Day 5 Part 2: Missing code

chicken.org

@@ -2068,24 +2068,27 @@ almanac. *What is the lowest location number that corresponds to any of the init
 
 *** Puzzle Solution
 
-All that is needed is to "expand" the seed numbers before continuing:
+All that is needed is to "expand" the seed numbers before continuing. Since we are dealing with such
+a huge number of seeds, I use ~delay-force~ to delay calculation of the later ones until they're
+actually needed. 
 
 #+NAME: day5-part2-expand-seeds
 #+begin_src scheme :tangle day5.scm
-  (define (expand-seed start size)
-    (let ((max (fx- (fx+ start size) 1)))
-      (do ((seedlist (list start)
-                     (cons (fx+ (car seedlist) 1)
-                           seedlist)))
-          ((fx< (fx- max 2) (car seedlist))
-           seedlist))))
+(define (expand-seed existing-seeds start size)
+  (let ((max (- (+ start size) 1)))
+    (do ((seedlist (cons start existing-seeds)
+                   (cons (+ (car seedlist) 1)
+                         seedlist)))
+        ((< (- max 2) (car seedlist))
+         seedlist))))
 
-  (define (expand-seeds seed-nums)
-    (if (fx<= 2 (length seed-nums))
-        (append
-         (expand-seed (car seed-nums) (cadr seed-nums))
-         (expand-seeds (cddr seed-nums)))
-        '()))
+(define (expand-seeds seed-nums #!optional (existing-seeds '()))
+  (if (< 2 (length seed-nums))
+      (expand-seeds (cddr seed-nums)
+                    (delay-force (expand-seed existing-seeds
+                                              (car seed-nums)
+                                              (cadr seed-nums))))
+      (expand-seed existing-seeds (car seed-nums) (cadr seed-nums))))
 #+end_src
 
 After that, it is /almost/ identical to part 1, but we have to replace the ~foldl~ with a recursive