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=act improve ByteString#take(...) (#21438)

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Currently, we use ByteStringBuilder to create a new ByteString instance,
which would not be quite efficient.

Instead of doing this, we can do as follows so that we can achieve better performance:
1. Seek the index of _last_ vector element we need to _take_
2. Find the number of characters left to take from the _last_ ByteString1 element.
3. Create ByteString based on the information we obtained from 1 and 2

Then we just need to create a new Vector[ByteString1] at most twice, which should be
better than the current implementation, i.e., _append_ a new element every time we check
bytestrings(Vector[ByteString1]) element, which ends up O(N) _append_ execution where _N_ is
the length of bytestrings.
This commit is contained in:
monkey-mas 2016-09-22 12:40:54 +09:00 • committed by Konrad Malawski
parent 94d7237d17
commit f8f8828451
2 changed files with 37 additions and 13 deletions
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34
akka-actor/src/main/scala/akka/util/ByteString.scala
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@ -249,8 +249,11 @@ object ByteString {
}
override def take(n: Int): ByteString =
if (n <= 0) ByteString.empty
else ByteString1(bytes, startIndex, Math.min(n, length))
if (n <= 0) ByteString.empty else take1(n)
private[akka] def take1(n: Int): ByteString1 =
if (n >= length) this
else ByteString1(bytes, startIndex, n)
override def slice(from: Int, until: Int): ByteString =
drop(from).take(until - Math.max(0, from))
@ -432,18 +435,23 @@ object ByteString {
bytestrings.foreach(_.writeToOutputStream(os))
}
override def take(n: Int): ByteString = {
@tailrec def take0(n: Int, b: ByteStringBuilder, bs: Vector[ByteString1]): ByteString =
if (bs.isEmpty || n <= 0) b.result
else {
val head = bs.head
if (n <= head.length) b.append(head.take(n)).result
else take0(n - head.length, b.append(head), bs.tail)
}
override def take(n: Int): ByteString =
if (n <= 0) ByteString.empty
else if (n >= length) this
else take0(n, ByteString.newBuilder, bytestrings)
else take0(n)
private[akka] def take0(n: Int): ByteString = {
@tailrec def go(last: Int, restToTake: Int): (Int, Int) = {
val bs = bytestrings(last)
if (bs.length > restToTake) (last, restToTake)
else go(last + 1, restToTake - bs.length)
}
val (last, restToTake) = go(0, n)
if (last == 0) bytestrings(last).take(restToTake)
else if (restToTake == 0) new ByteStrings(bytestrings.take(last), n)
else new ByteStrings(bytestrings.take(last) :+ bytestrings(last).take1(restToTake), n)
}
override def dropRight(n: Int): ByteString =
@ -469,7 +477,7 @@ object ByteString {
override def drop(n: Int): ByteString =
if (n <= 0) this
else if (n > length) ByteString.empty
else if (n >= length) ByteString.empty
else drop0(n)
private def drop0(n: Int): ByteString = {