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