From 988f4f31486b2b6a121a2da9316736bd0a585f50 Mon Sep 17 00:00:00 2001
From: Juan Marín Noguera <juan@mnpi.eu>
Date: Sat, 7 Dec 2024 18:39:27 +0100
Subject: Day 06

On the importance of not overengineering stuff
---
 day06.scm | 292 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 292 insertions(+)
 create mode 100644 day06.scm

(limited to 'day06.scm')

diff --git a/day06.scm b/day06.scm
new file mode 100644
index 0000000..6b55713
--- /dev/null
+++ b/day06.scm
@@ -0,0 +1,292 @@
+(define-module (day06)
+  #:use-module (ice-9 match)
+  #:use-module (ice-9 rdelim)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-9)
+  #:use-module (srfi srfi-43)
+  #:use-module (srfi srfi-69))
+
+(define-record-type <position> (%make-position x y dir) position?
+                    (x position-x)
+                    (y position-y)
+                    (dir position-dir))
+
+(define (make-position x y dir)
+  (unless (integer? x) (error "position: x should be an integer." x))
+  (unless (integer? y) (error "position: y should be an integer." y))
+  (unless (and (integer? dir) (<= 0 dir 3))
+    (error "position: dir should be an integer between 0 and 3." dir))
+  (%make-position x y dir))
+
+(define* (position-rotate pos #:optional (n 1))
+  (match-let ((($ <position> x y dir) pos))
+    (make-position x y (modulo (+ n (position-dir pos)) 4))))
+
+(define +num-directions+ 4)
+(define +visit-bits+ 15)
+(define +obstacle+ 16)
+(define +unvisited+ 0)
+
+(define (make-map strlist)
+  (define (char->spec c) (if (char=? c #\#) +obstacle+ +unvisited+))
+  (list->typed-array 'u8 2
+                     (map (lambda (str) (map char->spec (string->list str)))
+                          strlist)))
+
+(define (map-obstacle? map x y)
+  (logtest +obstacle+ (array-ref map y x)))
+
+(define (map-dimensions map)
+  (apply values (array-dimensions map)))
+
+(define (map-visited-dir? map pos)
+  (logbit? (position-dir pos)
+           (array-ref map (position-y pos) (position-x pos))))
+
+(define (map-visited? map x y)
+  (logtest +visit-bits+ (array-ref map y x)))
+
+(define (map-visit! map pos)
+  (define prev (array-ref map (position-y pos) (position-x pos)))
+  (when (logtest prev +obstacle+)
+    (error "map-visit!: Cannot visit a place with an obstacle."))
+  (array-set! map (logior prev (ash 1 (position-dir pos)))
+              (position-y pos) (position-x pos)))
+
+(define (rectangle-fold proc init x0 xpast y0 ypast)
+  (let yloop ((y y0) (result init))
+    (if (>= y ypast)
+        result
+        (let xloop ((x x0) (result result))
+          (if (>= x xpast)
+              (yloop (+ y 1) result)
+              (xloop (+ x 1) (proc x y result)))))))
+
+(define (find-guard strlist)
+  (let yloop ((rows strlist) (y 0) (result #f))
+    (if (null? rows)
+        result
+        (let ((row (car rows)))
+          (let xloop ((x 0) (result result))
+            (define (set-guard dir)
+              (define found-guard (make-position x y dir))
+              (if result
+                  (error "There are at least two guards." result found-guard)
+                  (make-position x y dir)))
+            (if (>= x (string-length row))
+                (yloop (cdr rows) (+ y 1) result)
+                (xloop (+ x 1)
+                       (case (string-ref row x)
+                         ((#\# #\.) result)
+                         ((#\^) (set-guard 0))
+                         ((#\>) (set-guard 1))
+                         ((#\v) (set-guard 2))
+                         ((#\<) (set-guard 3))
+                         (else (error "Invalid character in the map."
+                                      x y (string-ref row x)))))))))))
+
+(define* (read-map #:optional (port (current-input-port)))
+  (let loop ((acc '()))
+    (let ((line (read-line port)))
+      (if (or (eof-object? line) (string= "" line))
+          (let ((strlist (reverse! acc)))
+            (values (make-map strlist) (find-guard strlist)))
+          (loop (cons line acc))))))
+
+(define (position-next pos map)
+  (define x (position-x pos))
+  (define y (position-y pos))
+  (define-values (h w) (apply values (array-dimensions map)))
+  (case (position-dir pos)
+    ((0) (if (= y 0) #f (make-position x (- y 1) 0)))
+    ((1) (if (= (+ x 1) w) #f (make-position (+ x 1) y 1)))
+    ((2) (if (= (+ y 1) h) #f (make-position x (+ y 1) 2)))
+    ((3) (if (= x 0) #f (make-position (- x 1) y 3)))))
+
+(define (step-guard map guard)
+  (define next (position-next guard map))
+  (cond ((not next) #f)
+        ((map-obstacle? map (position-x next) (position-y next))
+         (make-position (position-x guard)
+                        (position-y guard)
+                        (modulo (+ 1 (position-dir guard)) 4)))
+        (else next)))
+
+(define* (part1 #:optional (port (current-input-port)))
+  (define-values (mp guard) (read-map port))
+  (let loop ((pos guard))
+    (when (map-visited-dir? mp pos) (error "Loop detected!"))
