postgresql/contrib/pg_trgm/trgm_regexp.c

/*-------------------------------------------------------------------------
 *
 * trgm_regexp.c
 *	  Regular expression matching using trigrams.
 *
 * The general idea of trigram index support for a regular expression (regex)
 * search is to transform the regex into a logical expression on trigrams.
 * For example:
 *
 *	 (ab|cd)efg  =>  ((abe & bef) | (cde & def)) & efg
 *
 * If a string matches the regex, then it must match the logical expression on
 * trigrams.  The opposite is not necessarily true, however: a string that
 * matches the logical expression might not match the original regex.  Such
 * false positives are removed via recheck, by running the regular regex match
 * operator on the retrieved heap tuple.
 *
 * Since the trigram expression involves both AND and OR operators, we can't
 * expect the core index machinery to evaluate it completely.  Instead, the
 * result of regex analysis is a list of trigrams to be sought in the index,
 * plus a simplified graph that is used by trigramsMatchGraph() to determine
 * whether a particular indexed value matches the expression.
 *
 * Converting a regex to a trigram expression is based on analysis of an
 * automaton corresponding to the regex.  The algorithm consists of four
 * stages:
 *
 * 1) Compile the regexp to NFA form.  This is handled by the PostgreSQL
 *	  regexp library, which provides accessors for its opaque regex_t struct
 *	  to expose the NFA state graph and the "colors" (sets of equivalent
 *	  characters) used as state transition labels.
 *
 * 2) Transform the original NFA into an expanded graph, where arcs
 *	  are labeled with trigrams that must be present in order to move from
 *	  one state to another via the arcs.  The trigrams used in this stage
 *	  consist of colors, not characters, as in the original NFA.
 *
 * 3) Expand the color trigrams into regular trigrams consisting of
 *	  characters.  If too many distinct trigrams are produced, trigrams are
 *	  eliminated and the graph is simplified until it's simple enough.
 *
 * 4) Finally, the resulting graph is packed into a TrgmPackedGraph struct,
 *	  and returned to the caller.
 *
 * 1) Compile the regexp to NFA form
 * ---------------------------------
 * The automaton returned by the regexp compiler is a graph where vertices
 * are "states" and arcs are labeled with colors.  Each color represents
 * a set of characters, so that all characters assigned to the same color
 * are interchangeable, so far as matching the regexp is concerned.  There
 * are two special states: "initial" and "final".  A state can have multiple
 * outgoing arcs labeled with the same color, which makes the automaton
 * non-deterministic, because it can be in many states simultaneously.
 *
 * Note that this NFA is already lossy compared to the original regexp,
 * since it ignores some regex features such as lookahead constraints and
 * backref matching.  This is OK for our purposes since it's still the case
 * that only strings matching the NFA can possibly satisfy the regexp.
 *
 * 2) Transform the original NFA into an expanded graph
 * ----------------------------------------------------
 * In the 2nd stage, the automaton is transformed into a graph based on the
 * original NFA.  Each state in the expanded graph represents a state from
 * the original NFA, plus a prefix identifying the last two characters
 * (colors, to be precise) seen before entering the state.  There can be
 * multiple states in the expanded graph for each state in the original NFA,
 * depending on what characters can precede it.  A prefix position can be
 * "unknown" if it's uncertain what the preceding character was, or "blank"
 * if the character was a non-word character (we don't need to distinguish
 * which non-word character it was, so just think of all of them as blanks).
 *
 * For convenience in description, call an expanded-state identifier
 * (two prefix colors plus a state number from the original NFA) an
 * "enter key".
 *
 * Each arc of the expanded graph is labelled with a trigram that must be
 * present in the string to match.  We can construct this from an out-arc of
 * the underlying NFA state by combining the expanded state's prefix with the
 * color label of the underlying out-arc, if neither prefix position is
 * "unknown".  But note that some of the colors in the trigram might be
 * "blank".  This is OK since we want to generate word-boundary trigrams as
 * the regular trigram machinery would, if we know that some word characters
 * must be adjacent to a word boundary in all strings matching the NFA.
 *
 * The expanded graph can also have fewer states than the original NFA,
 * because we don't bother to make a separate state entry unless the state
 * is reachable by a valid arc.  When an enter key is reachable from a state
 * of the expanded graph, but we do not know a complete trigram associated
 * with that transition, we cannot make a valid arc; instead we insert the
