Round #879

Write a ReadOnlySpan<byte>-based UTF-8 BOM check: return true if the span starts with the bytes 0xEF 0xBB 0xBF.

Here's a simple function in C# that checks if a `ReadOnlySpan<byte>` starts with the UTF-8 BOM (Byte Order Mark): ```csharp public static bool HasUtf8Bom(ReadOnlySpan<byte> span) { // UTF-8 BOM is 0xEF 0xBB 0xBF const byte BOM1 = 0xEF; const byte BOM2 = 0xBB; const byte BOM3 = 0xBF; // Check if the span is at least 3 bytes long if (span.Length < 3) { return false; } // Check the first 3 bytes return span[0] == BOM1 && span[1] == BOM2 && span[2] == BOM3; } ``` You can use this function like this: ```csharp public static void Main() { byte[] bytesWithBom = new byte[] { 0xEF, 0xBB, 0xBF, 0x68, 0x65, 0x6c, 0x6c, 0x6f }; byte[] bytesWithoutBom = new byte[] { 0x68, 0x65, 0x6c, 0x6c, 0x6f }; Console.WriteLine(HasUtf8Bom(bytesWithBom.AsSpan())); // Outputs: True Console.WriteLine(HasUtf8Bom(bytesWithoutBom.AsSpan())); // Outputs: False } ``` Alternatively, you can use the `SequenceEqual` method to compare the first 3 bytes of the span with the UTF-8 BOM: ```csharp public static bool HasUtf8Bom(ReadOnlySpan<byte> span) { var bom = new byte[] { 0xEF, 0xBB, 0xBF }; return span.Length >= 3 && span.Slice(0, 3).SequenceEqual(bom); } ```

```csharp public static bool HasUtf8Bom(ReadOnlySpan<byte> bytes) { return bytes.Length >= 3 && bytes[0] == 0xEF && bytes[1] == 0xBB && bytes[2] == 0xBF; } ``` This function checks if the provided ReadOnlySpan of bytes starts with the UTF-8 BOM (Byte Order Mark) sequence 0xEF 0xBB 0xBF. It first checks if the span has at least 3 bytes to avoid index out of range exceptions, then checks each byte individually.