From 5657157ffa2c63c62bddcfda068eb82685f47ab0 Mon Sep 17 00:00:00 2001
From: Greg Woods <greg.woods@moserit.com>
Date: Fri, 15 Dec 2023 09:32:56 -0500
Subject: [PATCH] Day 15

---
 examples/15.txt |  1 +
 lib/days/15.rb  | 54 +++++++++++++++++++++++++++++++++++++++++++++++++
 test/15_test.rb | 18 +++++++++++++++++
 3 files changed, 73 insertions(+)
 create mode 100644 examples/15.txt
 create mode 100644 lib/days/15.rb
 create mode 100644 test/15_test.rb

diff --git a/examples/15.txt b/examples/15.txt
new file mode 100644
index 0000000..4f58f74
--- /dev/null
+++ b/examples/15.txt
@@ -0,0 +1 @@
+rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7
diff --git a/lib/days/15.rb b/lib/days/15.rb
new file mode 100644
index 0000000..2d1eade
--- /dev/null
+++ b/lib/days/15.rb
@@ -0,0 +1,54 @@
+class Day15
+
+  def part1(input)
+    input.split(',').map { |s| compute_hash(s) }.sum
+  end
+
+  def compute_hash(str)
+    str.chars.reduce(0) do |value, char|
+      code = char.ord
+      value += code
+      value *= 17
+      value %= 256
+      value
+    end
+  end
+
+  Lens = Struct.new(:label, :focal_length)
+
+  def part2(input)
+    boxes = {}
+
+    input.split(',').each do |step|
+      if step.include?('=')
+        label, value = step.split('=')
+        box_num = compute_hash(label)
+
+        boxes[box_num] ||= []
+        new_lens = Lens.new(label, value.to_i)
+
+        index = boxes[box_num].index { |b| b.label == label }
+        if index
+          boxes[box_num][index] = new_lens
+        else
+          boxes[box_num] << new_lens
+        end
+      else
+        label = step[0...-1]
+        box_num = compute_hash(label)
+
+        if boxes[box_num]
+          index = boxes[box_num].index { |b| b.label == label }
+          boxes[box_num].delete_at(index) if index
+        end
+      end
+    end
+
+    boxes.sum do |box_num, box|
+      box.each_with_index.sum do |lens, index|
+        (box_num + 1) * (index + 1) * lens.focal_length
+      end
+    end
+  end
+
+end
diff --git a/test/15_test.rb b/test/15_test.rb
new file mode 100644
index 0000000..9fcab55
--- /dev/null
+++ b/test/15_test.rb
@@ -0,0 +1,18 @@
+require 'minitest/autorun'
+require 'minitest/pride'
+require_relative '../app'
+
+class TestDay15 < Minitest::Test
+  def setup
+    @data = File.read(File.join(APP_ROOT, 'examples', '15.txt')).rstrip
+    @day = Day15.new
+  end
+
+  def test_part1
+    assert_equal @day.part1(@data), 1320
+  end
+
+  def test_part2
+    assert_equal @day.part2(@data), 145
+  end
+end