From 7a591384668587a485a7a4eeb710f632d5741e1c Mon Sep 17 00:00:00 2001
From: Felix Martin <mail@felixm.de>
Date: Thu, 17 Dec 2020 16:34:24 -0500
Subject: [PATCH] Implement till 3.15

---
 ex-3_12-xx.scm | 98 +++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 97 insertions(+), 1 deletion(-)

diff --git a/ex-3_12-xx.scm b/ex-3_12-xx.scm
index d55c4ca..c72769b 100644
--- a/ex-3_12-xx.scm
+++ b/ex-3_12-xx.scm
@@ -1,3 +1,99 @@
 (load "util.scm")
 
-(display "\nex-3.12\n")
+(display "\nex-3.12 - append!\n")
+
+(define (append x y)
+  (if (null? x)
+      y
+      (cons (car x) (append (cdr x) y))))
+
+(define (append! x y)
+  (set-cdr! (last-pair x) y)
+  x)
+
+(define (last-pair x)
+  (if (null? (cdr x))
+      x
+      (last-pair (cdr x))))
+
+(define x (list 'a 'b))
+(define y (list 'c 'd))
+(define z (append x y))
+
+(assert z '(a b c d))
+(assert (cdr x) '(b))
+(define w (append! x y))
+(assert w '(a b c d))
+(assert (cdr x) '(b c d))
+
+(display "\nex-3.13 - make cycle\n")
+
+(define (make-cycle x)
+  (set-cdr! (last-pair x) x)
+  x)
+
+(define z (make-cycle (list 'a 'b 'c)))
+
+(display "[see comment]\n")
+; infinite loop
+; (last-pair z)
+
+(display "\nex-3.14 - mystery\n")
+
+; reverse
+(define (mystery x)
+  (define (loop x y)
+    (if (null? x)
+        y
+        (let ((temp (cdr x)))
+          (set-cdr! x y)
+          (loop temp x))))
+  (loop x '()))
+
+(define v (list 'a 'b 'c 'd))
+(define w (mystery v))
+
+(display v) (newline)
+(display w) (newline)
+
+(display "\nex-3.15\n")
+
+(define x (list 'a 'b))
+(define z1 (cons x x))
+(define z2 (cons (list 'a 'b) (list 'a 'b)))
+
+(define (set-to-wow! x)
+  (set-car! (car x) 'wow)
+  x)
+
+(assert z1 '((a b) a b))
+(set-to-wow! z1)
+(assert z1 '((wow b) wow b))
+(set-to-wow! z2)
+(assert z2 '((wow b) a b))
+
+
+(display "\nex-3.16 - count pairs\n")
+
+(define (count-pairs x)
+  (if (not (pair? x))
+      0
+      (+ (count-pairs (car x))
+         (count-pairs (cdr x))
+         1)))
+
+(define r3 '(1 2 3))
+;(define r4 '())
+;(define r7 '())
+;(define r-never '())
+
+(assert (count-pairs r3) 3)
+;(assert (count-pairs r4) 4)
+;(assert (count-pairs r7) 7)
+;(assert (count-pairs r-never) '())
+
+(display "\nex-3.17 - count pairs improved\n")
+
+
+(display "\nex-3.18\n")
+