Add support for tangent function

Author: rhannequin

lib/bigdecimal/math.rb

@@ -7,6 +7,7 @@
 #   sqrt(x, prec)
 #   sin (x, prec)
 #   cos (x, prec)
+#   tan (x, prec)
 #   atan(x, prec)  Note: |x|<1, x=0.9999 may not converge.
 #   PI  (prec)
 #   E   (prec) == exp(1.0,prec)
@@ -132,6 +133,24 @@ def cos(x, prec)
     y
   end
 
+  # call-seq:
+  #   tan(decimal, numeric) -> BigDecimal
+  #
+  # Computes the tangent of +decimal+ to the specified number of digits of
+  # precision, +numeric+.
+  #
+  # If +decimal+ is Infinity or NaN, returns NaN.
+  #
+  #   BigMath.tan(BigDecimal("0.0"), 4).to_s
+  #   #=> "0.0"
+  #
+  #   BigMath.tan(BigMath.PI(4) / 4, 4).to_s
+  #   #=> "0.999999999999999999999955588155008544487055622030633191403625599381672572e0"
+  #
+  def tan(x, prec)
+    sin(x, prec) / cos(x, prec)
+  end
+
   # call-seq:
   #   atan(decimal, numeric) -> BigDecimal
   #

test/bigdecimal/test_bigmath.rb

@@ -53,6 +53,16 @@ def test_cos
     assert_in_delta(0.0, cos(PI(N) * BigDecimal("301.5"), N))
   end
 
+  def test_tan
+    assert_in_delta(0.0, tan(BigDecimal("0.0"), N))
+    assert_in_delta(0.0, tan(PI(N), N))
+    assert_in_delta(1.0, tan(PI(N) / 4, N))
+    assert_in_delta(Math.sqrt(3.0), tan(PI(N) / 3, N))
+    assert_in_delta(0.0, tan(-PI(N), N))
+    assert_in_delta(-1.0, tan(-PI(N) / 4, N))
+    assert_in_delta(-Math.sqrt(3.0), tan(-PI(N) / 3, N))
+  end
+
   def test_atan
     assert_equal(0.0, atan(BigDecimal("0.0"), N))
     assert_in_delta(Math::PI/4, atan(BigDecimal("1.0"), N))