Return ±1 from (-1).Pow for an integer power beyond double range [patch]

SignificantNumber.Test/SignificantNumberRangeTests.cs

@@ -56,6 +56,26 @@ public void Exp_ResultBelowDoubleRange_ReturnsNonzeroMagnitude()
 		Assert.AreEqual(Parse("5.07595889755E-435"), result);
 	}
 
+	[TestMethod]
+	[DataRow("2E308", "1")]
+	[DataRow("-1E400", "1")]
+	[DataRow("1E308", "1")]
+	[DataRow("-3E400", "1")]
+	public void Pow_NegativeOneToAnEvenIntegerBeyondDoubleRange_ReturnsOne(string power, string expected) =>
+		Assert.AreEqual(Parse(expected), SignificantNumber.NegativeOne.Pow(Precise(power)));
+
+	[TestMethod]
+	public void Pow_NegativeOneToAnOddIntegerBeyondDoubleRange_ReturnsNegativeOne()
+	{
+		PreciseNumber oddPower = Precise("2" + new string('0', 310) + "1");
+		Assert.AreEqual(SignificantNumber.NegativeOne, SignificantNumber.NegativeOne.Pow(oddPower));
+		Assert.AreEqual(SignificantNumber.NegativeOne, SignificantNumber.NegativeOne.Pow(PreciseNumber.Negate(oddPower)));
+	}
+
+	[TestMethod]
+	public void Pow_NegativeOneToANonIntegerPower_StillThrowsArgumentException() =>
+		Assert.ThrowsExactly<ArgumentException>(() => SignificantNumber.NegativeOne.Pow(Precise("0.5")));
+
 	[TestMethod]
 	public void Pow_PowerOfTenBeyondExponentRange_ThrowsOverflowException()
 	{

SignificantNumber/SignificantNumber.cs

@@ -589,7 +589,7 @@
 	/// <exception cref="DivideByZeroException">Thrown when the current number is zero and <paramref name="power"/> is negative.</exception>
 	/// <exception cref="ArgumentException">Thrown when the current number is negative and <paramref name="power"/> is not an integer, as the result is not a real number.</exception>
 	/// <exception cref="OverflowException">Thrown when the result's power of ten is too large or too small to represent.</exception>
 	public SignificantNumber Pow(PreciseNumber power)
 	{
 		if (Equal(power, Zero))
 		{
@@ -605,6 +605,11 @@
 		{
 			return One;
 		}
+		else if (Equal(this, NegativeOne) && PreciseNumber.IsInteger(power))
+		{
+			// (-1)^n is exact for any integer n, however far beyond the range of a double it is.
+			return PreciseNumber.IsEvenInteger(power) ? One : NegativeOne;
+		}
 
 		bool isNegativeBase = PreciseNumber.IsNegative(Value);
 		if (isNegativeBase && !PreciseNumber.IsInteger(power))
@@ -709,7 +714,7 @@
 		double magnitude = Math.Abs(exponent);
 		int lostDigits = !double.IsFinite(magnitude)
 			? DoubleSignificantDigits
 			: magnitude > 1 ? (int)Math.Ceiling(Math.Log10(magnitude)) : 0;
 
 		return int.Max(1, int.Min(significantDigits, DoubleSignificantDigits - lostDigits));
 	}