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I found myself in a situation where I wanted to examine the layout of the mobile version of a particular website. I tend to use Ruby's OpenURI module and the Nokogiri gem for my webscraping needs, and it turns out it's really easy to get a mobile version of the site with a bit more effort:

require 'open-uri'
require 'nokogiri'

# let's look at my GitHub profile as an example
url = 'https://github.com/O-I'

# this opens the URL and parses it as XML 

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I'm trying to figure out if using counter_cache on a has_many :through association is still considered kosher in Rails 3.2. I've read several posts that distill the steps for setting this up (e.g., see 1 and 2). On the other hand, it seems like counter_cache on has_many :through was an unintended side effect that was never a supported feature of Rails and was ultimately removed.

Here's my specific scenario:

# Event model
has_many :event_attendees, dependent: :destroy
has_many :attendees, through: :event_attendees, source: :user

# EventAttendee model

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Quicksort

This is an implementation of the quicksort algorithm written in Ruby. Basically, the same code you'll find on Wikibook's entry for quicksort implementations. A gem from the 'Book'.

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module Damm
  # See http://en.wikipedia.org/wiki/Damm_algorithm
  TABLE=["0317598642","7092154863","4206871359","1750983426","6123045978",
         "3674209581","5869720134","8945362017","9438617205","2581436790"]
  
  def lookup number
    number.to_s.each_char.inject(0) do |m,v|
      TABLE[m][v.to_i].to_i
    end
  end

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{:user {:dependencies [[org.clojure/clojure "1.8.0"]
                       [org.clojure/tools.namespace "0.2.10"]
                       [spyscope "0.1.5"]
                       [criterium "0.4.3"]]
        :injections [(require '(clojure.tools.namespace repl find))
                     ; try/catch to workaround an issue where `lein repl`
                     ; outside a project dir will not load reader literal
                     ; definitions correctly:
                     (try (require 'spyscope.core)
                       (catch RuntimeException e))]

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The fundamental difficulty

Straight from the [wiki]:

Fundamentally, objects (instances) reference one another and therefore form a graph in the mathematical sense (a network including loops and cycles). Relational schemas are, in contrast, tabular and based on the relational algebra, which defines linked heterogeneous tuples (groupings of data fields into a "row" with different types for each field).

Converting linked tabular rows to graph structures is hard, and even described as the [Vietnam of Computer Science][vietnam-of-cs].

Further reading:

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• Why Most Unit Testing is Waste
• A Response to "Why Most Unit Testing is Waste"
• A Sequel to "Why Most Unit Testing is Waste"
• Is TDD Dead? Conversations between Kent Beck, Martin Fowler, and DHH

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def get_character(hexnum)
  char = ''
  char << hexnum.to_i(16)
end

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require 'matrix'

# Input: an array of the n coefficients [a_n, a_n-1,..., a_1] of 
# a univariate polynomial p of degree n ([a_n]x^n + [a_n-1]x^n-1 + ... + a_1)
#
# Output: an array of all n roots of p
#
# Exploits the fact that the eigenvalues of the companion matrix of the
# monic equivalent of p are the roots of p
#

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Note: As of Ruby 3.0.0, Hash#except is now [part][1] of the language. However, Ruby does not implement Hash#except!.

Sometimes I want to remove a specific key-value pair from a Ruby hash and get the resulting hash back. If you're using Rails or ActiveSupport you can accomplish this using Hash#except:

hash = { a: 1, b: 2, c: 3 }
hash.except(:a)     # => { b: 2, c: 3 }

# note, the original hash is not modified
hash # => { a: 1, b: 2, c: 3 }