 * enter key into the enterKeys list of the source state.  This effectively
 * means that the two expanded states are not reliably distinguishable based
 * on examining trigrams.
 *
 * So the expanded graph resembles the original NFA, but the arcs are
 * labeled with trigrams instead of individual characters, and there may be
 * more or fewer states.  It is a lossy representation of the original NFA:
 * any string that matches the original regexp must match the expanded graph,
 * but the reverse is not true.
 *
 * We build the expanded graph through a breadth-first traversal of states
 * reachable from the initial state.  At each reachable state, we identify the
 * states reachable from it without traversing a predictable trigram, and add
 * those states' enter keys to the current state.  Then we generate all
 * out-arcs leading out of this collection of states that have predictable
 * trigrams, adding their target states to the queue of states to examine.
 *
 * When building the graph, if the number of states or arcs exceed pre-defined
 * limits, we give up and simply mark any states not yet processed as final
 * states.  Roughly speaking, that means that we make use of some portion from
 * the beginning of the regexp.  Also, any colors that have too many member
 * characters are treated as "unknown", so that we can't derive trigrams
 * from them.
 *
 * 3) Expand the color trigrams into regular trigrams
 * --------------------------------------------------
 * The trigrams in the expanded graph are "color trigrams", consisting
 * of three consecutive colors that must be present in the string. But for
 * search, we need regular trigrams consisting of characters. In the 3rd
 * stage, the color trigrams are expanded into regular trigrams. Since each
 * color can represent many characters, the total number of regular trigrams
 * after expansion could be very large. Because searching the index for
 * thousands of trigrams would be slow, and would likely produce so many
 * false positives that we would have to traverse a large fraction of the
 * index, the graph is simplified further in a lossy fashion by removing
 * color trigrams. When a color trigram is removed, the states connected by
 * any arcs labelled with that trigram are merged.
 *
 * Trigrams do not all have equivalent value for searching: some of them are
 * more frequent and some of them are less frequent. Ideally, we would like
 * to know the distribution of trigrams, but we don't. But because of padding
 * we know for sure that the empty character is more frequent than others,
 * so we can penalize trigrams according to presence of whitespace. The
 * penalty assigned to each color trigram is the number of simple trigrams
 * it would produce, times the penalties[] multiplier associated with its
 * whitespace content. (The penalties[] constants were calculated by analysis
 * of some real-life text.) We eliminate color trigrams starting with the
 * highest-penalty one, until we get to a total penalty of no more than
 * WISH_TRGM_PENALTY. However, we cannot remove a color trigram if that would
 * lead to merging the initial and final states, so we may not be able to
 * reach WISH_TRGM_PENALTY. It's still okay so long as we have no more than
 * MAX_TRGM_COUNT simple trigrams in total, otherwise we fail.
 *
 * 4) Pack the graph into a compact representation
 * -----------------------------------------------
 * The 2nd and 3rd stages might have eliminated or merged many of the states
 * and trigrams created earlier, so in this final stage, the graph is
 * compacted and packed into a simpler struct that contains only the
 * information needed to evaluate it.
 *
 * ALGORITHM EXAMPLE:
 *
 * Consider the example regex "ab[cd]".  This regex is transformed into the
 * following NFA (for simplicity we show colors as their single members):
 *
 *					  4#
 *					c/
 *		 a	   b	/
 *	 1* --- 2 ---- 3
 *					\
 *					d\
 *					  5#
 *
 * We use * to mark initial state and # to mark final state. It's not depicted,
 * but states 1, 4, 5 have self-referencing arcs for all possible characters,
 * because this pattern can match to any part of a string.
 *
 * As the result of stage 2 we will have the following graph:
 *
 *		  abc	 abd
 *	 2# <---- 1* ----> 3#
 *
 * The process for generating this graph is:
 * 1) Create state 1 with enter key (UNKNOWN, UNKNOWN, 1).
 * 2) Add key (UNKNOWN, "a", 2) to state 1.
 * 3) Add key ("a", "b", 3) to state 1.
 * 4) Create new state 2 with enter key ("b", "c", 4).  Add an arc
 *	  from state 1 to state 2 with label trigram "abc".
 * 5) Mark state 2 final because state 4 of source NFA is marked as final.
 * 6) Create new state 3 with enter key ("b", "d", 5).  Add an arc
 *	  from state 1 to state 3 with label trigram "abd".
 * 7) Mark state 3 final because state 5 of source NFA is marked as final.
 *
 *
 * Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  contrib/pg_trgm/trgm_regexp.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "trgm.h"

