module.exports = function strlen(string) {
  //  discuss at: https://locutus.io/php/strlen/
  // original by: Kevin van Zonneveld (https://kvz.io)
  // improved by: Sakimori
  // improved by: Kevin van Zonneveld (https://kvz.io)
  //    input by: Kirk Strobeck
  // bugfixed by: Onno Marsman (https://twitter.com/onnomarsman)
  //  revised by: Brett Zamir (https://brett-zamir.me)
  //      note 1: May look like overkill, but in order to be truly faithful to handling all Unicode
  //      note 1: characters and to this function in PHP which does not count the number of bytes
  //      note 1: but counts the number of characters, something like this is really necessary.
  //   example 1: strlen('Kevin van Zonneveld')
  //   returns 1: 19
  //   example 2: ini_set('unicode.semantics', 'on')
  //   example 2: strlen('A\ud87e\udc04Z')
  //   returns 2: 3

  const str = string + ''

  const iniVal = (typeof require !== 'undefined' ? require('../info/ini_get')('unicode.semantics') : undefined) || 'off'
  if (iniVal === 'off') {
    return str.length
  }

  let i = 0
  let lgth = 0

  const getWholeChar = function (str, i) {
    const code = str.charCodeAt(i)
    let next = ''
    let prev = ''
    if (code >= 0xd800 && code <= 0xdbff) {
      // High surrogate (could change last hex to 0xDB7F to
      // treat high private surrogates as single characters)
      if (str.length <= i + 1) {
        throw new Error('High surrogate without following low surrogate')
      }
      next = str.charCodeAt(i + 1)
      if (next < 0xdc00 || next > 0xdfff) {
        throw new Error('High surrogate without following low surrogate')
      }
      return str.charAt(i) + str.charAt(i + 1)
    } else if (code >= 0xdc00 && code <= 0xdfff) {
      // Low surrogate
      if (i === 0) {
        throw new Error('Low surrogate without preceding high surrogate')
      }
      prev = str.charCodeAt(i - 1)
      if (prev < 0xd800 || prev > 0xdbff) {
        // (could change last hex to 0xDB7F to treat high private surrogates
        // as single characters)
        throw new Error('Low surrogate without preceding high surrogate')
      }
      // We can pass over low surrogates now as the second
      // component in a pair which we have already processed
      return false
    }
    return str.charAt(i)
  }

  for (i = 0, lgth = 0; i < str.length; i++) {
    if (getWholeChar(str, i) === false) {
      continue
    }
    // Adapt this line at the top of any loop, passing in the whole string and
    // the current iteration and returning a variable to represent the individual character;
    // purpose is to treat the first part of a surrogate pair as the whole character and then
    // ignore the second part
    lgth++
  }

  return lgth
}