// Basic Random Date Generator
function randomInt(a = 0, b = 0) { return (a + window.crypto.getRandomValues(new Uint8Array(1))[0] % Math.abs(b - a + 1)); }

function log(json) {
  for (key in arguments) {
		let string = JSON.stringify(arguments[key], null, '  ');
  	document.body.innerHTML += `<pre>${string}</pre>&nbsp;`;
  }
  console.log(...arguments);
  document.body.innerHTML += '<br>';
}

function getDaysInMonth (month, year) {
	const isLeapYear = year % 4 === 0;
  switch(month) {
    case 1: case 3: case 5: case 7: case 8: case 10: case 12: return 31; break;
    case 4: case 6: case 9: case 11: return 30; break;
    case 2: return isLeapYear ? 29 : 28;
  }
}

function getRandomDayInRange (start, end) {
	if (typeof start !== 'object' || typeof end !== 'object') { throw new Error('Incorrect parameter type. `start` and `end` need to be objects with properties `day`, `month` and `year`.'); }
	if (!start.day) { throw new Error('`start.day` needs to be defined.')}
	if (!start.month) { throw new Error('`start.month` needs to be defined.')}
	if (!start.year) { throw new Error('`start.year` needs to be defined.')}
	if (!end.day) { throw new Error('`end.day` needs to be defined.')}
	if (!end.month) { throw new Error('`end.month` needs to be defined.')}
	if (!end.year) { throw new Error('`end.year` needs to be defined.')}

  const year = randomInt(start.year, end.year);

  const isStartYear = year === start.year;
  const isEndYear = year === end.year;

  const isStartYearButNotSameAsEndYear = isStartYear && !isEndYear;
  const isEndYearButNotSameAsStartYear = isEndYear && !isStartYear;
  const isNeitherStartNorEndYear = !isStartYear && !isEndYear;

  if (isStartYearButNotSameAsEndYear) { end.month = 12; }
  if (isEndYearButNotSameAsStartYear) { start.month = 1; }
  if (isNeitherStartNorEndYear) { start.month = 1; end.month = 12; }

  const month = randomInt(start.month, end.month);

  const isStartMonth = month === start.month;
  const isEndMonth = month === end.month;

  const isFirstMonthButNotLast = isStartYear && isStartMonth && !isEndYear && !isEndMonth;
  const isLastMonthButNotFirst = isEndYear && isEndMonth && !isStartYear && !isStartMonth;
  const isNotFirstMonth = isStartYear && !isStartMonth || !isStartYear;
  const isNotLastMonth = isEndYear && !isEndMonth || !isEndYear;

  if (isFirstMonthButNotLast) { end.day = getDaysInMonth(month, year); }
  if (isLastMonthButNotFirst) { start.day = 1; }
  if (isNotFirstMonth) { start.day = 1; }
  if (isNotLastMonth) { end.day = getDaysInMonth(month, year); }
  if (isNeitherStartNorEndYear) { start.day = 1; end.day = getDaysInMonth(month, year); }

  const day = randomInt(start.day, end.day);
  return [day, month, year];
}

function parseDate (dates) {
	const DATE_REGEX = /\d{1,2}\-\d{1,2}\-\d{4}/;
	if (typeof startDate !== 'string' || typeof endDate !== 'string') { throw new Error('Incorrect input type'); }
  if (!(DATE_REGEX.test(startDate) && DATE_REGEX.test(endDate))) { throw new Error ('Incorrect date format'); }

	let [startDay, startMonth, startYear] = startDate.split('-').map(_ => parseInt(_));
	let [endDay, endMonth, endYear] = endDate.split('-').map(_ => parseInt(_));

  return [
  	{ day: startDay, month: startMonth, year: startYear },
  	{ day: endDay, month: endMonth, year: endYear }
  ];
}

// ===========================================================================

let [ startDate, endDate ] = ['10-06-2016', '20-06-2016'];

let [ start, end ] = parseDate(startDate, endDate);

const LIMIT = 20;
for (let i = 0; i < LIMIT; i++) {
  let [day, month, year] = getRandomDayInRange(start, end);
  log(day, month, year);
}

log('Thanks for trying this out!');