From 194a1d01e324516dd17133d6bd89d88fbaf68755 Mon Sep 17 00:00:00 2001
From: Peter McGoron <code@mcgoron.com>
Date: Sun, 22 Sep 2024 11:54:57 -0400
Subject: [PATCH] add linked list library

---
 linked-list.scm | 82 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 linked-list.scm

diff --git a/linked-list.scm b/linked-list.scm
new file mode 100644
index 0000000..356b2b2
--- /dev/null
+++ b/linked-list.scm
@@ -0,0 +1,82 @@
+;;; Copyright (C) Peter McGoron 2024
+;;; This program is free software: you can redistribute it and/or modify
+;;; it under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation, version 3 of the License.
+;;; 
+;;; This program is distributed in the hope that it will be useful,
+;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;; 
+;;; You should have received a copy of the GNU General Public License
+;;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Singly linked list with tail pointer, message passing style.
+
+;;; LINKED-LIST-ELEM:
+;;; 
+;;; (NEXT): Goes to the next element, if it exists. Does nothing if the
+;;; iterator is at the end of the list.
+;;; (GET): Gets the element at this list, signals error if at the end
+;;; of the list.
+;;; (EMPTY?): True if there are no more values to go to.
+
+(define linked-list-elem:new
+  (lambda (ptr)
+    (lambda args
+      (let ((op (car args)))
+        (cond
+          ((eq? op 'next)
+           (if (not (null? ptr))
+               (set! ptr (cdr ptr))
+               #f)
+           (null? ptr))
+          ((eq? op 'get) (car ptr))
+          ((eq? op 'empty?) (null? ptr)))))))
+
+;;; LINKED-LIST:
+;;; 
+;;; (PUSH): Pushes a value to the head of the list.
+;;; (PUSH-TAIL): Pushes a value to the end of the list.
+;;; (TO-LIST): Returns a list structure.
+;;; (TRAVERSE-FROM-HEAD): Returns an instance of LINKED-LIST-ELEM pointing
+;;; to the head of the list.
+
+(define linked-list:new
+  (lambda ()
+    (let ((head '())
+          (tail '()))
+      (letrec ((push
+                (lambda (val)
+                  (set! head (cons val head))
+                  (if (null? tail)
+                      (set! tail head)
+                      #f)))
+               (push-tail
+                (lambda (val)
+                  (if (null? tail)
+                      (push val)
+                      (begin
+                        (set-cdr! tail (cons val '()))
+                        (set! tail (cdr tail))))))
+               (%set-cdr!
+                (lambda (val)
+                  (if (null? tail)
+                      (error "cannot set cdr of empty list")
+                      (set-cdr! tail val)))))
+        (lambda args
+          (let ((op (car args)))
+            (cond
+              ((eq? op 'push) (apply push (cdr args)))
+              ((eq? op 'push-tail) (apply push-tail (cdr args)))
+              ((eq? op 'set-cdr!) (apply %set-cdr! (cdr args)))
+              ((eq? op 'to-list) head)
+              ((eq? op 'traverse-from-head) (linked-list-elem:new head))
+              (else (error (cons "invalid operation" args))))))))))
+
+(define x (linked-list:new))
+(x 'push-tail 1)
+(x 'push-tail 2)
+(x 'push-tail 3)
+(x 'to-list)
+