ListBuffer.size is O(n), but ListBuffer.length is O(1)

[scabug]

The length and size methods on ListBuffer differ. The former is overridden in ListBuffer.scala to have an O(1) implementation, but the latter forwards to "underlying", which uses the O(n ) implementation from TraversableOnce.

To reproduce,

def time[A](f: => A): A = {
  val t1 = System.currentTimeMillis
  val ret = f
  val t2 = System.currentTimeMillis
  println("%g secs".format((t2 - t1)/1000.))
  ret
}

val lb = collection.mutable.ListBuffer.tabulate[Int](1000*1000)(identity)

time(for (j <- 0 until 10000) { lb.length })
time(for (j <- 0 until 10000) { lb.size })

I get,

scala> time(for (j <- 0 until 1000) { lb.length })
0.00000 secs

scala> time(for (j <- 0 until 1000) { lb.size })
5.48300 secs

The issue came up on StackOverflow, where Daniel Sobral explained the behavior, http://stackoverflow.com/questions/7061842/what-is-the-running-time-for-size-on-scalas-listbuffer

It would also be nice to update the ScalaDoc for ListBuffer.length and size with performance characteristics.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4933?orig=1 Reporter: Kipton Barros (kbarros) Affected Versions: 2.9.1

[scabug]

Commit Message Bot (anonymous) said: (extempore in r25684) ListBuffer.size should be O(1).

Not O(n) like it was. Here's another good candidate for some mythical performance regression tests. Closes #4933, no review.