[API Proposal]: Utf8Formatter/Utf8Parser should have UInt128 and Int128 overloads

[krwq]

Background and motivation

As I started looking into #73500 I realized Utf8Formatter/Utf8Parser are missing UInt128/Int128 overloads and therefore we need to unnecessarily do UTF16 <--> UTF8 conversion.

cc: @tannergooding

API Proposal

namespace System.Buffers.Text;

public static partial class Utf8Formatter
{
    public static bool TryFormat(UInt128 value, System.Span<byte> destination, out int bytesWritten, System.Buffers.StandardFormat format = default(System.Buffers.StandardFormat)) { throw null; }
    public static bool TryFormat(Int128 value, System.Span<byte> destination, out int bytesWritten, System.Buffers.StandardFormat format = default(System.Buffers.StandardFormat)) { throw null; }
}

public static partial class Utf8Parser
{
    public static bool TryParse(System.ReadOnlySpan<byte> source, out UInt128 value, out int bytesConsumed, char standardFormat = '\0') { throw null; }
    public static bool TryParse(System.ReadOnlySpan<byte> source, out Int128 value, out int bytesConsumed, char standardFormat = '\0') { throw null; }
}

API Usage

if (Utf8Parser.TryParse(someUtf8Span, out UInt128 value, out int bytesConsumed) && someUtf8Span.Length == bytesConsumed)
{
    Console.WriteLine($"Successfully parsed UInt128 value: {value}");
}

UInt128 value = ...;
bool result = Utf8Formatter.TryFormat(value, utf8Output, out int bytesWritten);

// and similar for Int128

Alternative Designs

No response

Risks

No response

[ghost]

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Issue Details

---

Background and motivation

As I started looking into #73500 I realized Utf8Formatter/Utf8Parser are missing UInt128/Int128 overloads and therefore we need to unnecessarily do UTF16 <--> UTF8 conversion.

cc: @tannergooding

API Proposal

namespace System.Buffers.Text;

public static partial class Utf8Formatter
{
    public static bool TryFormat(UInt128 value, System.Span<byte> destination, out int bytesWritten, System.Buffers.StandardFormat format = default(System.Buffers.StandardFormat)) { throw null; }
    public static bool TryFormat(Int128 value, System.Span<byte> destination, out int bytesWritten, System.Buffers.StandardFormat format = default(System.Buffers.StandardFormat)) { throw null; }
}

public static partial class Utf8Parser
{
    public static bool TryParse(System.ReadOnlySpan<byte> source, out UInt128 value, out int bytesConsumed, char standardFormat = '\0') { throw null; }
    public static bool TryParse(System.ReadOnlySpan<byte> source, out Int128 value, out int bytesConsumed, char standardFormat = '\0') { throw null; }
}

API Usage

if (Utf8Parser.TryParse(someUtf8Span, out UInt128 value, out int bytesConsumed) && someUtf8Span.Length == bytesConsumed)
{
    Console.WriteLine($"Successfully parsed UInt128 value: {value}");
}

UInt128 value = ...;
bool result = Utf8Formatter.TryFormat(value, utf8Output, out int bytesWritten);

// and similar for Int128

Alternative Designs

No response

Risks

No response

Author:	krwq
Assignees:	-
Labels:	api-suggestion, area-System.Buffers
Milestone:	-

[bartonjs]

Video

Looks good as proposed

namespace System.Buffers.Text;

public static partial class Utf8Formatter
{
    public static bool TryFormat(UInt128 value, System.Span<byte> destination, out int bytesWritten, System.Buffers.StandardFormat format = default(System.Buffers.StandardFormat)) { throw null; }
    public static bool TryFormat(Int128 value, System.Span<byte> destination, out int bytesWritten, System.Buffers.StandardFormat format = default(System.Buffers.StandardFormat)) { throw null; }
}

public static partial class Utf8Parser
{
    public static bool TryParse(System.ReadOnlySpan<byte> source, out UInt128 value, out int bytesConsumed, char standardFormat = '\0') { throw null; }
    public static bool TryParse(System.ReadOnlySpan<byte> source, out Int128 value, out int bytesConsumed, char standardFormat = '\0') { throw null; }
}

[hrrrrustic]

@krwq @tannergooding
I'm trying to implement this API, but there are a lot of usages Convert.ChangeType and Convert.ToIntXX in the tests. Unfortunately, these methods are not support {U}Int128, is it by design or just nobody asked for this? Does this methods kind of obsolete and should be workarounded or I should create an issue with API proposal?

For Convert.ChangeType U{Int128} should implement IConvertible
For Convert.ToIntXX just add new methods

Btw, why U{Int128} returns a new instance every time in properties like {Max/Min}Value or NegativeOne? Other number types returns private consts or static readonly fields

runtime/src/libraries/System.Private.CoreLib/src/System/Int128.cs

Line 1191 in 7fdd4ee

public static Int128 MinValue => new Int128(0x8000_0000_0000_0000, 0);

[hrrrrustic]

@dotnet/area-system-buffers (does this tags work? 🤔)
This is kind of blocking linked PR, need some help for comment above

[tannergooding]

Much like with #53768 (comment), the plan is for APIs to simply implement IUtf8SpanFormattable and IUtf8SpanParsable moving forward.

For number types in particular, parsing where the first invalid character is treated as "end of string" rather than "invalid input" will be supported via #87171