From d1e05629a458bc56da70c6d86dd1e2647ba07b6e Mon Sep 17 00:00:00 2001
From: Jacopo De Simoi <wilderjds@protonmail.com>
Date: Sun, 30 Mar 2025 23:06:19 -0400
Subject: [PATCH] And take care of number 9 too

---
 p9/p9.org | 107 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)
 create mode 100644 p9/p9.org

diff --git a/p9/p9.org b/p9/p9.org
new file mode 100644
index 0000000..d0e51f7
--- /dev/null
+++ b/p9/p9.org
@@ -0,0 +1,107 @@
+#+title: Solution of 2024-p9
+
+Now, this was a bit more challenging than the others
+
+This binds ~data~ to the test input
+#+begin_src emacs-lisp
+  (setq data "2333133121414131402")
+#+end_src
+
+This instead binds ~data~ to the actual input
+#+begin_src emacs-lisp
+ (with-temp-buffer
+     (insert-file-contents "input")
+     (goto-char (point-min))
+     (replace-regexp "^" "\"")
+     (goto-char (point-min))
+     (replace-regexp "$" "\"")
+     (goto-char (point-min))
+     (insert "(setq data ")
+     (goto-char (point-max))
+     (insert ")")
+     (eval-buffer))
+#+end_src
+
+#+RESULTS:
+
+This creates a index of the disk: each element is ~(file index
+. length)~, where ~file index~ is ~nil~ if the node is empty
+#+begin_src emacs-lisp
+(setq disk-index (-map-indexed (lambda (index el)
+                               (if (eq 0 (% index 2))
+                                   (cons (/ index 2) el)
+                                 (cons nil el)))
+                             (-map #'string-to-number (split-string data "\\|.+" t))) )
+#+end_src
+
+This creates the disk image from the index
+#+begin_src emacs-lisp
+  (defun disk-from-index (d-index)
+    (-mapcat (lambda (x) (-repeat (cdr x) (car x))) d-index))
+
+  (setq disk (disk-from-image disk-index))
+#+end_src
+* Part 1:
+**  The recursive approach
+This works for the test input but stack overflows for the actual input
+since elisp does not have tail call optimization
+#+begin_src emacs-lisp
+(defun defrag (disk)
+  (let ((i (-find-index #'not disk))
+        (j (-find-last-index #'identity disk))        )
+    (if (> i j) disk
+      (defrag (-replace-at j nil (-replace-at i (nth j disk) disk))))))
+#+end_src
+** The while-loop approach
+This works, but is quite horrible
+#+begin_src emacs-lisp
+(defun defrag-loop (disk)
+  (let ((i (-find-index #'not disk))
+        (j (-find-last-index #'identity disk))        )
+    (while (<= i j)
+      (setq disk (-replace-at j nil (-replace-at i (nth j disk) disk)))
+      (setq i (-find-index #'not disk)
+            j (-find-last-index #'identity disk))))
+  disk  )
+#+end_src
+** The reduce-with-a-copy approach
+#+begin_src emacs-lisp
+  (defun defrag-reduce (disk)
+    (-reduce-r-from (lambda (el acc)
+                     (if (not el) acc
+                       (let ((i (-find-index #'not acc))
+                             (j (-find-last-index #'identity acc)))
+                         (if (> i j) acc
+                           (-replace-last el nil (-replace-at i el acc))))))
+                   disk disk))
+#+end_src
+And, compute!
+#+begin_src emacs-lisp
+  (-reduce #'+ (-map-indexed (lambda (i file) (* i file)) (-non-nil (defrag-reduce disk))))
+
+  6241633730082
+
+#+end_src
+
+* Part 2: defragging whole files
+This is fun, now we start from the index
+#+begin_src emacs-lisp
+  (defun defrag-index (d-index)
+    (-reduce-r-from (lambda (el acc)
+                     (if (not (car el)) acc
+                       (let ((i (-find-index (lambda (ell)
+                                               (and (not (car ell)) (>= (cdr ell) (cdr el)))) acc))
+                             (j (-find-last-index (lambda (e) (equal e el)) acc)))
+                         (if (or (not i) (> i j)) acc
+                           (-insert-at i el (-replace-at i (cons nil (- (cdr (nth i acc)) (cdr el))) (-replace-last el (cons nil (cdr el)) acc)))))))
+                   d-index d-index))
+#+end_src
+And compute
+#+begin_src emacs-lisp
+  (-reduce #'+ (-map-indexed (lambda (i file) (if file (* i file) 0)) (disk-from-index (defrag-index disk-index))))
+#+end_src
+
+#+RESULTS:
+: 6265268809555
+
+**