utf8 to wchar

void _GetTextFromFile(const char* fn, std::wstring* str)
{
    str->clear();
    FILE* file = fopen(fn, "rb");
    if (file)
    {
        fseek(file, 0, SEEK_END);
        int file_sz = ftell(file);
        fseek(file, 0, SEEK_SET);
        if (file_sz > 2)
        {
            unsigned char bom[3] = { 0, 0, 0 };
            fread(&bom[0], 3, 1, file);
            //printf("BOM [%#02X %#02X %#02X]\n", bom[0], bom[1], bom[2]);
            fseek(file, 0, SEEK_SET);

            if (bom[0] == 0xEF && bom[1] == 0xBB && bom[2] == 0xBF)
            {
                fseek(file, 3, SEEK_SET);
                file_sz -= 3;
            }

        }
        if (file_sz)
        {
            unsigned char* text_buffer = new unsigned char[file_sz + 1];
            fread(text_buffer, file_sz + 1, 1, file);
            text_buffer[file_sz] = 0;

            for (int i = 0; i < file_sz; ++i)
            {
                wchar_t wch = 0;

                unsigned char ch = text_buffer[i];

                if (ch >= 0 && ch < 0x80)
                {
                    wch = ch;
                }
                else if (ch >= 0xC0 && ch < 0xE0)
                {
                    if (i + 1 < file_sz)
                    {
                        unsigned char ch2 = text_buffer[i + 1];

                        if (ch2)
                        {
                            wchar_t char16 = 0;
                            char16 = ch;
                            char16 ^= 0xC0;
                            char16 <<= 6;
                            char16 |= (ch2 ^ 0x80);

                            wch = char16;
                        }
                    }
                }
                else if (ch >= 0xE0 && ch < 0xF0) // 3
                {
                    if (i + 1 < file_sz)
                    {
                        unsigned char ch2 = text_buffer[i + 1];

                        if (ch2)
                        {
                            if (i + 1 < file_sz)
                            {
                                unsigned char ch3 = text_buffer[i + 2];
                                if (ch3)
                                {
                                    wchar_t char16 = 0;
                                    char16 = ch;
                                    char16 ^= 0xE0;
                                    char16 <<= 12;
                                    char16 |= (ch2 ^ 0x80) << 6;
                                    char16 |= (ch3 ^ 0x80);

                                    wch = char16;
                                }
                            }
                        }
                    }
                }
                str->push_back(wch);
            }
            delete text_buffer;
        }
        fclose(file);
    }
}