FINALLY figured out the solution to puzzle 7

Easy problem once I figured out the trick to constructing an
inherently stateful structure with backwards pointer references (at
least the first way I would've made it) into a functional alist
solution.  Very cool.

2022/puzzle-7.lisp

@@ -9,7 +9,7 @@
           (split-completely (string-to-clist line) #\Space)))
      lines))
 
-(defun token-cmdp (token)
+(defun is-token-cmd? (token)
   "Checks if TOKEN is a command (by the first member)"
   (string= (car token) "$"))
 
@@ -18,65 +18,239 @@
 tokens (including command)."
   (loop
     for token-set on tokens
-    if (token-cmdp (car token-set))
+    if (is-token-cmd? (car token-set))
       return token-set))
 
-#| What is a directory structure? You have some root directory, with
+;;; Functions to manage a directory structure
-some entries.  Your current working directory (CWD), at the start of
+(progn
-the program, will be at the root but later on could be any descendant
+  #| What is a directory structure? You have some root directory, with
-of the root.  How do we manage moving around the directory structure
+  some entries.  Your current working directory (CWD), at the start of
-and ensure updates to the root?
+  the program, will be at the root but later on could be any descendant
+  of the root.  How do we manage moving around the directory structure
+  and ensure updates to the overall structure?
 
-We should maintain a HISTORY stack where
+  We should maintain a HISTORY stack where
-CAR(HISTORY) = (NAME of CWD . State of CWD when entering).
+  CAR(HISTORY) = (NAME of CWD . State of CWD according to parent).
 
-Firstly a function which updates the HISTORY on our current state.
+  So the top of the history stack represents the current directory
-UPDATE-CURRENT-DIRECTORY(CWD, HISTORY) {
+  structure in the form it'll inhabit in its parents.  We need a way to
-CWD-ENTRY = PEEK(HISTORY);
+  update the top of the history stack with the CWD.
-IF NULL(CWD-ENTRY)
-HISTORY = ((NIL . CWD))
-ELSE
-SETF((CDR CWD-ENTRY), CWD);
-}
 
-Say CWD = {... a = directory, b = file, ...}
+  UPDATE-CURRENT-DIRECTORY(CWD, HISTORY) {
-DOWN-DIRECTORY(a, CWD, HISTORY) {
+  CWD-ENTRY = PEEK(HISTORY);
-UPDATE-CURRENT-DIRECTORY(CWD, HISTORY); <-- This is so we don't lose information
+  IF NULL(CWD-ENTRY)
-HISTORY = ACONS(a, CWD@a, HISTORY);
+  HISTORY = ((NIL . CWD))
-CWD     = CWD@a;
+  ELSE
-}
+  SETF((CDR CWD-ENTRY), CWD);
+  }
+  |#
+  (defun update-current-directory (cwd history)
+    "Updates the top of HISTORY (structure of CWD) with CWD i.e. set the
+'state' of the top of the the HISTORY stack to CWD."
+    (if (null (car history))
+        (values
+         cwd
+         `((nil ,@cwd)))
+        (progn
+          (setf (cdr (car history)) cwd)
+          (values
+           cwd
+           history))))
+
+  #|
+  Say CWD = {... a = directory, ...}.  How do I go down to `a`?
+
+  DOWN-DIRECTORY(a, CWD, HISTORY) {
+
+  UPDATE-CURRENT-DIRECTORY(CWD, HISTORY); <-- This is so we don't lose
+  any current information
+  HISTORY = ACONS(a, CWD@a, HISTORY);
+  CWD     = CWD@a;
+  }
+  |#
+  (defun down-directory (name cwd history)
+    (multiple-value-bind (cwd history) (update-current-directory cwd history)
+      (let ((cwd@a (cdr (assoc name cwd :test #'string=))))
+        (values
+         cwd@a
+         (acons name cwd@a history)))))
+
+  #|
+  How do we ascend the directory structure?  We need to account for 3 cases:
+  1) At the root
+  2) Parent is the root
+  3) Parent is any directory
 
