Add underflow check to BigMath.erfc

Author: tompng

lib/bigdecimal/math.rb

@@ -637,7 +637,7 @@ def erfc(x, prec)
     return BigDecimal::Internal.nan_computation_result if x.nan?
     return BigDecimal(1 - x.infinite?) if x.infinite?
     return BigDecimal(1).sub(erf(x, prec + BigDecimal::Internal::EXTRA_PREC), prec) if x < 0.5
-    return BigDecimal(0) if x > 5000000000 # erfc(5000000000) < 1e-10000000000000000000 (underflow)
+    return BigDecimal::Internal.underflow_computation_result if x > 5000000000 # erfc(5000000000) < 1e-10000000000000000000 (underflow)
 
     if x >= 8
       y = _erfc_asymptotic(x, prec)

test/bigdecimal/helper.rb

@@ -83,12 +83,16 @@ def assert_negative_infinite_calculation(&block)
     assert_infinite_calculation(positive: false, &block)
   end
 
-  def assert_underflow_calculation
+  def assert_underflow_calculation(accept_overflow: false)
     BigDecimal.save_exception_mode do
       BigDecimal.mode(BigDecimal::EXCEPTION_UNDERFLOW, false)
+      BigDecimal.mode(BigDecimal::EXCEPTION_OVERFLOW, false)
       assert_equal(BigDecimal(0), yield)
       BigDecimal.mode(BigDecimal::EXCEPTION_UNDERFLOW, true)
-      assert_raise_with_message(FloatDomainError, /underflow/i) { yield }
+      BigDecimal.mode(BigDecimal::EXCEPTION_OVERFLOW, true)
+      # Accept internal overflow (e.g. overflow calculating denominator part)
+      pattern = accept_overflow ? /underflow|overflow/i : /underflow/i
+      assert_raise_with_message(FloatDomainError, pattern) { yield }
     end
   end
 

test/bigdecimal/test_bigmath.rb

@@ -521,7 +521,8 @@ def test_erfc
     end
     assert_equal(0, BigMath.erfc(PINF, N))
     assert_equal(2, BigMath.erfc(MINF, N))
-    assert_equal(0, BigMath.erfc(BigDecimal('1e400'), 10))
+    assert_underflow_calculation(accept_overflow: true) { BigMath.erfc(4999999999, 10) }
+    assert_underflow_calculation { BigMath.erfc(BigDecimal('1e400'), 10) }
     assert_equal(2, BigMath.erfc(BigDecimal('-1e400'), 10))
     assert_equal(1, BigMath.erfc(BigDecimal('1e-400'), N))