8000 Implement ListBuffer.isEmpty / nonEmpty / head efficiently · scala/scala@43f194d · GitHub
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jrudolphjrudolph
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Implement ListBuffer.isEmpty / nonEmpty / head efficiently
Uses the extra length information in comparison to list to provide efficient implementations. Evaluating these three methods turns up with about 6-7% of akka-http message parsing. From the profiles it looks the likely reason are polymorphic invokeinterface calls against List (i.e. just forwarding to `start.isEmpty` etc wouldn't help).
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‎src/library/scala/collection/mutable/ListBuffer.scala

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@@ -119,6 +119,11 @@ final class ListBuffer[A]
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// Don't use the inherited size, which forwards to a List and is O(n).
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override def size = length
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// Override with efficient implementations using the extra size information available to ListBuffer.
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override def isEmpty: Boolean = len == 0
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override def nonEmpty: Boolean = len > 0
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override def head: A = if (len > 0) start.asInstanceOf[::[A]].head else Nil.head
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// Implementations of abstract methods in Buffer
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override def apply(n: Int): A =

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