JarCastingcfor: fast java-style for and foreach loop in scala
This tiny project is a cfor macro inspired by the one originally implemented in Typelevel spire library. Supports Scala 2.12 and 2.13 and has zero dependencies.
Example
An index loop:
import io.github.metarank.cfor._
def sum(values: Array[Int]) = {
var result = 0
cfor(0)(_ < values.length, _ + 1) { i => result += values(i) }
result
}
A foreach loop:
import io.github.metarank.cfor._
def sum(values: Array[Int]) = {
var result = 0
cfor(values) { result += _ }
result
}
A range loop:
import io.github.metarank.cfor._
def sum(values: Array[Int]) = {
var result = 0
cfor(0 to values.length by 2) { i => result += values(i) }
result
}
All these cfor calls are expanded into a while loop with no boxing, no extra allocations and with the same performance as a pure java for loop.
Installation
Available on maven central for scala 2.12 and 2.13
libraryDependencies += "io.github.metarank" %% "cfor" % "0.2"
Purpose
In Java, there is a well-known for-loop:
public int sum(int[] values) {
int result = 0;
for (int i=0; i<values.length; i++) {
result += values[i];
}
return result;
}
It can be even simplified with a foreach loop:
public int sum(int[] values) {
int result = 0;
for (int value: values) {
result += value;
}
return result;
}
But the for construct in Scala is not the same thing as in Java and can do much more. The most straigthforward solution will be to have something like this:
// foreach style
def sumForeach(values: Array[Int]) = {
var sum = 0
for { value <- values } { sum += value }
sum
}
// scala collections style
def sum(values: Array[Int]) = values.sum
// old good while loop
def sumWhile(values: Array[Int]) = {
var sum = 0
var i = 0
while(i < values.length) {
sum += values(i)
i += 1
}
sum
}
But nice syntax comes with a price: performance and memory allocations:
[info] Benchmark length) Mode Cnt Score Error Units
[info] scalaForSum 1000 avgt 10 1783.558 ± 47.969 ns/op
[info] scalaSum 1000 avgt 10 5143.344 ± 242.712 ns/op
[info] scalaWhileSum 1000 avgt 10 253.662 ± 2.407 ns/op
[info] Benchmark (length) Mode Cnt Score Error Units
[info] scalaForSum:·gc.alloc.rate.norm 1000 avgt 10 32.001 ± 0.001 B/op
[info] scalaSum:·gc.alloc.rate.norm 1000 avgt 10 12352.002 ± 0.001 B/op
[info] scalaWhileSum:·gc.alloc.rate.norm 1000 avgt 10 ≈ 10⁻⁴ B/op
So if you write a performance-critical code, then you only can use while loops, which are much more verbose that Java for loops.
Here comes a cfor: a yet another syntax sugar macro which expands to a while loop:
def sum(values: Array[Int]) = {
var result = 0
cfor(values) { result += _ }
result
}
It has zero extra allocations and exactly the same performance as a while/for loop.
Performance
The benchmark is done on Scala 2.13.5, AdoptOpenJDK 11.0.10 x64.
[info] Benchmark (length) Mode Cnt Score Error Units
[info] cforForeachSum 1000 avgt 30 246.620 ± 0.836 ns/op
[info] cforSum 1000 avgt 30 248.275 ± 1.001 ns/op
[info] scalaCollectionsSum 1000 avgt 30 5058.811 ± 139.946 ns/op
[info] scalaForeachSum 1000 avgt 30 1769.533 ± 42.385 ns/op
[info] scalaWhileSum 1000 avgt 30 247.857 ± 0.358 ns/op
[info] cforForeachSum:·gc.alloc.rate.norm 1000 avgt 30 ≈ 10⁻³ B/op
[info] cforSum:·gc.alloc.rate.norm 1000 avgt 30 ≈ 10⁻³ B/op
[info] scalaCollectionsSum:·gc.alloc.rate.norm 1000 avgt 30 11813.341 ± 35.878 B/op
[info] scalaForeachSum:·gc.alloc.rate.norm 1000 avgt 30 32.003 ± 0.004 B/op
[info] scalaWhileSum:·gc.alloc.rate.norm 1000 avgt 30 ≈ 10⁻³ B/op
License
This project is released under the Apache 2.0 license, as specified in the LICENSE file.