52 lines
1.2 KiB
Swift
52 lines
1.2 KiB
Swift
//
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// Random.swift
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// NumericAnnex
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//
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// Created by Xiaodi Wu on 5/13/17.
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//
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/// A pseudo-random number generator that implements [`xorshift128+`][ref], an
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/// efficient linear-feedback shift register.
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///
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/// [ref]: http://vigna.di.unimi.it/ftp/papers/xorshiftplus.pdf
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public final class Random : PRNG {
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public var state: (UInt64, UInt64)
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public func next() -> UInt64? {
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#if true
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// An updated version of xorshift128+.
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//
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// This version adds the two halves of the current state, which allows for
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// better parallelization.
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var x = state.0
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let y = state.1
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let result = x &+ y
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state.0 = y
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x ^= x &<< 23
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state.1 = x ^ y ^ (x &>> 18) ^ (y &>> 5)
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return result
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#else
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// A previous version of xorshift128+.
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var x = state.0
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let y = state.1
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state.0 = y
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x ^= x &<< 23
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state.1 = x ^ y ^ (x &>> 17) ^ (y &>> 26)
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return state.1 &+ y
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#endif
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}
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public init(state: (UInt64, UInt64)) {
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self.state = state
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}
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public init?() {
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repeat {
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guard let entropy = Random._entropy(UInt64.self, count: 2) else {
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return nil
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}
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self.state = (entropy[0], entropy[1])
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} while self.state == (0, 0)
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}
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}
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