From a2e892891547cffcb46bbc4732d6eb1cef91113a Mon Sep 17 00:00:00 2001
From: Quint Daenen <me@di-wu.be>
Date: Fri, 19 Nov 2021 13:01:49 +0100
Subject: [PATCH] Add simple xor shifter.

---
 README.md       |  3 +++
 src/LFSR.mo     |  9 ++++-----
 src/XorShift.mo | 27 +++++++++++++++++++++++++++
 3 files changed, 34 insertions(+), 5 deletions(-)
 create mode 100644 src/XorShift.mo

diff --git a/README.md b/README.md
index d72a0c6..01d707d 100644
--- a/README.md
+++ b/README.md
@@ -34,3 +34,6 @@ let (v, _) = feed.next();
 // Iter
 let iter = LFSR.toIter(feed);
 ```
+## XorShift
+
+Reference: [prng](https://vigna.di.unimi.it/ftp/papers/xorshift.pdf).
diff --git a/src/LFSR.mo b/src/LFSR.mo
index 6d45e86..22babe5 100644
--- a/src/LFSR.mo
+++ b/src/LFSR.mo
@@ -1,5 +1,6 @@
 import Array "mo:base/Array";
 import Int "mo:base/Int";
+import IO "mo:io/IO";
 import Iter "mo:base/Iter";
 import Nat8 "mo:base/Nat8";
 import Nat16 "mo:base/Nat16";
@@ -7,8 +8,6 @@ import Nat32 "mo:base/Nat32";
 import Nat64 "mo:base/Nat64";
 import Time "mo:base/Time";
 
-import IO "mo:io/IO";
-
 module {
     public type LFSR<T> = {
         next() : (T, Bool);
@@ -17,7 +16,7 @@ module {
     // An 8-bit linear feedback shift register.
     public class LFSR8(
         s : ?Nat8, // Seed.
-    ) {
+    ) : LFSR<Nat8> {
         private let seed : Nat8 = switch (s) {
             case (null) { nat8(Int.abs(Time.now())); };
             case (? s)  { s;                         };
@@ -39,7 +38,7 @@ module {
     // An 16-bit linear feedback shift register.
     public class LFSR16(
         s : ?Nat16, // Seed.
-    ) {
+    ) : LFSR<Nat16> {
         private let seed : Nat16 = switch (s) {
             case (null) { nat16(Int.abs(Time.now())); };
             case (? s)  { s;                          };
@@ -61,7 +60,7 @@ module {
     // An 32-bit linear feedback shift register.
     public class LFSR32(
         s : ?Nat32, // Seed.
-    ) {
+    ) : LFSR<Nat32> {
         private let seed : Nat32 = switch (s) {
             case (null) { nat32(Int.abs(Time.now())); };
             case (? s)  { s;                          };
diff --git a/src/XorShift.mo b/src/XorShift.mo
new file mode 100644
index 0000000..4f98e23
--- /dev/null
+++ b/src/XorShift.mo
@@ -0,0 +1,27 @@
+import Int "mo:base/Int";
+import IO "mo:io/IO";
+import Nat64 "mo:base/Nat64";
+import Time "mo:base/Time";
+
+module {
+    public type XorShift = {
+        next() : Nat64;
+    };
+
+    public class XorShift64(
+        s : ?Nat64,
+    ) : XorShift {
+        private let seed : Nat64 = switch (s) {
+            case (null) Nat64.fromNat(Int.abs(Time.now()));
+            case (? s)  s;
+        };
+        var state = seed;
+
+        public func next() : Nat64 {
+            state ^= (state >> 12);
+            state ^= (state << 25);
+            state ^= (state >> 27);
+            state * 2685821657736338717;
+        };
+    };
+};