From cd6a52f02a3c3d7b8e2565c3199174ba1625e899 Mon Sep 17 00:00:00 2001
From: lamp
Date: Mon, 11 Dec 2023 14:07:09 +0000
Subject: add day 9 solutions

---
 9.scm | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)
 create mode 100755 9.scm

(limited to '9.scm')

diff --git a/9.scm b/9.scm
new file mode 100755
index 0000000..1f1eaec
--- /dev/null
+++ b/9.scm
@@ -0,0 +1,31 @@
+#!/usr/bin/env -S guile -s
+!#
+(use-modules (ice-9 textual-ports)
+             (srfi srfi-1))
+
+(define (read-lines port)
+  (letrec ((loop (lambda (l ls)
+                   (if (eof-object? l)
+                       ls
+                       (loop (get-line port) (cons l ls))))))
+    (reverse (loop (get-line port) '()))))
+
+(define (split-up nums)
+  (map string->number (filter (lambda (x) (not (string-null? x))) (string-split nums char-set:whitespace))))
+
+(define (seq-diff seq)
+  (map (lambda (x y) (- y x)) (drop-right seq 1) (cdr seq)))
+
+(define (all-seq-diffs seq)
+  (letrec ((loop (lambda (l ls)
+                   (if (every zero? l) ls
+                       (loop (seq-diff l) (cons l ls))))))
+    (loop seq '())))
+
+(define (predict seq-diffs)
+  (fold + 0 (map (lambda (x) (car (take-right x 1))) seq-diffs)))
+
+(let* ((port (open-input-file "9.txt"))
+       (lines (map split-up (read-lines port))))
+  (close-port port)
+  (display (fold + 0 (map (lambda (x) (predict (all-seq-diffs x))) lines))))
-- 
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