various updates

README.rst

@@ -7,6 +7,7 @@ topics.
 Projects
 --------
 
++ ``adsep/``: `Algorithms + Data Structures = Programs <https://en.wikipedia.org/wiki/Algorithms_%2B_Data_Structures_%3D_Programs>`_
 + ``blue-pill/``: blue pill experiments
 + ``lpn/``: `Learn Prolog Now <http://lpn.swi-prolog.org/>`_
 + ``misc/``: A scratchpad of random programs; a replacement for

misc/turing.rkt

@@ -1,18 +1,33 @@
 #lang racket
 
-;;; a turing machine implementation
-(define tape (make-hash))
+(define instruction
+  '(("B" "s1") . ("X" "R" "s2")))
 
-(define (lookup position)
+;;; The tape utility functions should hide the actual tape
+;;; representation.
+(define (tape-lookup tape position)
   (unless (hash-has-key? tape position)
-    (hash-set! tape position 0))
+    (hash-set! tape position "B"))
   (hash-ref tape position))
 
-(define turing-machine%
-  (class object%
-    (init)
-    (define tape (make-hash))
-    (define head 0)
-    (define current-state #f)
-    (define state-table)
-    (super-new)))
+(define (tape-write tape position sym)
+  (hash-set! tape position sym))
+
+(define (head-update head head_dir)
+  (cond
+   [(equal? head_dir "R") (+ head 1)]
+   [(equal? head_dir "L") (- head 1)]
+   [else (error "invalid head direction")]))
+
+(define (turing-update tape head state instruction)
+  (tape-write tape head (car instruction))
+  (values tape
+	  (head-update head (car (cdr instruction)))
+	  (car (cdr (cdr instruction)))))
+
+(define (turing-instruction tape instructions)
+  (for (instruction instructions)
+    ;; does the car match the tape and the head position?
+    ;; if it does, break the loop and return the instruction
+    ;; keep going
+    ))