Fix exp log power to raise "Computation results in Infinity/NaN" in EXCEPTION_INFINITY/EXCEPTION_NaN mode

Author: tompng

lib/bigdecimal.rb

@@ -46,29 +46,36 @@ def power(y, prec = nil)
     x = self
     y = BigMath._coerce_to_bigdecimal(y, :power)
 
-    return BigDecimal::NAN if x.nan? || y.nan?
+    return BigMath._nan_computation_result if x.nan? || y.nan?
+    return BigDecimal(1) if y.zero?
 
     if y.infinite?
       if x < 0
         return BigDecimal(0) if x < -1 && y.negative?
         return BigDecimal(0) if x > -1 && y.positive?
         raise Math::DomainError, 'Result undefined for negative base raised to infinite power'
       elsif x < 1
-        return y.positive? ? BigDecimal(0) : BigDecimal::INFINITY
+        return y.positive? ? BigDecimal(0) : BigMath._infinity_computation_result
       elsif x == 1
         return BigDecimal(1)
       else
-        return y.positive? ? BigDecimal::INFINITY : BigDecimal(0)
+        return y.positive? ? BigMath._infinity_computation_result : BigDecimal(0)
       end
     end
 
+    if x.infinite? && y < 0
+      # Computation result will be +0 or -0. Avoid overflow.
+      neg = x < 0 && y.frac.zero? && y % 2 == 1
+      return neg ? -BigDecimal(0) : BigDecimal(0)
+    end
+
     if x.zero?
       return BigDecimal(1) if y.zero?
       return BigDecimal(0) if y > 0
       if y.frac.zero? && y % 2 == 1 && x.sign == -1
-        return -BigDecimal::INFINITY
+        return -BigMath._infinity_computation_result
       else
-        return BigDecimal::INFINITY
+        return BigMath._infinity_computation_result
       end
     elsif x < 0
       if y.frac.zero?
@@ -101,7 +108,7 @@ def power(y, prec = nil)
         xn *= xn
         # Detect overflow/underflow before consuming infinite memory
         if (xn.exponent.abs - 1) * int_part / n >= 0x7FFFFFFFFFFFFFFF
-          return ((xn.exponent > 0) ^ neg ? BigDecimal::INFINITY : BigDecimal(0)) * (int_part.even? || x > 0 ? 1 : -1)
+          return ((xn.exponent > 0) ^ neg ? BigMath._infinity_computation_result : BigDecimal(0)) * (int_part.even? || x > 0 ? 1 : -1)
         end
       end
       return neg ? BigDecimal(1) / ans : ans
@@ -112,7 +119,7 @@ def power(y, prec = nil)
     if y < 0
       inv = x.power(-y, prec)
       return BigDecimal(0) if inv.infinite?
-      return BigDecimal::INFINITY if inv.zero?
+      return BigMath._infinity_computation_result if inv.zero?
       return BigDecimal(1).div(inv, prec)
     end
 
@@ -138,7 +145,7 @@ def power(y, prec = nil)
 # Core BigMath methods for BigDecimal (log, exp) are defined here.
 # Other methods (sin, cos, atan) are defined in 'bigdecimal/math.rb'.
 module BigMath
-  def self._coerce_to_bigdecimal(x, method_name, complex_domain_error = false)
+  def self._coerce_to_bigdecimal(x, method_name, complex_domain_error = false) # :nodoc:
     case x
     when BigDecimal
       return x
@@ -152,11 +159,25 @@ def self._coerce_to_bigdecimal(x, method_name, complex_domain_error = false)
     raise ArgumentError, "#{x.inspect} can't be coerced into BigDecimal"
   end
 
-  def self._validate_prec(prec, method_name)
+  def self._validate_prec(prec, method_name) # :nodoc:
     raise ArgumentError, 'precision must be an Integer' unless Integer === prec
     raise ArgumentError, "Zero or negative precision for #{method_name}" if prec <= 0
   end
 
+  def self._infinity_computation_result # :nodoc:
+    if BigDecimal.mode(BigDecimal::EXCEPTION_ALL).anybits?(BigDecimal::EXCEPTION_INFINITY)
+      raise FloatDomainError, "Computation results in 'Infinity'"
+    end
+    BigDecimal::INFINITY
+  end
+
+  def self._nan_computation_result # :nodoc:
+    if BigDecimal.mode(BigDecimal::EXCEPTION_ALL).anybits?(BigDecimal::EXCEPTION_NaN)
+      raise FloatDomainError, "Computation results to 'NaN'"
+    end
+    BigDecimal::NAN
+  end
+
   # call-seq:
   #   BigMath.log(decimal, numeric)    -> BigDecimal
   #
@@ -172,9 +193,9 @@ def self._validate_prec(prec, method_name)
   def self.log(x, prec)
     _validate_prec(prec, :log)
     x = _coerce_to_bigdecimal(x, :log, true)
-    return BigDecimal::NAN if x.nan?
+    return _nan_computation_result if x.nan?
     raise Math::DomainError, 'Zero or negative argument for log' if x <= 0
-    return BigDecimal::INFINITY if x.infinite?
+    return _infinity_computation_result if x.infinite?
     return BigDecimal(0) if x == 1
 
     if x > 10 || x < 0.1
@@ -238,8 +259,8 @@ def self.log(x, prec)
   def self.exp(x, prec)
     _validate_prec(prec, :exp)
     x = _coerce_to_bigdecimal(x, :exp)
-    return BigDecimal::NAN if x.nan?
-    return x.positive? ? BigDecimal::INFINITY : BigDecimal(0) if x.infinite?
+    return _nan_computation_result if x.nan?
+    return x.positive? ? _infinity_computation_result : BigDecimal(0) if x.infinite?
     return BigDecimal(1) if x.zero?
     return BigDecimal(1).div(exp(-x, prec), prec) if x < 0