/**
 * Manage access to data, be it to find, update or remove it
 */
const model = require('./model.js')

class Cursor {
  /**
   * Create a new cursor for this collection
   * @param {Datastore} db - The datastore this cursor is bound to
   * @param {Query} query - The query this cursor will operate on
   * @param {Function} execFn - Handler to be executed after cursor has found the results and before the callback passed to find/findOne/update/remove
   */
  constructor (db, query, execFn) {
    this.db = db
    this.query = query || {}
    if (execFn) { this.execFn = execFn }
  }

  /**
   * Set a limit to the number of results
   */
  limit (limit) {
    this._limit = limit
    return this
  }

  /**
   * Skip a the number of results
   */
  skip (skip) {
    this._skip = skip
    return this
  }

  /**
   * Sort results of the query
   * @param {SortQuery} sortQuery - SortQuery is { field: order }, field can use the dot-notation, order is 1 for ascending and -1 for descending
   */
  sort (sortQuery) {
    this._sort = sortQuery
    return this
  }

  /**
   * Add the use of a projection
   * @param {Object} projection - MongoDB-style projection. {} means take all fields. Then it's { key1: 1, key2: 1 } to take only key1 and key2
   *                              { key1: 0, key2: 0 } to omit only key1 and key2. Except _id, you can't mix takes and omits
   */
  projection (projection) {
    this._projection = projection
    return this
  }

  /**
   * Apply the projection
   */
  project (candidates) {
    const res = []
    let action

    if (this._projection === undefined || Object.keys(this._projection).length === 0) {
      return candidates
    }

    const keepId = this._projection._id !== 0
    const { _id, ...rest } = this._projection
    this._projection = rest

    // Check for consistency
    const keys = Object.keys(this._projection)
    keys.forEach(k => {
      if (action !== undefined && this._projection[k] !== action) throw new Error('Can\'t both keep and omit fields except for _id')
      action = this._projection[k]
    })

    // Do the actual projection
    candidates.forEach(candidate => {
      let toPush
      if (action === 1) { // pick-type projection
        toPush = { $set: {} }
        keys.forEach(k => {
          toPush.$set[k] = model.getDotValue(candidate, k)
          if (toPush.$set[k] === undefined) delete toPush.$set[k]
        })
        toPush = model.modify({}, toPush)
      } else { // omit-type projection
        toPush = { $unset: {} }
        keys.forEach(k => { toPush.$unset[k] = true })
        toPush = model.modify(candidate, toPush)
      }
      if (keepId) toPush._id = candidate._id
      else delete toPush._id
      res.push(toPush)
    })

    return res
  }

  /**
   * Get all matching elements
   * Will return pointers to matched elements (shallow copies), returning full copies is the role of find or findOne
   * This is an internal function, use exec which uses the executor
   *
   * @param {Function} callback - Signature: err, results
   */
  _exec (_callback) {
    let res = []
    let added = 0
    let skipped = 0
    let error = null
    let keys
    let key

    const callback = (error, res) => {
      if (this.execFn) return this.execFn(error, res, _callback)
      else return _callback(error, res)
    }

    this.db.getCandidates(this.query, (err, candidates) => {
      if (err) return callback(err)

      try {
        for (const candidate of candidates) {
          if (model.match(candidate, this.query)) {
            // If a sort is defined, wait for the results to be sorted before applying limit and skip
            if (!this._sort) {
              if (this._skip && this._skip > skipped) skipped += 1
              else {
                res.push(candidate)
                added += 1
                if (this._limit && this._limit <= added) break
              }
            } else res.push(candidate)
          }
        }
      } catch (err) {
        return callback(err)
      }

      // Apply all sorts
      if (this._sort) {
        keys = Object.keys(this._sort)

        // Sorting
        const criteria = []
        keys.forEach(item => {
          key = item
          criteria.push({ key: key, direction: this._sort[key] })
        })
        res.sort((a, b) => {
          for (const criterion of criteria) {
            const compare = criterion.direction * model.compareThings(model.getDotValue(a, criterion.key), model.getDotValue(b, criterion.key), this.db.compareStrings)
            if (compare !== 0) return compare
          }
          return 0
        })

        // Applying limit and skip
        const limit = this._limit || res.length
        const skip = this._skip || 0

        res = res.slice(skip, skip + limit)
      }

      // Apply projection
      try {
        res = this.project(res)
      } catch (e) {
        error = e
        res = undefined
      }

      return callback(error, res)
    })
  }

  exec () {
    this.db.executor.push({ this: this, fn: this._exec, arguments: arguments })
  }
}

// Interface
module.exports = Cursor