[Utf8Array] GetLengthメソッドを追加

[finphie]

SharpLab

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

public readonly partial struct Utf8Array
{
    readonly byte[] _value;
    
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public int GetLength1()
    {
        var count = 0;
        nuint i = 0;

        while ((int)i < _value.Length)
        {
            ref var valueStart = ref DangerousGetReference();
            var value = Unsafe.AddByteOffset(ref valueStart, (nint)i);
            i += (uint)UnicodeUtility.GetUtf8SequenceLength(value);
            count++;
        }

        return count;
    }
    
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public int GetLength_int()
    {
        var count = 0;
        var i = 0;

        while (i < _value.Length)
        {
            ref var valueStart = ref DangerousGetReference();
            var value = Unsafe.AddByteOffset(ref valueStart, (nint)(uint)i);
            i += UnicodeUtility.GetUtf8SequenceLength(value);
            count++;
        }

        return count;
    }
    
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public ref byte DangerousGetReference() => ref MemoryMarshal.GetArrayDataReference(_value);   
}

public static class UnicodeUtility
{
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static int GetUtf8SequenceLength(byte value)
    {
        ReadOnlySpan<byte> trailingBytesForUTF8 = new byte[]
        {
            1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
            1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
            1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
            1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
            1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
            1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
            1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
            1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
            0, 0, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
            2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
            3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
            4, 4, 4, 4, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
        };

        ref var table = ref MemoryMarshal.GetReference(trailingBytesForUTF8);
        return Unsafe.Add(ref table, (nint)value);
    }
}

Utf8Array.GetLength1()
    L0000: xor eax, eax
    L0002: xor edx, edx
    L0004: mov rcx, [rcx]
    L0007: cmp dword ptr [rcx+8], 0
    L000b: jle short L0035
    L000d: mov r8, rcx
    L0010: cmp [r8], r8d
    L0013: add r8, 0x10
    L0017: movzx r8d, byte ptr [r8+rdx]
    L001c: mov r9, 0x19def260a44
    L0026: movzx r8d, byte ptr [r8+r9]
    L002b: add rdx, r8
    L002e: inc eax
    L0030: cmp [rcx+8], edx
    L0033: jg short L000d
    L0035: ret

Utf8Array.GetLength_int()
    L0000: xor eax, eax
    L0002: xor edx, edx
    L0004: mov rcx, [rcx]
    L0007: cmp dword ptr [rcx+8], 0
    L000b: jle short L0038
    L000d: mov r8, rcx
    L0010: cmp [r8], r8d
    L0013: add r8, 0x10
    L0017: mov r9d, edx
    L001a: movzx r8d, byte ptr [r8+r9]
    L001f: mov r9, 0x19def260a44
    L0029: movzx r8d, byte ptr [r8+r9]
    L002e: add edx, r8d
    L0031: inc eax
    L0033: cmp [rcx+8], edx
    L0036: jg short L000d
    L0038: ret