Update copyright for 2022

[pgsql.git] / src / include / lib / simplehash.h

/*
 * simplehash.h
 *
 *        When included this file generates a "templated" (by way of macros)
 *        open-addressing hash table implementation specialized to user-defined
 *        types.
 *
 *        It's probably not worthwhile to generate such a specialized implementation
 *        for hash tables that aren't performance or space sensitive.
 *
 *        Compared to dynahash, simplehash has the following benefits:
 *
 *        - Due to the "templated" code generation has known structure sizes and no
 *          indirect function calls (which show up substantially in dynahash
 *          profiles). These features considerably increase speed for small
 *          entries.
 *        - Open addressing has better CPU cache behavior than dynahash's chained
 *          hashtables.
 *        - The generated interface is type-safe and easier to use than dynahash,
 *          though at the cost of more complex setup.
 *        - Allocates memory in a MemoryContext or another allocator with a
 *          malloc/free style interface (which isn't easily usable in a shared
 *          memory context)
 *        - Does not require the overhead of a separate memory context.
 *
 * Usage notes:
 *
 *        To generate a hash-table and associated functions for a use case several
 *        macros have to be #define'ed before this file is included.  Including
 *        the file #undef's all those, so a new hash table can be generated
 *        afterwards.
 *        The relevant parameters are:
 *        - SH_PREFIX - prefix for all symbol names generated. A prefix of 'foo'
 *              will result in hash table type 'foo_hash' and functions like
 *              'foo_insert'/'foo_lookup' and so forth.
 *        - SH_ELEMENT_TYPE - type of the contained elements
 *        - SH_KEY_TYPE - type of the hashtable's key
 *        - SH_DECLARE - if defined function prototypes and type declarations are
 *              generated
 *        - SH_DEFINE - if defined function definitions are generated
 *        - SH_SCOPE - in which scope (e.g. extern, static inline) do function
 *              declarations reside
 *        - SH_RAW_ALLOCATOR - if defined, memory contexts are not used; instead,
 *          use this to allocate bytes. The allocator must zero the returned space.
 *        - SH_USE_NONDEFAULT_ALLOCATOR - if defined no element allocator functions
 *              are defined, so you can supply your own
 *        The following parameters are only relevant when SH_DEFINE is defined:
 *        - SH_KEY - name of the element in SH_ELEMENT_TYPE containing the hash key
 *        - SH_EQUAL(table, a, b) - compare two table keys
 *        - SH_HASH_KEY(table, key) - generate hash for the key
 *        - SH_STORE_HASH - if defined the hash is stored in the elements
 *        - SH_GET_HASH(tb, a) - return the field to store the hash in
 *
 *        The element type is required to contain a "status" member that can store
 *        the range of values defined in the SH_STATUS enum.
 *
 *        While SH_STORE_HASH (and subsequently SH_GET_HASH) are optional, because
 *        the hash table implementation needs to compare hashes to move elements
 *        (particularly when growing the hash), it's preferable, if possible, to
 *        store the element's hash in the element's data type. If the hash is so
 *        stored, the hash table will also compare hashes before calling SH_EQUAL
 *        when comparing two keys.
 *
 *        For convenience the hash table create functions accept a void pointer
 *        that will be stored in the hash table type's member private_data. This
 *        allows callbacks to reference caller provided data.
 *
 *        For examples of usage look at tidbitmap.c (file local definition) and
 *        execnodes.h/execGrouping.c (exposed declaration, file local
 *        implementation).
 *
 * Hash table design:
 *
 *        The hash table design chosen is a variant of linear open-addressing. The
 *        reason for doing so is that linear addressing is CPU cache & pipeline
 *        friendly. The biggest disadvantage of simple linear addressing schemes
 *        are highly variable lookup times due to clustering, and deletions
 *        leaving a lot of tombstones around.  To address these issues a variant
 *        of "robin hood" hashing is employed.  Robin hood hashing optimizes
 *        chaining lengths by moving elements close to their optimal bucket
 *        ("rich" elements), out of the way if a to-be-inserted element is further
 *        away from its optimal position (i.e. it's "poor").  While that can make
 *        insertions slower, the average lookup performance is a lot better, and
 *        higher fill factors can be used in a still performant manner.  To avoid
 *        tombstones - which normally solve the issue that a deleted node's
 *        presence is relevant to determine whether a lookup needs to continue
 *        looking or is done - buckets following a deleted element are shifted
 *        backwards, unless they're empty or already at their optimal position.
 *
 * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/lib/simplehash.h
 */

#include "port/pg_bitutils.h"

/* helpers */
#define SH_MAKE_PREFIX(a) CppConcat(a,_)
#define SH_MAKE_NAME(name) SH_MAKE_NAME_(SH_MAKE_PREFIX(SH_PREFIX),name)
#define SH_MAKE_NAME_(a,b) CppConcat(a,b)

/* name macros for: */

/* type declarations */
#define SH_TYPE SH_MAKE_NAME(hash)
#define SH_STATUS SH_MAKE_NAME(status)
#define SH_STATUS_EMPTY SH_MAKE_NAME(SH_EMPTY)
#define SH_STATUS_IN_USE SH_MAKE_NAME(SH_IN_USE)
#define SH_ITERATOR SH_MAKE_NAME(iterator)

