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From viirya <...@git.apache.org>
Subject [GitHub] spark pull request #15018: [SPARK-17455][MLlib] Improve PAVA implementation ...
Date Wed, 14 Dec 2016 06:58:38 GMT
Github user viirya commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15018#discussion_r92331409
  
    --- Diff: mllib/src/main/scala/org/apache/spark/mllib/regression/IsotonicRegression.scala
---
    @@ -344,27 +344,30 @@ class IsotonicRegression private (private var isotonic: Boolean)
extends Seriali
         }
     
         var i = 0
    -    val len = input.length
    -    while (i < len) {
    -      var j = i
    -
    -      // Find monotonicity violating sequence, if any.
    -      while (j < len - 1 && input(j)._1 > input(j + 1)._1) {
    -        j = j + 1
    -      }
    +    val n = input.length - 1
    +    var notFinished = true
    +
    +    while (notFinished) {
    +      i = 0
    +      notFinished = false
    +
    +      // Iterate through the data, fix any monotonicity violations we find
    +      // We may need to do this multiple times, as pooling can introduce violations
    +      // at locations that were previously fine.
    +      while (i < n) {
    +        var j = i
    +
    +        // Find next monotonicity violating sequence, if any.
    +        while (j < n && input(j)._1 >= input(j + 1)._1) {
    --- End diff --
    
    I think the original one i.e., `input(j)._1 > input(j + 1)._1` is correct. Here it
is going to select out-of-order blocks.
    
    Quoted from the paper:
    > We refer to two blocks [p, q] and [q + 1, r] as consecutive. We refer to two consecutive
blocks [p, q] and [q +1, r] as in-order if  theta_pq <= theta_q+1, r and out-of-order otherwise.
    
    LEMMA 1 is pointing the how a merged block is also a single-valued block.


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