Merge pull request #104 from ktsu-dev/fix/103-integer-pow-precision

Author: matt-edmondson

SignificantNumber.Test/SignificantNumberTests.cs

@@ -716,6 +716,56 @@ public void Pow_NegativeBaseAndOddExponent_ReturnsNegativeResult()
 		Assert.AreEqual(SignificantNumber.CreateFromComponents(0, new BigInteger(-8)), result);
 	}
 
+	[TestMethod]
+	public void Pow_IntegerExponent_MatchesRepeatedMultiplication()
+	{
+		SignificantNumber baseNumber = SignificantNumber.CreateFromComponents(-3, new BigInteger(1234));
+		SignificantNumber two = SignificantNumber.CreateFromComponents(0, new BigInteger(2));
+		SignificantNumber three = SignificantNumber.CreateFromComponents(0, new BigInteger(3));
+
+		SignificantNumber squared = baseNumber.Pow(two);
+		SignificantNumber cubed = baseNumber.Pow(three);
+
+		// 1.234^2 = 1.522756 and 1.234^3 = 1.879080904, each kept to the base's 4 significant digits
+		Assert.AreEqual(baseNumber * baseNumber, squared);
+		Assert.AreEqual(1.523, squared.To<double>());
+		Assert.AreEqual(4, squared.SignificantDigits);
+		Assert.AreEqual(baseNumber * baseNumber * baseNumber, cubed);
+		Assert.AreEqual(1.879, cubed.To<double>());
+	}
+
+	[TestMethod]
+	public void Pow_NegativeBaseAndOddExponent_KeepsBasePrecision()
+	{
+		SignificantNumber baseNumber = SignificantNumber.CreateFromComponents(-1, new BigInteger(-25));
+		SignificantNumber power = SignificantNumber.CreateFromComponents(0, new BigInteger(3));
+		SignificantNumber result = baseNumber.Pow(power);
+
+		// (-2.5)^3 = -15.625, kept to the base's 2 significant digits
+		Assert.AreEqual(-16.0, result.To<double>());
+	}
+
+	[TestMethod]
+	public void Pow_NegativeIntegerExponent_MatchesReciprocalOfPower()
+	{
+		SignificantNumber baseNumber = SignificantNumber.CreateFromComponents(-3, new BigInteger(1234));
+		SignificantNumber power = SignificantNumber.CreateFromComponents(0, new BigInteger(-2));
+		SignificantNumber result = baseNumber.Pow(power);
+
+		// 1.234^-2 = 0.65670..., kept to the base's 4 significant digits
+		Assert.AreEqual(0.6567, result.To<double>());
+	}
+
+	[TestMethod]
+	public void Exp_IntegerExponent_KeepsMoreThanOneSignificantDigit()
+	{
+		SignificantNumber power = SignificantNumber.CreateFromComponents(0, new BigInteger(2));
+		SignificantNumber result = SignificantNumber.Exp(power);
+
+		Assert.IsGreaterThan(1, result.SignificantDigits);
+		Assert.AreEqual(Math.Exp(2), result.To<double>(), 1e-12);
+	}
+
 	[TestMethod]
 	public void Pow_NegativeOneAndOddExponent_ReturnsNegativeOne()
 	{

SignificantNumber/SignificantNumber.cs

@@ -586,7 +586,22 @@ public SignificantNumber Pow(PreciseNumber power)
 			throw new ArgumentException("Cannot raise a negative number to a non-integer power.", nameof(power));
 		}
 
-		int significantDigits = LowestSignificantDigits(this, power);
+		if (PreciseNumber.IsInteger(power) && Math.Abs(power.To<double>()) <= MaxExactIntegerPower)
+		{
+			// An integer power is an exact count, like the counting numbers in a multiplication, so the
+			// result keeps the base's significant digits. Computing it by exact repeated multiplication
+			// makes x.Pow(2) agree with x * x and keeps the sign of a negative base.
+			BigInteger exponent = IntegerValue(power);
+			PreciseNumber positivePower = ExactIntegerPower(Value, BigInteger.Abs(exponent));
+			PreciseNumber exact = exponent.Sign < 0 ? PreciseNumber.Divide(PreciseNumber.One, positivePower) : positivePower;
+			return exact.ToSignificantNumber(Value.SignificantDigits);
+		}
+
+		// An integer power too large to compute exactly is still exact, so it keeps the base's
+		// precision, bounded by the double the result is computed in.
+		int significantDigits = PreciseNumber.IsInteger(power)
+			? int.Min(Value.SignificantDigits, DoubleSignificantDigits)
+			: LowestSignificantDigits(this, power);
 
 		// Use logarithm and exponential to support decimal powers. This computes |x|^p, so the
 		// sign of a negative base is restored for odd integer powers.
@@ -612,12 +627,64 @@ public static SignificantNumber Exp(PreciseNumber power)
 			return E;
 		}
 
-		int significantDigits = LowestSignificantDigits(E, power);
+		// An integer power is exact, so the precision is bounded only by the double the result is
+		// computed in, not by the single significant digit an integer like 2 carries.
+		int significantDigits = PreciseNumber.IsInteger(power)
+			? int.Min(E.SignificantDigits, DoubleSignificantDigits)
+			: LowestSignificantDigits(E, power);
 
 		return Math.Exp(power.To<double>())
 			.ToSignificantNumber(significantDigits);
 	}
 
+	/// <summary>
+	/// The number of significant decimal digits a <see cref="double"/> always represents exactly.
+	/// </summary>
+	private const int DoubleSignificantDigits = 15;
+
+	/// <summary>
+	/// The largest integer power <see cref="Pow"/> computes exactly. Larger powers fall back to the
+	/// logarithm path, so that an extreme exponent cannot build an arbitrarily large intermediate.
+	/// </summary>
+	private const int MaxExactIntegerPower = 1024;
+
+	/// <summary>
+	/// Gets the value of an integer <see cref="PreciseNumber"/> as a <see cref="BigInteger"/>.
+	/// </summary>
+	/// <param name="value">A number that <see cref="PreciseNumber.IsInteger"/> reports as an integer.</param>
+	/// <returns>The integer value of <paramref name="value"/>.</returns>
+	private static BigInteger IntegerValue(PreciseNumber value) =>
+		value.Exponent >= 0
+			? value.Significand * BigInteger.Pow(10, value.Exponent)
+			: value.Significand / BigInteger.Pow(10, -value.Exponent);
+
+	/// <summary>
+	/// Raises a number to a non-negative integer power by exact repeated squaring.
+	/// </summary>
+	/// <param name="value">The base.</param>
+	/// <param name="exponent">The non-negative power.</param>
+	/// <returns><paramref name="value"/> raised to <paramref name="exponent"/>, with no rounding.</returns>
+	private static PreciseNumber ExactIntegerPower(PreciseNumber value, BigInteger exponent)
+	{
+		PreciseNumber result = PreciseNumber.One;
+		PreciseNumber square = value;
+		while (!exponent.IsZero)
+		{
+			if (!exponent.IsEven)
+			{
+				result = PreciseNumber.Multiply(result, square);
+			}
+
+			exponent >>= 1;
+			if (!exponent.IsZero)
+			{
+				square = PreciseNumber.Multiply(square, square);
+			}
+		}
+
+		return result;
+	}
+
 	/// <summary>
 	/// Compares the current instance with another <see cref="SignificantNumber"/> at the lower of their significant digit counts.
 	/// </summary>