Question

Given a set of strings, say:

"Alice"
"Bob"
"C"
"Ca"
"Car"
"Carol"
"Caroling"
"Carousel"

and given a single string, say:

"Carolers"

I would like a function that returns the smallest prefix not already inside the array.

For the above example, the function should return: “Caro”. (A subsequent call would return “Carole”)

I am very new to Ruby, and although I could probably hack out something ugly (using my C/C++/Objective-C brain), I would like to learn how to properly (elegantly?) code this up.

Answer 1

There’s a little known magical module in Ruby called Abbrev.

require 'abbrev'

abbreviations = Abbrev::abbrev([
  "Alice",
  "Bob",
  "C",
  "Ca",
  "Car",
  "Carol",
  "Caroling",
  "Carousel"
])
carolers = Abbrev::abbrev(%w[Carolers])
(carolers.keys - abbreviations.keys).sort.first # => "Caro"

Above I took the first element but this shows what else would be available.

pp (carolers.keys - abbreviations.keys).sort 
# >> ["Caro", "Carole", "Caroler", "Carolers"]

Wrap all the above in a function, compute the resulting missing elements, and then iterate over them yielding them to a block, or use an enumerator to return them one-by-one.

This is what is generated for a single word. For an array it is more complex.

require 'pp'
pp Abbrev::abbrev(['cat'])
# >> {"ca"=>"cat", "c"=>"cat", "cat"=>"cat"}

pp Abbrev::abbrev(['cat', 'car', 'cattle', 'carrier'])
# >> {"cattl"=>"cattle",
# >>  "catt"=>"cattle",
# >>  "cat"=>"cat",
# >>  "carrie"=>"carrier",
# >>  "carri"=>"carrier",
# >>  "carr"=>"carrier",
# >>  "car"=>"car",
# >>  "cattle"=>"cattle",
# >>  "carrier"=>"carrier"}