+    (map-visit! mp pos)
+    (and=> (step-guard mp pos) loop))
+  (define-values (h w) (map-dimensions mp))
+  (rectangle-fold (lambda (x y cnt) (if (map-visited? mp x y) (+ cnt 1) cnt))
+               0 0 h 0 w))
+
+(define (map-add-obstacle! mp x y)
+  (array-set! mp (logior (array-ref mp y x) +obstacle+) y x))
+
+(define (copy-map mp)
+  (define cp (apply make-array 0 (array-dimensions mp)))
+  (array-copy! mp cp)
+  cp)
+
+(define (do-guard! mp pos)
+  (if (map-visited-dir? mp pos)
+      'loop
+      (let ((next (step-guard mp pos)))
+        (map-visit! mp pos)
+        (if next
+            (do-guard! mp next)
+            'exit))))
+
+(define (would-loop? mp initpos x y)
+  (define new-map (copy-map mp))
+  (map-add-obstacle! new-map x y)
+  (eq? (do-guard! new-map initpos) 'loop))
+
+(define* (part2 #:optional (port (current-input-port)))
+  (define-values (mp initpos) (read-map port))
+  (define-values (h w) (map-dimensions mp))
+  (define visits (copy-map mp))
+  (do-guard! visits initpos)
+  (rectangle-fold
+   (lambda (x y cnt)
+     (when (= x 0)
+       (format #t "Parsing line ~a/~a...\n" (+ y 1) h))
+     (if (and (map-visited? visits x y)
+              (not (and (= x (position-x initpos)) (= y (position-y initpos))))
+              (would-loop? mp initpos x y))
+         (+ cnt 1)
+         cnt))
+   0 0 w 0 h))
+
+;;; EVERYTHING BELOW THIS LINE DOESN'T WORK AND IT'S ME TRYING TO BE
+;;; SMART RATHER THAN BRUTE-FORCING THE WHOLE THING
+;; (define-record-type <node> (%make-node tree depth parents loopy?) node?
+;;                     (tree node-tree)
+;;                     (depth node-depth)
+;;                     (parents node-parents)
+;;                     (loopy? node-loopy?))
+
+;; (define* (make-node tree depth parents #:optional (loopy? #f))
+;;   (%make-node tree depth (list->vector parents) loopy?))
+
+;; (define (graph-ref grph pos)
+;;   (array-ref grph (position-y pos) (position-x pos) (position-dir pos)))
+
+;; (define (%graph-set! grph pos node)
+;;   (array-set! grph node (position-y pos) (position-x pos) (position-dir pos)))
+
+;; (define (%parent-list grph acc idx)
+;;   (define plist (node-parents (car acc)))
+;;   (if (< idx (vector-length plist))
+;;       (%parent-list grph (cons (vector-ref plist idx) acc) (+ 1 idx))
+;;       (reverse! acc)))
+
+;; (define* (%child-node grph pnode #:optional (end-loop? #f))
+;;   (make-node (node-tree pnode)
+;;              (+ 1 (node-depth pnode))
+;;              (%parent-list grph (list pnode) 0)
+;;              (and (node-loopy? pnode) (not end-loop?))))
+
+;; (define (%dfs-data! grph mp pos revpath tree)
+;;   (if pos
+;;       (let ((prev (graph-ref grph pos)))
+;;         (case prev                      ; TODO Cleanup
+;;           ((#f)
+;;            (%graph-set! grph pos (make-node tree -1 '() #t))
+;;            (%dfs-data! grph mp (step-guard mp pos) (cons pos revpath) tree))
+;;           (else
+;;            (when (= tree (node-tree prev))
+;;              (%graph-set! grph pos 'start-loop))
+;;            (values revpath
+;;                    (if (null? revpath) prev (%child-node grph prev)))))) ; TODO Cleanup
+;;       (values revpath (make-node tree -1 '() #f))))
+
+;; (define (%dfs-place! grph revpath node)
+;;   ;; (format #t "%dfs-place! ~a (~a,~a)" (length revpath) (node-tree node) (node-depth node))
+;;   (define end-loop? (and (node-loopy? node)
+;;                          (eq? 'start-loop (graph-ref grph (car revpath)))))
+;;   (%graph-set! grph (car revpath) node)
+;;   (unless (null? (cdr revpath))
+;;     (%dfs-place! grph (cdr revpath) (%child-node grph node end-loop?))))
