calcUtf16LeLenImpl

pub fn calcUtf16LeLenImpl(utf8: []const u8, comptime surrogates: Surrogates) !usize {
    const utf8DecodeImpl = switch (surrogates) {
        .cannot_encode_surrogate_half => utf8Decode,
        .can_encode_surrogate_half => wtf8Decode,
    };
    var src_i: usize = 0;
    var dest_len: usize = 0;
    while (src_i < utf8.len) {
        const n = try utf8ByteSequenceLength(utf8[src_i]);
        const next_src_i = src_i + n;
        const codepoint = try utf8DecodeImpl(utf8[src_i..next_src_i]);
        if (codepoint < 0x10000) {
            dest_len += 1;
        } else {
            dest_len += 2;
        }
        src_i = next_src_i;
    }
    return dest_len;
}