Finished round 1 of puzzle 9

Pretty interesting, functional idioms kinda fail here as there are no
nice functions I can think of for the single-knot-execute-line
`dotimes` section.  I could use tail recursion with an optimisation
but this is pretty explicit.  You can compare execute-lines to
execute-line to see which type of iteration fits you.

2022/puzzle-9.lisp

@@ -0,0 +1,85 @@
+(load "lib")
+(defparameter input (uiop:read-file-string "9-input"))
+(defparameter lines (get-lines input))
+
+(defun move (P X Y)
+  (cons (+ (car P) X) (+ (cdr P) Y)))
+
+(defun -1unit (x)
+  (if (< x 0)
+      (+ x 1)
+      (- x 1)))
+
+(defun update-tail (TAIL HEAD)
+  (let ((xd (- (car HEAD) (car TAIL)))
+        (yd (- (cdr HEAD) (cdr TAIL))))
+    (if (and (<= (abs xd) 1) (<= (abs yd) 1))
+        ;; Leave as is
+        TAIL
+        (cond
+          ;; Diagonal movements
+          ((and (= 1 (abs xd)) (= 2 (abs yd)))
+           (move TAIL xd (-1unit yd)))
+          ((and (= 1 (abs yd)) (= 2 (abs xd)))
+           (move TAIL (-1unit xd) yd))
+          ;; Linear movements on an axis
+          ((= (abs xd) 2)
+           (move TAIL (-1unit xd) 0))
+          ((= (abs yd) 2)
+           (move TAIL 0 (-1unit yd)))))))
+
+(defun move-direction (direction P)
+  (case direction
+    (R (move P 1 0))
+    (L (move P -1 0))
+    (U (move P 0 1))
+    (D (move P 0 -1))))
+
+(defun print-grid (head tail)
+  (destructuring-bind (hx . hy) head
+    (destructuring-bind (tx . ty) tail
+      (loop for x from 0 to 6
+            do
+               (format t "-"))
+      (format t "~%")
+      (loop for y from 6 downto 0
+            do
+               (progn
+                 (format t "~d: " y)
+                 (loop for x from 0 to 6
+                       do
+                          (format t "~s"
+                                  (cond
+                                    ((and (= hx x) (= hy y)) 'H)
+                                    ((and (= tx x) (= ty y)) 'T)
+                                    (t '-))))
+                 (format t "~%"))))))
+
+(defun parse-line (line)
+  (cons (intern (subseq line 0 1))
+        (parse-integer (subseq line 2))))
+
+(defun coords-eq? (x y)
+  (and (= (car x) (car y)) (= (cdr x) (cdr y))))
+
+(defun single-knot-execute-line (line head tail points)
+  (destructuring-bind (direction . magnitude) (parse-line line)
+    (dotimes (x magnitude)
+      (setq head (move-direction direction head))
+      (setq tail (update-tail tail head))
+      (setq points (adjoin tail points :test #'coords-eq?))))
+  (values head tail points))
+
+(defun single-knot-execute-lines (lines &optional head tail points)
+  (if (null lines)
+      (values
+       head tail points)
+      (let ((line (car lines))
+            (head (or head `(0 . 0)))
+            (tail (or tail `(0 . 0)))
+            (points (or points nil)))
+        (multiple-value-bind (head tail points) (single-knot-execute-line line head tail points)
+          (single-knot-execute-lines (cdr lines) head tail points)))))
+
+(format t "round 1: ~s~%"
+        (length (car (last (multiple-value-list (single-knot-execute-lines lines))))))