MaxMagnitude(-2, 2) returns -2 and MinMagnitude(2, -2) returns 2, the opposite of the INumberBase contract

[matt-edmondson]

What's wrong

PreciseNumber.MaxMagnitude and MinMagnitude (PreciseNumber/PreciseNumber.cs, ~lines 1100-1108) break ties in favour of the first argument:

public static PreciseNumber MaxMagnitude(PreciseNumber x, PreciseNumber y) => x.Abs() >= y.Abs() ? x : y;
public static PreciseNumber MinMagnitude(PreciseNumber x, PreciseNumber y) => x.Abs() <= y.Abs() ? x : y;

The INumberBase<T> contract, as implemented by int, double, and decimal, says what happens on a tie:

• MaxMagnitude returns the positive value.
• MinMagnitude returns the negative value.

MaxMagnitudeNumber and MinMagnitudeNumber delegate to these methods, so they have the same problem.

Repro

Call	PreciseNumber	int / double
MaxMagnitude(-2, 2)	-2	2
MinMagnitude(2, -2)	2	-2

The current tests (TestStaticMaxMagnitude and related, PreciseNumberTests.cs ~588-620) only use 1 and -1 in the order that hides the problem.

Why it matters

PreciseNumber is sold as a drop-in INumber<T>. Generic math written against INumberBase<T>, such as a norm, clamping, or picking a pivot, gives different results when T is PreciseNumber instead of double. The result also depends on argument order, so MaxMagnitude(a, b) != MaxMagnitude(b, a) for a = -b.

Suggested fix

public static PreciseNumber MaxMagnitude(PreciseNumber x, PreciseNumber y)
{
    int c = x.Abs().CompareTo(y.Abs());
    return c > 0 ? x : c < 0 ? y : IsNegative(x) ? y : x;
}
public static PreciseNumber MinMagnitude(PreciseNumber x, PreciseNumber y)
{
    int c = x.Abs().CompareTo(y.Abs());
    return c < 0 ? x : c > 0 ? y : IsNegative(x) ? x : y;
}

Acceptance criteria

• MaxMagnitude(-2, 2) == 2 and MaxMagnitude(2, -2) == 2.
• MinMagnitude(-2, 2) == -2 and MinMagnitude(2, -2) == -2.
• Tests cover both argument orders for the tie case, for all four methods.