/* function declarations */
#define SH_CREATE SH_MAKE_NAME(create)
#define SH_DESTROY SH_MAKE_NAME(destroy)
#define SH_RESET SH_MAKE_NAME(reset)
#define SH_INSERT SH_MAKE_NAME(insert)
#define SH_INSERT_HASH SH_MAKE_NAME(insert_hash)
#define SH_DELETE_ITEM SH_MAKE_NAME(delete_item)
#define SH_DELETE SH_MAKE_NAME(delete)
#define SH_LOOKUP SH_MAKE_NAME(lookup)
#define SH_LOOKUP_HASH SH_MAKE_NAME(lookup_hash)
#define SH_GROW SH_MAKE_NAME(grow)
#define SH_START_ITERATE SH_MAKE_NAME(start_iterate)
#define SH_START_ITERATE_AT SH_MAKE_NAME(start_iterate_at)
#define SH_ITERATE SH_MAKE_NAME(iterate)
#define SH_ALLOCATE SH_MAKE_NAME(allocate)
#define SH_FREE SH_MAKE_NAME(free)
#define SH_STAT SH_MAKE_NAME(stat)

/* internal helper functions (no externally visible prototypes) */
#define SH_COMPUTE_PARAMETERS SH_MAKE_NAME(compute_parameters)
#define SH_NEXT SH_MAKE_NAME(next)
#define SH_PREV SH_MAKE_NAME(prev)
#define SH_DISTANCE_FROM_OPTIMAL SH_MAKE_NAME(distance)
#define SH_INITIAL_BUCKET SH_MAKE_NAME(initial_bucket)
#define SH_ENTRY_HASH SH_MAKE_NAME(entry_hash)
#define SH_INSERT_HASH_INTERNAL SH_MAKE_NAME(insert_hash_internal)
#define SH_LOOKUP_HASH_INTERNAL SH_MAKE_NAME(lookup_hash_internal)

/* generate forward declarations necessary to use the hash table */
#ifdef SH_DECLARE

/* type definitions */
typedef struct SH_TYPE
{
        /*
         * Size of data / bucket array, 64 bits to handle UINT32_MAX sized hash
         * tables.  Note that the maximum number of elements is lower
         * (SH_MAX_FILLFACTOR)
         */
        uint64          size;

        /* how many elements have valid contents */
        uint32          members;

        /* mask for bucket and size calculations, based on size */
        uint32          sizemask;

        /* boundary after which to grow hashtable */
        uint32          grow_threshold;

        /* hash buckets */
        SH_ELEMENT_TYPE *data;

#ifndef SH_RAW_ALLOCATOR
        /* memory context to use for allocations */
        MemoryContext ctx;
#endif

        /* user defined data, useful for callbacks */
        void       *private_data;
}                       SH_TYPE;

typedef enum SH_STATUS
{
        SH_STATUS_EMPTY = 0x00,
        SH_STATUS_IN_USE = 0x01
} SH_STATUS;

typedef struct SH_ITERATOR
{
        uint32          cur;                    /* current element */
        uint32          end;
        bool            done;                   /* iterator exhausted? */
}                       SH_ITERATOR;

/* externally visible function prototypes */
#ifdef SH_RAW_ALLOCATOR
/* <prefix>_hash <prefix>_create(uint32 nelements, void *private_data) */
SH_SCOPE        SH_TYPE *SH_CREATE(uint32 nelements, void *private_data);
#else
/*
 * <prefix>_hash <prefix>_create(MemoryContext ctx, uint32 nelements,
 *                                                               void *private_data)
 */
SH_SCOPE        SH_TYPE *SH_CREATE(MemoryContext ctx, uint32 nelements,
                                                           void *private_data);
#endif

/* void <prefix>_destroy(<prefix>_hash *tb) */
SH_SCOPE void SH_DESTROY(SH_TYPE * tb);

/* void <prefix>_reset(<prefix>_hash *tb) */
SH_SCOPE void SH_RESET(SH_TYPE * tb);

/* void <prefix>_grow(<prefix>_hash *tb, uint64 newsize) */
SH_SCOPE void SH_GROW(SH_TYPE * tb, uint64 newsize);

/* <element> *<prefix>_insert(<prefix>_hash *tb, <key> key, bool *found) */
SH_SCOPE        SH_ELEMENT_TYPE *SH_INSERT(SH_TYPE * tb, SH_KEY_TYPE key, bool *found);

/*
 * <element> *<prefix>_insert_hash(<prefix>_hash *tb, <key> key, uint32 hash,
 *                                                                bool *found)
 */
SH_SCOPE        SH_ELEMENT_TYPE *SH_INSERT_HASH(SH_TYPE * tb, SH_KEY_TYPE key,
                                                                                        uint32 hash, bool *found);

/* <element> *<prefix>_lookup(<prefix>_hash *tb, <key> key) */
SH_SCOPE        SH_ELEMENT_TYPE *SH_LOOKUP(SH_TYPE * tb, SH_KEY_TYPE key);

/* <element> *<prefix>_lookup_hash(<prefix>_hash *tb, <key> key, uint32 hash) */
SH_SCOPE        SH_ELEMENT_TYPE *SH_LOOKUP_HASH(SH_TYPE * tb, SH_KEY_TYPE key,
                                                                                        uint32 hash);

