Add constant across executions variant of hasher

Author: nerdsupremacist

Source/Shared/Extensions/Hasher+constantAccrossExecutions.swift

@@ -0,0 +1,26 @@
+
+import Foundation
+
+extension Hasher {
+    // Stolen from https://github.com/apple/swift/blob/master/stdlib/public/core/SipHash.swift
+    // in order to replicate the exact format in bytes
+    private struct _State {
+      private var v0: UInt64 = 0x736f6d6570736575
+      private var v1: UInt64 = 0x646f72616e646f6d
+      private var v2: UInt64 = 0x6c7967656e657261
+      private var v3: UInt64 = 0x7465646279746573
+      private var v4: UInt64 = 0
+      private var v5: UInt64 = 0
+      private var v6: UInt64 = 0
+      private var v7: UInt64 = 0
+    }
+
+    static func constantAccrossExecutions() -> Hasher {
+        let offset = MemoryLayout<Hasher>.size - MemoryLayout<_State>.size
+        var hasher = Hasher()
+        withUnsafeMutableBytes(of: &hasher) { pointer in
+            pointer.baseAddress!.storeBytes(of: _State(), toByteOffset: offset, as: _State.self)
+        }
+        return hasher
+    }
+}