Implement BigMath.frexp and ldexp with exponent of 10 (#448)

* Implement BigMath.frexp and ldexp with exponent of 10

Math.frexp and ldexp calculates with exponent of 2, BigDecimal version of frexp and ldexp calculates with exponent of 10

* Apply suggestion from @mrkn

Co-authored-by: Kenta Murata <3959+mrkn@users.noreply.github.com>

---------

Co-authored-by: Kenta Murata <3959+mrkn@users.noreply.github.com>

Author: tompng

lib/bigdecimal/math.rb

@@ -28,6 +28,8 @@
 #   erfc(x, prec)
 #   gamma(x, prec)
 #   lgamma(x, prec)
+#   frexp(x)
+#   ldexp(x, exponent)
 #   PI  (prec)
 #   E   (prec) == exp(1.0,prec)
 #
@@ -856,6 +858,36 @@ def lgamma(x, prec)
     sign * sin(x.mult(pi, prec), prec)
   end
 
+  # call-seq:
+  #   frexp(x) -> [BigDecimal, Integer]
+  #
+  # Decomposes +x+ into a normalized fraction and an integral power of ten.
+  #
+  #   BigMath.frexp(BigDecimal(123.456))
+  #   #=> [0.123456e0, 3]
+  #
+  def frexp(x)
+    x = BigDecimal::Internal.coerce_to_bigdecimal(x, 0, :frexp)
+    return [x, 0] unless x.finite?
+
+    exponent = x.exponent
+    [x._decimal_shift(-exponent), exponent]
+  end
+
+  # call-seq:
+  #   ldexp(fraction, exponent) -> BigDecimal
+  #
+  # Inverse of +frexp+.
+  # Returns the value of fraction * 10**exponent.
+  #
+  #   BigMath.ldexp(BigDecimal("0.123456e0"), 3)
+  #   #=> 0.123456e3
+  #
+  def ldexp(x, exponent)
+    x = BigDecimal::Internal.coerce_to_bigdecimal(x, 0, :ldexp)
+    x.finite? ? x._decimal_shift(exponent) : x
+  end
+
   # call-seq:
   #   PI(numeric) -> BigDecimal
   #

test/bigdecimal/test_bigmath.rb

@@ -585,4 +585,28 @@ def test_lgamma
     assert_converge_in_precision {|n| lgamma(BigDecimal('-3.143580888349980058694358781820227899566'), n).first }
     assert_converge_in_precision {|n| lgamma(BigDecimal('-4.991544640560047722345260122806465721667'), n).first }
   end
+
+  def test_frexp
+    BigDecimal.save_limit do
+      BigDecimal.limit(3)
+      assert_equal([BigDecimal("-0.123456"), 10], BigMath.frexp(BigDecimal("-0.123456e10")))
+      assert_equal([BigDecimal("0.123456"), -10], BigMath.frexp(BigDecimal("0.123456e-10")))
+      assert_equal([BigDecimal("0.123456789"), 9], BigMath.frexp(123456789))
+      assert_equal([BigDecimal(0), 0], BigMath.frexp(BigDecimal(0)))
+      assert_equal([BigDecimal::NAN, 0], BigMath.frexp(BigDecimal::NAN))
+      assert_equal([BigDecimal::INFINITY, 0], BigMath.frexp(BigDecimal::INFINITY))
+    end
+  end
+
+  def test_ldexp
+    BigDecimal.save_limit do
+      BigDecimal.limit(3)
+      assert_equal(BigDecimal("-0.123456e10"), BigMath.ldexp(BigDecimal("-0.123456"), 10))
+      assert_equal(BigDecimal("0.123456e20"), BigMath.ldexp(BigDecimal("0.123456e10"), 10.9))
+      assert_equal(BigDecimal("0.123456e-10"), BigMath.ldexp(BigDecimal("0.123456"), -10))
+      assert_equal(BigDecimal("0.123456789e19"), BigMath.ldexp(123456789, 10))
+      assert(BigMath.ldexp(BigDecimal::NAN, 10).nan?)
+      assert_equal(BigDecimal::INFINITY, BigMath.ldexp(BigDecimal::INFINITY, 10))
+    end
+  end
 end

test/bigdecimal/test_jruby.rb

@@ -75,6 +75,8 @@ def test_bigmath
     assert_in_delta(Math.erfc(10), BigMath.erfc(BigDecimal(10), N))
     assert_in_delta(Math.gamma(0.5), BigMath.gamma(BigDecimal('0.5'), N))
     assert_in_delta(Math.lgamma(0.5).first, BigMath.lgamma(BigDecimal('0.5'), N).first)
+    assert_equal([BigDecimal('0.123'), 4], BigMath.frexp(BigDecimal('0.123e4')))
+    assert_equal(BigDecimal('12.3e4'), BigMath.ldexp(BigDecimal('12.3'), 4))
     assert_in_delta(Math::PI, BigMath.PI(N))
     assert_in_delta(Math::E, BigMath.E(N))
   end