/* void <prefix>_delete_item(<prefix>_hash *tb, <element> *entry) */
SH_SCOPE void SH_DELETE_ITEM(SH_TYPE * tb, SH_ELEMENT_TYPE * entry);

/* bool <prefix>_delete(<prefix>_hash *tb, <key> key) */
SH_SCOPE bool SH_DELETE(SH_TYPE * tb, SH_KEY_TYPE key);

/* void <prefix>_start_iterate(<prefix>_hash *tb, <prefix>_iterator *iter) */
SH_SCOPE void SH_START_ITERATE(SH_TYPE * tb, SH_ITERATOR * iter);

/*
 * void <prefix>_start_iterate_at(<prefix>_hash *tb, <prefix>_iterator *iter,
 *                                                                uint32 at)
 */
SH_SCOPE void SH_START_ITERATE_AT(SH_TYPE * tb, SH_ITERATOR * iter, uint32 at);

/* <element> *<prefix>_iterate(<prefix>_hash *tb, <prefix>_iterator *iter) */
SH_SCOPE        SH_ELEMENT_TYPE *SH_ITERATE(SH_TYPE * tb, SH_ITERATOR * iter);

/* void <prefix>_stat(<prefix>_hash *tb */
SH_SCOPE void SH_STAT(SH_TYPE * tb);

#endif                                                  /* SH_DECLARE */


/* generate implementation of the hash table */
#ifdef SH_DEFINE

#ifndef SH_RAW_ALLOCATOR
#include "utils/memutils.h"
#endif

/* max data array size,we allow up to PG_UINT32_MAX buckets, including 0 */
#define SH_MAX_SIZE (((uint64) PG_UINT32_MAX) + 1)

/* normal fillfactor, unless already close to maximum */
#ifndef SH_FILLFACTOR
#define SH_FILLFACTOR (0.9)
#endif
/* increase fillfactor if we otherwise would error out */
#define SH_MAX_FILLFACTOR (0.98)
/* grow if actual and optimal location bigger than */
#ifndef SH_GROW_MAX_DIB
#define SH_GROW_MAX_DIB 25
#endif
/* grow if more than elements to move when inserting */
#ifndef SH_GROW_MAX_MOVE
#define SH_GROW_MAX_MOVE 150
#endif
#ifndef SH_GROW_MIN_FILLFACTOR
/* but do not grow due to SH_GROW_MAX_* if below */
#define SH_GROW_MIN_FILLFACTOR 0.1
#endif

#ifdef SH_STORE_HASH
#define SH_COMPARE_KEYS(tb, ahash, akey, b) (ahash == SH_GET_HASH(tb, b) && SH_EQUAL(tb, b->SH_KEY, akey))
#else
#define SH_COMPARE_KEYS(tb, ahash, akey, b) (SH_EQUAL(tb, b->SH_KEY, akey))
#endif

/*
 * Wrap the following definitions in include guards, to avoid multiple
 * definition errors if this header is included more than once.  The rest of
 * the file deliberately has no include guards, because it can be included
 * with different parameters to define functions and types with non-colliding
 * names.
 */
#ifndef SIMPLEHASH_H
#define SIMPLEHASH_H

#ifdef FRONTEND
#define sh_error(...) \
        do { pg_log_fatal(__VA_ARGS__); exit(1); } while(0)
#define sh_log(...) pg_log_info(__VA_ARGS__)
#else
#define sh_error(...) elog(ERROR, __VA_ARGS__)
#define sh_log(...) elog(LOG, __VA_ARGS__)
#endif

#endif

/*
 * Compute sizing parameters for hashtable. Called when creating and growing
 * the hashtable.
 */
static inline void
SH_COMPUTE_PARAMETERS(SH_TYPE * tb, uint64 newsize)
{
        uint64          size;

        /* supporting zero sized hashes would complicate matters */
        size = Max(newsize, 2);

        /* round up size to the next power of 2, that's how bucketing works */
        size = pg_nextpower2_64(size);
        Assert(size <= SH_MAX_SIZE);

        /*
         * Verify that allocation of ->data is possible on this platform, without
         * overflowing Size.
         */
        if (unlikely((((uint64) sizeof(SH_ELEMENT_TYPE)) * size) >= SIZE_MAX / 2))
                sh_error("hash table too large");

        /* now set size */
        tb->size = size;
        tb->sizemask = (uint32) (size - 1);

        /*
         * Compute the next threshold at which we need to grow the hash table
         * again.
         */
        if (tb->size == SH_MAX_SIZE)
                tb->grow_threshold = ((double) tb->size) * SH_MAX_FILLFACTOR;
        else
                tb->grow_threshold = ((double) tb->size) * SH_FILLFACTOR;
}

/* return the optimal bucket for the hash */
static inline uint32
SH_INITIAL_BUCKET(SH_TYPE * tb, uint32 hash)
{
        return hash & tb->sizemask;
}

/* return next bucket after the current, handling wraparound */
static inline uint32
SH_NEXT(SH_TYPE * tb, uint32 curelem, uint32 startelem)
{
        curelem = (curelem + 1) & tb->sizemask;

