diff --git a/.bundle/config b/.bundle/config
new file mode 100644
index 0000000..df11c75
--- /dev/null
+++ b/.bundle/config
@@ -0,0 +1,2 @@
+---
+BUNDLE_DISABLE_SHARED_GEMS: '1'
diff --git a/Gemfile b/Gemfile
new file mode 100644
index 0000000..d3d3f86
--- /dev/null
+++ b/Gemfile
@@ -0,0 +1,4 @@
+source 'https://rubygems.org'
+gem 'rspec'
+gem 'aruba'
+gem 'rake'
diff --git a/Gemfile.lock b/Gemfile.lock
new file mode 100644
index 0000000..cb05bed
--- /dev/null
+++ b/Gemfile.lock
@@ -0,0 +1,39 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    aruba (0.5.3)
+      childprocess (>= 0.3.6)
+      cucumber (>= 1.1.1)
+      rspec-expectations (>= 2.7.0)
+    builder (3.2.2)
+    childprocess (0.3.9)
+      ffi (~> 1.0, >= 1.0.11)
+    cucumber (1.3.10)
+      builder (>= 2.1.2)
+      diff-lcs (>= 1.1.3)
+      gherkin (~> 2.12)
+      multi_json (>= 1.7.5, < 2.0)
+      multi_test (>= 0.0.2)
+    diff-lcs (1.2.5)
+    ffi (1.9.3)
+    gherkin (2.12.2)
+      multi_json (~> 1.3)
+    multi_json (1.8.2)
+    multi_test (0.0.2)
+    rake (0.9.6)
+    rspec (2.14.1)
+      rspec-core (~> 2.14.0)
+      rspec-expectations (~> 2.14.0)
+      rspec-mocks (~> 2.14.0)
+    rspec-core (2.14.7)
+    rspec-expectations (2.14.4)
+      diff-lcs (>= 1.1.3, < 2.0)
+    rspec-mocks (2.14.4)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  aruba
+  rake
+  rspec
diff --git a/Rakefile b/Rakefile
new file mode 100644
index 0000000..9d3d473
--- /dev/null
+++ b/Rakefile
@@ -0,0 +1,8 @@
+require 'cucumber'
+require 'cucumber/rake/task'
+require 'Open3'
+
+Cucumber::Rake::Task.new(:default) do |t|
+  t.cucumber_opts = "features --format pretty -x"
+  t.fork = false
+end
diff --git a/features/parsing_a_sudoku.feature b/features/parsing_a_sudoku.feature
new file mode 100644
index 0000000..6187c0f
--- /dev/null
+++ b/features/parsing_a_sudoku.feature
@@ -0,0 +1,21 @@
+Feature: parsing a sudoku
+
+  Scenario: valid & complete
+    When I run `./sudoku-validator valid_complete.sudoku`
+    Then the stdout should contain "This sudoku is valid."
+
+  Scenario: valid & incomplete
+    When I run `./sudoku-validator valid_incomplete.sudoku`
+    Then the stdout should contain "This sudoku is valid, but incomplete."
+
+  Scenario: invalid & complete
+    When I run `./sudoku-validator invalid_complete.sudoku`
+    Then the stdout should contain "This sudoku is invalid."
+    And the stdout should contain "#4 contains duplicate 8s"
+    And the stdout should contain "#6 contains duplicate 2s"
+
+  Scenario: invalid & incomplete
+    When I run `./sudoku-validator invalid_incomplete.sudoku`
+    Then the stdout should contain "This sudoku is invalid."
+    And the stdout should contain "#2 contains duplicate 5s"
+    And the stdout should contain "#5 contains duplicate 8s"
diff --git a/features/support/env.rb b/features/support/env.rb
new file mode 100644
index 0000000..e52a59f
--- /dev/null
+++ b/features/support/env.rb
@@ -0,0 +1,5 @@
+require 'aruba/cucumber'
+
+Before do
+  @dirs = ["."]
+end
diff --git a/lib/assesses_sudoku_completeness.rb b/lib/assesses_sudoku_completeness.rb
new file mode 100644
index 0000000..eee1c54
--- /dev/null
+++ b/lib/assesses_sudoku_completeness.rb
@@ -0,0 +1,5 @@
+class AssessesSudokuCompleteness < Struct.new(:sudoku)
+  def is_complete?
+    sudoku.rows.all?(&:all?)
+  end
+end
diff --git a/lib/assesses_sudoku_validity.rb b/lib/assesses_sudoku_validity.rb
new file mode 100644
index 0000000..cc80cde
--- /dev/null
+++ b/lib/assesses_sudoku_validity.rb
@@ -0,0 +1,29 @@
+class AssessesSudokuValidity < Struct.new(:sudoku)
+  def is_valid?
