Commit 36a43c7

@@ -0,0 +1,86 @@
+require "spec_helper"
+
+# The Luhn test is used by some credit card companies to
+# distinguish valid credit card numbers from what could
+# be a random selection of digits.
+#
+# For example, if the trial number is 49927398716:
+#
+# 1. Reverse the digits:
+#   61789372994
+# 2. Sum the odd digits:
+#   6 + 7 + 9 + 7 + 9 + 4 = 42 = s1
+# 3. The even digits:
+#     1,  8,  3,  2,  9
+#   4. Two times each even digit:
+#     2, 16,  6,  4, 18
+#   5. Sum the digits of each multiplication:
+#     2,  7,  6,  4,  9
+#   6. Sum the last:
+#     2 + 7 + 6 + 4 + 9 = 28 = s2
+#
+# s1 + s2 = 70 which ends in zero which means {
+# that 49927398716 passes the Luhn test
+
+
+class Luhn
+  def self.valid?(cc)
+    reversed_cc = cc.to_s.reverse
+
+    step6 = sum(step_5(digits_from(reversed_cc, :odd?).map { |x| x.to_i * 2 }))
+    (step6.to_i + sum(digits_from(reversed_cc, :even?))).to_s[-1] == '0'
+  end
+
+  def self.sum(digits)
+    digits.map { |x| x.to_i }.sum
+  end
+
+  def self.digits_from(items, predicate)
+    results = []
+    items.chars.each_with_index do |item, index|
+      results << item if index.send(predicate)
+    end
+    results
+  end
+
+  def self.step_5(digits)
+    digits.map do |x|
+      sum(x.to_s.chars)
+    end
+  end
+end
+
+RSpec.describe Luhn do
+  subject { Luhn }
+
+  describe ".odd_digits" do
+    specify { expect(Luhn.digits_from("12345", :even?)).to match_array(["1", "3", "5"]) }
+    specify do
+      expect(Luhn.digits_from("61789372994", :even?)).to match_array(['6', '7', '9', '7', '9', '4'])
+    end
+  end
+
+  describe ".sum" do
+    specify { expect(subject.sum(['6', '7', '9', '7', '9', '4'])).to eql(42) }
+  end
+
+  describe ".step_5" do
+    specify { expect(subject.step_5([2, 16,  6,  4, 18])).to match_array([2,  7,  6,  4,  9]) }
+  end
+
+  it 'passes 49927398716' do
+    expect(Luhn.valid?(49927398716)).to be true
+  end
+
+  it 'fails 49927398717' do
+    expect(Luhn.valid?(49927398717)).to be false
+  end
+
+  it 'fails 1234567812345678' do
+    expect(Luhn.valid?(1234567812345678)).to be false
+  end
+
+  it 'passes 1234567812345670' do
+    expect(Luhn.valid?(1234567812345670)).to be true
+  end
+end