Add day 14: One-Time Pad

Author: petertseng

14_one_time_pad.rb

@@ -0,0 +1,47 @@
+require 'digest'
+
+WINDOW = 1000
+NUM_KEYS = 64
+
+def pads(inputs)
+  pads = []
+  stop = 1.0 / 0.0
+
+  triplets = Hash.new { |h, k| h[k] = [] }
+
+  0.step { |i|
+    md5 = inputs[i]
+
+    md5.scan(/(.)\1\1\1\1/).each { |(char)|
+      candidates = triplets.delete(char)
+      candidates.select { |n| n + WINDOW >= i }.each { |n|
+        pads << n
+        # Still need to check for any numbers undercutting n.
+        stop = n + WINDOW if pads.size == NUM_KEYS
+      }
+    }
+
+    m = (/(.)\1\1/.match(md5))
+    triplets[m[1]] << i if m
+
+    return pads.sort if i >= stop
+  }
+end
+
+real = ARGV.delete('-r')
+input = (!ARGV.empty? && !File.exist?(ARGV.first) ? ARGV.first : ARGF.read).freeze
+
+f2017 = ->i {
+  s = input + i.to_s
+  2017.times { s = Digest::MD5.hexdigest(s) }
+  s
+}
+if !real
+  precomputed_file = "#{__dir__}/hashes/md5-2017-#{Digest::SHA256.hexdigest(input)}"
+  f2017 = File.readlines(precomputed_file) if File.exist?(precomputed_file)
+end
+
+[
+  ->i { Digest::MD5.hexdigest(input + i.to_s) },
+  f2017,
+].each { |f| puts pads(f)[NUM_KEYS - 1] }

README.md

@@ -34,6 +34,9 @@ Some may additionally support other ways:
   Otherwise, pass the office designer's favourite number on ARGV.
   Changing the goal is not supported.
   The `-f` flag causes the flood-fill to be printed out.
+* 14 (One-Time Pad): Pass the salt in ARGV.
+  Due to long running time, the hashes have been precomputed.
+  To redo the computation, pass the `-r` flag.
 
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