From 3e50d2999949f4c7fa807fe289fb0b0d1a0a4512 Mon Sep 17 00:00:00 2001
From: Thomas Voss <mail@thomasvoss.com>
Date: Mon, 16 Dec 2024 20:16:29 +0100
Subject: Add 2024 day 16 solutions

---
 2024/16/heap.lisp | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)
 create mode 100644 2024/16/heap.lisp

(limited to '2024/16/heap.lisp')

diff --git a/2024/16/heap.lisp b/2024/16/heap.lisp
new file mode 100644
index 0000000..62d36fb
--- /dev/null
+++ b/2024/16/heap.lisp
@@ -0,0 +1,59 @@
+(defpackage #:heap
+  (:use :cl)
+  (:export :dequeue :emptyp :enqueue :make-heap))
+
+(in-package #:heap)
+
+(defun make-heap (&key (priority-function #'identity))
+  (cons (make-array 0 :fill-pointer t) priority-function))
+
+(defun enqueue (item heap)
+  (let ((heap-vec (car heap)))
+    (vector-push-extend item heap-vec)
+    (%sift-down heap 0 (1- (length heap-vec)))))
+
+(defun dequeue (heap)
+  (let* ((heap-vec (car heap))
+         (last-item (vector-pop heap-vec)))
+    (if (zerop (length heap-vec))
+        last-item
+        (prog1
+            (aref heap-vec 0)
+          (setf (aref heap-vec 0) last-item)
+          (%sift-up heap 0)))))
+
+(defun emptyp (heap)
+  (zerop (length (car heap))))
+
+(defun %sift-down (heap start-pos pos)
+  (let* ((heap-vec (car heap))
+         (heap-fn (cdr heap))
+         (new-item (aref heap-vec pos)))
+    (loop while (> pos start-pos) do
+      (let* ((parent-pos (ash (1- pos) -1))
+             (parent (aref heap-vec parent-pos)))
+        (unless (< (funcall heap-fn new-item)
+                   (funcall heap-fn parent))
+          (loop-finish))
+        (setf (aref heap-vec pos) parent)
+        (setq pos parent-pos)))
+    (setf (aref heap-vec pos) new-item)))
+
+(defun %sift-up (heap pos)
+  (let* ((heap-vec (car heap))
+         (heap-fn (cdr heap))
+         (end-pos (length heap-vec))
+         (start-pos pos)
+         (new-item (aref heap-vec pos))
+         (child-pos (1+ (* 2 pos))))
+    (loop while (< child-pos end-pos) do
+      (let ((right-pos (1+ child-pos)))
+        (when (and (< right-pos end-pos)
+                   (>= (funcall heap-fn (aref heap-vec child-pos))
+                       (funcall heap-fn (aref heap-vec right-pos))))
+          (setq child-pos right-pos))
+        (setf (aref heap-vec pos) (aref heap-vec child-pos))
+        (setq pos child-pos
+              child-pos (1+ (* 2 pos)))))
+    (setf (aref heap-vec pos) new-item)
+    (%sift-down heap start-pos pos)))
\ No newline at end of file
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