idx_cleanName() was called only from Doku_Indexer::addMetaKeys(), lookupKey(), getPages(), histogram()

This function is not called from elsewhere.

Note:  idx_listIndexLengths() is used in inc/infoutils.php file

Note: idx_getIndex() is used in inc/infoutils.php file

This function is not called from elsewhere.

This function is not called from elsewhere.

Note: idx_listIndexLengths() is used in iinc/TaskRunner.php and inc/Remote/ApiCore.php file

Also used in _test files.

class FulltextSearch
class MetaSearch

because used in inc/infoutils.php file

updateTuple() accept int for the second argument

This reverts commit 115f491.

provides convert($query), recert_simple(), and termParser().
No needs to pass $Indexer in method's arguments.

pageLookup() does not use fulltext index, but metadata index

Warning: Parameter 1 to dokuwiki\Search\MetaSearch::callback_pageLookup() expected to be a reference, value given in /path/to/dokuwiki/inc/Extension/Event.php on line 135

PageIndex, PagewordIndex, MetadataIndex inherit the AbstractIndex class

all extending acstract classes should use a static pidCache array

prepared for unit tests compatibility only

it is defined as namespace constant in Search/FulltextSearch.php file, and used in Remote/ApiCore.php and Ui/Search.php files.

Does it solve scrutinizer inspection?
The expression ``$this->addIndexKey('page', '', $page);`` of type ``false|integer|string`` adds the type ``string`` to the return on line 73 which is incompatible with the return type documented by ``dokuwiki\Search\AbstractIndex::getPID`` of type ``integer|false``.

make similar MetadataIndex::lookupKey() and PagewordIndex::lookup()

replace idx_* and ft_* functions with relevant class metods

getPages() is inherited to each subclass of AbstractIndex, but MetadataIndex::getPages() will override the inherited method.

getPID() : mark deleted page entry  in page.idx
getPIDNoLock() : obsoleted
getPages() : exclude deleted pages

INdexer::histogram() is only used in indexer_histogram.test.php file.

define deprecated functions in deprecated.php file.
fix load.php,  loading fulltext.php and indexer.php is nolonger necessary.

getPID() will be called only when the page exists.

According to the development manual, for [Metadata index](https://www.dokuwiki.org/devel:metadata#metadata_index),  indexer object supports `addMetaKeys()`, `lookupKey()`, and `getpages()`. This means that `idx_get_indexer()` should return  MetadataIndex object.

This reverts commit 4d04b7b.

Fixed inconsistent handling of falsy values on fperm setting

Third-party plugins may use this method. The [cloud plugin](https://github.com/dokufreaks/plugin-cloud) uses idx_indexLength().

* master: (111 commits)
  Update translation
  translation update
  don't crush tables too narrow. fixes #3250
  translation update
  Thorough tests for EO, DE, PT and ES
  translation update
  Optimized pageRestoreConfirm function
  Tests for Portuguese and Spanish
  Changes according to revisions in moisesbr-dw#2
  adjust callstack depth for deprecation message further
  better deprecation messages for self required plugin base files
  don't test on old PHP releases anymore
  increase minimum PHP version to 7.2
  fixed tests for cleanID and romanization for Greeklish
  Improved the transliteration from greek to latin.
  extension cli: do not try to upgrade bundled plugins
  Public access to patterns in external link parser
  test the collator fallback always
  cleanup for collator tests
  wrap sorting functions into their own class
  ...

Exceptions are better to handle than errors. What I don't like is that
we now have an unfortunate mix of return code and exception signalling
for errors. Some methods still return false for errors while others
now throw exceptions (always returning true otherwise).

getPID(), saveIndex(), saveIndexKey(), getPageWords() return always true, otherwise exceptions.

Just ignore $value argument if $key argument is array .
Ignore enpty key of $key argument.
Ensure to treat any null value of $key array as empty string.

Indexer, FulltextIndex, MetadataIndex uses common directory to store *.idx files, but this does not mean they should be singleton objects to avoid lock confrictions.

will reduce access to static $pidCache

frequently used in ajax call, singleton is not effective to reduce multiple instantiations.

singleton is not effective to reduce multiple instantiations, especially for MetadataSearch which is frequently used in ajax call.