1 files changed, 37 insertions, 0 deletions
diff --git a/spec/mspec/lib/mspec/matchers/be_computed_by.rb b/spec/mspec/lib/mspec/matchers/be_computed_by.rb
new file mode 100644
index 0000000000..c927eb7697
--- /dev/null
+++ b/spec/mspec/lib/mspec/matchers/be_computed_by.rb
@@ -0,0 +1,37 @@
+class BeComputedByMatcher
+  def initialize(sym, *args)
+    @method = sym
+    @args = args
+  end
+
+  def matches?(array)
+    array.each do |line|
+      @receiver = line.shift
+      @value = line.pop
+      @arguments = line
+      @arguments += @args
+      @actual = @receiver.send(@method, *@arguments)
+      return false unless @actual == @value
+    end
+
+    return true
+  end
+
+  def method_call
+    method_call = "#{@receiver.inspect}.#{@method}"
+    unless @arguments.empty?
+      method_call = "#{method_call} from #{@arguments.map { |x| x.inspect }.join(", ")}"
+    end
+    method_call
+  end
+
+  def failure_message
+    ["Expected #{@value.inspect}", "to be computed by #{method_call} (computed #{@actual.inspect} instead)"]
+  end
+end
+
+class Object
+  def be_computed_by(sym, *args)
+    BeComputedByMatcher.new(sym, *args)
+  end
+end