[swift-evolution] [Proposal] Random Unification

Jonathan Hull jhull at gbis.com
Thu Jan 11 03:12:35 CST 2018

> On Jan 10, 2018, at 4:42 PM, Nate Cook <natecook at apple.com> wrote:
>> Right. I guess my thought is that I would like them to be able to use a standard creation pattern so it doesn’t vary from type to type (that is the whole point of “unification” in my mind).  In my own code, I have a concept of constraint, of which a set are passed to the object being created. This allows me to random create colors which look good together, etc….  I then have some convenience methods which just automatically create an appropriate constraint from a range where appropriate.  I’d really like to see something standard which allows for constraints other than simple ranges.
> Is it possible for you to share some of this code?

Sure. Small disclaimer that this was originally written back in the Swift 1~2 days, so it is overdue for a simplifying rewrite.

Also, I should point out that the term “Source” has a special meaning in my code.  It basically means that something will provide an ~infinite collection of values of a type T.   I have what I call a “ConstantSource” which just wraps a T and gives it back when asked.  But then I have a bunch of other “sources" which let you create repeating patterns and do deferred calculations and things like that.  Finally I have a “RandomSource” which is part of what started this discussion.  You set up a RandomSource with a set of constraints, and then it gives you random values of T that adhere to those constraints (e.g. colors with a range of hues but the same saturation) whenever you ask for them.

This is really useful for doing things like graphic effects because, for example, I can ask for a source of colors and a source of line widths and then get out a large variety of interesting patterns from the same algorithm.  I can make simple stripes with ConstantSources, or I can make repeating patterns of lines with repeating sources, or I can have random colors which look good together by using a RandomSource.  I can take a BezierPath and make it look hand-drawn by breaking it into a bunch of lines and then offset the points a small amount using a RandomSource of CGVectors.

Not sure how useful this concept of randomness (and pattern) is to others, but I find it immensely useful!  Not sure of the best way to implement it.  The way I do it is a type erased protocol with private conforming structs and then public initializers on the type-erasing box.  The end result is that I can just say:

	let myConst = Source(1) //ConstantSource with 1 as a value
	let myPattern = Source([1, 2]) //OrderedSource which repeats 1, then 2 over and over forever
	let myMeta = Source([myConst, myPattern]) //Will alternate between sub-sources in order. Can be nested.
	//…and so on.

It is quite extensible and can make very complex/interesting patterns very easily.  What I like about it is that (well controlled) random values and patterns or constant values can be interchanged very easily.

The RandomSource has a RandomSourceCreatable Protocol that lets it take random bits and turn them into objects/structs of T adhering to the given constraints.  This is way more complex under the hood than it needs to be, but it works well in practice, and I haven’t gotten around to cleaning it up yet:

	public protocol RandomSourceCreatable {
    	associatedtype ConstraintType = Self
    	///This should be implimented by simple types without internal components
    	static func createRandom(rnd value:RandomSourceValue, constraint:RandomSourceConstraint<ConstraintType>)->Self
	///This should be implimented by complex types with multiple axis of constraints
    	static func createRandom(rnd value:RandomSourceValue, constraints:[String:RandomSourceConstraint<ConstraintType>])->Self
    	///Returns the proper dimension for the type given the constraints
    	static func dimension(given contraints:[String:RandomSourceConstraint<ConstraintType>])->RandomSourceDimension
    	///Validates the given contraints to make sure they can create valid objects. Only needs to be overridden for extremely complex types
    	static func validateConstraints(_ constraints:[String:RandomSourceConstraint<ConstraintType>])->Bool
    	///Convienience method which provides whitelist of keys for implicit validation of constraints
    	static var allowedConstraintKeys:Set<String> {get}

Most of these things also have default implementations so you only really have to deal with them for complex cases like colors or points.  The constraints are given using a dictionary with string keys and a RandomSourceConstraint value, which is defined like this:

	public enum RandomSourceConstraint<T> {
    	case none
    	case constant(T)
    	case min(T)
    	case max(T)
    	case range (T,T)
    	case custom ( (RandomSourceValue)->T )
	//A bunch of boring convenience code here that transforms values so I don’t always have to switch on the enum in other code that deals with this. I just ask for the bounds or constrained T (Note: T here refers to the type for a single axis as opposed to the generated type. e.g. CGFloat for a point) 

I have found that this handles pretty much all of the constraints I need, and the custom constraint is useful for anything exotic (e.g. sig-figs).  The RandomSource itself has convenience inits when T is Comparable that let you specify a range instead of having to create the constraints yourself.

