This has been on the web site since 2012, but never actually got
included in the code distribution.

This helps avoid overstemming.

Co-authored-by: Jim O’Regan <[email protected]>

The "aggressive" version is known to overstem.  According to the
original paper, the aggressive version performs slightly better, but
the difference isn't statistically significant and conflation from
overstemming can be problematic.

Co-authored-by: Jim O’Regan <[email protected]>

Co-authored-by: Jim O’Regan <[email protected]>

Avoids snowball and C compiler warnings.

The Java code removes this ending but it was missing from the Snowball
version.  Looking at the changes resulting from this, it seems a clear
improvement so I've concluded it was an accidental omission.

See snowballstem#151

Temporary addition to allow easy comparison with Snowball
implementation.

As downloaded, except for comment and whitespace tweaks, plus addition
of main() to allow testing.

The Java implementation removes če but has an incorrect comment
saying it removes č.  Compare before and after on the test vocabulary
this is a clear improvement.

The Java implementation removes the latter but has incorrect comments
saying it removes the former.

Changing the Snowball implementation makes no difference here (probably
due to the oddness around when to remove a character vs calling
do_palatalise) but changing Java to use the Snowball suffixes here leads
to a clear regression, so adjust the Snowball implementation to match
Java implementation.

This case was inconsistent with all the other cases where we call
palatalise as we remove the whole suffix here but leave the first
character in every over case.

Checking the vocabulary list, this means palatalise will almost never
match one of the suffixes, as the only words with this as an ending in
the list are these, which look like they're actually English words
(except "abies"):

abies
cookies
hippies
series
studies

This means palatalise will just remove the last character, which seems
odd.

This change changes a lot of stems but seems to be an improvement in
pretty much every instance I checked in google translate.

There are two issues here:

One seems clearly unintentional, which is that the cursor position from
do_case wasn't reset.

The other is that do_possessive was only called if do_case did something
which does not match the Java implementation.  It seems likely this
was not intended, and testing suggests it's not a helpful change.

For the test vocabulary, this results in 1877 merges of groups of
stems (all seem reasonable), 427 splits (all seem unhelpful) and
300 reshufflings of stems between existing groups (all seem
neutral).

Overall this seems a very clear improvement, but we should see if we can
address the splits.

Testing seems to show this was never helpful and sometimes harmful.

-es seems to be a valid suffix (e.g. diabetes) but there seem to be
more cases where it is harmful to remove.

-ich seems to only be a suffix for two pronouns.

-iho doesn't seem to be a valid suffix and removing it makes no
difference on the test vocabulary.

This is a valid Czech suffix and removing it seems beneficial (88
cases in the sample vocabulary, all seem to be improvements).

Use a definition of R1 more like the usual Snowball one, but take
syllabic consonants 'l' and 'r' into account.

It seems 'm' and 'n' can also be syllabic consonants but are much
rarer so we ignore these for now at least.

Testing suggests enforcing a minimum of 3 characters before R1 (like
the Danish, Dutch and German stemmers do) helps so we do that here
too.

See snowballstem#151

We can just handle the first character specially - after that we
know the previous character is a consonant because otherwise we'd
have already stopped.

See snowballstem#151

There seems no benefit from having a separate region we can remove
possessive suffixes in.

See snowballstem#151