From 15c6d8d79d9ec00a7abb14f90bd65547893c76f2 Mon Sep 17 00:00:00 2001
From: Marcin Chrzanowski <marcin.j.chrzanowski@gmail.com>
Date: Fri, 25 Dec 2020 15:25:06 +0100
Subject: Add day 9

---
 09/a.rb | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)
 create mode 100644 09/a.rb

(limited to '09/a.rb')

diff --git a/09/a.rb b/09/a.rb
new file mode 100644
index 0000000..ea90532
--- /dev/null
+++ b/09/a.rb
@@ -0,0 +1,32 @@
+preamble = 25
+
+def has_sum hash, sum
+  hash.keys.any? do |n|
+    hash[sum - n] > 0
+  end
+end
+
+def update array, hash, n
+  hash[n] += 1
+  hash[array.first] -= 1
+  array.shift
+  array.push n
+end
+
+lines = File.readlines('input.txt').map {|l| l.to_i}
+current25 = lines.slice(0, preamble)
+current25hash = Hash.new 0
+current25.each do |n|
+  current25hash[n] += 1
+end
+
+invalid = 0
+
+lines.slice(preamble, lines.size).each do |n|
+  if invalid == 0 && (!has_sum current25hash, n)
+    invalid = n
+  end 
+  update current25, current25hash, n
+end
+
+puts invalid
-- 
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