        Assert(curelem != startelem);

        return curelem;
}

/* return bucket before the current, handling wraparound */
static inline uint32
SH_PREV(SH_TYPE * tb, uint32 curelem, uint32 startelem)
{
        curelem = (curelem - 1) & tb->sizemask;

        Assert(curelem != startelem);

        return curelem;
}

/* return distance between bucket and its optimal position */
static inline uint32
SH_DISTANCE_FROM_OPTIMAL(SH_TYPE * tb, uint32 optimal, uint32 bucket)
{
        if (optimal <= bucket)
                return bucket - optimal;
        else
                return (tb->size + bucket) - optimal;
}

static inline uint32
SH_ENTRY_HASH(SH_TYPE * tb, SH_ELEMENT_TYPE * entry)
{
#ifdef SH_STORE_HASH
        return SH_GET_HASH(tb, entry);
#else
        return SH_HASH_KEY(tb, entry->SH_KEY);
#endif
}

/* default memory allocator function */
static inline void *SH_ALLOCATE(SH_TYPE * type, Size size);
static inline void SH_FREE(SH_TYPE * type, void *pointer);

#ifndef SH_USE_NONDEFAULT_ALLOCATOR

/* default memory allocator function */
static inline void *
SH_ALLOCATE(SH_TYPE * type, Size size)
{
#ifdef SH_RAW_ALLOCATOR
        return SH_RAW_ALLOCATOR(size);
#else
        return MemoryContextAllocExtended(type->ctx, size,
                                                                          MCXT_ALLOC_HUGE | MCXT_ALLOC_ZERO);
#endif
}

/* default memory free function */
static inline void
SH_FREE(SH_TYPE * type, void *pointer)
{
        pfree(pointer);
}

#endif

/*
 * Create a hash table with enough space for `nelements` distinct members.
 * Memory for the hash table is allocated from the passed-in context.  If
 * desired, the array of elements can be allocated using a passed-in allocator;
 * this could be useful in order to place the array of elements in a shared
 * memory, or in a context that will outlive the rest of the hash table.
 * Memory other than for the array of elements will still be allocated from
 * the passed-in context.
 */
#ifdef SH_RAW_ALLOCATOR
SH_SCOPE        SH_TYPE *
SH_CREATE(uint32 nelements, void *private_data)
#else
SH_SCOPE        SH_TYPE *
SH_CREATE(MemoryContext ctx, uint32 nelements, void *private_data)
#endif
{
        SH_TYPE    *tb;
        uint64          size;

#ifdef SH_RAW_ALLOCATOR
        tb = SH_RAW_ALLOCATOR(sizeof(SH_TYPE));
#else
        tb = MemoryContextAllocZero(ctx, sizeof(SH_TYPE));
        tb->ctx = ctx;
#endif
        tb->private_data = private_data;

        /* increase nelements by fillfactor, want to store nelements elements */
        size = Min((double) SH_MAX_SIZE, ((double) nelements) / SH_FILLFACTOR);

        SH_COMPUTE_PARAMETERS(tb, size);

        tb->data = SH_ALLOCATE(tb, sizeof(SH_ELEMENT_TYPE) * tb->size);

        return tb;
}

/* destroy a previously created hash table */
SH_SCOPE void
SH_DESTROY(SH_TYPE * tb)
{
        SH_FREE(tb, tb->data);
        pfree(tb);
}

/* reset the contents of a previously created hash table */
SH_SCOPE void
SH_RESET(SH_TYPE * tb)
{
        memset(tb->data, 0, sizeof(SH_ELEMENT_TYPE) * tb->size);
        tb->members = 0;
}

/*
 * Grow a hash table to at least `newsize` buckets.
 *
 * Usually this will automatically be called by insertions/deletions, when
 * necessary. But resizing to the exact input size can be advantageous
 * performance-wise, when known at some point.
 */
SH_SCOPE void
SH_GROW(SH_TYPE * tb, uint64 newsize)
{
        uint64          oldsize = tb->size;
        SH_ELEMENT_TYPE *olddata = tb->data;
        SH_ELEMENT_TYPE *newdata;
        uint32          i;
        uint32          startelem = 0;
        uint32          copyelem;

        Assert(oldsize == pg_nextpower2_64(oldsize));
        Assert(oldsize != SH_MAX_SIZE);
        Assert(oldsize < newsize);

        /* compute parameters for new table */
        SH_COMPUTE_PARAMETERS(tb, newsize);

        tb->data = SH_ALLOCATE(tb, sizeof(SH_ELEMENT_TYPE) * tb->size);

        newdata = tb->data;

        /*
         * Copy entries from the old data to newdata. We theoretically could use
         * SH_INSERT here, to avoid code duplication, but that's more general than
         * we need. We neither want tb->members increased, nor do we need to do
         * deal with deleted elements, nor do we need to compare keys. So a
         * special-cased implementation is lot faster. As resizing can be time
         * consuming and frequent, that's worthwhile to optimize.
         *
         * To be able to simply move entries over, we have to start not at the
         * first bucket (i.e olddata[0]), but find the first bucket that's either
         * empty, or is occupied by an entry at its optimal position. Such a
         * bucket has to exist in any table with a load factor under 1, as not all
         * buckets are occupied, i.e. there always has to be an empty bucket.  By
         * starting at such a bucket we can move the entries to the larger table,
         * without having to deal with conflicts.
         */

