function MemoryCache() {
  this.cache = {}
  this.size = 0
}
  
MemoryCache.prototype.add = function(key, value, time, timeoutCallback) {
  var old = this.cache[key]
  var instance = this
  
  var entry = {
    value: value,
    expire: time + Date.now(),
    timeout: setTimeout(function() {
      instance.delete(key)
      return timeoutCallback && typeof timeoutCallback === 'function' && timeoutCallback(value, key)
    }, time)
  }
  
  this.cache[key] = entry
  this.size = Object.keys(this.cache).length
  
  return entry
}
  
MemoryCache.prototype.delete = function(key) {
  var entry = this.cache[key]
  
  if (entry) {
    clearTimeout(entry.timeout)
  }
  
  delete this.cache[key]
  
  this.size = Object.keys(this.cache).length
  
  return null
}
  
MemoryCache.prototype.get = function(key) {
  var entry = this.cache[key]
  
  return entry
}
  
MemoryCache.prototype.getValue = function(key) {
  var entry = this.get(key)
  
  return entry && entry.value
}
  
MemoryCache.prototype.clear = function() {
  Object.keys(this.cache).forEach(function(key) {
    this.delete(key)
  }, this)
  
  return true
}
  
module.exports = MemoryCache