(some of the) conjugations for Japanese verbs

conjugations.js

var conjugations = (function() {
  var godan_map = 
    { "う": ["わ", "い","う","え","お"]
    , "く": ["か", "き","く","け","こ"]
    , "ぐ": ["が", "ぎ","ぐ","げ","ご"]
    , "す": ["さ", "し","す","せ","そ"]
    , "つ": ["た", "ち","つ","て","と"]
    , "ぬ": ["な", "に","ぬ","ね","の"]
    , "ぶ": ["ば", "び","ぶ","べ","ぼ"]
    , "む": ["ま", "み","む","め","も"]
    , "る": ["ら", "り","る","れ","ろ"]
    };
  var romaji_vowels = {"a": 0, "i": 1, "u": 2, "e": 3, "o": 4};
  var hiragana_vowels = {"あ": 0, "い": 1, "う": 2, "え": 3, "お": 4};

  function changeVowel(u_char, vowel) {
    if (vowel in romaji_vowels) {
      return godan_map[u_char][romaji_vowels[vowel]];
    } else if (vowel in hiragana_vowels) {
      return godan_map[u_char][hiragana_vowels[vowel]];
    }
  }

  var API = {
    negative: function(dict, type, ruby) {
      switch (dict) {
        case "来る": return ruby ? "<ruby><rb>来</rb><rt>こ</rt></ruby>ない"
                                 : "来（こ）ない";
        case "ある": return "ない";
        default: 
          root = dict.slice(0, -1);
          switch (type) {
            case "する": return dict.slice(0, -2) + "しない";
            case "る": return root + "ない";
            case "う": 
              c = changeVowel(dict.slice(-1), "あ");
              return root + c + "ない";
          }
      }
    },
    past: function(dict, type, ruby) {
      switch (dict) {
        case "来る": return ruby ? "<ruby><rb>来</rb><rt>き</rt></ruby>た"
                                 : "来（き）た";
        case "行く": return "行った";
        default: 
          var root = dict.slice(0, -1);
          switch (type) {
            case "する": return dict.slice(0, -2) + "した";
            case "る": return dict.slice(0, -1) + "た";
            case "う": switch (dict.slice(-1)) {
              case "す": return root + "した";
              case "く": return root + "いた";
              case "ぐ": return root + "いだ";
              case "む": 
              case "ぶ":
              case "ぬ": return root + "んだ";
              case "る": 
              case "う":
              case "つ": return root + "った";
            }
          }
      }
    },
    past_negative: function() {
      return API.negative.apply(this, arguments).slice(0, -1) + "かった";
    },
    stem: function(dict, type, ruby) {
      switch (dict) {
        case "来る": return ruby ? "<ruby><rb>来</rb><rt>き</rt></ruby>"
                                 : "来（き）";
        default: 
          root = dict.slice(0, -1);
          switch (type) {
            case "する": return dict.slice(0, -2) + "し";
            case "る": return root;
            case "う": return root + changeVowel(dict.slice(-1), "い");
          }
      }
    },
    masu: function() {
      return API.stem.apply(this, arguments) + "ます";
    },
    masu_negative: function() {
      return API.stem.apply(this, arguments) + "ません";
    },
    masu_past: function() {
      return API.stem.apply(this, arguments) + "ました";
    },
    masu_past_negative: function() {
      return API.stem.apply(this, arguments) + "ませんでした";
    },
    te: function() {
      var past = API.past.apply(this, arguments);
      switch (past.slice(-1)) {
        case "た": return past.slice(0, -1) + "て";
        case "だ": return past.slice(0, -1) + "で";
      }
    },
    te_negative: function() {
      return API.negative.apply(this, arguments).slice(0, -1) + "くて";
    },
    potential: function(dict, type, ruby) {
      switch (dict) {
        case "来る": return ruby ? "<ruby><rb>来</rb><rt>こ</rt></ruby>られる"
                                 : "来（こ）られる";
        default: 
          root = dict.slice(0, -1);
          switch (type) {
            case "する": 
              var suffix = ruby ? "<ruby>"
                                  + "<rb>出</rb><rt>で</rt>"
                                  + "<rb>来</rb><rt>き</rt>"
                                  + "</ruby>る"
                                : "出来る（できる）";
              return dict.slice(0, -2) + suffix;
            case "る": return root + "られる";
            case "う": return root + changeVowel(dict.slice(-1), "え") + "る";
          }
      }
    }
  }
  return API;
})();