Public API for counting digits?

[thfsilvab]

I was looking for functions in the framework or runtime to count the digits of uint, I know I can just write the function myself but I'd like to use something baked in already if possible.

To give context, I need to allocate a buffer for a string that I don't want to trim at the end because of overallocation.

I came across FormattingHelpers.CountDigits.cs but it's internal and used at uint.Format, does anyone know of any public API that gives access to this?

Thanks in advance!

[huoyaoyuan]

There is no such public API. Even if exposed, it's likely to get a different name.

[stephentoub]

Using the default format, formatting a uint will never exceed ten digits, so you can just format into temporary stack space, e.g.

Span<char> formatted = stackalloc char[10];
theUInt32Value.TryFormat(formatted, out int numDigits);

then, bonus, you already have the digits, which you can just copy from formatted.Slice(0, numDigits) into your destination buffer.

[thfsilvab]

Shortly after asking the question I came up with something similar, though I'm glad we align with the solution, thanks a lot!

[tannergooding]

Notably the exact number of digits is logically ceil(log10(x)). But computing this is often more expensive than simply statically determining the maximum number of digits based on ceil(log10(MaxValue)).

[neon-sunset]

To expand on Tanner's point, there is an implementation that does this inside CoreLib, which you can carry over if you need to know the exact length in advance:

runtime/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs

Lines 64 to 108 in 212d7de

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static int CountDigits(uint value)
	{
	// Algorithm based on https://lemire.me/blog/2021/06/03/computing-the-number-of-digits-of-an-integer-even-faster.
	ReadOnlySpan<long> table =
	[
	4294967296,
	8589934582,
	8589934582,
	8589934582,
	12884901788,
	12884901788,
	12884901788,
	17179868184,
	17179868184,
	17179868184,
	21474826480,
	21474826480,
	21474826480,
	21474826480,
	25769703776,
	25769703776,
	25769703776,
	30063771072,
	30063771072,
	30063771072,
	34349738368,
	34349738368,
	34349738368,
	34349738368,
	38554705664,
	38554705664,
	38554705664,
	41949672960,
	41949672960,
	41949672960,
	42949672960,
	42949672960,
	];
	Debug.Assert(table.Length == 32, "Every result of uint.Log2(value) needs a long entry in the table.");
	// TODO: Replace with table[uint.Log2(value)] once https://github.com/dotnet/runtime/issues/79257 is fixed
	long tableValue = Unsafe.Add(ref MemoryMarshal.GetReference(table), uint.Log2(value));
	return (int)((value + tableValue) >> 32);
	}