Matrix: Add #reflexive? method. [Fix GH-1730]

Adapted from a patch by Yilo

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64796 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

NEWS

@@ -264,10 +264,12 @@ sufficient information, see the ChangeLog file or Redmine
 
 * `Matrix`
 
-    * New method:
+    * New methods:
 
         * `Matrix#antisymmetric?`
 
+        * `Matrix#reflexive?`
+
 * `Net`
 
     * New options:

lib/matrix.rb

@@ -813,6 +813,15 @@ class Matrix
     true
   end
 
+  #
+  # Returns +true+ if this is a reflexive matrix.
+  # Raises an error if matrix is not square.
+  #
+  def reflexive?
+    Matrix.Raise ErrDimensionMismatch unless square?
+    each(:diagonal).all? { |e| e == 1 }
+  end
+
   #
   # Returns +true+ if this is a unitary matrix
   # Raises an error if matrix is not square.

spec/ruby/library/matrix/reflexive_spec.rb

@@ -0,0 +1,31 @@
+require File.expand_path('../../../spec_helper', __FILE__)
+require 'matrix'
+
+ruby_version_is '2.6' do
+  describe "Matrix.reflexive?" do
+    it "returns true for a reflexive Matrix" do
+      Matrix[[1, 2, 3], [4, 1, 3], [5, 3, 1]].reflexive?.should be_true
+    end
+
+    it "returns true for a 0x0 empty matrix" do
+      Matrix.empty.reflexive?.should be_true
+    end
+
+    it "returns false for a non-reflexive Matrix" do
+      Matrix[[1, 1],[2, 2]].reflexive?.should be_false
+    end
+
+    it "raises an error for non-square matrices" do
+      [
+        Matrix[[0], [0]],
+        Matrix[[0, 0]],
+        Matrix.empty(0, 2),
+        Matrix.empty(2, 0),
+      ].each do |rectangular_matrix|
+        lambda {
+          rectangular_matrix.reflexive?
+        }.should raise_error(Matrix::ErrDimensionMismatch)
+      end
+    end
+  end
+end