'use strict'

function id (e) { return e }
var prop = require('../util/prop')
var filter = require('./filter')

//drop items you have already seen.
module.exports = function unique (field, invert) {
  field = prop(field) || id
  var seen = {}
  return filter(function (data) {
    var key = field(data)
    if(seen[key]) return !!invert //false, by default
    else seen[key] = true
    return !invert //true by default
  })
}