+
+;; (define (map->graph mp)
+;;   (define-values (h w) (map-dimensions mp))
+;;   (define grph (make-array #f h w 4))
+;;   (define (ensure-node! pos tree)
+;;     (define-values (revpath root-node) (%dfs-data! grph mp pos '() tree))
+;;     (unless (null? revpath) (%dfs-place! grph revpath root-node))
+;;     (if (= tree (node-tree root-node)) (+ 1 tree) tree))
+;;   (define* (fill-cell! x y tree #:optional (dir 0))
+;;     (if (= dir 4)
+;;         tree
+;;         (fill-cell! x y (ensure-node! (make-position x y dir) tree) (+ 1 dir))))
+;;   (values grph (rectangle-fold fill-cell! 0 0 w 0 h)))
+
+;; (define (graph-in-bounds? grph pos)
+;;   (array-in-bounds? grph (position-y pos) (position-x pos) (position-dir pos)))
+
+;; (define (node-ancestor child n)
+;;   (if (zero? n)
+;;       child
+;;       (let ((idx (- (integer-length n) 1)))
+;;         (node-ancestor (vector-ref (node-parents child) idx)
+;;                        (logand n (lognot (ash 1 idx)))))))
+
+;; (define (node-ancestor? child ancestor)
+;;   (and (= (node-tree child) (node-tree ancestor))
+;;        (or (node-loopy? ancestor)
+;;            (let ((diff (- (node-depth child) (node-depth ancestor))))
+;;              (and (not (negative? diff))
+;;                   (eq? ancestor (node-ancestor child diff)))))))
+
+;; (define (ends-with-loop? node)
+;;   (or (node-loopy? node)
+;;       (let ((len (vector-length (node-parents node))))
+;;         (and (> len 0)
+;;              (ends-with-loop? (vector-ref (node-parents node) (- len 1)))))))
+
+;; (define (would-loop? grph init block-x block-y)
+;;   (define (position-redirection pos)
+;;     (if (graph-in-bounds? grph pos)
+;;         (let ((node (graph-ref grph pos)))
+;;           (if (node? node)
+;;               (cons node (graph-ref grph (position-rotate pos)))
+;;               #f))
+;;         #f))
+;;   (define redirections
+;;     (filter-map position-redirection
+;;                 (list (make-position block-x (+ block-y 1) 0)
+;;                       (make-position (- block-x 1) block-y 1)
+;;                       (make-position block-x (- block-y 1) 2)
+;;                       (make-position (+ block-x 1) block-y 3))))
+;;   ;; (for-each (lambda (r) (display (cons (cons (node-tree (car r)) (node-depth (car r)))
+;;   ;;                                      (cons (node-tree (cdr r)) (node-depth (cdr r))))))
+;;   ;;           redirections)
+;;   (let iter ((node init) (rounds-left 4))
+;;     (define appliable-redirections
+;;       (filter (lambda (red) (node-ancestor? node (car red))) redirections))
+;;     (define (red< r1 r2)
+;;       (< (modulo (- (node-depth node) (node-depth (car r1))) 1000000000)
+;;          (modulo (- (node-depth node) (node-depth (car r2))) 1000000000)))
+;;     (cond ((null? appliable-redirections) (ends-with-loop? node))
+;;           ((= rounds-left 0) #t)
+;;           (else (iter (cdar (sort appliable-redirections red<))
+;;                       (- rounds-left 1))))))
+
+;; (define* (part2 #:optional (port (current-input-port)))
+;;   (define-values (mp initpos) (read-map port))
+;;   (define-values (h w) (map-dimensions mp))
+;;   (define-values (grph trees) (map->graph mp))
+;;   (format #t "Node preprocessed successfully with ~a trees.\n" trees)
+;;   ;;  (let ((x  (make-array '() h w 4))) (array-map! x (lambda (node) (vector-map (lambda (i n) (cons (node-tree n) (node-depth n))) (node-parents node))) grph) (display x))
+;;   (define initnode (graph-ref grph initpos))
+;;   (define (count-cell x y)
+;;     (if (and (not (and (= x (position-x initpos)) (= y (position-y initpos))))
+;;              (not (map-obstacle? mp x y))
+;;              (would-loop? grph initnode x y))
+;;         (begin (display (cons x y)) 1)
+;;         0))
+;;   (rectangle-fold (lambda (x y cnt) (+ cnt (count-cell x y))) 0 0 h 0 w))
+;; ;;; (1949, 2343)
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