Fix very long string

[mennodekker]

I hope this fixes the very long string implementation. I had some trouble since spss seems picky about the extra variables names. there can be a clash when names conflict. I did nto address that (yet)

[SamMousa]

According to docs the old implementation is correct (from https://www.gnu.org/software/pspp/pspp-dev/html_node/Very-Long-String-Record.html#Very-Long-String-Record)

char string_lengths[];

A list of key–value tuples, where key is the name of a variable, and value is its length. The key field is at most 8 bytes long and must match the name of a variable which appears in the variable record (see Variable Record). The value field is exactly 5 bytes long. It is a zero-padded, ASCII-encoded string that is the length of the variable. The key and value fields are separated by a ‘=’ byte. Tuples are delimited by a two-byte sequence {00, 09}. After the last tuple, there may be a single byte 00, or {00, 09}. The total length is count bytes.

[SamMousa]

SPSS needs the segment names of variables that are segments to have a 5 character overlap:
For example a var named LONG can have a segment named LONG1 but a var named LONGER can not, it needs segments to start with LONGE.

I have discovered that collisions in a segment name are not a problem, using question names like LONGER1 and LONGER2 which lead to segment names that collide works just fine.

Deciding the primary variable name like this will make sure there are no collisions for primary variable names:

$variable->name = strtoupper(substr($var->name, 0, 5) . strtr(base64_encode($idx), ['=' => '']));

This allows us to use the base64 alphabet to encode indexes allowing us to uniquely encode 64^3 variables with a common 5 letter prefix..

[mennodekker]

Closing this one since there is a better pull to fix this now