Week3: TweetSet Class

最近一直在跟Coursera上的scala和cloud computing课，干脆开博客记录一下遇到的问题和解决方法。Scala作为函数式编程，如何熟练掌握function的传递，tail recursion的设计是这门课前半部分的重点。这次的作业是设计一个TweetSet，Tweet是一个记录了user,string,Retweeted三个field的class，要求TweetSet以binary tree形式存储，同时满足特定函数。

Aim: To implement a TweetSet Class, including its fundamental functions.Eg: union, filter(p: Tweet => Boolean), mostRetweeted, descendingByRetweet

Note:
filter and mostRetweeted are both implemented using tail recursion(filterAcc and mostAcc)
注意tail recursion的逻辑，一般都需要传递一个中间结果（1作为累加值或者2作为比较值），从而使下次递归可以直接比较之前完成部分的结果。注意递归顺序，这里因为是binary tree，一般都是先elem，再left，最后right。
filterAcc records a TweetSet acc that represents the accumulative tweetSet which satisfy the predicate pif (p(elem)) then include current tweet into the tweetSet
mostAcc records a tweet object with maximum retweets. At beginning, it is initialized as new tweet("ini","ini",0)retweet number is 0 at first!

Difficulties:
Remember how to use function to implement traverse of the list!!! Just as follows.
google.exists(x => y.text.contains(x))

val google = List("android", "Android", "galaxy", "Galaxy", "nexus", "Nexus")
val apple = List("ios", "iOS", "iphone", "iPhone", "ipad", "iPad")lazy 
val googleTweets: TweetSet = TweetReader.allTweets.filter(y => google.exists(x => y.text.contains(x)))lazy 
val appleTweets: TweetSet = TweetReader.allTweets.filter(y => apple.exists(x => y.text.contains(x)))

----------Codes-------------

package objsets

import TweetReader._

/**
 * A class to represent tweets.
 */
class Tweet(val user: String, val text: String, val retweets: Int) {
  override def toString: String =
    "User: " + user + "\n" +
    "Text: " + text + " [" + retweets + "]"
}

/**
 * This represents a set of objects of type `Tweet` in the form of a binary search
 * tree. Every branch in the tree has two children (two `TweetSet`s). There is an
 * invariant which always holds: for every branch `b`, all elements in the left
 * subtree are smaller than the tweet at `b`. The elements in the right subtree are
 * larger.
 *
 * Note that the above structure requires us to be able to compare two tweets (we
 * need to be able to say which of two tweets is larger, or if they are equal). In
 * this implementation, the equality / order of tweets is based on the tweet's text
 * (see `def incl`). Hence, a `TweetSet` could not contain two tweets with the same
 * text from different users.
 *
 *
 * The advantage of representing sets as binary search trees is that the elements
 * of the set can be found quickly. If you want to learn more you can take a look
 * at the Wikipedia page [1], but this is not necessary in order to solve this
 * assignment.
 *
 * [1] http://en.wikipedia.org/wiki/Binary_search_tree
 */
abstract class TweetSet {

  /**
   * This method takes a predicate and returns a subset of all the elements
   * in the original set for which the predicate is true.
   *
   * Question: Can we implment this method here, or should it remain abstract
   * and be implemented in the subclasses?
   */
    def filter(p: Tweet => Boolean): TweetSet =
      this.filterAcc(p, new Empty())
  
  /**
   * This is a helper method for `filter` that propagetes the accumulated tweets.
   */
  def filterAcc(p: Tweet => Boolean, acc: TweetSet): TweetSet

  /**
   * Returns a new `TweetSet` that is the union of `TweetSet`s `this` and `that`.
   *
   * Question: Should we implment this method here, or should it remain abstract
   * and be implemented in the subclasses?
   */
    def union(that: TweetSet): TweetSet 
  
  /**
   * Returns the tweet from this set which has the greatest retweet count.
   *
   * Calling `mostRetweeted` on an empty set should throw an exception of
   * type `java.util.NoSuchElementException`.
   *
   * Question: Should we implment this method here, or should it remain abstract
   * and be implemented in the subclasses?
   */
    def mostRetweeted: Tweet = mostAcc(new Tweet("ini_max", "ini_max", 0))
    def mostAcc(max: Tweet): Tweet
  
  /**
   * Returns a list containing all tweets of this set, sorted by retweet count
   * in descending order. In other words, the head of the resulting list should
   * have the highest retweet count.
   *
   * Hint: the method `remove` on TweetSet will be very useful.
   * Question: Should we implment this method here, or should it remain abstract
   * and be implemented in the subclasses?
   */
    def descendingByRetweet: TweetList 
  
