Converting an NVARCHAR string to a UTF-8 VARBINARY data block in pure Transact-SQL

ToUTF8.sql

create function [dbo].[ToUTF8](@s nvarchar(max))
returns varbinary(max)
as
begin
    declare @i int = 1, @n int = datalength(@s)/2, @r varbinary(max) = 0x, @c int, @c2 int, @d varbinary(4)
    while @i <= @n
    begin
        set @c = unicode(substring(@s, @i, 1))
        if (@c & 0xFC00) = 0xD800
        begin
            set @i += 1
            if @i > @n
                return cast(cast('Malformed UTF-16 - two nchar sequence cut short' as int) as varbinary)
            set @c2 = unicode(substring(@s, @i, 1))
            if (@c2 & 0xFC00) <> 0xDC00
                return cast(cast('Malformed UTF-16 - continuation missing in a two nchar sequence' as int) as varbinary)
            set @c = (((@c & 0x3FF) * 0x400) | (@c2 & 0x3FF)) + 0x10000
        end

        if @c < 0x80
            set @d = cast(@c as binary(1))
        if @c >= 0x80 and @c < 0x800 
            set @d = cast(((@c * 4) & 0xFF00) | (@c & 0x3F) | 0xC080 as binary(2))
        if @c >= 0x800 and @c < 0x10000
            set @d = cast(((@c * 0x10) & 0xFF0000) | ((@c * 4) & 0x3F00) | (@c & 0x3F) | 0xe08080 as binary(3))
        if @c >= 0x10000
            set @d = cast(((@c * 0x40) & 0xFF000000) | ((@c * 0x10) & 0x3F0000) | ((@c * 4) & 0x3F00) | (@c & 0x3F) | 0xf0808080 as binary(4))
            
        set @r += @d
        set @i += 1
    end
    return @r
end

Here's a gist for the converse operation - from a UTF-8 VARBINARY to a NVARCHAR string: https://gist.github.com/sevaa/5a50ba98762762ae960b87fb148a7df2

Works perfectly 👍

Isn't the handling of surrogate pairs incorrect here? According to https://en.wikipedia.org/wiki/UTF-16#Code_points_from_U+010000_to_U+10FFFF , 0x10000 should be added to the combination produced from two surrogate code units, rather than OR'ed with this combination.

Also, this code doesn't check that the second code unit is actually surrogate (or whether it exists at all), so it can produce unexpected/broken results if unpaired surrogate code units are encountered.

I've added some checks for malformed UTF-16 surrogate pairs. The function will now throw a reasonably descriptive error instead of producing garbage outputs.

Since MSSQL 2019, there is native support for UTF-8. The following will do the same job, most likely much faster:

declare @s nvarchar(100) = N'Москва 東京 🙃', @t varchar(200) 
set @t = CAST(@s AS varchar) collate Latin1_General_100_CI_AI_SC_UTF8
print @t