Spark如何实现PageRank

object SparkPageRank {

 def showWarning() {
   System.err.println(
     """WARN: This is a naive implementation of PageRank and is given as an example!
       |Please use the PageRank implementation found in org.apache.spark.graphx.lib.PageRank
       |for more conventional use.
     """.stripMargin)
 }

 def main(args: Array[String]) {
   if (args.length < 1) {
     System.err.println("Usage: SparkPageRank <file> <iter>")
     System.exit(1)
   }

   showWarning()

   val spark = SparkSession
     .builder
     .appName("SparkPageRank")
     .getOrCreate()

   val iters = if (args.length > 1) args(1).toInt else 10
   val lines = spark.read.textFile(args(0)).rdd
   val links = lines.map{ s =>
     val parts = s.split("\\s+")
     (parts(0), parts(1))
   }.distinct().groupByKey().cache()
   var ranks = links.mapValues(v => 1.0)

   for (i <- 1 to iters) {
     val contribs = links.join(ranks).values.flatMap{ case (urls, rank) =>
       val size = urls.size
       urls.map(url => (url, rank / size))
     }
     ranks = contribs.reduceByKey(_ + _).mapValues(0.15 + 0.85 * _)
   }

   val output = ranks.collect()
   output.foreach(tup => println(s"${tup._1} has rank:  ${tup._2} ."))

   spark.stop()
 }
}