  /**
   * The following methods are already implemented
   */

  /**
   * Returns a new `TweetSet` which contains all elements of this set, and the
   * the new element `tweet` in case it does not already exist in this set.
   *
   * If `this.contains(tweet)`, the current set is returned.
   */
  def incl(tweet: Tweet): TweetSet

  /**
   * Returns a new `TweetSet` which excludes `tweet`.
   */
  def remove(tweet: Tweet): TweetSet

  /**
   * Tests if `tweet` exists in this `TweetSet`.
   */
  def contains(tweet: Tweet): Boolean

  /**
   * This method takes a function and applies it to every element in the set.
   */
  def foreach(f: Tweet => Unit): Unit
}

class Empty extends TweetSet {
    def filterAcc(p: Tweet => Boolean, acc: TweetSet): TweetSet = acc
    
    def union(that: TweetSet): TweetSet = that
    
    def mostAcc(max: Tweet): Tweet = max
    
     def descendingByRetweet: TweetList = Nil
  
  /**
   * The following methods are already implemented
   */

  def contains(tweet: Tweet): Boolean = false

  def incl(tweet: Tweet): TweetSet = new NonEmpty(tweet, new Empty, new Empty)

  def remove(tweet: Tweet): TweetSet = this

  def foreach(f: Tweet => Unit): Unit = ()
}

class NonEmpty(elem: Tweet, left: TweetSet, right: TweetSet) extends TweetSet {

    def filterAcc(p: Tweet => Boolean, acc: TweetSet): TweetSet = 
      if  (p(elem)) right.filterAcc(p, left.filterAcc(p, acc.incl(elem)))
      else  right.filterAcc(p, left.filterAcc(p, acc))
  
    def union(that: TweetSet): TweetSet = 
      ((left union right) union that) incl elem
      
       def mostAcc(max: Tweet): Tweet = 
         if (elem.retweets > max.retweets) right.mostAcc(left.mostAcc(elem)) 
         else right.mostAcc(left.mostAcc(max)) 
    
    def descendingByRetweet: TweetList = 
      new Cons(this.mostRetweeted, this.remove(this.mostRetweeted).descendingByRetweet)
      
         
  /**
   * The following methods are already implemented
   */

  def contains(x: Tweet): Boolean =
    if (x.text < elem.text) left.contains(x)
    else if (elem.text < x.text) right.contains(x)
    else true

  def incl(x: Tweet): TweetSet = {
    if (x.text < elem.text) new NonEmpty(elem, left.incl(x), right)
    else if (elem.text < x.text) new NonEmpty(elem, left, right.incl(x))
    else this
  }

  def remove(tw: Tweet): TweetSet =
    if (tw.text < elem.text) new NonEmpty(elem, left.remove(tw), right)
    else if (elem.text < tw.text) new NonEmpty(elem, left, right.remove(tw))
    else left.union(right)

  def foreach(f: Tweet => Unit): Unit = {
    f(elem)
    left.foreach(f)
    right.foreach(f)
  }
}

trait TweetList {
  def head: Tweet
  def tail: TweetList
  def isEmpty: Boolean
  def foreach(f: Tweet => Unit): Unit =
    if (!isEmpty) {
      f(head)
      tail.foreach(f)
    }
}

object Nil extends TweetList {
  def head = throw new java.util.NoSuchElementException("head of EmptyList")
  def tail = throw new java.util.NoSuchElementException("tail of EmptyList")
  def isEmpty = true
}

class Cons(val head: Tweet, val tail: TweetList) extends TweetList {
  def isEmpty = false
}


object GoogleVsApple {
  val google = List("android", "Android", "galaxy", "Galaxy", "nexus", "Nexus")
  val apple = List("ios", "iOS", "iphone", "iPhone", "ipad", "iPad")

    lazy val googleTweets: TweetSet = TweetReader.allTweets.filter(y => google.exists(x => y.text.contains(x)))
  lazy val appleTweets: TweetSet = TweetReader.allTweets.filter(y => apple.exists(x => y.text.contains(x)))
  
  /**
   * A list of all tweets mentioning a keyword from either apple or google,
   * sorted by the number of retweets.
   */
     lazy val trending: TweetList = (googleTweets.union(appleTweets)).descendingByRetweet
  }

object Main extends App {
  // Print the trending tweets
  GoogleVsApple.trending foreach println
}