ruby: minimal graph, topological sort

Author: jethrodaniel

ruby/lib/dsa.rb

@@ -6,6 +6,7 @@
 require_relative "dsa/binary_tree"
 require_relative "dsa/trie"
 require_relative "dsa/array_list"
+require_relative "dsa/graph"
 
 # Data structures and algorithms practice.
 module DSA

ruby/lib/dsa/graph.rb

@@ -0,0 +1,72 @@
+module DSA
+  # A graph, implemented as an adjacency list.
+  #
+  class Graph
+    class Vertex
+      # @return [T]
+      attr_accessor :value
+
+      # @param value [T]
+      def initialize value:
+        @value = value
+      end
+    end
+
+    def initialize
+      @adjacency_list = {}
+      @in_degrees = {}
+    end
+
+    attr_reader :adjacency_list
+
+    def add_vertex value:
+      vertex = Vertex.new(value:)
+      @adjacency_list[vertex] = SinglyLinkedList.new
+      @in_degrees[vertex] ||= 0
+      vertex
+    end
+
+    def add_edge from:, to:
+      # TODO: ensure vertices are already present in the graph
+      @adjacency_list.fetch(from).prepend(to)
+      @in_degrees[to] += 1
+    end
+
+    # Kahn’s algorithm
+    #
+    # - Time: O(V + E)
+    # - Space: O(V)
+    #
+    def topological_sort
+      sources = DSA::Queue.new
+      result = []
+
+      @in_degrees.select { |k, v| v.zero? }.keys.each do |source|
+        sources.enqueue source
+      end
+
+      until sources.empty?
+        source = sources.dequeue
+        result << source
+
+        children = @adjacency_list[source]
+
+        children.each do |node|
+          child = node.value
+
+          @in_degrees[child] -= 1
+
+          if @in_degrees[child].zero?
+            sources.enqueue child
+          end
+        end
+      end
+
+      if result.size < @adjacency_list.keys.size
+        raise ArgumentError, "graph cannot contain cycles"
+      end
+
+      result
+    end
+  end
+end

ruby/spec/dsa/graph_spec.rb

@@ -0,0 +1,80 @@
+RSpec.describe DSA::Graph do
+  describe "#add_vertex" do
+    it "creates a new vertex" do
+      graph = described_class.new
+
+      vertex = graph.add_vertex(value: 42)
+
+      expect(vertex).to be_a described_class::Vertex
+      expect(vertex.value).to eq 42
+    end
+  end
+
+  describe "#add_edge" do
+    it "creates a new edge" do
+      graph = described_class.new
+
+      one = graph.add_vertex(value: 1)
+      two = graph.add_vertex(value: 2)
+
+      graph.add_edge(from: one, to: two)
+
+      edges = graph.adjacency_list[one]
+      expect(edges).to be_a DSA::SinglyLinkedList
+      expect(edges.length).to eq 1
+      expect(edges[0].value).to be_a described_class::Vertex
+      expect(edges[0].value.value).to eq 2
+    end
+  end
+
+  describe "#topological_sort" do
+    it "returns a sorted list of vertices" do
+      graph = described_class.new
+
+      cook_veggies = graph.add_vertex(value: :cook_veggies)
+      cook_meat = graph.add_vertex(value: :cook_meat)
+      prep_veggies = graph.add_vertex(value: :prep_veggies)
+      prep_meat = graph.add_vertex(value: :prep_meat)
+      cook_rice = graph.add_vertex(value: :cook_rice)
+      serve_meal = graph.add_vertex(value: :serve_meal)
+      buy_food = graph.add_vertex(value: :buy_food)
+
+      graph.add_edge(from: buy_food, to: prep_veggies)
+      graph.add_edge(from: buy_food, to: prep_meat)
+      graph.add_edge(from: buy_food, to: cook_rice)
+
+      graph.add_edge(from: prep_veggies, to: cook_veggies)
+      graph.add_edge(from: prep_meat, to: cook_meat)
+
+      graph.add_edge(from: cook_veggies, to: serve_meal)
+      graph.add_edge(from: cook_meat, to: serve_meal)
+      graph.add_edge(from: cook_rice, to: serve_meal)
+
+      result = graph.topological_sort
+
+      expect(result).to be_a Array
+      expect(result[0]).to eq buy_food
+      expect(result[1]).to eq cook_rice
+      expect(result[2]).to eq prep_meat
+      expect(result[3]).to eq prep_veggies
+      expect(result[4]).to eq cook_meat
+      expect(result[5]).to eq cook_veggies
+      expect(result[6]).to eq serve_meal
+    end
+
+    it "errors if graph contains a cycle" do
+      graph = described_class.new
+
+      one = graph.add_vertex(value: 1)
+      two = graph.add_vertex(value: 2)
+      three = graph.add_vertex(value: 3)
+
+      graph.add_edge(from: one, to: two)
+      graph.add_edge(from: two, to: three)
+      graph.add_edge(from: three, to: one)
+
+      expect { graph.topological_sort }
+        .to raise_error(ArgumentError, "graph cannot contain cycles")
+    end
+  end
+end

ruby/spec/dsa_spec.rb

@@ -5,7 +5,7 @@
     it { is_expected.to eq("0.0.0") }
   end
 
-  it "has the correct gemspec info" do
+  it "has the correct gemspec info" do # rubocop:disable RSpec/ExampleLength
     path = File.expand_path("../../dsa.gemspec", __FILE__)
     gemspec = Gem::Specification.load path
 
@@ -20,6 +20,7 @@
         "lib/dsa/binary_tree.rb",
         "lib/dsa/deque.rb",
         "lib/dsa/doubly_linked_list.rb",
+        "lib/dsa/graph.rb",
         "lib/dsa/queue.rb",
         "lib/dsa/singly_linked_list.rb",
         "lib/dsa/stack.rb",