index.js

'use strict';

var HIGH_SURROGATE_START = 0xd800;
var HIGH_SURROGATE_END = 0xdbff;
var LOW_SURROGATE_START = 0xdc00;
var REGIONAL_INDICATOR_START = 0x1f1e6;
var REGIONAL_INDICATOR_END = 0x1f1ff;
var FITZPATRICK_MODIFIER_START = 0x1f3fb;
var FITZPATRICK_MODIFIER_END = 0x1f3ff;
var VARIATION_MODIFIER_START = 0xfe00;
var VARIATION_MODIFIER_END = 0xfe0f;
var DIACRITICAL_MARKS_START = 0x20d0;
var DIACRITICAL_MARKS_END = 0x20ff;
var ZWJ = 0x200d;
var GRAPHEMS = [0x0308, // ( ◌̈ ) COMBINING DIAERESIS
0x0937, // ( ष ) DEVANAGARI LETTER SSA
0x0937, // ( ष ) DEVANAGARI LETTER SSA
0x093F, // ( ि ) DEVANAGARI VOWEL SIGN I
0x093F, // ( ि ) DEVANAGARI VOWEL SIGN I
0x0BA8, // ( ந ) TAMIL LETTER NA
0x0BBF, // ( ி ) TAMIL VOWEL SIGN I
0x0BCD, // ( ◌்) TAMIL SIGN VIRAMA
0x0E31, // ( ◌ั ) THAI CHARACTER MAI HAN-AKAT
0x0E33, // ( ำ ) THAI CHARACTER SARA AM
0x0E40, // ( เ ) THAI CHARACTER SARA E
0x0E49, // ( เ ) THAI CHARACTER MAI THO
0x1100, // ( ᄀ ) HANGUL CHOSEONG KIYEOK
0x1161, // ( ᅡ ) HANGUL JUNGSEONG A
0x11A8 // ( ᆨ ) HANGUL JONGSEONG KIYEOK
];

function runes(string) {
  if (typeof string !== 'string') {
    throw new Error('string cannot be undefined or null');
  }

  var result = [];
  var i = 0;
  var increment = 0;

  while (i < string.length) {
    increment += nextUnits(i + increment, string);

    if (isGraphem(string[i + increment])) {
      increment++;
    }

    if (isVariationSelector(string[i + increment])) {
      increment++;
    }

    if (isDiacriticalMark(string[i + increment])) {
      increment++;
    }

    if (isZeroWidthJoiner(string[i + increment])) {
      increment++;
      continue;
    }

    result.push(string.substring(i, i + increment));
    i += increment;
    increment = 0;
  }

  return result;
} // Decide how many code units make up the current character.
// BMP characters: 1 code unit
// Non-BMP characters (represented by surrogate pairs): 2 code units
// Emoji with skin-tone modifiers: 4 code units (2 code points)
// Country flags: 4 code units (2 code points)
// Variations: 2 code units


function nextUnits(i, string) {
  var current = string[i]; // If we don't have a value that is part of a surrogate pair, or we're at
  // the end, only take the value at i

  if (!isFirstOfSurrogatePair(current) || i === string.length - 1) {
    return 1;
  }

  var currentPair = current + string[i + 1];
  var nextPair = string.substring(i + 2, i + 5); // Country flags are comprised of two regional indicator symbols,
  // each represented by a surrogate pair.
  // See http://emojipedia.org/flags/
  // If both pairs are regional indicator symbols, take 4

  if (isRegionalIndicator(currentPair) && isRegionalIndicator(nextPair)) {
    return 4;
  } // If the next pair make a Fitzpatrick skin tone
  // modifier, take 4
  // See http://emojipedia.org/modifiers/
  // Technically, only some code points are meant to be
  // combined with the skin tone modifiers. This function
  // does not check the current pair to see if it is
  // one of them.


  if (isFitzpatrickModifier(nextPair)) {
    return 4;
  }

  return 2;
}

function isFirstOfSurrogatePair(string) {
  return string && betweenInclusive(string[0].charCodeAt(0), HIGH_SURROGATE_START, HIGH_SURROGATE_END);
}

function isRegionalIndicator(string) {
  return betweenInclusive(codePointFromSurrogatePair(string), REGIONAL_INDICATOR_START, REGIONAL_INDICATOR_END);
}

function isFitzpatrickModifier(string) {
  return betweenInclusive(codePointFromSurrogatePair(string), FITZPATRICK_MODIFIER_START, FITZPATRICK_MODIFIER_END);
}

function isVariationSelector(string) {
  return typeof string === 'string' && betweenInclusive(string.charCodeAt(0), VARIATION_MODIFIER_START, VARIATION_MODIFIER_END);
}

function isDiacriticalMark(string) {
  return typeof string === 'string' && betweenInclusive(string.charCodeAt(0), DIACRITICAL_MARKS_START, DIACRITICAL_MARKS_END);
}

function isGraphem(string) {
  return typeof string === 'string' && GRAPHEMS.indexOf(string.charCodeAt(0)) !== -1;
}

function isZeroWidthJoiner(string) {
  return typeof string === 'string' && string.charCodeAt(0) === ZWJ;
}

function codePointFromSurrogatePair(pair) {
  var highOffset = pair.charCodeAt(0) - HIGH_SURROGATE_START;
  var lowOffset = pair.charCodeAt(1) - LOW_SURROGATE_START;
  return (highOffset << 10) + lowOffset + 0x10000;
}

function betweenInclusive(value, lower, upper) {
  return value >= lower && value <= upper;
}

function substring(string, start, width) {
  var chars = runes(string);

  if (start === undefined) {
    return string;
  }

  if (start >= chars.length) {
    return '';
  }

  var rest = chars.length - start;
  var stringWidth = width === undefined ? rest : width;
  var endIndex = start + stringWidth;

  if (endIndex > start + rest) {
    endIndex = undefined;
  }

  return chars.slice(start, endIndex).join('');
}

module.exports = runes;
module.exports.substr = substring;