From 53c1f2ca21484b732f646787898e68cbbf731972 Mon Sep 17 00:00:00 2001
From: Nat Lasseter <Nat Lasseter nathan@bytemark.co.uk>
Date: Sun, 10 Dec 2017 21:14:34 +0000
Subject: Day 10

---
 day10/part2 | 85 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)
 create mode 100755 day10/part2

(limited to 'day10/part2')

diff --git a/day10/part2 b/day10/part2
new file mode 100755
index 0000000..5a55d7d
--- /dev/null
+++ b/day10/part2
@@ -0,0 +1,85 @@
+#!/usr/bin/env ruby
+
+class KnotHash
+  def initialize
+    @list = (0...256).to_a
+    @skip = 0
+    @ptr = 0
+  end
+
+  def reverse_sublist(len)
+    if @ptr + len > @list.length then
+      reverse_wrapping_sublist(len)
+    else
+      reverse_contained_sublist(len)
+    end
+
+    inc_ptr(len)
+  end
+
+  def compress
+    p = 0
+    out = []
+
+    16.times do
+      out << @list[p...p+16].inject(:^)
+      p += 16
+    end
+
+    return out
+  end
+
+  def hexdigest
+    dense = self.compress
+    hex = dense.map { |i| "%02x" % i }
+    return hex.join
+  end
+
+  def to_a
+    return @list
+  end
+
+  def to_s
+    l = @list.dup
+    l[@ptr] = "[#{l[@ptr]}]"
+    return "#{l.join(' ')} skip = #{@skip}"
+  end
+
+  private
+
+  def reverse_wrapping_sublist(len)
+    dbl = @list.dup + @list.dup
+    sublist = dbl[@ptr...@ptr+len].reverse
+
+    (@ptr...@ptr+len).each do |i|
+      @list[i % @list.length] = sublist[i-@ptr]
+    end
+  end
+
+  def reverse_contained_sublist(len)
+    sublist = @list[@ptr...@ptr+len].reverse
+
+    (@ptr...@ptr+len).each do |i|
+      @list[i] = sublist[i-@ptr]
+    end
+  end
+
+  def inc_ptr(len)
+    @ptr += len
+    @ptr += @skip
+    @ptr = @ptr % @list.length
+    @skip += 1
+  end
+end
+
+input = gets.chomp.chars.map(&:ord) + [17,31,73,47,23]
+
+hash = KnotHash.new
+
+64.times do
+  input.each do |len|
+    hash.reverse_sublist(len)
+  end
+end
+
+puts hash.hexdigest
-- 
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