var level = require('level')
var AutoIndex = require('level-auto-index')
var sub = require('subleveldown')
var keyReducer = AutoIndex.keyReducer

var db = level()

var posts = sub(db, 'posts', {valueEncoding: 'json'})
var idx = {
  title: sub(db, 'title'),
  length: sub(db, 'length'),
  tag: sub(db, 'tag')
}

// add a title index
posts.byTitle = AutoIndex(posts, idx.title, keyReducer('title'))

// add a length index
// append the post.id for unique indexes with possibly overlapping values
posts.byLength = AutoIndex(posts, idx.length, function (post) {
  return post.body.length + '!' + post.id
})

// Create multiple index keys on an index
posts.byTag = AutoIndex(posts, idx.tag, function (post) {
    if (!post || !post.tags || !Array.isArray(post.tags)) return
    return post.tags.map(function (tag) {
      return [tag, post.id].join('!')
    })
  }, { multi: true })

posts.put('1337', {
  id: '1337',
  title: 'a title',
  body: 'lorem ipsum',
  tags: [ 'foo', 'bar', 'baz' ]
}, function (err) {
  if (err) throw err

  posts.byTitle.get('a title', function (err, post) {
    if (err) throw err
    console.log('get', post)
    // => get: { id: '1337', title: 'a title', body: 'lorem ipsum' }

    posts.del('1337', function (err) {
      if (err) throw err
      posts.byTitle.get('a title', function (err) {
        console.log(err.name)
        // => NotFoundError
      })
    })
  })

  posts.byLength.createReadStream({
    start: 10,
    end: 20
  }).on('data', console.log.bind(console, 'read'))
  // => read { key: '1337', value: { id: '1337', title: 'a title', body: 'lorem ipsum' } }

  posts.byLength.createKeyStream({
    start: 10,
    end: 20
  }).on('data', console.log.bind(console, 'key'))
  // => key 1337

  posts.byLength.createValueStream({
    start: 10,
    end: 20
  }).on('data', console.log.bind(console, 'value'))
  // => value { id: '1337', title: 'a title', body: 'lorem ipsum' }
})

function reducer (value) {
  var idxKey = value.foo + '!' + value.bar
  return idxKey
}

{
  multi: false // Reducer returns an array of keys to associate with the primary key
}

function keyReducer (reducerString) {
  function keyRdc (value) {
    return value[reducerString]
  }
  return keyRdc
}

function multiKeyReducer (multiFieldName, primaryKeyFieldName) {
  return function multiKeyrdc (document) {
    if (!document || !document[multiFieldName] || !Array.isArray(document[multiFieldName])) return
    return document[multiFieldName].map(function (tag) {
      return [tag, document[primaryKeyFieldName]].join('!')
    })
  }