+    [sudoku.rows, sudoku.columns, sudoku.blocks].all? { |superset|
+      superset.all? { |set| set_is_valid?(set) }
+    }
+  end
+
+  def errors
+    [:rows, :columns, :blocks].each_with_object({}) { |set_name, errors_hash|
+      errors_hash[set_name] = errors_for_superset(set_name)
+    }
+  end
+
+  private
+  def set_is_valid?(set)
+    set.compact.uniq.length == set.compact.length
+  end
+
+  def errors_for_superset(set_name)
+    sudoku.send(set_name).each_with_index.each_with_object({}) { |(set, index), errors|
+      duplicates = duplicates_for_set(set)
+      errors[index + 1] = duplicates unless duplicates.empty?
+    }
+  end
+
+  def duplicates_for_set(set)
+    set.group_by{ |number| number }.select{ |number, count| count.size > 1 }.keys.compact
+  end
+end
diff --git a/lib/sudoku.rb b/lib/sudoku.rb
new file mode 100644
index 0000000..82161a3
--- /dev/null
+++ b/lib/sudoku.rb
@@ -0,0 +1,32 @@
+require 'matrix'
+
+class Sudoku
+  def initialize(file)
+    @matrix = Matrix.rows(parse_rows_from_file(file))
+  end
+
+  def rows
+    @matrix.row_vectors.map(&:to_a)
+  end
+
+  def columns
+    @matrix.column_vectors.map(&:to_a)
+  end
+
+  def blocks
+    [0, 3, 6].repeated_permutation(2).map { |x, y|
+      @matrix.minor(x, 3, y, 3).to_a.flatten
+    }
+  end
+
+  private
+  def parse_rows_from_file(file)
+    file.each_line.reject{ |line| line.include?('-') }.map { |line|
+      numbers_row_from_line(line)
+    }
+  end
+
+  def numbers_row_from_line(line)
+    line.split(/ \||\s/).map { |value| value.to_i unless value == '.' }
+  end
+end
diff --git a/spec/lib/assesses_sudoku_completeness_spec.rb b/spec/lib/assesses_sudoku_completeness_spec.rb
new file mode 100644
index 0000000..8f160c4
--- /dev/null
+++ b/spec/lib/assesses_sudoku_completeness_spec.rb
@@ -0,0 +1,36 @@
+require_relative '../../lib/assesses_sudoku_completeness'
+
+describe AssessesSudokuCompleteness do
+  let(:sudoku) { double('sudoku') }
+  subject { AssessesSudokuCompleteness.new(sudoku) }
+
+  it "returns true for complete sudokus" do
+    expect(sudoku).to receive(:rows).and_return([
+        [1,2,3,4,5,6,7,8,9],
+        [2,3,4,5,6,7,8,9,1],
+        [3,4,5,6,7,8,9,1,2],
+        [4,5,6,7,8,9,1,2,3],
+        [5,6,7,8,9,1,2,3,4],
+        [6,7,8,9,1,2,3,4,5],
+        [7,8,9,1,2,3,4,5,6],
+        [8,9,1,2,3,4,5,6,7],
+        [9,1,2,3,4,5,6,7,8]
+      ])
+    expect(subject.is_complete?).to be_true
+  end
+
+  it "returns false for incomplete sudokus" do
+    expect(sudoku).to receive(:rows).and_return([
+        [nil,2,3,4,5,6,7,8,9],
+        [2,3,4,5,6,7,8,9,1],
+        [3,4,5,6,7,8,9,1,2],
+        [4,5,6,7,8,9,1,2,3],
+        [5,6,7,8,9,1,2,3,4],
+        [6,7,8,9,1,2,3,4,5],
+        [7,8,9,1,2,3,4,5,6],
+        [8,9,1,2,3,4,5,6,7],
+        [9,1,2,3,4,5,6,7,8]
+      ])
+    expect(subject.is_complete?).to be_false
+  end
+end
diff --git a/spec/lib/assesses_sudoku_validity_spec.rb b/spec/lib/assesses_sudoku_validity_spec.rb
new file mode 100644
index 0000000..207ba41
--- /dev/null
+++ b/spec/lib/assesses_sudoku_validity_spec.rb
@@ -0,0 +1,166 @@
+require_relative '../../lib/assesses_sudoku_validity'
+
+describe AssessesSudokuValidity do
+  let(:sudoku) { double('sudoku') }
+  subject { AssessesSudokuValidity.new(sudoku) }
+
+  describe "#is_valid?" do
+    it "returns true for valid sudokus" do
+      expect(sudoku).to receive(:rows).and_return([
