Ruby Weekly Challenge - Make Acronym

make_acronym.rb

# Implement a function called make_acronym that returns the first letters 
# of each word in a passed in string. Make sure the letters returned are 
# uppercase.

# If the value passed in is not a string return 'Not a string'
# If the value passed in is a string which contains only characters 
# other than spaces and alphabet letters, return 'Not letters'

def make_acronym(input_string)
  return nil unless input_string
  return 'Not a string' unless input_string.is_a? String
  return 'Not letters' unless (input_string =~ /^[a-z\s]*$/i)

  input_string.split(" ").each_with_object(''){|e, m| m << e[0]}.upcase
end

make_acronym_spec.rb

require_relative '../make_acronym'

describe "make_acronym" do
  context "when the input is a valid string" do
    it "returns the first letter of each word, capitalized" do
      expect(make_acronym('Hello codewarrior')).to eql 'HC'
    end
  end

  context "when the input is not a valid string" do
    it "returns 'Not a string'" do
      expect(make_acronym(42)).to eql "Not a string"
    end

    it "returns 'Not a string'" do
      expect(make_acronym([2,12])).to eql "Not a string"
    end

    it "returns 'Not a string'" do
      expect(make_acronym({name: 'Abraham'})).to eql "Not a string"
    end

    context "when the input contains only non-alpha characters" do
      it "returns 'Not letters'" do
        expect(make_acronym('42')).to eql "Not letters"
      end
    end
  end
end