#include "regex/regexport.h"
#include "tsearch/ts_locale.h"
#include "utils/hsearch.h"
#include "utils/memutils.h"


/*
 * Uncomment to print intermediate stages, for exploring and debugging the
 * algorithm implementation.  This produces three graph files in /tmp,
 * in Graphviz .dot format.
 */
/* #define TRGM_REGEXP_DEBUG */

/*
 * These parameters are used to limit the amount of work done.
 * Otherwise regex processing could be too slow and memory-consuming.
 *
 *	MAX_EXPANDED_STATES - How many states we allow in expanded graph
 *	MAX_EXPANDED_ARCS - How many arcs we allow in expanded graph
 *	MAX_TRGM_COUNT - How many simple trigrams we allow to be extracted
 *	WISH_TRGM_PENALTY - Maximum desired sum of color trigram penalties
 *	COLOR_COUNT_LIMIT - Maximum number of characters per color
 */
#define MAX_EXPANDED_STATES 128
#define MAX_EXPANDED_ARCS	1024
#define MAX_TRGM_COUNT		256
#define WISH_TRGM_PENALTY	16
#define COLOR_COUNT_LIMIT	256

/*
 * Penalty multipliers for trigram counts depending on whitespace contents.
 * Numbers based on analysis of real-life texts.
 */
const float4 penalties[8] = {
	1.0f,						/* "aaa" */
	3.5f,						/* "aa " */
	0.0f,						/* "a a" (impossible) */
	0.0f,						/* "a  " (impossible) */
	4.2f,						/* " aa" */
	2.1f,						/* " a " */
	25.0f,						/* "  a" */
	0.0f						/* "   " (impossible) */
};

/* Struct representing a single pg_wchar, converted back to multibyte form */
typedef struct
{
	char		bytes[MAX_MULTIBYTE_CHAR_LEN];
} trgm_mb_char;

/*
 * Attributes of NFA colors:
 *
 *	expandable				- we know the character expansion of this color
 *	containsNonWord			- color contains non-word characters
 *							  (which will not be extracted into trigrams)
 *	wordCharsCount			- count of word characters in color
 *	wordChars				- array of this color's word characters
 *							  (which can be extracted into trigrams)
 *
 * When expandable is false, the other attributes don't matter; we just
 * assume this color represents unknown character(s).
 */
typedef struct
{
	bool		expandable;
	bool		containsNonWord;
	int			wordCharsCount;
	trgm_mb_char *wordChars;
} TrgmColorInfo;

/*
 * A "prefix" is information about the colors of the last two characters read
 * before reaching a specific NFA state.  These colors can have special values
 * COLOR_UNKNOWN and COLOR_BLANK.  COLOR_UNKNOWN means that we have no
 * information, for example because we read some character of an unexpandable
 * color.  COLOR_BLANK means that we read a non-word character.
 *
 * We call a prefix ambiguous if at least one of its colors is unknown.  It's
 * fully ambiguous if both are unknown, partially ambiguous if only the first
 * is unknown.  (The case of first color known, second unknown is not valid.)
 *
 * Wholly- or partly-blank prefixes are mostly handled the same as regular
 * color prefixes.  This allows us to generate appropriate partly-blank
 * trigrams when the NFA requires word character(s) to appear adjacent to
 * non-word character(s).
 */
typedef int TrgmColor;

/* We assume that colors returned by the regexp engine cannot be these: */
#define COLOR_UNKNOWN	(-1)
#define COLOR_BLANK		(-2)

typedef struct
{
	TrgmColor	colors[2];
} TrgmPrefix;

/*
 * Color-trigram data type.  Note that some elements of the trigram can be
 * COLOR_BLANK, but we don't allow COLOR_UNKNOWN.
 */
typedef struct
{
	TrgmColor	colors[3];
} ColorTrgm;

