lib/matrix: Fix potential bug of Vector#angle_with

Could happen for some linearly dependent vectors.
Patch by Vasiliy Petrov. [Fix GH-1803]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64761 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

2 files changed, 9 insertions, 3 deletions

diff --git a/lib/matrix.rb b/lib/matrix.rb
index 5cc9697714..7639b1de6d 100644
--- a/lib/matrix.rb
+++ b/lib/matrix.rb
@@ -2080,7 +2080,7 @@ class Vector
   end
 
   #
-  # Returns an angle with another vector. Result is within the [0...Math::PI].
+  # Returns an angle with another vector. Result is within the [0..Math::PI].
   #   Vector[1,0].angle_with(Vector[0,1])
   #   # => Math::PI / 2
   #
@@ -2089,8 +2089,12 @@ class Vector
     Vector.Raise ErrDimensionMismatch if size != v.size
     prod = magnitude * v.magnitude
     raise ZeroVectorError, "Can't get angle of zero vector" if prod == 0
-
-    Math.acos( inner_product(v) / prod )
+    dot = inner_product(v)
+    if dot.abs >= prod
+      dot.positive? ? 0 : Math::PI
+    else
+      Math.acos(dot / prod)
+    end
   end
 
   #--
diff --git a/test/matrix/test_vector.rb b/test/matrix/test_vector.rb
index 52785120b1..631b4d809a 100644
--- a/test/matrix/test_vector.rb
+++ b/test/matrix/test_vector.rb
@@ -223,6 +223,8 @@ class TestVector < Test::Unit::TestCase
     assert_in_epsilon(Math::PI/2, Vector[1, 0].angle_with(Vector[0, -1]))
     assert_in_epsilon(Math::PI/4, Vector[2, 2].angle_with(Vector[0, 1]))
     assert_in_delta(0.0, Vector[1, 1].angle_with(Vector[1, 1]), 0.00001)
+    assert_equal(Vector[6, 6].angle_with(Vector[7, 7]), 0.0)
+    assert_equal(Vector[6, 6].angle_with(Vector[-7, -7]), Math::PI)
 
     assert_raise(Vector::ZeroVectorError) { Vector[1, 1].angle_with(Vector[0, 0]) }
     assert_raise(Vector::ZeroVectorError) { Vector[0, 0].angle_with(Vector[1, 1]) }