BigMath.log(0,n)==-Infinity just like Math.log(0) (#430)

Author: tompng

lib/bigdecimal.rb

@@ -233,7 +233,8 @@ def self.log(x, prec)
 
     x = BigDecimal::Internal.coerce_to_bigdecimal(x, prec, :log)
     return BigDecimal::Internal.nan_computation_result if x.nan?
-    raise Math::DomainError, 'Zero or negative argument for log' if x <= 0
+    raise Math::DomainError, 'Negative argument for log' if x < 0
+    return -BigDecimal::Internal.infinity_computation_result if x.zero?
     return BigDecimal::Internal.infinity_computation_result if x.infinite?
     return BigDecimal(0) if x == 1
 

test/bigdecimal/test_bigdecimal.rb

@@ -2352,8 +2352,9 @@ def test_BigMath_log_with_complex
   end
 
   def test_BigMath_log_with_zero_arg
-    assert_raise(Math::DomainError) do
-      BigMath.log(0, 20)
+    BigDecimal.save_exception_mode do
+      BigDecimal.mode(BigDecimal::EXCEPTION_INFINITY, false)
+      assert_equal(Math.log(0), BigMath.log(0, 20))
     end
   end
 

test/bigdecimal/test_bigmath.rb

@@ -149,8 +149,7 @@ def test_log
     assert_converge_in_precision {|n| BigMath.log(BigDecimal("1e-30"), n) }
     assert_converge_in_precision {|n| BigMath.log(BigDecimal("1e30"), n) }
     assert_converge_in_precision {|n| BigMath.log(SQRT2, n) }
-    assert_raise(Math::DomainError) {BigMath.log(BigDecimal("0"), 10)}
-    assert_raise(Math::DomainError) {BigMath.log(BigDecimal("-1"), 10)}
+    assert_raise(Math::DomainError) {BigMath.log(BigDecimal("-0.1"), 10)}
     assert_separately(%w[-rbigdecimal], <<-SRC)
     begin
       x = BigMath.log(BigDecimal("1E19999999999999"), 10)