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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift Collections open source project
//
// Copyright (c) 2021 - 2024 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
//
//===----------------------------------------------------------------------===//
public struct RepeatableRandomNumberGenerator: RandomNumberGenerator {
public static var globalSeed: Int {
#if COLLECTIONS_RANDOMIZED_TESTING
42.hashValue
#else
0
#endif
}
// This uses the same linear congruential generator as rand48.
// FIXME: Replace with something better.
internal static let _m: UInt64 = 1 << 48
internal static let _a: UInt64 = 25214903917
internal static let _c: UInt64 = 11
internal var _state: UInt64
public init(seed: Int) {
self.init(seed: UInt64(truncatingIfNeeded: seed))
}
public init(seed: UInt64) {
// Perturb the seed a little so that the sequence doesn't start with a
// zero value in the common case of seed == 0. (Using a zero seed is a
// rather silly thing to do, but it's the easy thing.)
let global = UInt64(truncatingIfNeeded: Self.globalSeed)
_state = seed ^ global ^ 0x536f52616e646f6d // "SoRandom"
}
private mutating func _next() -> UInt64 {
_state = (Self._a &* _state &+ Self._c) & (Self._m - 1)
return _state &>> 16
}
public mutating func next() -> UInt64 {
return (_next() &<< 32) | _next()
}
}
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