Hash vs. case

string_lookup_case.rb

module StringLookup

  def self.lookup(string)
    case string
    when /^Foo.*$/ then 'group A'
    when /^F.*$/   then 'group B'
    when /^Bar.*$/ then 'group C'
    when /^Baz.*$/ then string.gsub('a', '0')
    end
  end

end

string_lookup_hash.rb

module RegexAndLambdaHash
  def value_for_key_matching(string)
    pattern = self.keys.find { |key| key =~ string }

    value_for(pattern, string)
  end

  def value_for(pattern_key, original_key)
    value = self[pattern_key]

    if value.respond_to? :call
      value.call(original_key)
    else
      value
    end
  end
end

module StringLookup
  LOOKUP = {
    /^Foo.*$/ => 'group A',
    /^F.*$/ => 'group B',
    /^Bar.*$/ => 'group C',
    /^Baz.*$/ => lambda { |x| x.gsub('a','0') },
  }.extend RegexAndLambdaHash

  def self.lookup(string)
    LOOKUP.value_for_key_matching(string)
  end
end

string_lookup_spec.rb

require 'string_lookup'

describe StringLookup do
  subject { StringLookup }

  it "assigns strings beginning with 'Foo' to 'group A'" do
    expect(subject.lookup('Foobar')).to eq 'group A'
  end

  it "assigns strings beginning with 'F' but not 'Foo' to 'group A'" do
    expect(subject.lookup('Fbar')).to eq 'group B'
  end

  it "assigns strings beginning with 'Foo' to 'group A'" do
    expect(subject.lookup('Bar')).to eq 'group C'
  end

  it "strings beginning with 'Baz' have 'a's changed to '0's" do
    expect(subject.lookup('Bazaz')).to eq 'B0z0z'
  end
end