IsNormal(Zero) returns true, unlike int/decimal/BigInteger, so generic math treats PreciseNumber zero as a normal number

[matt-edmondson]

What's wrong

PreciseNumber/PreciseNumber.cs:1094:

public static bool IsNormal(PreciseNumber value) => true;

INumberBase<T>.IsNormal is false for zero on every built-in numeric type:

int.IsNormal(0)                        // false
decimal.IsNormal(0m)                   // false
BigInteger.IsNormal(BigInteger.Zero)   // false
PreciseNumber.IsNormal(PreciseNumber.Zero) // true  <-- inconsistent

Why it matters

Generic math code written against INumberBase<T> often uses IsNormal as a "non-zero, finite, not subnormal" guard, for example before dividing or taking a log. With PreciseNumber, zero passes that guard, so the same generic algorithm behaves differently than it does with decimal or BigInteger.

Suggested fix

public static bool IsNormal(PreciseNumber value) => !value.Significand.IsZero;

Note that IsSubnormal (line 1114) is currently written as !IsNormal(value). Once IsNormal is fixed, that definition would make IsSubnormal(Zero) true, which is also wrong. Change it to => false, which is what BigInteger and decimal do.

Acceptance: IsNormal(Zero) == false, IsNormal(x) == true for any non-zero x, and IsSubnormal(x) == false for every x, with tests added.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. It is a semantic mismatch with INumberBase<T>: generic math that uses IsNormal as a "safe to divide or log" guard lets zero through for PreciseNumber, when it wouldn't for int, decimal or BigInteger. The fix is trivial, but it changes behavior that callers can observe.
• Area / suggested assignee: IsNormal and IsSubnormal in PreciseNumber.cs (:1094, :1114). Maintainer: @matt-edmondson.
• Duplicates: None.
• In progress: No open PR in this repo.

Next steps: Make IsNormal return !value.Significand.IsZero and IsSubnormal return false, which also stops IsSubnormal(Zero) from flipping to true. Add tests for zero, non-zero and negative values, and note the behavior change in the changelog.

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