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Numeric trait for Scala which is specialized and has other improvements.

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README.md

NUMERIC

This package (com.azavea.math.Numeric) is based on scala.math.Numeric, which is a type trait designed to allow abstraction across numeric types (e.g. Int, Double, etc). I (Erik Osheim) began this work as part of the specialized-numeric Scala Incubator project.

This package is a much-improved version. For one, it is way faster, thanks to Scala's specialization system, as well as some restructuring and an optional compiler plugin. It is also more flexible, allowing the user to operate on generic numeric types, concrete numeric types, and literals without requiring ugly casting.

Ultimately my hope is to push these changes back into Scala's library.

REQUIREMENTS

Build using SBT 10. Will port to SBT 11 soon.

EXAMPLES

Basic example of addition:

import com.azavea.math.Numeric
import com.azavea.math.FastImplicits._

def adder[A:Numeric](a:A, b:A) = a + b

Creating a Point3 object with coordinates of any numeric type:

import com.azavea.math.Numeric
import com.azavea.math.FastImplicits._

case class Point3[T:Numeric](x:T, y:T, z:T) {
  def +(rhs:Point3[T]) = Point(x + rhs.x, y + rhs.y, z + rhs.z)

  def distance(rhs:Point3[T]) = {
    val dx = x - rhs.x
    val dy = y - rhs.y
    val dz = z - rhs.z
    val d = dx * dx + dy * dy + dz * dz
    math.sqrt(d.toDouble)
  }
}

Mixing literals and variables:

import com.azavea.math.Numeric
import com.azavea.math.EasyImplicits._
import Predef.{any2stringadd => _, _}

def foo[T:Numeric](a:T, b:T):T = a * 100 + b

Currently there are two different ways to use Numeric: EasyImplicits allows you to operate on mixed numeric types (e.g. T + U + Int). FastImplicits sacrifices this ability in exchange for speed. Both can be made equally fast (as fast as operating on direct types) with the optimized-numeric compiler plugin.

If you plan to use the compiler plugin, or aren't worried about speed, you will probably want to use EasyImplicits.

ANNOYING PREDEF

You may have noticed this ugly-looking import:

import Predef.{any2stringadd => _, _}

This is to work around a design problem in Scala. You may not always need to use this, but if you notice problems with + not working correctly you should add this top-level import. Sorry! :(

PROJECT STRUCTURE

Here is a list of the SBT projects:

  • root: contains the library code itself ("package" builds the library jar)
  • plugin: contains the compiler plugin code ("package" builds the plugin jar)
  • perf: contains the performance test

Use "projects" to view them, and "project XYZ" to switch to XYZ.

USING THE PLUGIN

The optimized-numeric plugin is able to speed things up by rewring certain constructions into other, faster ones. Here's an example:

// written
def foo[T:Numeric](a:T, b:T) = a + b

// compiled
def foo[T](a:T, b:T)(implicit ev:Numeric[T]) = new FastNumericOps(a).+(b)

// compiled with plugin
def foo[T](a:T, b:T)(implicit ev:Numeric[T]) = ev.add(a, b)

In the future scalac might be able to do this for us (or hotspot might be able to optimized it away). But in the absence of these things the plugin helps make Numeric much faster (especially when using EasyImplicits).

At the most basic level, you can add "-Xplugin:path/to/optimized-numeric.jar" to your scalac invocation to compile things with the plugin.

When running the perf project, here are the steps you can take to build and enable the plugin:

  1. in sbt: "project plugin", then "package"
  2. cp plugin/target/scala-2.9.1.final/optimized-numeric-plugin_2.9.1-0.1.jar perf/lib/
  3. uncomment perf/build.sbt line involving -Xplugin
  4. in sbt: "project perf", then "run"

The plugin is enabled by default. To disable the plugin just revert step #3.

BENCHMARKS

To run the benchmarks, do the following:

  1. optionally build and install the plugin
  2. in sbt: "project perf", then "run"

The output shows the speed (in milliseconds) of a direct implementation (without generics), the new implementation (com.azavea.math.Numeric) and the old implementation (the built-in scala.math.Numeric). In some cases there is no old implementation--in those cases the test tries to hem as closely as possible to the direct implementation.

  • n:d is how new compares to direct
  • o:d is how old compares to direct
  • o:n is how old compares to new

It also creates a benchmark.html file which colors the output.

RESULTS

There are some interesting results:

  1. Both scala.math.Numeric and com.azavea.math.Numeric seem to perform worse on integral types than fractional ones. this is very pronounced for scala.math.Numeric and only slight for com.azavea.math.Numeric.

  2. com.azavea.math.Numeric mostly* performs as well as direct implementations except when using infix operators without the compiler plugin. The current Numeric is clearly inappropriate for any application where performance is important.