/*
 * Key identifying a state of our expanded graph: color prefix, and number
 * of the corresponding state in the underlying regex NFA.  The color prefix
 * shows how we reached the regex state (to the extent that we know it).
 */
typedef struct
{
	TrgmPrefix	prefix;
	int			nstate;
} TrgmStateKey;

/*
 * One state of the expanded graph.
 *
 *	stateKey - ID of this state
 *	arcs	 - outgoing arcs of this state (List of TrgmArc)
 *	enterKeys - enter keys reachable from this state without reading any
 *			   predictable trigram (List of TrgmStateKey)
 *	fin		 - flag indicating this state is final
 *	init	 - flag indicating this state is initial
 *	parent	 - parent state, if this state has been merged into another
 *	children - child states (states that have been merged into this one)
 *	number	 - number of this state (used at the packaging stage)
 */
typedef struct TrgmState
{
	TrgmStateKey stateKey;		/* hashtable key: must be first field */
	List	   *arcs;
	List	   *enterKeys;
	bool		fin;
	bool		init;
	struct TrgmState *parent;
	List	   *children;
	int			number;
} TrgmState;

/*
 * One arc in the expanded graph.
 */
typedef struct
{
	ColorTrgm	ctrgm;			/* trigram needed to traverse arc */
	TrgmState  *target;			/* next state */
} TrgmArc;

/*
 * Information about arc of specific color trigram (used in stage 3)
 *
 * Contains pointers to the source and target states.
 */
typedef struct
{
	TrgmState  *source;
	TrgmState  *target;
} TrgmArcInfo;

/*
 * Information about color trigram (used in stage 3)
 *
 * ctrgm	- trigram itself
 * number	- number of this trigram (used in the packaging stage)
 * count	- number of simple trigrams created from this color trigram
 * expanded - indicates this color trigram is expanded into simple trigrams
 * arcs		- list of all arcs labeled with this color trigram.
 */
typedef struct
{
	ColorTrgm	ctrgm;
	int			number;
	int			count;
	float4		penalty;
	bool		expanded;
	List	   *arcs;
} ColorTrgmInfo;

/*
 * Data structure representing all the data we need during regex processing.
 *
 *	regex			- compiled regex
 *	colorInfo		- extracted information about regex's colors
 *	ncolors			- number of colors in colorInfo[]
 *	states			- hashtable of TrgmStates (states of expanded graph)
 *	initState		- pointer to initial state of expanded graph
 *	queue			- queue of to-be-processed TrgmStates
 *	keysQueue		- queue of to-be-processed TrgmStateKeys
 *	arcsCount		- total number of arcs of expanded graph (for resource
 *					  limiting)
 *	overflowed		- we have exceeded resource limit for transformation
 *	colorTrgms		- array of all color trigrams present in graph
 *	colorTrgmsCount - count of those color trigrams
 *	totalTrgmCount	- total count of extracted simple trigrams
 */
typedef struct
{
	/* Source regexp, and color information extracted from it (stage 1) */
	regex_t    *regex;
	TrgmColorInfo *colorInfo;
	int			ncolors;

	/* Expanded graph (stage 2) */
	HTAB	   *states;
	TrgmState  *initState;

	/* Workspace for stage 2 */
	List	   *queue;
	List	   *keysQueue;
	int			arcsCount;
	bool		overflowed;

	/* Information about distinct color trigrams in the graph (stage 3) */
	ColorTrgmInfo *colorTrgms;
	int			colorTrgmsCount;
	int			totalTrgmCount;
} TrgmNFA;

/*
 * Final, compact representation of expanded graph.
 */
typedef struct
{
	int			targetState;	/* index of target state (zero-based) */
	int			colorTrgm;		/* index of color trigram for transition */
} TrgmPackedArc;

typedef struct
{
	int			arcsCount;		/* number of out-arcs for this state */
	TrgmPackedArc *arcs;		/* array of arcsCount packed arcs */
} TrgmPackedState;

