// Adapted from: https://github.com/sebastiansandqvist/s-ago (MIT)

function format(
  diff: number,
  divisor: number,
  unit: string,
  past: string,
  future: string,
  isInTheFuture: boolean
) {
  var val = Math.floor(Math.abs(diff) / divisor);
  if (isInTheFuture) return val <= 1 ? future : 'in ' + val + ' ' + unit + 's';
  return val <= 1 ? past : val + ' ' + unit + 's ago';
}

const units = [
  { max: 60000, value: 1000, name: 'second', past: 'a second ago', future: 'in a second' }, // max: 60 seconds
  { max: 3600000, value: 60000, name: 'minute', past: 'a minute ago', future: 'in a minute' }, // max: 60 minutes
  { max: 72000000, value: 3600000, name: 'hour', past: 'an hour ago', future: 'in an hour' }, // max: 20 hours
  { max: 518400000, value: 86400000, name: 'day', past: 'yesterday', future: 'tomorrow' }, // max: 6 days
  { max: 2419200000, value: 604800000, name: 'week', past: 'last week', future: 'in a week' }, // max: 28 days
  { max: 28512000000, value: 2592000000, name: 'month', past: 'last month', future: 'in a month' } // max: 11 months
];

export default function ago(
  date: Date,
  now: Date = new Date(),
  max?: 'second' | 'minute' | 'hour' | 'day' | 'week' | 'month'
): string {
  const diff = now.getTime() - date.getTime();

  // less than a second
  if (Math.abs(diff) < 1000) return 'just now';

  for (var i = 0; i < units.length; i++) {
    if (Math.abs(diff) < units[i].max || (max && units[i].name === max)) {
      return format(diff, units[i].value, units[i].name, units[i].past, units[i].future, diff < 0);
    }
  }

  return format(diff, 31536000000, 'year', 'last year', 'in a year', diff < 0);
}