add RNG

Sources/EcgSynKit/SplitMix64.swift

@@ -0,0 +1,21 @@
+
+struct SplitMix64 : RandomNumberGenerator {
+    public typealias State = UInt64
+    public private(set) var state: State
+
+    init(state: UInt64) {
+        self.state = state
+    }
+
+    public mutating func next() -> UInt64 {
+        state &+= 0x9E3779B97F4A7C15
+        var z = state
+        z = (z ^ (z >> 30)) &* 0xBF58476D1CE4E5B9
+        z = (z ^ (z >> 27)) &* 0x94D049BB133111EB
+        return z ^ (z >> 31)
+    }
+
+    public mutating func nextDouble() -> Double {
+        Double(next() >> 11) * 0x1.0p-53
+    }
+}

Sources/EcgSynKit/Xoshiro256Plus.swift

@@ -0,0 +1,63 @@
+import Foundation
+
+public enum Xoshiro256: Equatable {
+    public typealias State = (UInt64, UInt64, UInt64, UInt64)
+
+    internal static var invalidState: State { (0, 0, 0, 0) }
+
+    internal static func isValid(state: State) -> Bool {
+        state != invalidState
+    }
+}
+
+@inlinable
+@inline(__always)
+internal func rotl(_ x: UInt64, _ k: UInt64) -> UInt64 {
+    (x << k) | (x >> (64 &- k))
+}
+
+struct Xoshiro256Plus: RandomNumberGenerator {
+    public typealias State = Xoshiro256.State
+
+    private var state: State
+
+    public init() {
+        var generator = SystemRandomNumberGenerator()
+        self.init(seed: generator.next())
+    }
+
+    public init(seed: UInt64) {
+        var generator = SplitMix64(state: seed)
+        var state = Xoshiro256.invalidState
+
+        repeat {
+            state = (generator.next(), generator.next(), generator.next(), generator.next())
+        } while !Xoshiro256.isValid(state: state)
+
+        self.init(state: state)
+    }
+
+    public init(state: State) {
+        precondition(Xoshiro256.isValid(state: state), "The state must not be zero")
+        self.state = state
+    }
+
+    public mutating func next() -> UInt64 {
+        let result = state.0 &+ state.3
+        let t = state.1 << 17
+
+        state.2 ^= state.0
+        state.3 ^= state.1
+        state.1 ^= state.2
+        state.0 ^= state.3
+
+        state.2 ^= t
+        state.3 = rotl(state.3, 45)
+
+        return result
+    }
+
+    public mutating func nextDouble() -> Double {
+        Float64(next() >> 11) * 0x1.0p-53
+    }
+}