How can I install a new dictionary on OS X for use with the "Word of the Day" screensaver?
On a rather obscure comment thread, someone explained where you might find the word list that Apple uses to power the screensaver. It is at /System/Library/Graphics/Quartz\ Composer\ Plug-Ins/WOTD.plugin/Contents/Resources/NOAD_wotd_list.txt. The file looks like this:

m_en_us1282510  quinsy
m_en_us1273791  orbicular
m_en_us1220945  alimony
m_en_us1250517  genome


It is a list of tab-separated entries. On the right you have the word, and on the left, what looks like an ID. But what's it an ID for, and how would you be able to find it for another word not already on the list?

As you might expect, the ID refers to an entry in Apple's default dictionary, the "New Oxford American Dictionary". (That's what "NOAD" stands for in the word-list path above.)

How to find the IDs for other words?

A fellow named Joseph Gentle, in a series of blog posts, shows how to get at the data underlying Apple's dictionaries. In his "Apple dictionaries, part 2" post, he points to code that unpacks the relevant binary file (stored in /Library/Dictionaries) into XML. Using the dedict.c and strip.c files found here, and following Gentle's example, I used the following bash commands to get at the NOAD's XML (these commands are run from the directory where you downloaded the dedict.c and strip.c files):

clang dedict.c -Wall -lz -o dedict
clang strip.c -Wall -lz -o strip
./dedict "New Oxford American Dictionary" | ./strip &gt; dict.xml


When I head-ed the first few lines of the dict.xml file, I saw something promising, with entries that looked like this:

&lt;d:entry xmlns:d="http://www.apple.com/DTDs/DictionaryService-1.0.rng" id="m_en_us1219333" d:title="abode" class="entry"&gt;


Notice that id param... it's an identifier that looks exactly like the one in the screensaver word list!

I wrote the following Ruby script to parse the XML, and then, using my own word list, create a new screensaver file that maps my own words to their IDs in the dictionary:

raw = File.open("./dict.xml").read
my_words = File.open("./word_list.csv").readlines.map { |line| line.split(',')[1] }

word_id_map = {}

raw.scan(/&lt;d:entry .*? id="(.*?)" d:title="(.*?)" class="entry"&gt;/).each do |entry|
   word_id_map[entry[1]] = entry[0]
end

my_words.each do |word|
  if id = word_id_map[word]
    puts [id, word].join('  ')
  end
end


When I replaced the original screensaver file with this new one, it worked. You can now have a "Word of the Day" screensaver that gives definitions for the words you choose.