        /* search for the first element in the hash that's not wrapped around */
        for (i = 0; i < oldsize; i++)
        {
                SH_ELEMENT_TYPE *oldentry = &olddata[i];
                uint32          hash;
                uint32          optimal;

                if (oldentry->status != SH_STATUS_IN_USE)
                {
                        startelem = i;
                        break;
                }

                hash = SH_ENTRY_HASH(tb, oldentry);
                optimal = SH_INITIAL_BUCKET(tb, hash);

                if (optimal == i)
                {
                        startelem = i;
                        break;
                }
        }

        /* and copy all elements in the old table */
        copyelem = startelem;
        for (i = 0; i < oldsize; i++)
        {
                SH_ELEMENT_TYPE *oldentry = &olddata[copyelem];

                if (oldentry->status == SH_STATUS_IN_USE)
                {
                        uint32          hash;
                        uint32          startelem;
                        uint32          curelem;
                        SH_ELEMENT_TYPE *newentry;

                        hash = SH_ENTRY_HASH(tb, oldentry);
                        startelem = SH_INITIAL_BUCKET(tb, hash);
                        curelem = startelem;

                        /* find empty element to put data into */
                        while (true)
                        {
                                newentry = &newdata[curelem];

                                if (newentry->status == SH_STATUS_EMPTY)
                                {
                                        break;
                                }

                                curelem = SH_NEXT(tb, curelem, startelem);
                        }

                        /* copy entry to new slot */
                        memcpy(newentry, oldentry, sizeof(SH_ELEMENT_TYPE));
                }

                /* can't use SH_NEXT here, would use new size */
                copyelem++;
                if (copyelem >= oldsize)
                {
                        copyelem = 0;
                }
        }

        SH_FREE(tb, olddata);
}

/*
 * This is a separate static inline function, so it can be reliably be inlined
 * into its wrapper functions even if SH_SCOPE is extern.
 */
static inline SH_ELEMENT_TYPE *
SH_INSERT_HASH_INTERNAL(SH_TYPE * tb, SH_KEY_TYPE key, uint32 hash, bool *found)
{
        uint32          startelem;
        uint32          curelem;
        SH_ELEMENT_TYPE *data;
        uint32          insertdist;

restart:
        insertdist = 0;

        /*
         * We do the grow check even if the key is actually present, to avoid
         * doing the check inside the loop. This also lets us avoid having to
         * re-find our position in the hashtable after resizing.
         *
         * Note that this also reached when resizing the table due to
         * SH_GROW_MAX_DIB / SH_GROW_MAX_MOVE.
         */
        if (unlikely(tb->members >= tb->grow_threshold))
        {
                if (unlikely(tb->size == SH_MAX_SIZE))
                        sh_error("hash table size exceeded");

                /*
                 * When optimizing, it can be very useful to print these out.
                 */
                /* SH_STAT(tb); */
                SH_GROW(tb, tb->size * 2);
                /* SH_STAT(tb); */
        }

        /* perform insert, start bucket search at optimal location */
        data = tb->data;
        startelem = SH_INITIAL_BUCKET(tb, hash);
        curelem = startelem;
        while (true)
        {
                uint32          curdist;
                uint32          curhash;
                uint32          curoptimal;
                SH_ELEMENT_TYPE *entry = &data[curelem];

                /* any empty bucket can directly be used */
                if (entry->status == SH_STATUS_EMPTY)
                {
                        tb->members++;
                        entry->SH_KEY = key;
#ifdef SH_STORE_HASH
                        SH_GET_HASH(tb, entry) = hash;
#endif
                        entry->status = SH_STATUS_IN_USE;
                        *found = false;
                        return entry;
                }

                /*
                 * If the bucket is not empty, we either found a match (in which case
                 * we're done), or we have to decide whether to skip over or move the
                 * colliding entry. When the colliding element's distance to its
                 * optimal position is smaller than the to-be-inserted entry's, we
                 * shift the colliding entry (and its followers) forward by one.
                 */

                if (SH_COMPARE_KEYS(tb, hash, key, entry))
                {
                        Assert(entry->status == SH_STATUS_IN_USE);
                        *found = true;
                        return entry;
                }

                curhash = SH_ENTRY_HASH(tb, entry);
                curoptimal = SH_INITIAL_BUCKET(tb, curhash);
                curdist = SH_DISTANCE_FROM_OPTIMAL(tb, curoptimal, curelem);

                if (insertdist > curdist)
                {
                        SH_ELEMENT_TYPE *lastentry = entry;
                        uint32          emptyelem = curelem;
                        uint32          moveelem;
                        int32           emptydist = 0;

                        /* find next empty bucket */
                        while (true)
                        {
                                SH_ELEMENT_TYPE *emptyentry;

                                emptyelem = SH_NEXT(tb, emptyelem, startelem);
                                emptyentry = &data[emptyelem];

                                if (emptyentry->status == SH_STATUS_EMPTY)
                                {
                                        lastentry = emptyentry;
                                        break;
                                }