+          [8,5,9,6,1,2,4,3,7],
+          [7,2,3,8,5,4,1,6,9],
+          [1,6,4,3,7,9,5,2,8],
+          [9,8,6,1,4,7,3,5,2],
+          [3,7,5,2,6,8,9,1,4],
+          [2,4,1,5,9,3,7,8,6],
+          [4,3,2,9,8,1,6,7,5],
+          [6,1,7,4,2,5,8,9,3],
+          [5,9,8,7,3,6,2,4,1]
+        ])
+      expect(sudoku).to receive(:columns).and_return([
+          [8,7,1,9,3,2,4,6,5],
+          [5,2,6,8,7,4,3,1,9],
+          [9,3,4,6,5,1,2,7,8],
+          [6,8,3,1,2,5,9,4,7],
+          [1,5,7,4,6,9,8,2,3],
+          [2,4,9,7,8,3,1,5,6],
+          [4,1,5,3,9,7,6,8,2],
+          [3,6,2,5,1,8,7,9,4],
+          [7,9,8,2,4,6,5,3,1]
+      ])
+      expect(sudoku).to receive(:blocks).and_return([
+          [8,5,9,7,2,3,1,6,4],
+          [6,1,2,8,5,4,3,7,9],
+          [4,3,7,1,6,9,5,2,8],
+          [9,8,6,3,7,5,2,4,1],
+          [1,4,7,2,6,8,5,9,3],
+          [3,5,2,9,1,4,7,8,6],
+          [4,3,2,6,1,7,5,9,8],
+          [9,8,1,4,2,5,7,3,6],
+          [6,7,5,8,9,3,2,4,1]
+        ])
+      expect(subject.is_valid?).to be_true
+    end
+
+    it "returns false for invalid rows" do
+      expect(sudoku).to receive(:rows).and_return([
+          [1,5,9,6,1,2,4,3,7],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil]
+        ])
+      allow(sudoku).to receive(:columns)
+      allow(sudoku).to receive(:blocks)
+      expect(subject.is_valid?).to be_false
+    end
+
+    it "returns false for invalid columns" do
+      expect(sudoku).to receive(:rows).and_return([
+          [8,5,9,6,1,2,4,3,7],
+          [8,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil]
+        ])
+      expect(sudoku).to receive(:columns).and_return([
+          [8,8,nil,nil,nil,nil,nil,nil,nil],
+          [5,nil,nil,nil,nil,nil,nil,nil,nil],
+          [9,nil,nil,nil,nil,nil,nil,nil,nil],
+          [6,nil,nil,nil,nil,nil,nil,nil,nil],
+          [1,nil,nil,nil,nil,nil,nil,nil,nil],
+          [2,nil,nil,nil,nil,nil,nil,nil,nil],
+          [3,nil,nil,nil,nil,nil,nil,nil,nil],
+          [7,nil,nil,nil,nil,nil,nil,nil,nil]
+      ])
+      allow(sudoku).to receive(:blocks)
+      expect(subject.is_valid?).to be_false
+    end
+
+    it "returns false for invalid blocks" do
+      expect(sudoku).to receive(:rows).and_return([
+          [8,5,9,nil,nil,nil,nil,nil,nil],
+          [6,8,2,nil,nil,nil,nil,nil,nil],
+          [4,3,7,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil]
+        ])
+      expect(sudoku).to receive(:columns).and_return([
+          [8,6,4,nil,nil,nil,nil,nil,nil],
+          [5,8,3,nil,nil,nil,nil,nil,nil],
+          [9,2,7,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil]
+      ])
+      expect(sudoku).to receive(:blocks).and_return([
+          [8,5,9,6,8,2,4,3,7],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil]
+        ])
+      expect(subject.is_valid?).to be_false
+    end
+  end
+
+  describe "#errors" do
+    it "returns a list of errors and their locations" do
+      expect(sudoku).to receive(:rows).and_return([
+          [8,5,9,nil,nil,nil,nil,nil,nil],
+          [6,8,2,nil,nil,nil,nil,nil,nil],
+          [4,7,7,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil]
+        ])
+      expect(sudoku).to receive(:columns).and_return([
+          [8,6,4,nil,nil,nil,nil,nil,nil],
+          [5,8,7,nil,nil,nil,nil,nil,nil],
+          [9,2,7,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil]