/* "typedef struct TrgmPackedGraph TrgmPackedGraph" appears in trgm.h */
struct TrgmPackedGraph
{
	/*
	 * colorTrigramsCount and colorTrigramsGroups contain information about
	 * how trigrams are grouped into color trigrams.  "colorTrigramsCount" is
	 * the count of color trigrams and "colorTrigramGroups" contains number of
	 * simple trigrams for each color trigram.  The array of simple trigrams
	 * (stored separately from this struct) is ordered so that the simple
	 * trigrams for each color trigram are consecutive, and they're in order
	 * by color trigram number.
	 */
	int			colorTrigramsCount;
	int		   *colorTrigramGroups;		/* array of size colorTrigramsCount */

	/*
	 * The states of the simplified NFA.  State number 0 is always initial
	 * state and state number 1 is always final state.
	 */
	int			statesCount;
	TrgmPackedState *states;	/* array of size statesCount */

	/* Temporary work space for trigramsMatchGraph() */
	bool	   *colorTrigramsActive;	/* array of size colorTrigramsCount */
	bool	   *statesActive;	/* array of size statesCount */
	int		   *statesQueue;	/* array of size statesCount */
};

/*
 * Temporary structure for representing an arc during packaging.
 */
typedef struct
{
	int			sourceState;
	int			targetState;
	int			colorTrgm;
} TrgmPackArcInfo;


/* prototypes for private functions */
static TRGM *createTrgmNFAInternal(regex_t *regex, TrgmPackedGraph **graph,
					  MemoryContext rcontext);
static void RE_compile(regex_t *regex, text *text_re,
		   int cflags, Oid collation);
static void getColorInfo(regex_t *regex, TrgmNFA *trgmNFA);
static bool convertPgWchar(pg_wchar c, trgm_mb_char *result);
static void transformGraph(TrgmNFA *trgmNFA);
static void processState(TrgmNFA *trgmNFA, TrgmState *state);
static void addKey(TrgmNFA *trgmNFA, TrgmState *state, TrgmStateKey *key);
static void addKeyToQueue(TrgmNFA *trgmNFA, TrgmStateKey *key);
static void addArcs(TrgmNFA *trgmNFA, TrgmState *state);
static void addArc(TrgmNFA *trgmNFA, TrgmState *state, TrgmStateKey *key,
	   TrgmColor co, TrgmStateKey *destKey);
static bool validArcLabel(TrgmStateKey *key, TrgmColor co);
static TrgmState *getState(TrgmNFA *trgmNFA, TrgmStateKey *key);
static bool prefixContains(TrgmPrefix *prefix1, TrgmPrefix *prefix2);
static bool selectColorTrigrams(TrgmNFA *trgmNFA);
static TRGM *expandColorTrigrams(TrgmNFA *trgmNFA, MemoryContext rcontext);
static void fillTrgm(trgm *ptrgm, trgm_mb_char s[3]);
static void mergeStates(TrgmState *state1, TrgmState *state2);
static int	colorTrgmInfoCmp(const void *p1, const void *p2);
static int	colorTrgmInfoPenaltyCmp(const void *p1, const void *p2);
static TrgmPackedGraph *packGraph(TrgmNFA *trgmNFA, MemoryContext rcontext);
static int	packArcInfoCmp(const void *a1, const void *a2);

#ifdef TRGM_REGEXP_DEBUG
static void printSourceNFA(regex_t *regex, TrgmColorInfo *colors, int ncolors);
static void printTrgmNFA(TrgmNFA *trgmNFA);
static void printTrgmColor(StringInfo buf, TrgmColor co);
static void printTrgmPackedGraph(TrgmPackedGraph *packedGraph, TRGM *trigrams);
#endif


/*
 * Main entry point to process a regular expression.
 *
 * Returns an array of trigrams required by the regular expression, or NULL if
 * the regular expression was too complex to analyze.  In addition, a packed
 * graph representation of the regex is returned into *graph.  The results
 * must be allocated in rcontext (which might or might not be the current
 * context).
 */
TRGM *
createTrgmNFA(text *text_re, Oid collation,
			  TrgmPackedGraph **graph, MemoryContext rcontext)
{
	TRGM	   *trg;
	regex_t		regex;
	MemoryContext tmpcontext;
	MemoryContext oldcontext;