I then have conformed many standard types to RandomSourceCreatable so that I can create Sources out of them.  Here is CGPoint for reference:

	extension CGPoint:RandomSourceCreatable {
    	public static func dimension(given contraints:[String:RandomSourceConstraint<CGFloat>])->RandomSourceDimension {
        	return RandomSourceDimension.manyWord(2)
    	public typealias ConstraintType = CGFloat
    	public static var allowedConstraintKeys:Set<String>{
        	return ["x","y"]
    	public static func createRandom(rnd value:RandomSourceValue, constraints:[String:RandomSourceConstraint<CGFloat>])->CGPoint {
        	let xVal = value.value(at: 0)
        	let yVal = value.value(at: 1)
        	//Note: Ints have a better distribution for normal use cases of points
        	let x = CGFloat(Int.createRandom(rnd: xVal, constraint: constraints["x"]?.asType({Int($0 * 1000)}) ?? .none))/1000
        	let y = CGFloat(Int.createRandom(rnd: yVal, constraint: constraints["y"]?.asType({Int($0 * 1000)}) ?? .none))/1000
        	return CGPoint(x: x, y: y)

Notice that I have a RandomSourceValue type that provides the random bits of the requested dimension. When I get around to updating this, I might do something closer to the proposal, where I would just pass the generator and grab bits as needed.  The main reason I did it the way I did is that it lets me have random access to the source very easily.  

The ‘asType’ method converts a constraint to work with another type (in this case Ints).

Colors are a bit more complicated, mainly because I allow a bunch of different constraints, and I also have validation code to make sure the constraints fit together properly. I also ask for different amounts of randomness based on whether it is greyscale or contains alpha. Just to give you a sense, here are the allowed constraint keys for a CGColor:
	public static var allowedConstraintKeys:Set<String>{
        return ["alpha","gray","red","green","blue", "hue", "saturation", "brightness"]

and here is the creation method when the keys are for RGBA (I have similar sections for HSBA and greyscale):

	let rVal = value.value(at: 0)
    let gVal = value.value(at: 1)
    let bVal = value.value(at: 2)
    let aVal = value.value(at: 3)
    let r = CGFloat.createRandom(rnd: rVal, constraint: constraints["red"] ?? .range(0,1))
    let g = CGFloat.createRandom(rnd: gVal, constraint: constraints["green"] ?? .range(0,1))
    let b = CGFloat.createRandom(rnd: bVal, constraint: constraints["blue"] ?? .range(0,1))
    let a = CGFloat.createRandom(rnd: aVal, constraint: constraints["alpha"] ?? .constant(1.0))
    return self.init(colorSpace: CGColorSpaceCreateDeviceRGB(), components: [r,g,b,a])!

The end result is that initializing a source of CGColors looks like this (either parameter can be omitted if desired):

	let colorSource:Source<CGColor> = Source(seed: optionalSeed, constraints:["saturation": .constant(0.4), "brightness": .constant(0.6)])

Anyway, I hope this was useful/informative.  I know the code is a bit messy, but I still find it enormously useful in practice.  I plan to clean it up when I find time, simplifying the RandomSourceValue stuff and moving from String Keys to a Struct with static functions for the constraints.  The new constraints will probably end up looking like this:

	let colorSource:Source<CGColor> = Source(seed: optionalSeed, constraints:[.saturation(0.4), .brightness(0.4...0.6)])


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