From 7f25d2bde6bd2b344a8e63595a214db85cd03a33 Mon Sep 17 00:00:00 2001
From: Alpha Chen <alpha@kejadlen.dev>
Date: Sat, 9 Dec 2023 10:46:51 -0800
Subject: [PATCH] [2023][ruby][9.x]

---
 2023/ruby/day_09.rb | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)
 create mode 100644 2023/ruby/day_09.rb

diff --git a/2023/ruby/day_09.rb b/2023/ruby/day_09.rb
new file mode 100644
index 0000000..3461a7b
--- /dev/null
+++ b/2023/ruby/day_09.rb
@@ -0,0 +1,26 @@
+histories = ARGF.readlines.map { _1.scan(/-?\d+/).map(&:to_i) }
+
+# part one
+p histories
+  .map {|history|
+    diffs = [history]
+    until diffs.last.all?(&:zero?)
+      diffs << diffs.last.each_cons(2).map { _2 - _1 }
+    end
+
+    diffs.sum(&:last)
+  }
+  .sum
+
+# part two
+p histories
+  .map {|history|
+    diffs = [history]
+    until diffs.last.all?(&:zero?)
+      diffs << diffs.last.each_cons(2).map { _2 - _1 }
+    end
+
+    diffs.map(&:first).reverse.inject { _2 - _1 }
+  }
+  .sum
+