Tag: geocaching

The Puzzle

The other day I needed to solve a puzzle¹. Here’s the essence of it: there was a grid of 16 words. They needed to be organised into four thematic “groups” of four words each; then each group needed to be sorted alphabetically.

Each item in each group had a two-character code associated with it: these were to be concatenated together into a string and added to a pastebin.com/... URL. The correct four URLs would each contain a quarter of the answer to the puzzle.

I’m sure I could have solved the puzzle. But I figured it’d be more satisfying to solve a different puzzle, with the same answer: how to write a program that finds the correct URLs for me.

Here’s what I came up with:

#!/usr/bin/env ruby
require 'set'
require 'net/http'
require 'uri'
FRAGMENTS = [
  'Salsa|mO',
  'Henry|J2',
  'Primate|Q4',
  'Brine|M6',
  'Trinidad|1Z',
  'Pile|8g',
  'Crock|mO',
  'Howitzer|wT',
  'Light|DC',
  'Argo|Lt',
  'Juniper|2I',
  'Hamlet|wN',
  'Warden|xU',
  'Joker|1H',
  'Warner|9s',
  'Bargain|Cq',
]

result = FRAGMENTS.permutation(4).to_a

sorted_results = result.map { |combination| combination.sort }.uniq
shortened_results = sorted_results.map { |combination| combination.map { |fragment| fragment.split('|').last } }
urls = shortened_results.map { |combination| "https://pastebin.com/#{combination.join('')}" }

START_AT = 0

urls[START_AT..].each_with_index do |url, idx|
  uri = URI(url)
  response = Net::HTTP.get_response(uri)
  print '.'
  if response.code != '404'
    puts "\nHIT at index #{idx + START_AT}: #{url}"
  end
  sleep(rand(0..3) * 0.1)
end

Here’s how it works:

1. It creates an array containing the 43,680 possible permutations of 4 from the 16 words.
2. If sorts the permutations and removes duplicates, reducing the set to just 1,820.
3. It removes the bit of each that isn’t the two digit code at the end and concatenates them into a URL.
4. It tries each URL, with short random gaps between them, listing each one that isn’t a 404 “Not found” response.⁴

I kicked off the program and got on with some work. Meanwhile, in the background, it permuted the puzzle for me. Within a few minutes, I had four working pastebin URLs, which collectively gave me the geocache’s coordinates. Tada!

Was this cheating?

I still solved a puzzle. It probably took me, as a strong programmer, about as long as it would have taken me to solve the puzzle the conventional way were I a strong… “only connect”-er⁵. But I adapted the puzzle into a programming puzzle and solved it a completely different way, . Here’s the arguments, as I see them:

Click on a 😡 or a 🧠 to let me know whether you think I cheated or not, or drop me a comment if you’ve got a more-nuanced opinion.