elasticlunr.Index = function () {
  this._fields = [];
  this._ref = 'id';
  this.pipeline = new elasticlunr.Pipeline;
  this.documentStore = new elasticlunr.DocumentStore;
  this.index = {};
  this.eventEmitter = new elasticlunr.EventEmitter;
  this._idfCache = {};

  this.on('add', 'remove', 'update', (function () {
    this._idfCache = {};
  }).bind(this));
};

elasticlunr.Index.prototype.on = function () {
  var args = Array.prototype.slice.call(arguments);
  return this.eventEmitter.addListener.apply(this.eventEmitter, args);
};

elasticlunr.Index.prototype.off = function (name, fn) {
  return this.eventEmitter.removeListener(name, fn);
};

elasticlunr.Index.load = function (serialisedData) {
  if (serialisedData.version !== elasticlunr.version) {
    elasticlunr.utils.warn('version mismatch: current '
                    + elasticlunr.version + ' importing ' + serialisedData.version);
  }

  var idx = new this;

  idx._fields = serialisedData.fields;
  idx._ref = serialisedData.ref;
  idx.documentStore = elasticlunr.DocumentStore.load(serialisedData.documentStore);
  idx.pipeline = elasticlunr.Pipeline.load(serialisedData.pipeline);
  idx.index = {};
  for (var field in serialisedData.index) {
    idx.index[field] = elasticlunr.InvertedIndex.load(serialisedData.index[field]);
  }

  return idx;
};

elasticlunr.Index.prototype.addField = function (fieldName) {
  this._fields.push(fieldName);
  this.index[fieldName] = new elasticlunr.InvertedIndex;
  return this;
};

elasticlunr.Index.prototype.setRef = function (refName) {
  this._ref = refName;
  return this;
};

elasticlunr.Index.prototype.saveDocument = function (save) {
  this.documentStore = new elasticlunr.DocumentStore(save);
  return this;
};

elasticlunr.Index.prototype.addDoc = function (doc, emitEvent) {
  if (!doc) return;
  var emitEvent = emitEvent === undefined ? true : emitEvent;

  var docRef = doc[this._ref];

  this.documentStore.addDoc(docRef, doc);
  this._fields.forEach(function (field) {
    var fieldTokens = this.pipeline.run(elasticlunr.tokenizer(doc[field]));
    this.documentStore.addFieldLength(docRef, field, fieldTokens.length);

    var tokenCount = {};
    fieldTokens.forEach(function (token) {
      if (token in tokenCount) tokenCount[token] += 1;
      else tokenCount[token] = 1;
    }, this);

    for (var token in tokenCount) {
      var termFrequency = tokenCount[token];
      termFrequency = Math.sqrt(termFrequency);
      this.index[field].addToken(token, { ref: docRef, tf: termFrequency });
    }
  }, this);

  if (emitEvent) this.eventEmitter.emit('add', doc, this);
};

elasticlunr.Index.prototype.removeDocByRef = function (docRef, emitEvent) {
  if (!docRef) return;
  if (this.documentStore.isDocStored() === false) {
    return;
  }

  if (!this.documentStore.hasDoc(docRef)) return;
  var doc = this.documentStore.getDoc(docRef);
  this.removeDoc(doc, false);
};

elasticlunr.Index.prototype.removeDoc = function (doc, emitEvent) {
  if (!doc) return;

  var emitEvent = emitEvent === undefined ? true : emitEvent;

  var docRef = doc[this._ref];
  if (!this.documentStore.hasDoc(docRef)) return;

  this.documentStore.removeDoc(docRef);

  this._fields.forEach(function (field) {
    var fieldTokens = this.pipeline.run(elasticlunr.tokenizer(doc[field]));
    fieldTokens.forEach(function (token) {
      this.index[field].removeToken(token, docRef);
    }, this);
  }, this);

  if (emitEvent) this.eventEmitter.emit('remove', doc, this);
};

elasticlunr.Index.prototype.updateDoc = function (doc, emitEvent) {
  var emitEvent = emitEvent === undefined ? true : emitEvent;

  this.removeDocByRef(doc[this._ref], false);
  this.addDoc(doc, false);

  if (emitEvent) this.eventEmitter.emit('update', doc, this);
};

elasticlunr.Index.prototype.getFields = function () {
  return this._fields.slice();
};

elasticlunr.Index.prototype.search = function (query, userConfig) {
  if (!query) return [];

  var configStr = null;
  if (userConfig != null) {
    configStr = JSON.stringify(userConfig);
  }

  var config = new elasticlunr.Configuration(configStr, this.getFields()).get();

  var queryTokens = this.pipeline.run(elasticlunr.tokenizer(query));

  var queryResults = {};

  for (var field in config) {
    var fieldSearchResults = this.fieldSearch(queryTokens, field, config);
    var fieldBoost = config[field].boost;

    for (var docRef in fieldSearchResults) {
      fieldSearchResults[docRef] = fieldSearchResults[docRef] * fieldBoost;
    }

    for (var docRef in fieldSearchResults) {
      if (docRef in queryResults) {
        queryResults[docRef] += fieldSearchResults[docRef];
      } else {
        queryResults[docRef] = fieldSearchResults[docRef];
      }
    }
  }

  var results = [];
  for (var docRef in queryResults) {
    results.push({ref: docRef, score: queryResults[docRef]});
  }

  results.sort(function (a, b) { return b.score - a.score; });
  return results;
};

elasticlunr.Index.prototype.fieldSearch = function (queryTokens, fieldName, config) {
  var booleanType = config[fieldName].bool;
  var expand = config[fieldName].expand;
  var boost = config[fieldName].boost;
  var scores = null;
  var docTokens = {};

