function checkForCloseMatch(longString, shortString) {
  // too many false positives with very short strings
  if (shortString.length < 3) return '';

  // test if the shortString is in the string (so everything is fine)
  if (longString.includes(shortString)) return '';

  // split the shortString string into two at each postion e.g. g|mail gm|ail gma|il gmai|l
  // and test that each half exists with one gap
  for (let i = 1; i < shortString.length; i++) {
    const firstPart = shortString.substring(0, i);
    const secondPart = shortString.substring(i);

    // test for wrong letter
    const wrongLetterRegEx = new RegExp(`${firstPart}.${secondPart.substring(1)}`);
    if (wrongLetterRegEx.test(longString)) {
      return longString.replace(wrongLetterRegEx, shortString);
    }

    // test for extra letter
    const extraLetterRegEx = new RegExp(`${firstPart}.${secondPart}`);
    if (extraLetterRegEx.test(longString)) {
      return longString.replace(extraLetterRegEx, shortString);
    }

    // test for missing letter
    if (secondPart !== 'mail') {
      const missingLetterRegEx = new RegExp(`${firstPart}{0}${secondPart}`);
      if (missingLetterRegEx.test(longString)) {
        return longString.replace(missingLetterRegEx, shortString);
      }
    }

    // test for switched letters
    const switchedLetters = [
     shortString.substring(0, i - 1),
     shortString.charAt(i),
     shortString.charAt(i - 1),
     shortString.substring(i + 1),
    ].join('');

    if (longString.includes(switchedLetters)) {
      return longString.replace(switchedLetters, shortString);
    }
  }

  // if nothing was close, then there wasn't a typo
  return '';
}

function checkForDomainTypo(userEmail) {
  const domains = ['gmail', 'hotmail', 'outlook', 'yahoo', 'icloud', 'mail', 'zoho'];
  const [leftPart, rightPart] = userEmail.split('@');

  for (let i = 0; i < domains.length; i++) {
    const domain = domains[i];

    const result = checkForCloseMatch(rightPart, domain);

    if (result) return `${leftPart}@${result}`;
  }

  return '';
}

function checkForNameTypo(userEmail, name) {
  const [leftPart, rightPart] = userEmail.split('@');
  const result = checkForCloseMatch(leftPart, name);

  if (result) return `${result}@${rightPart}`;

  return '';
}

function checkForCommonTypos(userInput) {
  const commonTypos = [
    {
      pattern: /,com$/,
      fix: str => str.replace(/,com$/, '.com'),
    },
    {
      pattern: /,co\.\w{2}$/,
      fix: str => str.replace(/,(co\.\w{2}$)/, '.$1'),
    },
    {
      pattern: /@\w*$/,
      fix: str => str + '.com',
    },
  ];

  typo = commonTypos.find(typo => typo.pattern.test(userInput));

  if (typo) return typo.fix(userInput);

  return '';
}

function checkForTypo(userInput) {
    const email = userInput.email.trim().toLowerCase();

    return checkForCommonTypos(email)
      || checkForDomainTypo(email)
      || checkForNameTypo(email, userInput.firstName.trim().toLowerCase())
      || checkForNameTypo(email, userInput.lastName.trim().toLowerCase());
}

function runTests() {
  const testInputs = [
    // correct
    {firstName: '', lastName: '', email: 'first.last@example.com', expectedResult: ''},
    {firstName: 'yo', lastName: 'shortname', email: 'david.gilbertson@yahoo.com', expectedResult: ''},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.GILBERTSON@gmail.com', expectedResult: ''},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertson@SOME+THING-ODD!!.com', expectedResult: ''},
    {firstName: 'david', lastName: 'gilbertson', email: 'DAVId.gilbertson@outlook.com', expectedResult: ''},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertson@yahoo.com', expectedResult: ''},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertson@hotmail.com', expectedResult: ''},
    {firstName: 'david', lastName: 'gilbertson', email: 'blah@unknown.com', expectedResult: ''},
    {firstName: 'david', lastName: 'gilbertson', email: 'blah@hotmail.co.uk', expectedResult: ''},

    // common mistakes
    {firstName: 'a', lastName: 'b', email: 'a.b@example,com', expectedResult: 'a.b@example.com'},
    {firstName: 'a', lastName: 'b', email: 'a.b@example,co.de', expectedResult: 'a.b@example.co.de'},
    {firstName: 'a', lastName: 'b', email: 'a.b@noDotCom', expectedResult: 'a.b@nodotcom.com'},

    // domain typo
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertson@gmial.com', expectedResult: 'david.gilbertson@gmail.com'},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertson@gmal.com', expectedResult: 'david.gilbertson@gmail.com'},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertson@outloik.com', expectedResult: 'david.gilbertson@outlook.com'},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertson@outloook.com', expectedResult: 'david.gilbertson@outlook.com'},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertson@yohoo.com', expectedResult: 'david.gilbertson@yahoo.com'},

    // name typo
    {firstName: 'david', lastName: 'gilbertson', email: 'daivd.gilbertson@xxxxx.com', expectedResult: 'david.gilbertson@xxxxx.com'},
    {firstName: 'david', lastName: 'gilbertson', email: 'daavid.gilbertson@xxxxx.com', expectedResult: 'david.gilbertson@xxxxx.com'},
    {firstName: 'david', lastName: 'gilbertson', email: 'davd.gilbertson@xxxxx.com', expectedResult: 'david.gilbertson@xxxxx.com'},
    {firstName: 'david', lastName: 'gilbertson', email: 'daivd.gilbertson@xxxxx.com', expectedResult: 'david.gilbertson@xxxxx.com'},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertsin@xxxxx.com', expectedResult: 'david.gilbertson@xxxxx.com'},
    {firstName: 'david', lastName: 'gilbertson', email: 'david.gilbertsson@xxxxx.com', expectedResult: 'david.gilbertson@xxxxx.com'},
  ];

  testInputs.forEach(testInput => {
    console.assert(
      testInput.expectedResult === checkForTypo(testInput),
      `${testInput.email} was not given the suggestion: ${testInput.expectedResult}`
    );
  });
}

runTests();