Ruby Weekly Challenge - String Prefixes

longest_common_prefix.rb

# Two strings have a common prefix that consists of the longest prefix of the strings that is the same. 
# For instance, the strings “I love cats” and “I love dogs” have the common prefix “I love ” 
# (including a trailing space at the end of love, which doesn’t appear properly in some browsers).
# Your task is to write a program that finds the common prefix of a list of strings (possibly more than two strings).

# Algo: 
# 1. find length of shortest sting in arr since this is the max longest-prefix length
# 2. iterate over each string in arr, comparing against the substring
# 3. if we reach the end of arr, return the calculated prefix as the longest.
def shortest_string(arr)
  return '' if arr.nil? || arr.empty?
  arr.compact.min { |a, b| a.size <=> b.size }
end

def longest_common_prefix(arr)
  return '' if arr.nil? || arr.empty? || arr.compact.empty?
  arr.compact!
  (0..shortest_string(arr).size-1).reverse_each do |i|
    arr.each_with_index do |s, arr_idx|
      break unless s && s[0..i] == arr[0][0..i]
      return arr[0][0..i] if s[0..i] == arr[0][0..i] && arr_idx == arr.size-1
    end
  end
  ''
end

longest_common_prefix_spec.rb

require_relative '../longest_common_prefix'

describe "#longest_common_prefix" do
  context "when arr is nil" do
    it "returns an empty string" do
      expect(longest_common_prefix nil).to eq('')
    end
  end

  context "when arr has one element" do
    it "returns the string" do
      expect(longest_common_prefix(["i love"])).to eq('i love')
    end

    context "when arr has a single nil element" do
      it "returns an empty string" do
        expect(longest_common_prefix([nil])).to eq('')
      end
    end
  end

  context "when arr has more than one element" do
    context "when there is a common prefix" do
      it "returns the common prefix" do
        expect(longest_common_prefix(['i lo', 'i love cats', 'i l ', 'i love dogs', 'i love my daughter'])).to eq('i l')
      end
    end

    context "when there is not a common prefix" do
      it "returns the empty string" do
        expect(longest_common_prefix(['love', 'hate'])).to eq('')
      end
    end

    context "when there is a nil element" do
      it "ignores nil" do
        expect(longest_common_prefix(['i lo', 'i love cats', 'i l', 'i love dogs', 'i love my daughter', nil])).to eq('i l')
      end
    end
  end
end