Round to negative decimal places correctly for integers

[matt-edmondson]

Fixes #105

What was wrong

Round(int decimalDigits) worked out how many digits to drop from CountDecimalDigits(), then moved the exponent by CopySign(dropped, Exponent). That is only correct when the stored exponent is negative. Integers keep their trailing zeros in a positive exponent: 1234 is stored at exponent 0, and 1230 as 123 at exponent 1. For those, a negative decimalDigits dropped the wrong number of digits and also moved the exponent the wrong way:

Call	Before	After
1234.5.Round(-2)	1200	1200
1234.Round(-2)	0.12	1200
1230.Round(-2)	0.1	1200
1500.Round(-3)	0	2000

Decision: support negative digits, don't throw

The triage note left the choice open. I went with support because 1234.5.Round(-2) already returns 1200, so negative digits already work for any value with a fractional part. Throwing would break that existing behaviour. This change makes the result independent of how the value happens to be stored. If you'd rather reject negative digits the way Math.Round does, that is a small follow-up.

Change

This is the rewrite suggested in the issue. It computes targetExponent = -decimalDigits, drops targetExponent - Exponent digits when the value has any below that place (still capped at SignificantDigits + 1), and sets the new exponent to checked(Exponent + dropped). For negative exponents this is the same arithmetic as before, and every existing rounding test passes unchanged. The static Round(PreciseNumber, int) forwards to this method, so it is fixed too.

Tests

TestRoundToNegativeDecimalsDoesNotDependOnHowTheValueIsStored checks the instance and static overloads on:

• the four inputs from the issue
• half away from zero on both signs (1250, ±1500)
• rounding down (1499) and rounding to zero (400)
• a value already at the requested place
• non-negative digits on integers, which must be no-ops

Verification

• dotnet test: 386/386 pass.
• With the source change reverted, the new test fails and all others pass.
• dotnet build PreciseNumber/PreciseNumber.csproj succeeds with no warnings.
• This PR and Count trailing zeros in IsEvenInteger and IsOddInteger #107 (IsEvenInteger(10) returns false and IsOddInteger(10) returns true for any integer with a trailing zero #104) change different methods, so they don't conflict.

🤖 Generated with Claude Code

https://claude.ai/code/session_018AEYBNCRr6chFLiZDv1MVj

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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