implemented inverse operation for xoshiro because why not

zephyrtronium · Jun 10, 2019 · 17fab01 · 17fab01
1 parent 39265ec
commit 17fab01
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Showing 2 changed files with 29 additions and 0 deletions.
diff --git a/xoshiro.go b/xoshiro.go
@@ -326,3 +326,19 @@ func (xoshi *Xoshiro) Jump() {
 var xoshiroJump = [4]uint64{
 	0x76e15d3efefdcbbf, 0xc5004e441c522fb3, 0x77710069854ee241, 0x39109bb02acbe635,
 }
+
+// Reverse moves the generator backward one step.
+func (xoshi *Xoshiro) Reverse() {
+	xa := xoshi.x
+	xa ^= xa << 17
+	xa ^= xa << 34
+	ya := xoshi.y
+	ya ^= ya << 17
+	ya ^= ya << 34
+	xoshi.z = bits.RotateLeft64(xoshi.z, 19)
+	xoshi.y = xoshi.w ^ xoshi.x
+	xoshi.w ^= xoshi.z
+	xoshi.x = xa ^ ya
+	xoshi.z ^= xoshi.x
+	xoshi.y ^= xoshi.z
+}
diff --git a/xoshiro_test.go b/xoshiro_test.go
@@ -62,6 +62,19 @@ func TestXoshiCopy(t *testing.T) {
 	}
 }
 
+func TestXoshiReverse(t *testing.T) {
+	xoshi := CryptoSeeded(NewXoshiro(), 32).(*Xoshiro)
+	for i := 0; i < 1024; i++ {
+		a := xoshi.Uint64()
+		xoshi.Reverse()
+		b := xoshi.Uint64()
+		if a != b {
+			t.Fail()
+		}
+		xoshi.Uint64()
+	}
+}
+
 func BenchmarkXoshiro(b *testing.B) {
 	xoshi := CryptoSeeded(NewXoshiro(), 32).(*Xoshiro)
 	f := func(p []byte) func(b *testing.B) {