Entropy methods for String

entropy.swift

import Foundation

extension String {
    /// Returns the minimum number of bits required to encode the given string
    func bitFloor() -> Int { Int(entropy().rounded(.up)) }
    
    /// Returns the metric entropy of the given string, as a measure of randomness
    func randomness() -> Double { entropy() / Double(self.count)}
    
    /// Returns the approximate Shannon entropy of the given string
    func entropy() -> Double {
        var freq = [Character: Double]()
        self.map { freq[$0, default: 0] += 1 }
        let prob = Array(freq.values.map { Double($0 / Double(self.count)) })
        let ce = prob.map { Double($0) * log2(Double($0)) / log2(2.0) }
        return Double(abs(ce.reduce(0, +)))
    }
}

assert("2A2D84FD-CAE9-4753-93AC-F1E0BBE14D43".bitFloor() == 4)
assert("DamienStanton".bitFloor() == 4)
assert("abc".bitFloor() == 2)
assert("Bb".bitFloor() == 1)
assert("x".bitFloor() == 0)