'use strict'

const co = require('co')
const compose = require('koa-compose')

module.exports = convert

function convert (mw) {
  if (typeof mw !== 'function') {
    throw new TypeError('middleware must be a function')
  }
  if (mw.constructor.name !== 'GeneratorFunction') {
    // assume it's Promise-based middleware
    return mw
  }
  const converted = function (ctx, next) {
    return co.call(ctx, mw.call(ctx, createGenerator(next)))
  }
  converted._name = mw._name || mw.name
  return converted
}

function * createGenerator (next) {
  return yield next()
}

// convert.compose(mw, mw, mw)
// convert.compose([mw, mw, mw])
convert.compose = function (arr) {
  if (!Array.isArray(arr)) {
    arr = Array.from(arguments)
  }
  return compose(arr.map(convert))
}

convert.back = function (mw) {
  if (typeof mw !== 'function') {
    throw new TypeError('middleware must be a function')
  }
  if (mw.constructor.name === 'GeneratorFunction') {
    // assume it's generator middleware
    return mw
  }
  const converted = function * (next) {
    let ctx = this
    let called = false
    // no need try...catch here, it's ok even `mw()` throw exception
    yield Promise.resolve(mw(ctx, function () {
      if (called) {
        // guard against multiple next() calls
        // https://github.com/koajs/compose/blob/4e3e96baf58b817d71bd44a8c0d78bb42623aa95/index.js#L36
        return Promise.reject(new Error('next() called multiple times'))
      }
      called = true
      return co.call(ctx, next)
    }))
  }
  converted._name = mw._name || mw.name
  return converted
}