'use strict'
const EventEmitter = require('events')

 // default expiry 3 mins
const DEFAULT_EXPIRY = 3 * 60 * 1000

const create = ({ keys, ssm, keyPrefix = '', expiryMs = DEFAULT_EXPIRY }) => {
  let isRefreshing = false
  if (!keys || !Array.isArray(keys) || keys.length === 0) {
    throw new Error('Provide a non-empty array of config keys')
  }

  if (expiryMs <= 0) {
    throw new Error('Specify an expiry (ms) greater than 0')
  }

  let cache = {
    expiration: new Date(0),
    items: {}
  }

  let eventEmitter = new EventEmitter()

  let validate = (keys, params) => {
    let missing = keys.filter(k => params[k] === undefined)
    if (missing.length > 0) {
      throw new Error(`Missing SSM Parameter Store keys: ${missing}`)
    }
  }

  let fetchParameters = async () => {
    if (process.env.NODE_ENV === 'production') {
      let req = {
        Names: keys.map(k => `${keyPrefix}${k}`),
        WithDecryption: true
      }

      let {
        Parameters
      } = await ssm.getParameters(req).promise()

      let params = {}
      for (let p of Parameters) {
        let name = p.Name.replace(keyPrefix, '')
        params[name] = p.Value
      }
      return params
    } else {
      let params = {}
      for (let key of keys) {
        params[key] = process.env[key]
      }
      return params
    }
  }

  let refresh = async () => {
    if (isRefreshing) { return }

    isRefreshing = true
    console.log(`Refreshing SSM Parameter Store keys: ${keys}`)
    try {
      let params = await fetchParameters()
      validate(keys, params)
      console.log(`Successfully refreshed SSM Parameter Store keys: ${keys}`)
      let now = new Date()
      cache.expiration = new Date(now.getTime() + expiryMs)
      cache.items = params
      eventEmitter.emit('refresh')
    } finally {
      isRefreshing = false
    }
  }

  let getValue = (key) => {
    let now = new Date()
    if (now >= cache.expiration) {
      refresh()
    }
    return cache.items[key]
  }

  let config = {
    refresh: refresh,
    onRefresh: listener => eventEmitter.addListener('refresh', listener),
    onRefreshError: listener => eventEmitter.addListener('refreshError', listener)
  }
  let defaultKeys = Object.keys(config)

  for (let key of keys) {
    key = key.replace(keyPrefix, '')

    Object.defineProperty(config, key, {
      get() { return getValue(key) },
      enumerable: true,
      configurable: false
    })
  }

  // catch usage of undefined keys
  if (process.env.NODE_ENV !== 'production') {
    return new Proxy(config, {
      get(target, key) {
        if (keys.includes(key) === false && defaultKeys.includes(key) === false) {
          throw new Error(`Key '${key}' is not defined in config keys`)
        }
        return target[key]
      }
    })
  }

  return config
}

module.exports = {
  create
}