From b20c05d6dfc962b6e9b4c868b1236a750148cfe6 Mon Sep 17 00:00:00 2001
From: Felix Martin <mail@felixm.de>
Date: Wed, 13 Jan 2021 04:23:34 -0500
Subject: [PATCH] Implement 3.81

---
 ex-3_73-82.scm | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/ex-3_73-82.scm b/ex-3_73-82.scm
index 9d6d201..764c1a3 100644
--- a/ex-3_73-82.scm
+++ b/ex-3_73-82.scm
@@ -148,7 +148,38 @@
 (define RLC1 (RLC 1 1 0.2 0.1))
 (display "[ok]\n")
 
-(display "\nex-3.81\n")
+(display "\nex-3.81 - random-stream\n")
+
+(define (make-random-stream request-stream)
+  (define a 1664525)
+  (define c 1013904223)
+  (define m (expt 2 32))
+  (define new-random-stream
+    (cons-stream
+      0
+      (stream-map (lambda (x) (modulo (+ (* a x) c) m))
+                  new-random-stream)))
+  (define (result-stream request-stream random-stream)
+    (cond
+      ((eq? (stream-car request-stream) 'gen)
+       (cons-stream (stream-car random-stream)
+                    (result-stream (stream-cdr request-stream)
+                                   (stream-cdr random-stream))))
+      ((eq? (stream-car request-stream) 'reset)
+       (result-stream (stream-cdr request-stream) new-random-stream))
+      (else (error "Unsupported request"))))
+  (result-stream request-stream new-random-stream))
+
+(define requests
+  (cons-stream
+    'gen (cons-stream 'gen (cons-stream 'gen (cons-stream 'reset (cons-stream 'gen
+            (cons-stream 'gen (cons-stream 'gen requests))))))))
+
+(define rs (make-random-stream requests))
+
+(assert (take 10 rs)
+        (take 10 rs))
 
 (display "\nex-3.82\n")
 
+