 
-Then UP-DIRECTORY(CWD, HISTORY) {
+  Then UP-DIRECTORY(CWD, HISTORY) {
-PAIR   = POP(HISTORY);               <-- (CWD-NAME . PREVIOUS-CWD-STATE)
+  PAIR   = POP(HISTORY);               <-- (CWD-NAME . PREVIOUS-CWD-STATE)
-PARENT = PEEK(HISTORY);              <-- Parent of CWD (PARENT-NAME . PARENT-STATE)
+  if NULL(CAR(PAIR))
-ENTRY = ASSOC(CAR(PAIR) CDR(PARENT)) <-- Gives us the CWD entry in the parent
+  then error(AT ROOT)
-SETF CDR(ENTRY) CWD                  <-- Update the parent
+  else
-CWD = CDR(PARENT)                    <-- Update CWD
+  PARENT = PEEK(HISTORY);              <-- Parent of CWD (PARENT-NAME . PARENT-STATE)
-}
+  ENTRY = ASSOC(CAR(PAIR) CDR(PARENT)) <-- Gives us the CWD entry in the parent
-|#
+  SETF CDR(ENTRY) CWD                  <-- Update the parent
+  CWD = CDR(PARENT)                    <-- Update CWD
+  }
+  |#
+  (defun up-directory (cwd history)
+    (when (null (caar history))
+      ;; At root, so we can't go further up
+      (error "Directory already at root"))
+    ;; We can't use names as in the pseudocode because we need to mutate
+    ;; this specific pointer: Lisp copies by value so we won't be
+    ;; mutating it if we use names.
+    (setf
+     ;; CDR(ENTRY)
+     (cdr
+      ;; ENTRY
+      (assoc
+       ;; CAR(PAIR)
+       (car (car history))
+       ;; CDR(PARENT)
+       (cdr (cadr history))
+       :test #'string=))
+     cwd)
+    (values
+     (cdr (cadr history))
+     (cdr history)))
 
-(defun parse-ls (tokens)
+  #|
-  "Converts the following TOKENS till a command (via TOKEN-CMDP) into a
+  Finally, how do we go from some child all the way back to the root?
+  UP-TO-ROOT(CWD, HISTORY) {
+  IF (NULL (CAAR HISTORY)) <-- Means we're at the root
+  RETURN
+  ELSE
+  UP-DIRECTORY(CWD, HISTORY)
+  UP-TO-ROOT(CWD, HISTORY)
+  }
+  |#
+  (defun up-to-root (cwd history)
+    (if (null (caar history))
+        (values
+         cwd
+         history)
+        (multiple-value-bind (cwd history) (up-directory cwd history)
+          (up-to-root cwd history)))))
+
+;;; Functions to parse the shell output
+(progn
+  (defun parse-ls (tokens)
+    "Converts the following TOKENS till a command (via IS-TOKEN-CMD?) into a
 directory structure i.e. an alist of string names by their content."
-  (loop for token in tokens
+    (loop for token in tokens
-        until (token-cmdp token)
+          until (is-token-cmd? token)
-        collect
+          collect
-        (let ((size (car token))
+          (let ((size (car token))
-              (name (cadr token)))
+                (name (cadr token)))
-          (if (string= size "dir")
+            (if (string= size "dir")
-              `(,name . nil)
+                `(,name . nil)
-              `(,name . ,(parse-integer size))))))
+                `(,name . ,(parse-integer size))))))
 
