+** CAPI3REF: Custom Page Cache Object
+** EXPERIMENTAL
+**
+** The sqlite3_pcache type is opaque. It is implemented by
+** the pluggable module. The SQLite core has no knowledge of
+** its size or internal structure and never deals with the
+** sqlite3_pcache object except by holding and passing pointers
+** to the object.
+**
+** See [sqlite3_pcache_methods] for additional information.
+*/
+typedef struct sqlite3_pcache sqlite3_pcache;
+
+/*
+** CAPI3REF: Application Defined Page Cache.
+** EXPERIMENTAL
+**
+** The [sqlite3_config]([SQLITE_CONFIG_PCACHE], ...) interface can
+** register an alternative page cache implementation by passing in an
+** instance of the sqlite3_pcache_methods structure. The majority of the
+** heap memory used by sqlite is used by the page cache to cache data read
+** from, or ready to be written to, the database file. By implementing a
+** custom page cache using this API, an application can control more
+** precisely the amount of memory consumed by sqlite, the way in which
+** said memory is allocated and released, and the policies used to
+** determine exactly which parts of a database file are cached and for
+** how long.
+**
+** The contents of the structure are copied to an internal buffer by sqlite
+** within the call to [sqlite3_config].
+**
+** The xInit() method is called once for each call to [sqlite3_initialize()]
+** (usually only once during the lifetime of the process). It is passed
+** a copy of the sqlite3_pcache_methods.pArg value. It can be used to set
+** up global structures and mutexes required by the custom page cache
+** implementation. The xShutdown() method is called from within
+** [sqlite3_shutdown()], if the application invokes this API. It can be used
+** to clean up any outstanding resources before process shutdown, if required.
+**
+** The xCreate() method is used to construct a new cache instance. The
+** first parameter, szPage, is the size in bytes of the pages that must
+** be allocated by the cache. szPage will not be a power of two. The
+** second argument, bPurgeable, is true if the cache being created will
+** be used to cache database pages read from a file stored on disk, or
+** false if it is used for an in-memory database. The cache implementation
+** does not have to do anything special based on the value of bPurgeable,
+** it is purely advisory.
+**
+** The xCachesize() method may be called at any time by SQLite to set the
+** suggested maximum cache-size (number of pages stored by) the cache
+** instance passed as the first argument. This is the value configured using
+** the SQLite "[PRAGMA cache_size]" command. As with the bPurgeable parameter,
+** the implementation is not required to do anything special with this
+** value, it is advisory only.
+**
+** The xPagecount() method should return the number of pages currently
+** stored in the cache supplied as an argument.
+**
+** The xFetch() method is used to fetch a page and return a pointer to it.
+** A 'page', in this context, is a buffer of szPage bytes aligned at an
+** 8-byte boundary. The page to be fetched is determined by the key. The
+** mimimum key value is 1. After it has been retrieved using xFetch, the page
+** is considered to be pinned.
+**
+** If the requested page is already in the page cache, then a pointer to
+** the cached buffer should be returned with its contents intact. If the
+** page is not already in the cache, then the expected behaviour of the
+** cache is determined by the value of the createFlag parameter passed
+** to xFetch, according to the following table:
+**
+** <table border=1 width=85% align=center>
+** <tr><th>createFlag<th>Expected Behaviour
+** <tr><td>0<td>NULL should be returned. No new cache entry is created.
+** <tr><td>1<td>If createFlag is set to 1, this indicates that
+** SQLite is holding pinned pages that can be unpinned
+** by writing their contents to the database file (a
+** relatively expensive operation). In this situation the
+** cache implementation has two choices: it can return NULL,
+** in which case SQLite will attempt to unpin one or more
+** pages before re-requesting the same page, or it can
+** allocate a new page and return a pointer to it. If a new
+** page is allocated, then it must be completely zeroed before
+** it is returned.
+** <tr><td>2<td>If createFlag is set to 2, then SQLite is not holding any
+** pinned pages associated with the specific cache passed
+** as the first argument to xFetch() that can be unpinned. The
+** cache implementation should attempt to allocate a new
+** cache entry and return a pointer to it. Again, the new
+** page should be zeroed before it is returned. If the xFetch()
+** method returns NULL when createFlag==2, SQLite assumes that
+** a memory allocation failed and returns SQLITE_NOMEM to the
+** user.
+** </table>
+**
+** xUnpin() is called by SQLite with a pointer to a currently pinned page
+** as its second argument. If the third parameter, discard, is non-zero,
+** then the page should be evicted from the cache. In this case SQLite
+** assumes that the next time the page is retrieved from the cache using
+** the xFetch() method, it will be zeroed. If the discard parameter is
+** zero, then the page is considered to be unpinned. The cache implementation
+** may choose to reclaim (free or recycle) unpinned pages at any time.
+** SQLite assumes that next time the page is retrieved from the cache
+** it will either be zeroed, or contain the same data that it did when it
+** was unpinned.
+**
+** The cache is not required to perform any reference counting. A single
+** call to xUnpin() unpins the page regardless of the number of prior calls
+** to xFetch().
+**
+** The xRekey() method is used to change the key value associated with the
+** page passed as the second argument from oldKey to newKey. If the cache
+** previously contains an entry associated with newKey, it should be
+** discarded. Any prior cache entry associated with newKey is guaranteed not
+** to be pinned.
+**
+** When SQLite calls the xTruncate() method, the cache must discard all
+** existing cache entries with page numbers (keys) greater than or equal
+** to the value of the iLimit parameter passed to xTruncate(). If any
+** of these pages are pinned, they are implicitly unpinned, meaning that
+** they can be safely discarded.
+**
+** The xDestroy() method is used to delete a cache allocated by xCreate().
+** All resources associated with the specified cache should be freed. After
+** calling the xDestroy() method, SQLite considers the [sqlite3_pcache*]
+** handle invalid, and will not use it with any other sqlite3_pcache_methods
+** functions.
+*/
+typedef struct sqlite3_pcache_methods sqlite3_pcache_methods;
+struct sqlite3_pcache_methods {
+ void *pArg;
+ int (*xInit)(void*);
+ void (*xShutdown)(void*);
+ sqlite3_pcache *(*xCreate)(int szPage, int bPurgeable);
+ void (*xCachesize)(sqlite3_pcache*, int nCachesize);
+ int (*xPagecount)(sqlite3_pcache*);
+ void *(*xFetch)(sqlite3_pcache*, unsigned key, int createFlag);
+ void (*xUnpin)(sqlite3_pcache*, void*, int discard);
+ void (*xRekey)(sqlite3_pcache*, void*, unsigned oldKey, unsigned newKey);
+ void (*xTruncate)(sqlite3_pcache*, unsigned iLimit);
+ void (*xDestroy)(sqlite3_pcache*);
+};
+
+/*