/**
 * Contains and collects metadata about a matching document.
 * A single instance of lunr.MatchData is returned as part of every
 * lunr.Index~Result.
 *
 * @constructor
 * @param {string} term - The term this match data is associated with
 * @param {string} field - The field in which the term was found
 * @param {object} metadata - The metadata recorded about this term in this field
 * @property {object} metadata - A cloned collection of metadata associated with this document.
 * @see {@link lunr.Index~Result}
 */
lunr.MatchData = function (term, field, metadata) {
  var clonedMetadata = Object.create(null),
      metadataKeys = Object.keys(metadata || {})

  // Cloning the metadata to prevent the original
  // being mutated during match data combination.
  // Metadata is kept in an array within the inverted
  // index so cloning the data can be done with
  // Array#slice
  for (var i = 0; i < metadataKeys.length; i++) {
    var key = metadataKeys[i]
    clonedMetadata[key] = metadata[key].slice()
  }

  this.metadata = Object.create(null)

  if (term !== undefined) {
    this.metadata[term] = Object.create(null)
    this.metadata[term][field] = clonedMetadata
  }
}

/**
 * An instance of lunr.MatchData will be created for every term that matches a
 * document. However only one instance is required in a lunr.Index~Result. This
 * method combines metadata from another instance of lunr.MatchData with this
 * objects metadata.
 *
 * @param {lunr.MatchData} otherMatchData - Another instance of match data to merge with this one.
 * @see {@link lunr.Index~Result}
 */
lunr.MatchData.prototype.combine = function (otherMatchData) {
  var terms = Object.keys(otherMatchData.metadata)

  for (var i = 0; i < terms.length; i++) {
    var term = terms[i],
        fields = Object.keys(otherMatchData.metadata[term])

    if (this.metadata[term] == undefined) {
      this.metadata[term] = Object.create(null)
    }

    for (var j = 0; j < fields.length; j++) {
      var field = fields[j],
          keys = Object.keys(otherMatchData.metadata[term][field])

      if (this.metadata[term][field] == undefined) {
        this.metadata[term][field] = Object.create(null)
      }

      for (var k = 0; k < keys.length; k++) {
        var key = keys[k]

        if (this.metadata[term][field][key] == undefined) {
          this.metadata[term][field][key] = otherMatchData.metadata[term][field][key]
        } else {
          this.metadata[term][field][key] = this.metadata[term][field][key].concat(otherMatchData.metadata[term][field][key])
        }

      }
    }
  }
}

/**
 * Add metadata for a term/field pair to this instance of match data.
 *
 * @param {string} term - The term this match data is associated with
 * @param {string} field - The field in which the term was found
 * @param {object} metadata - The metadata recorded about this term in this field
 */
lunr.MatchData.prototype.add = function (term, field, metadata) {
  if (!(term in this.metadata)) {
    this.metadata[term] = Object.create(null)
    this.metadata[term][field] = metadata
    return
  }

  if (!(field in this.metadata[term])) {
    this.metadata[term][field] = metadata
    return
  }

  var metadataKeys = Object.keys(metadata)

  for (var i = 0; i < metadataKeys.length; i++) {
    var key = metadataKeys[i]

    if (key in this.metadata[term][field]) {
      this.metadata[term][field][key] = this.metadata[term][field][key].concat(metadata[key])
    } else {
      this.metadata[term][field][key] = metadata[key]
    }
  }
}