var SENTENCE_SIZE,
    WEIGHT,
    BASE;

SENTENCE_SIZE = 30;
WEIGHT = 1.0430;
BASE = 3.1291;

/**
 * Get the grade level of a given value according to the SMOG formula.
 * More information is available at WikiPedia:
 *
 *   http://en.wikipedia.org/wiki/SMOG
 *
 * @param {Object} counts
 * @param {number} counts.sentence - Number of sentences.
 * @param {number} counts.polysillabicWord - Number of polysillabic words
 *   (three or more syllables).
 * @return {number}
 */

function smog(counts) {
    if (!counts || !counts.sentence) {
        return NaN;
    }

    return WEIGHT * Math.sqrt(
            (counts.polysillabicWord || 0) * (SENTENCE_SIZE / counts.sentence)
        ) + BASE;
}

var area = document.querySelector('textarea');
var readability_el = document.getElementById('readability');

var ages = [
  'Empty',
  'Toddler','Toddler','Toddler', // 1–3
  'Kindergarten', 'Kindergarten', // 4–5
  'First Grade', // 6
  'Second Grade', // 7
  'Third Grade', // 8
  'Fourth Grade', // 9
  'Fifth Grade', // 10
  'Sixth Grade', // 11
  'Seventh Grade', // 12
  'Eighth Grade', // 13
  'Ninth Grade', // 14
  'Tenth Grade', // 15
  'Eleventh grade', // 16
  'Twelfth grade', // 17
  'College', 'College', 'College', 'College', 'College',
  'Grad School & Beyond'
];

var sentence = /[^.!?\s][^.!?]*(?:[.!?](?!['"]?\s|$)[^.!?]*)*[.!?]?['"]?(?=\s|$)/g;
var word = /\w+/g;
var syllable = /[aeiouy]+[^$e(,.:;!?)]/g;

var displayReadability = area.oninput = function(){
  var sentences = area.value.match(sentence);
  var words = area.value.match(word);
  var polysillabicWord = 0;
  for(var i=0; i<words.length; i++){
    var wordSyllables = words[i].match(syllable);
    if(wordSyllables !== null && wordSyllables.length >= 3){
      polysillabicWord++;
    }
  }
  var counts = {
    word:words.length,
    sentence:sentences.length,
    polysillabicWord:polysillabicWord
  };
  var readability = smog(counts);
  readability = Math.ceil(readability);
  readability = Math.max(0,readability);
  readability = Math.min(readability,ages.length-1);
  readability_el.innerHTML = readability + ' years ('+ages[readability]+')';
}

displayReadability();