	/*
	 * This processing generates a great deal of cruft, which we'd like to
	 * clean up before returning (since this function may be called in a
	 * query-lifespan memory context).  Make a temp context we can work in so
	 * that cleanup is easy.
	 */
	tmpcontext = AllocSetContextCreate(CurrentMemoryContext,
									   "createTrgmNFA temporary context",
									   ALLOCSET_DEFAULT_MINSIZE,
									   ALLOCSET_DEFAULT_INITSIZE,
									   ALLOCSET_DEFAULT_MAXSIZE);
	oldcontext = MemoryContextSwitchTo(tmpcontext);

	/*
	 * Stage 1: Compile the regexp into a NFA, using the regexp library.
	 */
#ifdef IGNORECASE
	RE_compile(&regex, text_re, REG_ADVANCED | REG_ICASE, collation);
#else
	RE_compile(&regex, text_re, REG_ADVANCED, collation);
#endif

	/*
	 * Since the regexp library allocates its internal data structures with
	 * malloc, we need to use a PG_TRY block to ensure that pg_regfree() gets
	 * done even if there's an error.
	 */
	PG_TRY();
	{
		trg = createTrgmNFAInternal(&regex, graph, rcontext);
	}
	PG_CATCH();
	{
		pg_regfree(&regex);
		PG_RE_THROW();
	}
	PG_END_TRY();

	pg_regfree(&regex);

	/* Clean up all the cruft we created */
	MemoryContextSwitchTo(oldcontext);
	MemoryContextDelete(tmpcontext);

	return trg;
}

/*
 * Body of createTrgmNFA, exclusive of regex compilation/freeing.
 */
static TRGM *
createTrgmNFAInternal(regex_t *regex, TrgmPackedGraph **graph,
					  MemoryContext rcontext)
{
	TRGM	   *trg;
	TrgmNFA		trgmNFA;

	trgmNFA.regex = regex;

	/* Collect color information from the regex */
	getColorInfo(regex, &trgmNFA);

#ifdef TRGM_REGEXP_DEBUG
	printSourceNFA(regex, trgmNFA.colorInfo, trgmNFA.ncolors);
#endif

	/*
	 * Stage 2: Create an expanded graph from the source NFA.
	 */
	transformGraph(&trgmNFA);

#ifdef TRGM_REGEXP_DEBUG
	printTrgmNFA(&trgmNFA);
#endif

	/*
	 * Fail if we were unable to make a nontrivial graph, ie it is possible to
	 * get from the initial state to the final state without reading any
	 * predictable trigram.
	 */
	if (trgmNFA.initState->fin)
		return NULL;

	/*
	 * Stage 3: Select color trigrams to expand.  Fail if too many trigrams.
	 */
	if (!selectColorTrigrams(&trgmNFA))
		return NULL;

	/*
	 * Stage 4: Expand color trigrams and pack graph into final
	 * representation.
	 */
	trg = expandColorTrigrams(&trgmNFA, rcontext);

	*graph = packGraph(&trgmNFA, rcontext);

#ifdef TRGM_REGEXP_DEBUG
	printTrgmPackedGraph(*graph, trg);
#endif

	return trg;
}

/*
 * Main entry point for evaluating a graph during index scanning.
 *
 * The check[] array is indexed by trigram number (in the array of simple
 * trigrams returned by createTrgmNFA), and holds TRUE for those trigrams
 * that are present in the index entry being checked.
 */
bool
trigramsMatchGraph(TrgmPackedGraph *graph, bool *check)
{
	int			i,
				j,
				k,
				queueIn,
				queueOut;

	/*
	 * Reset temporary working areas.
	 */
	memset(graph->colorTrigramsActive, 0,
		   sizeof(bool) * graph->colorTrigramsCount);
	memset(graph->statesActive, 0, sizeof(bool) * graph->statesCount);

	/*
	 * Check which color trigrams were matched.  A match for any simple
	 * trigram associated with a color trigram counts as a match of the color
	 * trigram.
	 */
	j = 0;
	for (i = 0; i < graph->colorTrigramsCount; i++)
	{
		int			cnt = graph->colorTrigramGroups[i];

		for (k = j; k < j + cnt; k++)
		{
			if (check[k])
			{
				/*
				 * Found one matched trigram in the group. Can skip the rest
				 * of them and go to the next group.
				 */
				graph->colorTrigramsActive[i] = true;
				break;
			}
		}
		j = j + cnt;
	}