                                /*
                                 * To avoid negative consequences from overly imbalanced
                                 * hashtables, grow the hashtable if collisions would require
                                 * us to move a lot of entries.  The most likely cause of such
                                 * imbalance is filling a (currently) small table, from a
                                 * currently big one, in hash-table order.  Don't grow if the
                                 * hashtable would be too empty, to prevent quick space
                                 * explosion for some weird edge cases.
                                 */
                                if (unlikely(++emptydist > SH_GROW_MAX_MOVE) &&
                                        ((double) tb->members / tb->size) >= SH_GROW_MIN_FILLFACTOR)
                                {
                                        tb->grow_threshold = 0;
                                        goto restart;
                                }
                        }

                        /* shift forward, starting at last occupied element */

                        /*
                         * TODO: This could be optimized to be one memcpy in many cases,
                         * excepting wrapping around at the end of ->data. Hasn't shown up
                         * in profiles so far though.
                         */
                        moveelem = emptyelem;
                        while (moveelem != curelem)
                        {
                                SH_ELEMENT_TYPE *moveentry;

                                moveelem = SH_PREV(tb, moveelem, startelem);
                                moveentry = &data[moveelem];

                                memcpy(lastentry, moveentry, sizeof(SH_ELEMENT_TYPE));
                                lastentry = moveentry;
                        }

                        /* and fill the now empty spot */
                        tb->members++;

                        entry->SH_KEY = key;
#ifdef SH_STORE_HASH
                        SH_GET_HASH(tb, entry) = hash;
#endif
                        entry->status = SH_STATUS_IN_USE;
                        *found = false;
                        return entry;
                }

                curelem = SH_NEXT(tb, curelem, startelem);
                insertdist++;

                /*
                 * To avoid negative consequences from overly imbalanced hashtables,
                 * grow the hashtable if collisions lead to large runs. The most
                 * likely cause of such imbalance is filling a (currently) small
                 * table, from a currently big one, in hash-table order.  Don't grow
                 * if the hashtable would be too empty, to prevent quick space
                 * explosion for some weird edge cases.
                 */
                if (unlikely(insertdist > SH_GROW_MAX_DIB) &&
                        ((double) tb->members / tb->size) >= SH_GROW_MIN_FILLFACTOR)
                {
                        tb->grow_threshold = 0;
                        goto restart;
                }
        }
}

/*
 * Insert the key key into the hash-table, set *found to true if the key
 * already exists, false otherwise. Returns the hash-table entry in either
 * case.
 */
SH_SCOPE        SH_ELEMENT_TYPE *
SH_INSERT(SH_TYPE * tb, SH_KEY_TYPE key, bool *found)
{
        uint32          hash = SH_HASH_KEY(tb, key);

        return SH_INSERT_HASH_INTERNAL(tb, key, hash, found);
}

/*
 * Insert the key key into the hash-table using an already-calculated
 * hash. Set *found to true if the key already exists, false
 * otherwise. Returns the hash-table entry in either case.
 */
SH_SCOPE        SH_ELEMENT_TYPE *
SH_INSERT_HASH(SH_TYPE * tb, SH_KEY_TYPE key, uint32 hash, bool *found)
{
        return SH_INSERT_HASH_INTERNAL(tb, key, hash, found);
}

/*
 * This is a separate static inline function, so it can be reliably be inlined
 * into its wrapper functions even if SH_SCOPE is extern.
 */
static inline SH_ELEMENT_TYPE *
SH_LOOKUP_HASH_INTERNAL(SH_TYPE * tb, SH_KEY_TYPE key, uint32 hash)
{
        const uint32 startelem = SH_INITIAL_BUCKET(tb, hash);
        uint32          curelem = startelem;

        while (true)
        {
                SH_ELEMENT_TYPE *entry = &tb->data[curelem];

                if (entry->status == SH_STATUS_EMPTY)
                {
                        return NULL;
                }

                Assert(entry->status == SH_STATUS_IN_USE);

                if (SH_COMPARE_KEYS(tb, hash, key, entry))
                        return entry;

                /*
                 * TODO: we could stop search based on distance. If the current
                 * buckets's distance-from-optimal is smaller than what we've skipped
                 * already, the entry doesn't exist. Probably only do so if
                 * SH_STORE_HASH is defined, to avoid re-computing hashes?
                 */

                curelem = SH_NEXT(tb, curelem, startelem);
        }
}

/*
 * Lookup up entry in hash table.  Returns NULL if key not present.
 */
SH_SCOPE        SH_ELEMENT_TYPE *
SH_LOOKUP(SH_TYPE * tb, SH_KEY_TYPE key)
{
        uint32          hash = SH_HASH_KEY(tb, key);

        return SH_LOOKUP_HASH_INTERNAL(tb, key, hash);
}

/*
 * Lookup up entry in hash table using an already-calculated hash.
 *
 * Returns NULL if key not present.
 */
SH_SCOPE        SH_ELEMENT_TYPE *
SH_LOOKUP_HASH(SH_TYPE * tb, SH_KEY_TYPE key, uint32 hash)
{
        return SH_LOOKUP_HASH_INTERNAL(tb, key, hash);
}

/*
 * Delete entry from hash table by key.  Returns whether to-be-deleted key was
 * present.
 */
SH_SCOPE bool
SH_DELETE(SH_TYPE * tb, SH_KEY_TYPE key)
{
        uint32          hash = SH_HASH_KEY(tb, key);
        uint32          startelem = SH_INITIAL_BUCKET(tb, hash);
        uint32          curelem = startelem;

        while (true)
        {
                SH_ELEMENT_TYPE *entry = &tb->data[curelem];

                if (entry->status == SH_STATUS_EMPTY)
                        return false;

                if (entry->status == SH_STATUS_IN_USE &&
                        SH_COMPARE_KEYS(tb, hash, key, entry))
                {
                        SH_ELEMENT_TYPE *lastentry = entry;

                        tb->members--;

                        /*
                         * Backward shift following elements till either an empty element
                         * or an element at its optimal position is encountered.
                         *
                         * While that sounds expensive, the average chain length is short,
                         * and deletions would otherwise require tombstones.
                         */
                        while (true)
                        {
                                SH_ELEMENT_TYPE *curentry;
                                uint32          curhash;
                                uint32          curoptimal;

                                curelem = SH_NEXT(tb, curelem, startelem);
                                curentry = &tb->data[curelem];

                                if (curentry->status != SH_STATUS_IN_USE)
                                {
                                        lastentry->status = SH_STATUS_EMPTY;
                                        break;
                                }

                                curhash = SH_ENTRY_HASH(tb, curentry);
                                curoptimal = SH_INITIAL_BUCKET(tb, curhash);

                                /* current is at optimal position, done */
                                if (curoptimal == curelem)
                                {
                                        lastentry->status = SH_STATUS_EMPTY;
                                        break;
                                }

                                /* shift */
                                memcpy(lastentry, curentry, sizeof(SH_ELEMENT_TYPE));

                                lastentry = curentry;
                        }

                        return true;
                }

                /* TODO: return false; if distance too big */

                curelem = SH_NEXT(tb, curelem, startelem);
        }
}

/*
 * Delete entry from hash table by entry pointer
 */
SH_SCOPE void
SH_DELETE_ITEM(SH_TYPE * tb, SH_ELEMENT_TYPE * entry)
{
        SH_ELEMENT_TYPE *lastentry = entry;
        uint32          hash = SH_ENTRY_HASH(tb, entry);
        uint32          startelem = SH_INITIAL_BUCKET(tb, hash);
        uint32          curelem;

        /* Calculate the index of 'entry' */
        curelem = entry - &tb->data[0];

        tb->members--;

        /*
         * Backward shift following elements till either an empty element or an
         * element at its optimal position is encountered.
         *
         * While that sounds expensive, the average chain length is short, and
         * deletions would otherwise require tombstones.
         */
        while (true)
        {
                SH_ELEMENT_TYPE *curentry;
                uint32          curhash;
                uint32          curoptimal;

                curelem = SH_NEXT(tb, curelem, startelem);
                curentry = &tb->data[curelem];

                if (curentry->status != SH_STATUS_IN_USE)
                {
                        lastentry->status = SH_STATUS_EMPTY;
                        break;
                }

                curhash = SH_ENTRY_HASH(tb, curentry);
                curoptimal = SH_INITIAL_BUCKET(tb, curhash);

                /* current is at optimal position, done */
                if (curoptimal == curelem)
                {
                        lastentry->status = SH_STATUS_EMPTY;
                        break;
                }

                /* shift */
                memcpy(lastentry, curentry, sizeof(SH_ELEMENT_TYPE));

                lastentry = curentry;
        }
}

/*
 * Initialize iterator.
 */
SH_SCOPE void
SH_START_ITERATE(SH_TYPE * tb, SH_ITERATOR * iter)
{
        int                     i;
        uint64          startelem = PG_UINT64_MAX;

        /*
         * Search for the first empty element. As deletions during iterations are
         * supported, we want to start/end at an element that cannot be affected
         * by elements being shifted.
         */
        for (i = 0; i < tb->size; i++)
        {
                SH_ELEMENT_TYPE *entry = &tb->data[i];

                if (entry->status != SH_STATUS_IN_USE)
                {
                        startelem = i;
                        break;
                }
        }

        Assert(startelem < SH_MAX_SIZE);

        /*
         * Iterate backwards, that allows the current element to be deleted, even
         * if there are backward shifts
         */
        iter->cur = startelem;
        iter->end = iter->cur;
        iter->done = false;
}

/*
 * Initialize iterator to a specific bucket. That's really only useful for
 * cases where callers are partially iterating over the hashspace, and that
 * iteration deletes and inserts elements based on visited entries. Doing that
 * repeatedly could lead to an unbalanced keyspace when always starting at the
 * same position.
 */
SH_SCOPE void
SH_START_ITERATE_AT(SH_TYPE * tb, SH_ITERATOR * iter, uint32 at)
{
        /*
         * Iterate backwards, that allows the current element to be deleted, even
         * if there are backward shifts.
         */
        iter->cur = at & tb->sizemask;  /* ensure at is within a valid range */
        iter->end = iter->cur;
        iter->done = false;
}

