var isWindows = process.platform === 'win32'
var windir = isWindows ? process.env.windir || 'C:\\Windows' : null
var path = require('path')
var exec = require('child_process').exec

// looking up envs is a bit costly.
// Also, sometimes we want to have a fallback
// Pass in a callback to wait for the fallback on failures
// After the first lookup, always returns the same thing.
function memo (key, lookup, fallback) {
  var fell = false
  var falling = false
  exports[key] = function (cb) {
    var val = lookup()
    if (!val && !fell && !falling && fallback) {
      fell = true
      falling = true
      exec(fallback, function (er, output, stderr) {
        falling = false
        if (er) return // oh well, we tried
        val = output.trim()
      })
    }
    exports[key] = function (cb) {
      if (cb) process.nextTick(cb.bind(null, null, val))
      return val
    }
    if (cb && !falling) process.nextTick(cb.bind(null, null, val))
    return val
  }
}

memo('user', function () {
  return ( isWindows
         ? process.env.USERDOMAIN + '\\' + process.env.USERNAME
         : process.env.USER
         )
}, 'whoami')

memo('prompt', function () {
  return isWindows ? process.env.PROMPT : process.env.PS1
})

memo('hostname', function () {
  return isWindows ? process.env.COMPUTERNAME : process.env.HOSTNAME
}, 'hostname')

memo('tmpdir', function () {
  var t = isWindows ? 'temp' : 'tmp'
  return process.env.TMPDIR ||
         process.env.TMP ||
         process.env.TEMP ||
         ( exports.home() ? path.resolve(exports.home(), t)
         : isWindows ? path.resolve(windir, t)
         : '/tmp'
         )
})

memo('home', function () {
  return ( isWindows ? process.env.USERPROFILE
         : process.env.HOME
         )
})

memo('path', function () {
  return (process.env.PATH ||
          process.env.Path ||
          process.env.path).split(isWindows ? ';' : ':')
})

memo('editor', function () {
  return process.env.EDITOR ||
         process.env.VISUAL ||
         (isWindows ? 'notepad.exe' : 'vi')
})

memo('shell', function () {
  return isWindows ? process.env.ComSpec || 'cmd'
         : process.env.SHELL || 'bash'
})
