Add trailing zero check to `NumberBuffer.CheckConsistency`

[pgovind]

For floating point numbers, we don't keep track of trailing zeros that occur after a decimal point. This check needs to be added to the NumberBuffer.CheckConsistency method. My first attempt to do this was

                uint totalDigits = (uint)(DigitsCount);
                uint positiveExponent = (uint)(Math.Max(0, Scale));
                uint integerDigitsPresent = Math.Min(positiveExponent, totalDigits);
                uint fractionalDigitsPresent = totalDigits - integerDigitsPresent;

                // For a number like 1.23000, verify that we don't store trailing zeros in Digits
                // However, if the number of digits exceeds maxDigCount and rounding is required, we store the trailing zeros in the buffer.
                if (fractionalDigitsPresent > 0 && !HasNonZeroTail)
                {
                    Debug.Assert(Digits[DigitsCount - 1] != '0', ToString());
                }

However while this works for double.Parse, it doesn't work for double.Format because of how the number buffer is populated. Example error below:

["21474836470000000", Length = 17, Scale = 10, IsNegative = True, HasNonZeroTail = False, Kind = FloatingPoint]
   at System.Number.FormatDouble(ValueStringBuilder& sb, Double value, ReadOnlySpan`1 format, NumberFormatInfo info) in /_/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs:line 554
   at System.Number.FormatDouble(Double value, String format, NumberFormatInfo info) in /_/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs:line 383
   at System.Double.ToString(String format) in /_/src/libraries/System.Private.CoreLib/src/System/Double.cs:line 266
   at System.Numerics.Tests.BigIntegerConstructorTest.VerifyCtorDouble(Double value) in /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/ctor.cs:line 548
   at System.Numerics.Tests.BigIntegerConstructorTest.RunCtorDoubleTests() in /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/ctor.cs:line 402

The likely fix here is to either turn on the trailing zero check only for the *Parse methods, or to improve the trailing zero check to better track a decimal point in the input/format.

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Issue Details

---

For floating point numbers, we don't keep track of trailing zeros that occur after a decimal point. This check needs to be added to the NumberBuffer.CheckConsistency method. My first attempt to do this was

                uint totalDigits = (uint)(DigitsCount);
                uint positiveExponent = (uint)(Math.Max(0, Scale));
                uint integerDigitsPresent = Math.Min(positiveExponent, totalDigits);
                uint fractionalDigitsPresent = totalDigits - integerDigitsPresent;

                // For a number like 1.23000, verify that we don't store trailing zeros in Digits
                // However, if the number of digits exceeds maxDigCount and rounding is required, we store the trailing zeros in the buffer.
                if (fractionalDigitsPresent > 0 && !HasNonZeroTail)
                {
                    Debug.Assert(Digits[DigitsCount - 1] != '0', ToString());
                }

However while this works for double.Parse, it doesn't work for double.Format because of how the number buffer is populated. Example error below:

["21474836470000000", Length = 17, Scale = 10, IsNegative = True, HasNonZeroTail = False, Kind = FloatingPoint]
   at System.Number.FormatDouble(ValueStringBuilder& sb, Double value, ReadOnlySpan`1 format, NumberFormatInfo info) in /_/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs:line 554
   at System.Number.FormatDouble(Double value, String format, NumberFormatInfo info) in /_/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs:line 383
   at System.Double.ToString(String format) in /_/src/libraries/System.Private.CoreLib/src/System/Double.cs:line 266
   at System.Numerics.Tests.BigIntegerConstructorTest.VerifyCtorDouble(Double value) in /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/ctor.cs:line 548
   at System.Numerics.Tests.BigIntegerConstructorTest.RunCtorDoubleTests() in /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/ctor.cs:line 402

The likely fix here is to either turn on the trailing zero check only for the *Parse methods, or to improve the trailing zero check to better track a decimal point in the input/format.

Author:	pgovind
Assignees:	-
Labels:	area-System.Numerics, untriaged
Milestone:	-