  // Do nothing if the boost is 0
  if (boost === 0) {
    return;
  }

  queryTokens.forEach(function (token) {
    var tokens = [token];
    if (expand == true) {
      tokens = this.index[fieldName].expandToken(token);
    }
    // Consider every query token in turn. If expanded, each query token
    // corresponds to a set of tokens, which is all tokens in the 
    // index matching the pattern queryToken* .
    // For the set of tokens corresponding to a query token, find and score
    // all matching documents. Store those scores in queryTokenScores, 
    // keyed by docRef.
    // Then, depending on the value of booleanType, combine the scores
    // for this query token with previous scores.  If booleanType is OR,
    // then merge the scores by summing into the accumulated total, adding
    // new document scores are required (effectively a union operator). 
    // If booleanType is AND, accumulate scores only if the document 
    // has previously been scored by another query token (an intersection
    // operation0. 
    // Furthermore, since when booleanType is AND, additional 
    // query tokens can't add new documents to the result set, use the
    // current document set to limit the processing of each new query 
    // token for efficiency (i.e., incremental intersection).
    
    var queryTokenScores = {};
    tokens.forEach(function (key) {
      var docs = this.index[fieldName].getDocs(key);
      var idf = this.idf(key, fieldName);
      
      if (scores && booleanType == 'AND') {
          // special case, we can rule out documents that have been
          // already been filtered out because they weren't scored
          // by previous query token passes.
          var filteredDocs = {};
          for (var docRef in scores) {
              if (docRef in docs) {
                  filteredDocs[docRef] = docs[docRef];
              }
          }
          docs = filteredDocs;
      }
      // only record appeared token for retrieved documents for the
      // original token, not for expaned token.
      // beause for doing coordNorm for a retrieved document, coordNorm only care how many
      // query token appear in that document.
      // so expanded token should not be added into docTokens, if added, this will pollute the
      // coordNorm
      if (key == token) {
        this.fieldSearchStats(docTokens, key, docs);
      }

      for (var docRef in docs) {
        var tf = this.index[fieldName].getTermFrequency(key, docRef);
        var fieldLength = this.documentStore.getFieldLength(docRef, fieldName);
        var fieldLengthNorm = 1;
        if (fieldLength != 0) {
          fieldLengthNorm = 1 / Math.sqrt(fieldLength);
        }

        var penality = 1;
        if (key != token) {
          // currently I'm not sure if this penality is enough,
          // need to do verification
          penality = (1 - (key.length - token.length) / key.length) * 0.15;
        }

        var score = tf * idf * fieldLengthNorm * penality;

        if (docRef in queryTokenScores) {
          queryTokenScores[docRef] += score;
        } else {
          queryTokenScores[docRef] = score;
        }
      }
    }, this);
    
    scores = this.mergeScores(scores, queryTokenScores, booleanType);
  }, this);

  scores = this.coordNorm(scores, docTokens, queryTokens.length);
  return scores;
};

elasticlunr.Index.prototype.mergeScores = function (accumScores, scores, op) {
    if (!accumScores) {
        return scores; 
    }
    if (op == 'AND') {
        var intersection = {};
        for (var docRef in scores) {
            if (docRef in accumScores) {
                intersection[docRef] = accumScores[docRef] + scores[docRef];
            }
        }
        return intersection;
    } else {
        for (var docRef in scores) {
            if (docRef in accumScores) {
                accumScores[docRef] += scores[docRef];
            } else {
                accumScores[docRef] = scores[docRef];
            }
        }
        return accumScores;
    }
};

elasticlunr.Index.prototype.fieldSearchStats = function (docTokens, token, docs) {
  for (var doc in docs) {
    if (doc in docTokens) {
      docTokens[doc].push(token);
    } else {
      docTokens[doc] = [token];
    }
  }
};

elasticlunr.Index.prototype.coordNorm = function (scores, docTokens, n) {
  for (var doc in scores) {
    if (!(doc in docTokens)) continue;
    var tokens = docTokens[doc].length;
    scores[doc] = scores[doc] * tokens / n;
  }

  return scores;
};

elasticlunr.Index.prototype.toJSON = function () {
  var indexJson = {};
  this._fields.forEach(function (field) {
    indexJson[field] = this.index[field].toJSON();
  }, this);

  return {
    version: elasticlunr.version,
    fields: this._fields,
    ref: this._ref,
    documentStore: this.documentStore.toJSON(),
    index: indexJson,
    pipeline: this.pipeline.toJSON()
  };
};

var myPlugin = function (idx, arg1, arg2) {
  // `this` is the index to be extended
  // apply any extensions etc here.
}

var idx = elasticlunr(function () {
  this.use(myPlugin, 'arg1', 'arg2')
})

elasticlunr.Index.prototype.use = function (plugin) {
  var args = Array.prototype.slice.call(arguments, 1);
  args.unshift(this);
  plugin.apply(this, args);
};