day 21, part 2 backward computation. that's hot

day21.lisp

@@ -33,3 +33,66 @@
   for line in (uiop:read-file-lines filename)
   for definition = (line-to-quoted-operation line)
   do (eval definition)))
+
+(defun reverse-operation (op)
+  (case op
+    ('*  '/)
+    ('/  '*)
+    ('+ '-)
+    ('- '+)))
+
+(defun back-symbol (symb)
+  (intern (format nil "BACK-~a" symb)))
+
+;; adding BACK-symbol computations.
+(defun line-to-quoted-operation-2 (line)
+  (let* ((words (ppcre:split " " (ppcre:regex-replace ":" line "")))
+         (symbols (mapcar #'parse-integer-or-symbol words)))
+    (cond
+      ((= 4 (length symbols))
+       ;; with operation
+       (destructuring-bind (name operand1 op operand2)
+           symbols
+         (if (eq name 'ROOT)
+             `(progn (defparameter ,(back-symbol operand1) '(eval ,operand2))
+                      (defparameter ,(back-symbol operand2) '(eval ,operand1)))
+             (let ((forward-calc
+                     `(defparameter ,name '(,op (eval ,operand1) (eval ,operand2))))
+                   (backward-calc
+                     (case op
+                       ('+ `((defparameter
+                                 ,(back-symbol operand1)
+                               '(- (eval ,(back-symbol name)) (eval ,operand2)))
+                             (defparameter
+                                 ,(back-symbol operand2)
+                               '(- (eval ,(back-symbol name)) (eval ,operand1)))))
+                       ('- `((defparameter
+                                 ,(back-symbol operand1)
+                               '(+ (eval ,(back-symbol name)) (eval ,operand2)))
+                             (defparameter
+                                 ,(back-symbol operand2)
+                               '(- (eval ,operand1) (eval ,(back-symbol name))))))
+                       (t `((defparameter
+                                ,(back-symbol operand1)
+                              '(,(reverse-operation op) (eval ,(back-symbol name)) (eval ,operand2)))
+                            (defparameter
+                                ,(back-symbol operand2)
+                              '(,(reverse-operation op) (eval ,(back-symbol name)) (eval ,operand1)))))
+                       )))
+               `(progn
+                  ,forward-calc
+                  ,@backward-calc
+                  ;; (defparameter ,(innern (format t "BACK~a" name) ))
+                  )))))
+
+      ((= 2 (length symbols))
+       ;; just number
+       (destructuring-bind (name value)
+           symbols
+         `(defparameter ,name ,value))))))
+
+(defun load-file-defs-2 (filename)
+  (loop
+  for line in (uiop:read-file-lines filename)
+  for definitions = (line-to-quoted-operation-2 line)
+  do (eval definitions)))