+      ])
+      expect(sudoku).to receive(:blocks).and_return([
+          [8,5,9,6,8,2,4,7,7],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil],
+          [nil,nil,nil,nil,nil,nil,nil,nil,nil]
+        ])
+      expect(subject.errors).to eq({
+        :rows => { 3 => [7] },
+        :columns => {},
+        :blocks => { 1 => [8, 7] }
+      })
+    end
+  end
+end
diff --git a/spec/lib/sudoku_spec.rb b/spec/lib/sudoku_spec.rb
new file mode 100644
index 0000000..998eb36
--- /dev/null
+++ b/spec/lib/sudoku_spec.rb
@@ -0,0 +1,71 @@
+require_relative '../../lib/sudoku'
+
+class AssessSudokuCompleteness; end
+
+describe Sudoku do
+  describe "#rows" do
+    it "returns the rows" do
+      sudoku = Sudoku.new(File.open('valid_complete.sudoku'))
+      expect(sudoku.rows).to eq([
+        [8,5,9,6,1,2,4,3,7],
+        [7,2,3,8,5,4,1,6,9],
+        [1,6,4,3,7,9,5,2,8],
+        [9,8,6,1,4,7,3,5,2],
+        [3,7,5,2,6,8,9,1,4],
+        [2,4,1,5,9,3,7,8,6],
+        [4,3,2,9,8,1,6,7,5],
+        [6,1,7,4,2,5,8,9,3],
+        [5,9,8,7,3,6,2,4,1]
+      ])
+    end
+
+    it "replaces uses nil for blank values" do
+      sudoku = Sudoku.new(File.open('valid_incomplete.sudoku'))
+      expect(sudoku.rows).to eq([
+        [8,5,nil,nil,nil,2,4,nil,nil],
+        [7,2,nil,nil,nil,nil,nil,nil,9],
+        [nil,nil,4,nil,nil,nil,nil,nil,nil],
+        [nil,nil,nil,1,nil,7,nil,nil,2],
+        [3,nil,5,nil,nil,nil,9,nil,nil],
+        [nil,4,nil,nil,nil,nil,nil,nil,nil],
+        [nil,nil,nil,nil,8,nil,nil,7,nil],
+        [nil,1,7,nil,nil,nil,nil,nil,nil],
+        [nil,nil,nil,nil,3,6,nil,4,nil]
+      ])
+    end
+  end
+
+  describe "#columns" do
+    it "returns the columns" do
+      sudoku = Sudoku.new(File.open('valid_complete.sudoku'))
+      expect(sudoku.columns).to eq([
+        [8,7,1,9,3,2,4,6,5],
+        [5,2,6,8,7,4,3,1,9],
+        [9,3,4,6,5,1,2,7,8],
+        [6,8,3,1,2,5,9,4,7],
+        [1,5,7,4,6,9,8,2,3],
+        [2,4,9,7,8,3,1,5,6],
+        [4,1,5,3,9,7,6,8,2],
+        [3,6,2,5,1,8,7,9,4],
+        [7,9,8,2,4,6,5,3,1]
+      ])
+    end
+  end
+
+  describe "#blocks" do
+    it "returns the blocks" do
+      sudoku = Sudoku.new(File.open('valid_complete.sudoku'))
+      expect(sudoku.blocks).to eq([
+        [8,5,9,7,2,3,1,6,4],
+        [6,1,2,8,5,4,3,7,9],
+        [4,3,7,1,6,9,5,2,8],
+        [9,8,6,3,7,5,2,4,1],
+        [1,4,7,2,6,8,5,9,3],
+        [3,5,2,9,1,4,7,8,6],
+        [4,3,2,6,1,7,5,9,8],
+        [9,8,1,4,2,5,7,3,6],
+        [6,7,5,8,9,3,2,4,1]
+      ])
+    end
+  end
+end
diff --git a/sudoku-validator b/sudoku-validator
new file mode 100755
index 0000000..1ad36de
--- /dev/null
+++ b/sudoku-validator
@@ -0,0 +1,27 @@
+#!/usr/bin/env ruby
+
+Dir.glob(Dir.pwd + '/lib/*', &method(:require))
+
+sudoku = Sudoku.new(File.open(ARGV[0]))
+
+validator = AssessesSudokuValidity.new(sudoku)
+
+if validator.is_valid?
+  if AssessesSudokuCompleteness.new(sudoku).is_complete?
+    puts "This sudoku is valid."
+  else
+    puts "This sudoku is valid, but incomplete."
+  end
+else
+  puts "This sudoku is invalid."
+
+  puts "Errors :-"
+  validator.errors.each do |set, errors|
+    puts "\n#{set.upcase}" unless errors.empty?
+    errors.each do |index, duplicates|
+      duplicates.each do |duplicate|
+        puts "##{index} contains duplicate #{duplicate}s"
+      end
+    end
+  end
+end