StringIO#seek

static VALUE
strio_seek(int argc, VALUE *argv, VALUE self)
{
    VALUE whence;
    struct StringIO *ptr = StringIO(self);
    rb_len_t amount, offset;

    rb_scan_args(argc, argv, "11", NULL, &whence);
    amount = NUM2LEN(argv[0]);
    if (CLOSED(self)) {
        rb_raise(rb_eIOError, "closed stream");
    }
    switch (NIL_P(whence) ? 0 : NUM2LONG(whence)) {
      case 0:
        offset = 0;
        break;
      case 1:
        offset = ptr->pos;
        break;
      case 2:
        if (NIL_P(ptr->string)) {
            offset = 0;
        }
        else {
            offset = RSTRING_LEN(ptr->string);
        }
        break;
      default:
        error_inval("invalid whence");
    }
    if (amount > RB_LEN_MAX - offset || amount + offset < 0) {
        error_inval(0);
    }
    ptr->pos = amount + offset;
    return INT2FIX(0);
}