[ruby/matrix] Add `Matrix#rotate_entries` [#19]

Co-authored-by: Marc-André Lafortune <github@marc-andre.ca>

1 files changed, 29 insertions, 0 deletions

diff --git a/lib/matrix.rb b/lib/matrix.rb
index 8c636f61e8..0a17678972 100644
--- a/lib/matrix.rb
+++ b/lib/matrix.rb
@@ -1458,6 +1458,35 @@ class Matrix
     rank
   end
 
+  #
+  # Returns a new matrix with rotated elements.
+  # The argument specifies the rotation (defaults to `:clockwise`):
+  # * :clockwise, 1, -3, etc.: "turn right" - first row becomes last column
+  # * :half_turn, 2, -2, etc.: first row becomes last row, elements in reverse order
+  # * :counter_clockwise, -1, 3: "turn left" - first row becomes first column
+  #   (but with elements in reverse order)
+  #
+  #   m = Matrix[ [1, 2], [3, 4] ]
+  #   r = m.rotate_entries(:clockwise)
+  #   #  => Matrix[[3, 1], [4, 2]]
+  #
+  def rotate_entries(rotation = :clockwise)
+    rotation %= 4 if rotation.respond_to? :to_int
+
+    case rotation
+    when 0
+      dup
+    when 1, :clockwise
+      new_matrix @rows.transpose.each(&:reverse!), row_count
+    when 2, :half_turn
+      new_matrix @rows.map(&:reverse).reverse!, column_count
+    when 3, :counter_clockwise
+      new_matrix @rows.transpose.reverse!, row_count
+    else
+      raise ArgumentError, "expected #{rotation.inspect} to be one of :clockwise, :counter_clockwise, :half_turn or an integer"
+    end
+  end
+
   # Returns a matrix with entries rounded to the given precision
   # (see Float#round)
   #