-(defun parse-tokens (tokens)
+  (defun parse-cd (name cwd history)
-  (if (token-cmdp (car tokens))
+    "Converts TOKEN into an action on CWD and HISTORY then returns them"
-      (cond
+    (cond
-        ((string= (cadar tokens) "ls")
+      ((string= "/" name)
+       (up-to-root cwd history))
+      ((string= ".." name)
+       (up-directory cwd history))
+      (t
+       (down-directory name cwd history))))
+
+  (defun parse-next-command (tokens cwd history)
+    "Parses the next command available in TOKENS, managing CWD and HISTORY.
+Assumes CAR(TOKENS) is a command (see IS-TOKEN-CMD?)"
+    (cond
+      ((null tokens)
+       (multiple-value-bind (cwd history) (up-to-root cwd history)
+         (values
+          tokens
+          cwd
+          history)))
+      ((string= (cadar tokens) "ls")
+       (values
+        (till-next-cmd (cdr tokens))
+        (parse-ls (cdr tokens))
+        history))
+      ((string= (cadar tokens) "cd")
+       (multiple-value-bind (cwd history) (parse-cd (caddar tokens) cwd history)
          (values
           (till-next-cmd (cdr tokens))
-          (parse-ls (cdr tokens))))
+          cwd
-        ((string= (cadar tokens) "cd")
+          history)))))
-         ;; TODO: Actually figure out what I should do here
+
-         (values
+  (defun parse-all-tokens (tokens &optional cwd history)
-          (cdr tokens)
+    "Recursively parse all TOKENS, returning (CWD . HISTORY) from the
-          nil)))))
+perspective of the root."
+    (if (null tokens)
+        (multiple-value-bind (cwd history) (up-to-root cwd history)
+          (values
+           cwd
+           history))
+        (multiple-value-bind (tokens cwd history)
+            (parse-next-command tokens cwd history)
+          (parse-all-tokens tokens cwd history)))))
+
+(defun is-record-directory? (record)
+  "Checks if the record (output from parse-tokens) is a directory by
+checking if the CDR is a list."
+  (listp (cdr record)))
+
+(defun strip-file-names (record)
+  "Deletes file names, leaving the corresponding file sizes and
+directory names (and the wrapping list structure)."
+  (if (is-record-directory? record)
+      (cons (car record)
+            (loop
+              for entry in (cdr record)
+              collect
+              (strip-file-names entry)))
+      (cdr record)))
+
+(defun get-directory-sizes (dir &optional current)
+  "Computes the size of the directory dir, as well as all subdirectories.
+Returns both the overall size of dir as well as an alist of sizes for
+subdirectories."
+  (if (not (listp dir))
+      (values
+       dir
+       nil)
+      (let ((name (car dir))
+            (size (loop
+                    for record in (cdr dir)
+                    sum
+                    (multiple-value-bind (size dir-alist) (get-directory-sizes record nil)
+                      ;; This is for the alist of subdirectory sizes
+                      (setq current (concatenate 'list dir-alist current))
+                      size))))
+        (values
+         size
+         (acons name size current)))))
+
+(defparameter TOTAL-SIZE 70000000)
+(defparameter REQUESTED-SIZE 30000000)
+(multiple-value-bind (_ history)
+    (parse-all-tokens (tokenise-lines lines))
+  ;; Since we're at the root, CAR(history) is the entire file structure
+  (declare (ignore _))
+  (multiple-value-bind (sum size-list)
+      (get-directory-sizes
+       (strip-file-names
+        ;; normalize the name of root
+        (cons "/" (cdr (car history)))))
+    (format t "Round 1: ~s~%"
+            (reduce #'+ (loop for rec in size-list
+                              if (< (cdr rec) 100000) ; only records that are at most 100000
+                                collect (cdr rec))))
+    (let* ((root-size sum)
+           (free-space (- TOTAL-SIZE root-size))
+           (extra-space-req (- REQUESTED-SIZE free-space)))
+      (format t "Round 2: ~s~%"
+              (cdar
+               (sort (loop
+                       for rec in size-list
+                       ;; only records whose size is at least the space required
+                       if (>= (cdr rec) extra-space-req)
+                         collect rec)
+                     (lambda (rec1 rec2)
+                       (< (cdr rec1) (cdr rec2)))))))))