Last active
February 19, 2019 23:09
-
-
Save erica/627531342c0394205219 to your computer and use it in GitHub Desktop.
Probability-Weighted Enumeration
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
I have an enum, and I am trying to randomly return one of the members of the enum where each member can be given a frequencey of how often it is chosen (e.g. a member with a percentage of 0.05 would be returned 5% of the time). | |
I will post my best solution below, but I am hoping someone else has a better swift-2-ier way of doing it (e.g. filter/map/reduce, etc...), or a simpler solution. | |
Some notes: | |
ListableEnum is a protocol which adds the allMembers() function in an extension (it returns an array of all members) | |
init?(index:) is required by the ListableEnum to work its magic | |
random() is added to Double via extension and it returns a Double between 0 and 1 (inclusive) | |
I make an assumption that the frequencies add to 1.0 | |
See https://forums.developer.apple.com/message/11944 |
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
import UIKit | |
enum MyEnumeration : Int { | |
case A = 10, B = 85, C = 5 | |
// Raw values per member | |
static let memberValues = stride(from: 0, through: 100, by: 1).filter{ | |
MyEnumeration(rawValue: $0) != nil} | |
// Sum of raw values | |
static let memberSum = memberValues.reduce(0, combine: +) | |
// Multiplier offset from a true 1.0 sum | |
static let memberMultiplier = 1.0 / Double(memberSum) | |
// Members -- sorted by raw values | |
static let members = memberValues.flatMap{MyEnumeration(rawValue: $0)} | |
// Expected frequency per member based on percent distribution | |
static let expectedFrequencies = zip(members, memberValues.map{Double($0) * memberMultiplier}).map{($0, $1)} | |
// Return a probability-weighted member | |
static var randomMember : MyEnumeration { | |
var values = ArraySlice(memberValues) | |
var roll = Double(memberSum) * (Double(arc4random()) / Double(UINT32_MAX)) // weight by sum | |
repeat { | |
guard let first = values.first else {fatalError()} | |
guard let firstItem = MyEnumeration(rawValue: first) else {fatalError()} | |
if roll < Double(first) {return firstItem} | |
roll -= Double(first); values = dropFirst(values) | |
} while !values.isEmpty | |
guard let defaultEnumeration = MyEnumeration(rawValue: 0) else {fatalError()} | |
return defaultEnumeration | |
} | |
} | |
// Monte Carlo the results | |
var aCount = 0; var bCount = 0; var cCount = 0 | |
let num = 1000 | |
for _ in 0...num { | |
switch MyEnumeration.randomMember { | |
case .A: aCount++ | |
case .B: bCount++ | |
case .C: cCount++ | |
} | |
} | |
MyEnumeration.members // .C, .A, .B, not in expected order | |
print("Expected ", false) | |
print(MyEnumeration.expectedFrequencies) | |
print("Actual ", false) | |
print(zip(MyEnumeration.members, [cCount, aCount, bCount].map{Double($0) / Double(num)}).map{($0, $1)}) |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment