postgresql/contrib/pg_trgm/trgm_gist.c

/*
 * contrib/pg_trgm/trgm_gist.c
 */
#include "postgres.h"

#include "trgm.h"

#include "access/stratnum.h"
#include "fmgr.h"


typedef struct
{
	/* most recent inputs to gtrgm_consistent */
	StrategyNumber strategy;
	text	   *query;
	/* extracted trigrams for query */
	TRGM	   *trigrams;
	/* if a regex operator, the extracted graph */
	TrgmPackedGraph *graph;

	/*
	 * The "query" and "trigrams" are stored in the same palloc block as this
	 * cache struct, at MAXALIGN'ed offsets.  The graph however isn't.
	 */
} gtrgm_consistent_cache;

#define GETENTRY(vec,pos) ((TRGM *) DatumGetPointer((vec)->vector[(pos)].key))


PG_FUNCTION_INFO_V1(gtrgm_in);
PG_FUNCTION_INFO_V1(gtrgm_out);
PG_FUNCTION_INFO_V1(gtrgm_compress);
PG_FUNCTION_INFO_V1(gtrgm_decompress);
PG_FUNCTION_INFO_V1(gtrgm_consistent);
PG_FUNCTION_INFO_V1(gtrgm_distance);
PG_FUNCTION_INFO_V1(gtrgm_union);
PG_FUNCTION_INFO_V1(gtrgm_same);
PG_FUNCTION_INFO_V1(gtrgm_penalty);
PG_FUNCTION_INFO_V1(gtrgm_picksplit);

/* Number of one-bits in an unsigned byte */
static const uint8 number_of_ones[256] = {
	0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
	1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
	1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
	2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
	1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
	2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
	2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
	3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
	1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
	2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
	2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
	3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
	2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
	3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
	3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
	4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8
};


Datum
gtrgm_in(PG_FUNCTION_ARGS)
{
	elog(ERROR, "not implemented");
	PG_RETURN_DATUM(0);
}

Datum
gtrgm_out(PG_FUNCTION_ARGS)
{
	elog(ERROR, "not implemented");
	PG_RETURN_DATUM(0);
}

static void
makesign(BITVECP sign, TRGM *a)
{
	int32		k,
				len = ARRNELEM(a);
	trgm	   *ptr = GETARR(a);
	int32		tmp = 0;

	MemSet((void *) sign, 0, sizeof(BITVEC));
	SETBIT(sign, SIGLENBIT);	/* set last unused bit */
	for (k = 0; k < len; k++)
	{
		CPTRGM(((char *) &tmp), ptr + k);
		HASH(sign, tmp);
	}
}

Datum
gtrgm_compress(PG_FUNCTION_ARGS)
{
	GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
	GISTENTRY  *retval = entry;

	if (entry->leafkey)
	{							/* trgm */
		TRGM	   *res;
		text	   *val = DatumGetTextP(entry->key);

		res = generate_trgm(VARDATA(val), VARSIZE(val) - VARHDRSZ);
		retval = (GISTENTRY *) palloc(sizeof(GISTENTRY));
		gistentryinit(*retval, PointerGetDatum(res),
					  entry->rel, entry->page,
					  entry->offset, FALSE);
	}
	else if (ISSIGNKEY(DatumGetPointer(entry->key)) &&
			 !ISALLTRUE(DatumGetPointer(entry->key)))
	{
		int32		i,
					len;
		TRGM	   *res;
		BITVECP		sign = GETSIGN(DatumGetPointer(entry->key));

		LOOPBYTE
		{
			if ((sign[i] & 0xff) != 0xff)
				PG_RETURN_POINTER(retval);
		}

		len = CALCGTSIZE(SIGNKEY | ALLISTRUE, 0);
		res = (TRGM *) palloc(len);
		SET_VARSIZE(res, len);
		res->flag = SIGNKEY | ALLISTRUE;

		retval = (GISTENTRY *) palloc(sizeof(GISTENTRY));
		gistentryinit(*retval, PointerGetDatum(res),
					  entry->rel, entry->page,
					  entry->offset, FALSE);
	}
	PG_RETURN_POINTER(retval);
}

Datum
gtrgm_decompress(PG_FUNCTION_ARGS)
{
	GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
	GISTENTRY  *retval;
	text	   *key;

	key = DatumGetTextP(entry->key);

	if (key != (text *) DatumGetPointer(entry->key))
	{
		/* need to pass back the decompressed item */
		retval = palloc(sizeof(GISTENTRY));
		gistentryinit(*retval, PointerGetDatum(key),
					  entry->rel, entry->page, entry->offset, entry->leafkey);
		PG_RETURN_POINTER(retval);
	}
	else
	{
		/* we can return the entry as-is */
		PG_RETURN_POINTER(entry);
	}
}

static int32
cnt_sml_sign_common(TRGM *qtrg, BITVECP sign)
{
	int32		count = 0;
	int32		k,
				len = ARRNELEM(qtrg);
	trgm	   *ptr = GETARR(qtrg);
	int32		tmp = 0;

	for (k = 0; k < len; k++)
	{
		CPTRGM(((char *) &tmp), ptr + k);
		count += GETBIT(sign, HASHVAL(tmp));
	}

	return count;
}

Datum
gtrgm_consistent(PG_FUNCTION_ARGS)
{
	GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
	text	   *query = PG_GETARG_TEXT_P(1);
	StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);

	/* Oid		subtype = PG_GETARG_OID(3); */
	bool	   *recheck = (bool *) PG_GETARG_POINTER(4);
	TRGM	   *key = (TRGM *) DatumGetPointer(entry->key);
	TRGM	   *qtrg;
	bool		res;
	Size		querysize = VARSIZE(query);
	gtrgm_consistent_cache *cache;
	double		nlimit;

	/*
	 * We keep the extracted trigrams in cache, because trigram extraction is
	 * relatively CPU-expensive.  When trying to reuse a cached value, check
	 * strategy number not just query itself, because trigram extraction
	 * depends on strategy.
	 *
	 * The cached structure is a single palloc chunk containing the
	 * gtrgm_consistent_cache header, then the input query (starting at a
	 * MAXALIGN boundary), then the TRGM value (also starting at a MAXALIGN
	 * boundary).  However we don't try to include the regex graph (if any) in
	 * that struct.  (XXX currently, this approach can leak regex graphs
	 * across index rescans.  Not clear if that's worth fixing.)
	 */
	cache = (gtrgm_consistent_cache *) fcinfo->flinfo->fn_extra;
	if (cache == NULL ||
		cache->strategy != strategy ||
		VARSIZE(cache->query) != querysize ||
		memcmp((char *) cache->query, (char *) query, querysize) != 0)
	{
		gtrgm_consistent_cache *newcache;
		TrgmPackedGraph *graph = NULL;
		Size		qtrgsize;

		switch (strategy)
		{
			case SimilarityStrategyNumber:
			case WordSimilarityStrategyNumber:
				qtrg = generate_trgm(VARDATA(query),
									 querysize - VARHDRSZ);
				break;
			case ILikeStrategyNumber:
#ifndef IGNORECASE
				elog(ERROR, "cannot handle ~~* with case-sensitive trigrams");
#endif
				/* FALL THRU */
			case LikeStrategyNumber:
				qtrg = generate_wildcard_trgm(VARDATA(query),
											  querysize - VARHDRSZ);
				break;
			case RegExpICaseStrategyNumber:
#ifndef IGNORECASE
				elog(ERROR, "cannot handle ~* with case-sensitive trigrams");
#endif
				/* FALL THRU */
			case RegExpStrategyNumber:
				qtrg = createTrgmNFA(query, PG_GET_COLLATION(),
									 &graph, fcinfo->flinfo->fn_mcxt);
				/* just in case an empty array is returned ... */
				if (qtrg && ARRNELEM(qtrg) <= 0)
				{
					pfree(qtrg);
					qtrg = NULL;
				}
				break;
			default:
				elog(ERROR, "unrecognized strategy number: %d", strategy);
				qtrg = NULL;	/* keep compiler quiet */
				break;
		}

		qtrgsize = qtrg ? VARSIZE(qtrg) : 0;

		newcache = (gtrgm_consistent_cache *)
			MemoryContextAlloc(fcinfo->flinfo->fn_mcxt,
							   MAXALIGN(sizeof(gtrgm_consistent_cache)) +
							   MAXALIGN(querysize) +
							   qtrgsize);

		newcache->strategy = strategy;
		newcache->query = (text *)
			((char *) newcache + MAXALIGN(sizeof(gtrgm_consistent_cache)));
		memcpy((char *) newcache->query, (char *) query, querysize);
		if (qtrg)
		{
			newcache->trigrams = (TRGM *)
				((char *) newcache->query + MAXALIGN(querysize));
			memcpy((char *) newcache->trigrams, (char *) qtrg, qtrgsize);
			/* release qtrg in case it was made in fn_mcxt */
			pfree(qtrg);
		}
		else
			newcache->trigrams = NULL;
		newcache->graph = graph;

		if (cache)
			pfree(cache);
		fcinfo->flinfo->fn_extra = (void *) newcache;
		cache = newcache;
	}

	qtrg = cache->trigrams;

	switch (strategy)
	{
		case SimilarityStrategyNumber:
		case WordSimilarityStrategyNumber:
			/* Similarity search is exact. Word similarity search is inexact */
			*recheck = (strategy == WordSimilarityStrategyNumber);
			nlimit = (strategy == SimilarityStrategyNumber) ?
				similarity_threshold : word_similarity_threshold;

			if (GIST_LEAF(entry))
			{					/* all leafs contains orig trgm */
				double		tmpsml = cnt_sml(qtrg, key, *recheck);

				res = (tmpsml >= nlimit);
			}
			else if (ISALLTRUE(key))
			{					/* non-leaf contains signature */
				res = true;
			}
			else
			{					/* non-leaf contains signature */
				int32		count = cnt_sml_sign_common(qtrg, GETSIGN(key));
				int32		len = ARRNELEM(qtrg);

				if (len == 0)
					res = false;
				else
					res = (((((float8) count) / ((float8) len))) >= nlimit);
			}
			break;
		case ILikeStrategyNumber:
#ifndef IGNORECASE
			elog(ERROR, "cannot handle ~~* with case-sensitive trigrams");
#endif
			/* FALL THRU */
		case LikeStrategyNumber:
			/* Wildcard search is inexact */
			*recheck = true;

			/*
			 * Check if all the extracted trigrams can be present in child
			 * nodes.
			 */
			if (GIST_LEAF(entry))
			{					/* all leafs contains orig trgm */
				res = trgm_contained_by(qtrg, key);
			}
			else if (ISALLTRUE(key))
			{					/* non-leaf contains signature */
				res = true;
			}
			else
			{					/* non-leaf contains signature */
				int32		k,
							tmp = 0,
							len = ARRNELEM(qtrg);
				trgm	   *ptr = GETARR(qtrg);
				BITVECP		sign = GETSIGN(key);

				res = true;
				for (k = 0; k < len; k++)
				{
					CPTRGM(((char *) &tmp), ptr + k);
					if (!GETBIT(sign, HASHVAL(tmp)))
					{
						res = false;
						break;
					}
				}
			}
			break;
		case RegExpICaseStrategyNumber:
#ifndef IGNORECASE
			elog(ERROR, "cannot handle ~* with case-sensitive trigrams");
#endif
			/* FALL THRU */
		case RegExpStrategyNumber:
			/* Regexp search is inexact */
			*recheck = true;

			/* Check regex match as much as we can with available info */
			if (qtrg)
			{
				if (GIST_LEAF(entry))
				{				/* all leafs contains orig trgm */
					bool	   *check;

					check = trgm_presence_map(qtrg, key);
					res = trigramsMatchGraph(cache->graph, check);
					pfree(check);
				}
				else if (ISALLTRUE(key))
				{				/* non-leaf contains signature */
					res = true;
				}
				else
				{				/* non-leaf contains signature */
					int32		k,
								tmp = 0,
								len = ARRNELEM(qtrg);
					trgm	   *ptr = GETARR(qtrg);
					BITVECP		sign = GETSIGN(key);
					bool	   *check;

					/*
					 * GETBIT() tests may give false positives, due to limited
					 * size of the sign array.  But since trigramsMatchGraph()
					 * implements a monotone boolean function, false positives
					 * in the check array can't lead to false negative answer.
					 * So we can apply trigramsMatchGraph despite uncertainty,
					 * and that usefully improves the quality of the search.
					 */
					check = (bool *) palloc(len * sizeof(bool));
					for (k = 0; k < len; k++)
					{
						CPTRGM(((char *) &tmp), ptr + k);
						check[k] = GETBIT(sign, HASHVAL(tmp));
					}
					res = trigramsMatchGraph(cache->graph, check);
					pfree(check);
				}
			}
			else
			{
				/* trigram-free query must be rechecked everywhere */
				res = true;
			}
			break;
		default:
			elog(ERROR, "unrecognized strategy number: %d", strategy);
			res = false;		/* keep compiler quiet */
			break;
	}

	PG_RETURN_BOOL(res);
}

Datum
gtrgm_distance(PG_FUNCTION_ARGS)
{
	GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
	text	   *query = PG_GETARG_TEXT_P(1);
	StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);

	/* Oid		subtype = PG_GETARG_OID(3); */
	bool	   *recheck = (bool *) PG_GETARG_POINTER(4);
	TRGM	   *key = (TRGM *) DatumGetPointer(entry->key);
	TRGM	   *qtrg;
	float8		res;
	Size		querysize = VARSIZE(query);
	char	   *cache = (char *) fcinfo->flinfo->fn_extra;

	/*
	 * Cache the generated trigrams across multiple calls with the same query.
	 */
	if (cache == NULL ||
		VARSIZE(cache) != querysize ||
		memcmp(cache, query, querysize) != 0)
	{
		char	   *newcache;

		qtrg = generate_trgm(VARDATA(query), querysize - VARHDRSZ);

		newcache = MemoryContextAlloc(fcinfo->flinfo->fn_mcxt,
									  MAXALIGN(querysize) +
									  VARSIZE(qtrg));

		memcpy(newcache, query, querysize);
		memcpy(newcache + MAXALIGN(querysize), qtrg, VARSIZE(qtrg));

		if (cache)
			pfree(cache);
		fcinfo->flinfo->fn_extra = newcache;
		cache = newcache;
	}

	qtrg = (TRGM *) (cache + MAXALIGN(querysize));

	switch (strategy)
	{
		case DistanceStrategyNumber:
		case WordDistanceStrategyNumber:
			*recheck = strategy == WordDistanceStrategyNumber;
			if (GIST_LEAF(entry))
			{					/* all leafs contains orig trgm */

				/*
				 * Prevent gcc optimizing the sml variable using volatile
				 * keyword. Otherwise res can differ from the
				 * word_similarity_dist_op() function.
				 */
				float4 volatile sml = cnt_sml(qtrg, key, *recheck);

				res = 1.0 - sml;
			}
			else if (ISALLTRUE(key))
			{					/* all leafs contains orig trgm */
				res = 0.0;
			}
			else
			{					/* non-leaf contains signature */
				int32		count = cnt_sml_sign_common(qtrg, GETSIGN(key));
				int32		len = ARRNELEM(qtrg);

				res = (len == 0) ? -1.0 : 1.0 - ((float8) count) / ((float8) len);
			}
			break;
		default:
			elog(ERROR, "unrecognized strategy number: %d", strategy);
			res = 0;			/* keep compiler quiet */
			break;
	}

	PG_RETURN_FLOAT8(res);
}

static int32
unionkey(BITVECP sbase, TRGM *add)
{
	int32		i;

	if (ISSIGNKEY(add))
	{
		BITVECP		sadd = GETSIGN(add);

		if (ISALLTRUE(add))
			return 1;

		LOOPBYTE
			sbase[i] |= sadd[i];
	}
	else
	{
		trgm	   *ptr = GETARR(add);
		int32		tmp = 0;

		for (i = 0; i < ARRNELEM(add); i++)
		{
			CPTRGM(((char *) &tmp), ptr + i);
			HASH(sbase, tmp);
		}
	}
	return 0;
}


Datum
gtrgm_union(PG_FUNCTION_ARGS)
{
	GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
	int32		len = entryvec->n;
	int		   *size = (int *) PG_GETARG_POINTER(1);
	BITVEC		base;
	int32		i;
	int32		flag = 0;
	TRGM	   *result;

	MemSet((void *) base, 0, sizeof(BITVEC));
	for (i = 0; i < len; i++)
	{
		if (unionkey(base, GETENTRY(entryvec, i)))
		{
			flag = ALLISTRUE;
			break;
		}
	}

	flag |= SIGNKEY;
	len = CALCGTSIZE(flag, 0);
	result = (TRGM *) palloc(len);
	SET_VARSIZE(result, len);
	result->flag = flag;
	if (!ISALLTRUE(result))
		memcpy((void *) GETSIGN(result), (void *) base, sizeof(BITVEC));
	*size = len;

	PG_RETURN_POINTER(result);
}

Datum
gtrgm_same(PG_FUNCTION_ARGS)
{
	TRGM	   *a = (TRGM *) PG_GETARG_POINTER(0);
	TRGM	   *b = (TRGM *) PG_GETARG_POINTER(1);
	bool	   *result = (bool *) PG_GETARG_POINTER(2);

	if (ISSIGNKEY(a))
	{							/* then b also ISSIGNKEY */
		if (ISALLTRUE(a) && ISALLTRUE(b))
			*result = true;
		else if (ISALLTRUE(a))
			*result = false;
		else if (ISALLTRUE(b))
			*result = false;
		else
		{
			int32		i;
			BITVECP		sa = GETSIGN(a),
						sb = GETSIGN(b);

			*result = true;
			LOOPBYTE
			{
				if (sa[i] != sb[i])
				{
					*result = false;
					break;
				}
			}
		}
	}
	else
	{							/* a and b ISARRKEY */
		int32		lena = ARRNELEM(a),
					lenb = ARRNELEM(b);

		if (lena != lenb)
			*result = false;
		else
		{
			trgm	   *ptra = GETARR(a),
					   *ptrb = GETARR(b);
			int32		i;

			*result = true;
			for (i = 0; i < lena; i++)
				if (CMPTRGM(ptra + i, ptrb + i))
				{
					*result = false;
					break;
				}
		}
	}

	PG_RETURN_POINTER(result);
}

static int32
sizebitvec(BITVECP sign)
{
	int32		size = 0,
				i;

	LOOPBYTE
		size += number_of_ones[(unsigned char) sign[i]];
	return size;
}

static int
hemdistsign(BITVECP a, BITVECP b)
{
	int			i,
				diff,
				dist = 0;

	LOOPBYTE
	{
		diff = (unsigned char) (a[i] ^ b[i]);
		dist += number_of_ones[diff];
	}
	return dist;
}

static int
hemdist(TRGM *a, TRGM *b)
{
	if (ISALLTRUE(a))
	{
		if (ISALLTRUE(b))
			return 0;
		else
			return SIGLENBIT - sizebitvec(GETSIGN(b));
	}
	else if (ISALLTRUE(b))
		return SIGLENBIT - sizebitvec(GETSIGN(a));

	return hemdistsign(GETSIGN(a), GETSIGN(b));
}

Datum
gtrgm_penalty(PG_FUNCTION_ARGS)
{
	GISTENTRY  *origentry = (GISTENTRY *) PG_GETARG_POINTER(0); /* always ISSIGNKEY */
	GISTENTRY  *newentry = (GISTENTRY *) PG_GETARG_POINTER(1);
	float	   *penalty = (float *) PG_GETARG_POINTER(2);
	TRGM	   *origval = (TRGM *) DatumGetPointer(origentry->key);
	TRGM	   *newval = (TRGM *) DatumGetPointer(newentry->key);
	BITVECP		orig = GETSIGN(origval);

	*penalty = 0.0;

	if (ISARRKEY(newval))
	{
		char	   *cache = (char *) fcinfo->flinfo->fn_extra;
		TRGM	   *cachedVal = (TRGM *) (cache + MAXALIGN(sizeof(BITVEC)));
		Size		newvalsize = VARSIZE(newval);
		BITVECP		sign;

		/*
		 * Cache the sign data across multiple calls with the same newval.
		 */
		if (cache == NULL ||
			VARSIZE(cachedVal) != newvalsize ||
			memcmp(cachedVal, newval, newvalsize) != 0)
		{
			char	   *newcache;

			newcache = MemoryContextAlloc(fcinfo->flinfo->fn_mcxt,
										  MAXALIGN(sizeof(BITVEC)) +
										  newvalsize);

			makesign((BITVECP) newcache, newval);

			cachedVal = (TRGM *) (newcache + MAXALIGN(sizeof(BITVEC)));
			memcpy(cachedVal, newval, newvalsize);

			if (cache)
				pfree(cache);
			fcinfo->flinfo->fn_extra = newcache;
			cache = newcache;
		}

		sign = (BITVECP) cache;

		if (ISALLTRUE(origval))
			*penalty = ((float) (SIGLENBIT - sizebitvec(sign))) / (float) (SIGLENBIT + 1);
		else
			*penalty = hemdistsign(sign, orig);
	}
	else
		*penalty = hemdist(origval, newval);
	PG_RETURN_POINTER(penalty);
}

typedef struct
{
	bool		allistrue;
	BITVEC		sign;
} CACHESIGN;

static void
fillcache(CACHESIGN *item, TRGM *key)
{
	item->allistrue = false;
	if (ISARRKEY(key))
		makesign(item->sign, key);
	else if (ISALLTRUE(key))
		item->allistrue = true;
	else
		memcpy((void *) item->sign, (void *) GETSIGN(key), sizeof(BITVEC));
}

#define WISH_F(a,b,c) (double)( -(double)(((a)-(b))*((a)-(b))*((a)-(b)))*(c) )
typedef struct
{
	OffsetNumber pos;
	int32		cost;
} SPLITCOST;

static int
comparecost(const void *a, const void *b)
{
	if (((const SPLITCOST *) a)->cost == ((const SPLITCOST *) b)->cost)
		return 0;
	else
		return (((const SPLITCOST *) a)->cost > ((const SPLITCOST *) b)->cost) ? 1 : -1;
}


static int
hemdistcache(CACHESIGN *a, CACHESIGN *b)
{
	if (a->allistrue)
	{
		if (b->allistrue)
			return 0;
		else
			return SIGLENBIT - sizebitvec(b->sign);
	}
	else if (b->allistrue)
		return SIGLENBIT - sizebitvec(a->sign);

	return hemdistsign(a->sign, b->sign);
}

Datum
gtrgm_picksplit(PG_FUNCTION_ARGS)
{
	GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
	OffsetNumber maxoff = entryvec->n - 2;
	GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1);
	OffsetNumber k,
				j;
	TRGM	   *datum_l,
			   *datum_r;
	BITVECP		union_l,
				union_r;
	int32		size_alpha,
				size_beta;
	int32		size_waste,
				waste = -1;
	int32		nbytes;
	OffsetNumber seed_1 = 0,
				seed_2 = 0;
	OffsetNumber *left,
			   *right;
	BITVECP		ptr;
	int			i;
	CACHESIGN  *cache;
	SPLITCOST  *costvector;

	/* cache the sign data for each existing item */
	cache = (CACHESIGN *) palloc(sizeof(CACHESIGN) * (maxoff + 2));
	for (k = FirstOffsetNumber; k <= maxoff; k = OffsetNumberNext(k))
		fillcache(&cache[k], GETENTRY(entryvec, k));

	/* now find the two furthest-apart items */
	for (k = FirstOffsetNumber; k < maxoff; k = OffsetNumberNext(k))
	{
		for (j = OffsetNumberNext(k); j <= maxoff; j = OffsetNumberNext(j))
		{
			size_waste = hemdistcache(&(cache[j]), &(cache[k]));
			if (size_waste > waste)
			{
				waste = size_waste;
				seed_1 = k;
				seed_2 = j;
			}
		}
	}

	/* just in case we didn't make a selection ... */
	if (seed_1 == 0 || seed_2 == 0)
	{
		seed_1 = 1;
		seed_2 = 2;
	}

	/* initialize the result vectors */
	nbytes = (maxoff + 2) * sizeof(OffsetNumber);
	v->spl_left = left = (OffsetNumber *) palloc(nbytes);
	v->spl_right = right = (OffsetNumber *) palloc(nbytes);
	v->spl_nleft = 0;
	v->spl_nright = 0;

	/* form initial .. */
	if (cache[seed_1].allistrue)
	{
		datum_l = (TRGM *) palloc(CALCGTSIZE(SIGNKEY | ALLISTRUE, 0));
		SET_VARSIZE(datum_l, CALCGTSIZE(SIGNKEY | ALLISTRUE, 0));
		datum_l->flag = SIGNKEY | ALLISTRUE;
	}
	else
	{
		datum_l = (TRGM *) palloc(CALCGTSIZE(SIGNKEY, 0));
		SET_VARSIZE(datum_l, CALCGTSIZE(SIGNKEY, 0));
		datum_l->flag = SIGNKEY;
		memcpy((void *) GETSIGN(datum_l), (void *) cache[seed_1].sign, sizeof(BITVEC));
	}
	if (cache[seed_2].allistrue)
	{
		datum_r = (TRGM *) palloc(CALCGTSIZE(SIGNKEY | ALLISTRUE, 0));
		SET_VARSIZE(datum_r, CALCGTSIZE(SIGNKEY | ALLISTRUE, 0));
		datum_r->flag = SIGNKEY | ALLISTRUE;
	}
	else
	{
		datum_r = (TRGM *) palloc(CALCGTSIZE(SIGNKEY, 0));
		SET_VARSIZE(datum_r, CALCGTSIZE(SIGNKEY, 0));
		datum_r->flag = SIGNKEY;
		memcpy((void *) GETSIGN(datum_r), (void *) cache[seed_2].sign, sizeof(BITVEC));
	}

	union_l = GETSIGN(datum_l);
	union_r = GETSIGN(datum_r);
	maxoff = OffsetNumberNext(maxoff);
	fillcache(&cache[maxoff], GETENTRY(entryvec, maxoff));
	/* sort before ... */
	costvector = (SPLITCOST *) palloc(sizeof(SPLITCOST) * maxoff);
	for (j = FirstOffsetNumber; j <= maxoff; j = OffsetNumberNext(j))
	{
		costvector[j - 1].pos = j;
		size_alpha = hemdistcache(&(cache[seed_1]), &(cache[j]));
		size_beta = hemdistcache(&(cache[seed_2]), &(cache[j]));
		costvector[j - 1].cost = abs(size_alpha - size_beta);
	}
	qsort((void *) costvector, maxoff, sizeof(SPLITCOST), comparecost);

	for (k = 0; k < maxoff; k++)
	{
		j = costvector[k].pos;
		if (j == seed_1)
		{
			*left++ = j;
			v->spl_nleft++;
			continue;
		}
		else if (j == seed_2)
		{
			*right++ = j;
			v->spl_nright++;
			continue;
		}

		if (ISALLTRUE(datum_l) || cache[j].allistrue)
		{
			if (ISALLTRUE(datum_l) && cache[j].allistrue)
				size_alpha = 0;
			else
				size_alpha = SIGLENBIT - sizebitvec(
													(cache[j].allistrue) ? GETSIGN(datum_l) : GETSIGN(cache[j].sign)
					);
		}
		else
			size_alpha = hemdistsign(cache[j].sign, GETSIGN(datum_l));

		if (ISALLTRUE(datum_r) || cache[j].allistrue)
		{
			if (ISALLTRUE(datum_r) && cache[j].allistrue)
				size_beta = 0;
			else
				size_beta = SIGLENBIT - sizebitvec(
												   (cache[j].allistrue) ? GETSIGN(datum_r) : GETSIGN(cache[j].sign)
					);
		}
		else
			size_beta = hemdistsign(cache[j].sign, GETSIGN(datum_r));

		if (size_alpha < size_beta + WISH_F(v->spl_nleft, v->spl_nright, 0.1))
		{
			if (ISALLTRUE(datum_l) || cache[j].allistrue)
			{
				if (!ISALLTRUE(datum_l))
					MemSet((void *) GETSIGN(datum_l), 0xff, sizeof(BITVEC));
			}
			else
			{
				ptr = cache[j].sign;
				LOOPBYTE
					union_l[i] |= ptr[i];
			}
			*left++ = j;
			v->spl_nleft++;
		}
		else
		{
			if (ISALLTRUE(datum_r) || cache[j].allistrue)
			{
				if (!ISALLTRUE(datum_r))
					MemSet((void *) GETSIGN(datum_r), 0xff, sizeof(BITVEC));
			}
			else
			{
				ptr = cache[j].sign;
				LOOPBYTE
					union_r[i] |= ptr[i];
			}
			*right++ = j;
			v->spl_nright++;
		}
	}

	*right = *left = FirstOffsetNumber;
	v->spl_ldatum = PointerGetDatum(datum_l);
	v->spl_rdatum = PointerGetDatum(datum_r);

	PG_RETURN_POINTER(v);
}