  3. The asterisk in the previous item has to do with Quicksort. Basically, scala.util.Sorting uses Ordering[A] which is not specialized and which implements all its own (non-specialized) comparison operators in terms of compare().

    This ends up being really slow, so my Numeric trait doesn't extend it, but instead provides a getOrdering() method (which builds a separate Ordering instance wrapping the Numeric instance). As a result, it doesn't perform any better than scala.math.Numeric on this test (and in fact does a bit worse).

    I don't know how likely it is that Ordering will be specialized, but huge performance gains seem possible.

  4. scala.util.Sorting.quickSort lacks a direct Long implementation, so using it with Longs is ~5x slower than Int, Float or Double.

  5. It seems like scala.util.Sorting could use some love. My naive direct implementation of merge sort seems to beat Sorting.quickSort for Long (obviously), Float and Double. That said, optimizing sort algorithms can be tricky. But gains seem possible.

DIFFERENCES

While very similar to scala.math.Numeric, com.azavea.math.Numeric has some differences. The most significant ones are:

  1. It does not inherit from the Ordering type class, but rather directly implements the comparison methods. I will try to do some cleanup on this and make it more compatible with Ordering, but it was important to me that the comparison methods are also specialized.

  2. It does not implement Integral/Fractional. I think that leaving division/modulo off of Numeric is a mistake, and don't think that forcing users to use Integral/Fractional is a good idea. Given that Scala uses the same symbol (/) to mean both "integer division" and "true division" it seems clear that Numeric can too.

  3. It's in a different package.

  4. It adds some operators that I thought would be nice to have:

    • <=> as an alias for compare
    • === as an alias for equiv
    • !== as an alias for !equiv
    • ** as an alias for math.pow
  1. It adds a full-suite of conversions. Unlike the existing Numeric, you can convert directly to/from any numeric type. This is useful since you might be going from a generic type to a known type (e.g. A -> Int) or a known type to a geneirc one (Int -> A). In both cases it is important not to do any unnecessary work (when A is an Int, you should not have to do any copying/casting).

    These conversions can be used from the instance of Numeric[T] or directly on the values of T themselves:

    def foo[T:Numeric](t:T):Double = {
        val i:Int = numeric.toInt(t)
        val d:Double = t.toDouble
        d - i
    }
    

CAVEATS

This section is just for "known problems" with the Numeric type class approach in Scala.

Precision

Given the signatures of Numeric's functions, it's possible to mix literals and generic types in a way which will lose precision. Consider:

def foo[T:Numeric](t:T) = t + 9.2

The author might expect that foo[Int](5) will return a Double (14.2), but in fact foo[T] returns T and so Foo[Int] will return an Int (14). The solution in cases like this (if you know you want a double) is to convert T to a Double first:

 def foo[T:Numeric](t:T) = t.toDouble + 9.2

You could imagine that + could "figure out" whether T has more precision than Double (e.g. BigDecimal) or less precisoin (e.g. Int) and "do the right thing". Sadly, this is not possible using the current strategy.

If you are interested in implementing a numeric tower in Scala (presumably by writing some pretty intense compiler plugins if not modifying Scala's type system) please get in touch with the author. :)

Clunky syntax

Unfortunately in order to benefit from the speed of this library you must annotate all your numeric with @specialization. Also, you often need a manifest (for instance when you want to allocate Arrays of that type), so you have to provide those too. Your code will end up looking similar to this:

def handleInt(a:Int) = ...

def handleA[@specialized A:Numeric:Manifest](a:A) = ...

When you compare these visually the second is obviously terrible. In many of the examples I have omitted the @specialized annotation and the Manifest type bound for clarity. Hopefully this is not too deceptive.

It would be great if there was some way to create a type bound that "included" specialization. For instance:

// written
def bar[T:SNumeric](a:T, b:T, c:T) = a * b + c

// compiled
def bar[@specialized(Int,Long,Float,Double) T:Numeric](a:T, b:T, c:T) = a * b + c

The current design of specialization works well when a library author expects users to call her generic functions with concrete types. But if her users are themselves defining generic functions, the library author is powerless to help.

Obviously, this could pose real problem in terms of a bytecode explosion, which is why it should not be a default behavior. In fact, Numeric may be one of the few places where this behavior would be desirable.

Non-extensibility

While you should be able to implement your own instances of Numeric for user-generated types (e.g. Complex or BigRational) you will not be able to use the full power of this library due to the need to add methods to the ConvertableFrom and ConvertableTo traits.

This is unfortunate, and I'm is exploring a pluggable numeric conversion system. However, in the interested of getting a fast, working implementation out there, I have shelved that for now.

Lack of range support

I am still working on implementing a corresponding NumericRange. For now you'll need to convert to a known type (e.g. Long), or use while loops. This is definitely possible and will hopefuly be done soon.

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