make tests pass

lib/topo_sort.rb

@@ -1,5 +1,54 @@
 require "topo_sort/version"
+require 'ostruct'
 
 module TopoSort
-  # Your code goes here...
+  class SelfDependency < StandardError; end
+  class Cyclic < StandardError;         end
+
+  extend self
+
+  #Take a hash of elements and their dependencies and sort
+  #them in an order where each element required is before
+  #the elements that require it.
+  #
+  #Example usage:
+  #
+  #   nodes  = { :a => nil, :b => :c, :c => :f, :d => :a, :e => :b, :f => nil }
+  #   TopoSort.sort(nodes)
+  #   #=> [:a, :f, :c, :b, :d, :e]
+  #
+  def sort(nodes)
+    @sorted   = []
+
+    @vertices = nodes.inject({}) do |memo, (node, dependency)|
+      memo[node] = OpenStruct.new(
+        name:       node,
+        dependency: dependency,
+        marked:     false,       #store the vertex has been visited by dfs
+        on_stack:   false        #to detect cycle
+      )
+      memo
+    end
+
+    nodes.each{ |(node, dep)| visit(@vertices[node]) unless @vertices[node].marked }
+
+    @sorted
+  end
+
+  protected
+
+  def visit(vertice)
+    vertice.marked  = true
+    vertice.on_stack = true
+    if vertice.dependency && dependency = @vertices[vertice.dependency]
+      raise SelfDependency.new("sort failed: detected self-dependency for '#{vertice.name}'")             if dependency == vertice
+      raise Cyclic.new("sort failed: detected cyclic dependency '#{vertice.name} => #{dependency.name}'") if dependency.on_stack
+      visit(dependency) unless dependency.marked
+    end
+    vertice.on_stack = false
+
+    @sorted.push vertice.name
+  end
+
 end
+

spec/lib/topo_sort_spec.rb

@@ -24,7 +24,7 @@
     }
 
     sorted = TopoSort.sort(nodes)
-    sorted.should == nodes
+    sorted.should == [:a, :b, :c]
   end
 
   it "should sort several nodes with one that has dependency" do
@@ -49,7 +49,7 @@
     }
 
     sorted = TopoSort.sort(nodes)
-    sorted.should == [:f, :c, :b, :e, :a, :d]
+    sorted.should == [:a, :f, :c, :b, :d, :e]
   end
 
   it "should raise error when node depends on themselve" do
@@ -58,7 +58,7 @@
       b: nil,
       c: :c
     }
-    expect { TopoSort.sort(nodes) }.to raise_error(TopoSort::SelfDependency, /sort failed: detected self dependency/)
+    expect { TopoSort.sort(nodes) }.to raise_error(TopoSort::SelfDependency, /sort failed: detected self-dependency/)
   end
 
   it "should raise error when circular dependency is presented" do
@@ -70,6 +70,6 @@
       e: nil,
       f: :b
     }
-    expect { TopoSort.sort(nodes) }.to raise_error(TopoSort::Cyclic, /sort failed: detcted cyclic dependency/)
+    expect { TopoSort.sort(nodes) }.to raise_error(TopoSort::Cyclic, /sort failed: detected cyclic dependency/)
   end
 end