/*
 * Iterate over all entries in the hash-table. Return the next occupied entry,
 * or NULL if done.
 *
 * During iteration the current entry in the hash table may be deleted,
 * without leading to elements being skipped or returned twice.  Additionally
 * the rest of the table may be modified (i.e. there can be insertions or
 * deletions), but if so, there's neither a guarantee that all nodes are
 * visited at least once, nor a guarantee that a node is visited at most once.
 */
SH_SCOPE        SH_ELEMENT_TYPE *
SH_ITERATE(SH_TYPE * tb, SH_ITERATOR * iter)
{
        while (!iter->done)
        {
                SH_ELEMENT_TYPE *elem;

                elem = &tb->data[iter->cur];

                /* next element in backward direction */
                iter->cur = (iter->cur - 1) & tb->sizemask;

                if ((iter->cur & tb->sizemask) == (iter->end & tb->sizemask))
                        iter->done = true;
                if (elem->status == SH_STATUS_IN_USE)
                {
                        return elem;
                }
        }

        return NULL;
}

/*
 * Report some statistics about the state of the hashtable. For
 * debugging/profiling purposes only.
 */
SH_SCOPE void
SH_STAT(SH_TYPE * tb)
{
        uint32          max_chain_length = 0;
        uint32          total_chain_length = 0;
        double          avg_chain_length;
        double          fillfactor;
        uint32          i;

        uint32     *collisions = palloc0(tb->size * sizeof(uint32));
        uint32          total_collisions = 0;
        uint32          max_collisions = 0;
        double          avg_collisions;

        for (i = 0; i < tb->size; i++)
        {
                uint32          hash;
                uint32          optimal;
                uint32          dist;
                SH_ELEMENT_TYPE *elem;

                elem = &tb->data[i];

                if (elem->status != SH_STATUS_IN_USE)
                        continue;

                hash = SH_ENTRY_HASH(tb, elem);
                optimal = SH_INITIAL_BUCKET(tb, hash);
                dist = SH_DISTANCE_FROM_OPTIMAL(tb, optimal, i);

                if (dist > max_chain_length)
                        max_chain_length = dist;
                total_chain_length += dist;

                collisions[optimal]++;
        }

        for (i = 0; i < tb->size; i++)
        {
                uint32          curcoll = collisions[i];

                if (curcoll == 0)
                        continue;

                /* single contained element is not a collision */
                curcoll--;
                total_collisions += curcoll;
                if (curcoll > max_collisions)
                        max_collisions = curcoll;
        }

        if (tb->members > 0)
        {
                fillfactor = tb->members / ((double) tb->size);
                avg_chain_length = ((double) total_chain_length) / tb->members;
                avg_collisions = ((double) total_collisions) / tb->members;
        }
        else
        {
                fillfactor = 0;
                avg_chain_length = 0;
                avg_collisions = 0;
        }

        sh_log("size: " UINT64_FORMAT ", members: %u, filled: %f, total chain: %u, max chain: %u, avg chain: %f, total_collisions: %u, max_collisions: %u, avg_collisions: %f",
                   tb->size, tb->members, fillfactor, total_chain_length, max_chain_length, avg_chain_length,
                   total_collisions, max_collisions, avg_collisions);
}

#endif                                                  /* SH_DEFINE */


/* undefine external parameters, so next hash table can be defined */
#undef SH_PREFIX
#undef SH_KEY_TYPE
#undef SH_KEY
#undef SH_ELEMENT_TYPE
#undef SH_HASH_KEY
#undef SH_SCOPE
#undef SH_DECLARE
#undef SH_DEFINE
#undef SH_GET_HASH
#undef SH_STORE_HASH
#undef SH_USE_NONDEFAULT_ALLOCATOR
#undef SH_EQUAL

/* undefine locally declared macros */
#undef SH_MAKE_PREFIX
#undef SH_MAKE_NAME
#undef SH_MAKE_NAME_
#undef SH_FILLFACTOR
#undef SH_MAX_FILLFACTOR
#undef SH_GROW_MAX_DIB
#undef SH_GROW_MAX_MOVE
#undef SH_GROW_MIN_FILLFACTOR
#undef SH_MAX_SIZE

/* types */
#undef SH_TYPE
#undef SH_STATUS
#undef SH_STATUS_EMPTY
#undef SH_STATUS_IN_USE
#undef SH_ITERATOR

/* external function names */
#undef SH_CREATE
#undef SH_DESTROY
#undef SH_RESET
#undef SH_INSERT
#undef SH_INSERT_HASH
#undef SH_DELETE_ITEM
#undef SH_DELETE
#undef SH_LOOKUP
#undef SH_LOOKUP_HASH
#undef SH_GROW
#undef SH_START_ITERATE
#undef SH_START_ITERATE_AT
#undef SH_ITERATE
#undef SH_ALLOCATE
#undef SH_FREE
#undef SH_STAT

/* internal function names */
#undef SH_COMPUTE_PARAMETERS
#undef SH_COMPARE_KEYS
#undef SH_INITIAL_BUCKET
#undef SH_NEXT
#undef SH_PREV
#undef SH_DISTANCE_FROM_OPTIMAL
#undef SH_ENTRY_HASH
#undef SH_INSERT_HASH_INTERNAL
#undef SH_LOOKUP_HASH_INTERNAL