
    } else {
      result.push(`Century result: ${centuryNum} + ${diff7} = ${centurySum} -> ${centurySum} % 7 = ${centuryResult} <---`);
    }
  }

  const month = date.getUTCMonth() + 1;
  const day = date.getUTCDate();

  let isLeapYear = false;

  if (month <= 2) {
    result.push(`${month === 1 ? "January" : "February"}: Leap year?`);
    if (lastTwo === 0) {
      if (firstTwo % 4 === 0) {
        result.push(`Year 00 – be careful! ${firstTwo} is divisible by 4 -> Is leap year!`);
        isLeapYear = true;
      } else {
        result.push(`Year 00 – be careful! ${firstTwo} is NOT divisible by 4 -> Not leap year`);
      }
    } else if (lastTwo % 4 === 0) {
      result.push(`Year: ${lastTwo} is divisible by 4 -> Is leap year!`);
      isLeapYear = true;
    } else {
      result.push(`Year: ${lastTwo} is NOT divisible by 4 -> Not leap year`);
    }
  }

  const doomsday = getDoomsday(month, day, isLeapYear);
  result.push(`Doomsday: ${month} -> ${doomsday}`);
  let final;
  if (day === doomsday) {
    final = centuryResult % 7;
    result.push(`Is doomsday!`);
    if (centuryResult < 7) {
      result.push(`Final: ${final} <---`);
    } else {
      result.push(`Final: ${centuryResult} % 7 = ${final} <---`);
    }
  } else {
    const diff = day - doomsday;
    result.push(`Diff: ${day} - ${doomsday} = ${diff}`);
    const centuryDiff = centuryResult + diff;
    result.push(`Century + diff: ${centuryResult} + ${diff} = ${centuryDiff}`);
    let positiveCenturyDiff = centuryDiff;
    while (positiveCenturyDiff < 0) {
      positiveCenturyDiff += 7;
      result.push(`Positive: ${centuryDiff} + 7 = ${positiveCenturyDiff}`);
    }
    final = positiveCenturyDiff % 7;
    if (positiveCenturyDiff < 7) {
      result.push(`Final: ${final} <---`);
    } else {
      result.push(`Final: ${positiveCenturyDiff} % 7 = ${final} <---`);
    }
  }

  const expected = date.getUTCDay();
  if (final !== expected) {
    result.push(`ERROR: Expected ${expected} but got ${final}`)
  }
  result.push("");
  result.push(dayNames[final]);

  return result.join("\n");
}

// console.log(printDetails(randomDate()));
// console.log(printDetails(new Date(`${Math.floor(9999 * Math.random())}-01-01`)));
// console.log(printDetails(new Date(`2022-12-01`)));
// process.exit(0);

let date;

function run() {
  if (date === undefined) {
    date = randomDate();
    console.log(date.toISOString().slice(0, 10));
    console.time("time");
  } else {
    // console.log(date.toUTCString().split(",")[0]);
    console.log(printDetails(date));
    console.timeEnd("time");
    date = undefined;
  }
}

run();

for await (const _ of rl) {
  run();
}