'use strict';

var _       = require('lodash')
var Promise = require('./promise')
var helpers = require('./helpers')
var debug   = require('debug')
var assign  = require('lodash/object/assign');

var PassThrough;

// The "Runner" constructor takes a "builder" (query, schema, or raw)
// and runs through each of the query statements, calling any additional
// "output" method provided alongside the query and bindings.
function Runner(client, builder) {
  this.client  = client
  this.builder = builder
  this.queries = []

  // The "connection" object is set on the runner when
  // "run" is called.
  this.connection = void 0
}

assign(Runner.prototype, {

  // "Run" the target, calling "toSQL" on the builder, returning
  // an object or array of queries to run, each of which are run on
  // a single connection.
  run: function() {
    var runner = this

    return Promise.using(this.ensureConnection(), function(connection) {
      runner.connection = connection;

      // Emit a "start" event on both the builder and the client,
      // allowing us to listen in on any events. We fire on the "client"
      // before building the SQL, and on the builder after building the SQL
      // in case we want to determine at how long it actually
      // took to build the query.
      runner.client.emit('start', runner.builder);
      var sql = runner.builder.toSQL();
      runner.builder.emit('start', runner.builder);

      if (_.isArray(sql)) {
        return runner.queryArray(sql);
      }
      return runner.query(sql);

    })

    // If there are any "error" listeners, we fire an error event
    // and then re-throw the error to be eventually handled by
    // the promise chain. Useful if you're wrapping in a custom `Promise`.
    .catch(function(err) {
      if (runner.builder._events && runner.builder._events.error) {
        runner.builder.emit('error', err);
      }
      throw err;
    })

    // Fire a single "end" event on the builder when
    // all queries have successfully completed.
    .tap(function() {
      runner.builder.emit('end');
    })

  },

  // Stream the result set, by passing through to the dialect's streaming
  // capabilities. If the options are
  stream: function(options, handler) {
    
    // If we specify stream(handler).then(...
    if (arguments.length === 1) {
      if (typeof options === 'function') {
        handler = options;
        options = {};
      }
    }

    // Determines whether we emit an error or throw here.
    var hasHandler = typeof handler === 'function';

    // Lazy-load the "PassThrough" dependency.
    PassThrough = PassThrough || require('readable-stream').PassThrough;
    
    var runner = this;
    var stream  = new PassThrough({objectMode: true});
    var promise = Promise.using(this.ensureConnection(), function(connection) {
      runner.connection = connection;
      var sql = runner.builder.toSQL()
      var err = new Error('The stream may only be used with a single query statement.');
      if (_.isArray(sql)) {
        if (hasHandler) throw err;
        stream.emit('error', err);
      }
      return sql;
    })
    .then(function(sql) {
      return runner.client.stream(runner.connection, sql, stream, options);
    })

    // If a function is passed to handle the stream, send the stream
    // there and return the promise, otherwise just return the stream
    // and the promise will take care of itsself.
    if (hasHandler) {
      handler(stream);
      return promise;
    }
    return stream;
  },

  // Allow you to pipe the stream to a writable stream.
  pipe: function(writable) {
    return this.stream().pipe(writable);
  },

  // "Runs" a query, returning a promise. All queries specified by the builder are guaranteed
  // to run in sequence, and on the same connection, especially helpful when schema building
  // and dealing with foreign key constraints, etc.
  query: Promise.method(function(obj) {
    this.builder.emit('query', assign({__knexUid: this.connection.__knexUid}, obj))
    var runner = this
    return this.client.query(this.connection, obj)
      .then(function(resp) {
        return runner.client.processResponse(resp, runner)
      });
  }),

  // In the case of the "schema builder" we call `queryArray`, which runs each
  // of the queries in sequence.
  queryArray: function(queries) {
    return queries.length === 1 ? this.query(queries[0]) : Promise.bind(this)
      .return(queries)
      .reduce(function(memo, query) {
        return this.query(query).then(function(resp) {
          memo.push(resp)
          return memo;
        });
      }, [])
  },

  // Check whether there's a transaction flag, and that it has a connection.
  ensureConnection: function() {
    var runner = this
    return Promise.try(function() {
      return runner.connection || runner.client.acquireConnection()
    }).disposer(function() {
      runner.client.releaseConnection(runner.connection)
    })
  }

})

module.exports = Runner;