	/*
	 * Initialize the statesQueue to hold just the initial state.  Note:
	 * statesQueue has room for statesCount entries, which is certainly enough
	 * since no state will be put in the queue more than once. The
	 * statesActive array marks which states have been queued.
	 */
	graph->statesActive[0] = true;
	graph->statesQueue[0] = 0;
	queueIn = 0;
	queueOut = 1;

	/* Process queued states as long as there are any. */
	while (queueIn < queueOut)
	{
		int			stateno = graph->statesQueue[queueIn++];
		TrgmPackedState *state = &graph->states[stateno];
		int			cnt = state->arcsCount;

		/* Loop over state's out-arcs */
		for (i = 0; i < cnt; i++)
		{
			TrgmPackedArc *arc = &state->arcs[i];

			/*
			 * If corresponding color trigram is present then activate the
			 * corresponding state.  We're done if that's the final state,
			 * otherwise queue the state if it's not been queued already.
			 */
			if (graph->colorTrigramsActive[arc->colorTrgm])
			{
				int			nextstate = arc->targetState;

				if (nextstate == 1)
					return true;	/* success: final state is reachable */

				if (!graph->statesActive[nextstate])
				{
					graph->statesActive[nextstate] = true;
					graph->statesQueue[queueOut++] = nextstate;
				}
			}
		}
	}

	/* Queue is empty, so match fails. */
	return false;
}

/*
 * Compile regex string into struct at *regex.
 * NB: pg_regfree must be applied to regex if this completes successfully.
 */
static void
RE_compile(regex_t *regex, text *text_re, int cflags, Oid collation)
{
	int			text_re_len = VARSIZE_ANY_EXHDR(text_re);
	char	   *text_re_val = VARDATA_ANY(text_re);
	pg_wchar   *pattern;
	int			pattern_len;
	int			regcomp_result;
	char		errMsg[100];

	/* Convert pattern string to wide characters */
	pattern = (pg_wchar *) palloc((text_re_len + 1) * sizeof(pg_wchar));
	pattern_len = pg_mb2wchar_with_len(text_re_val,
									   pattern,
									   text_re_len);

	/* Compile regex */
	regcomp_result = pg_regcomp(regex,
								pattern,
								pattern_len,
								cflags,
								collation);

	pfree(pattern);

	if (regcomp_result != REG_OKAY)
	{
		/* re didn't compile (no need for pg_regfree, if so) */
		pg_regerror(regcomp_result, regex, errMsg, sizeof(errMsg));
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_REGULAR_EXPRESSION),
				 errmsg("invalid regular expression: %s", errMsg)));
	}
}


/*---------------------
 * Subroutines for pre-processing the color map (stage 1).
 *---------------------
 */

/*
 * Fill TrgmColorInfo structure for each color using regex export functions.
 */
static void
getColorInfo(regex_t *regex, TrgmNFA *trgmNFA)
{
	int			colorsCount = pg_reg_getnumcolors(regex);
	int			i;

	trgmNFA->ncolors = colorsCount;
	trgmNFA->colorInfo = (TrgmColorInfo *)
		palloc0(colorsCount * sizeof(TrgmColorInfo));

	/*
	 * Loop over colors, filling TrgmColorInfo about each.
	 */
	for (i = 0; i < colorsCount; i++)
	{
		TrgmColorInfo *colorInfo = &trgmNFA->colorInfo[i];
		int			charsCount = pg_reg_getnumcharacters(regex, i);
		pg_wchar   *chars;
		int			j;

		if (charsCount < 0 || charsCount > COLOR_COUNT_LIMIT)
		{
			/* Non expandable, or too large to work with */
			colorInfo->expandable = false;
			continue;
		}

		colorInfo->expandable = true;
		colorInfo->containsNonWord = false;
		colorInfo->wordChars = (trgm_mb_char *)
			palloc(sizeof(trgm_mb_char) * charsCount);
		colorInfo->wordCharsCount = 0;

		/* Extract all the chars in this color */
		chars = (pg_wchar *) palloc(sizeof(pg_wchar) * charsCount);
		pg_reg_getcharacters(regex, i, chars, charsCount);

		/*
		 * Convert characters back to multibyte form, and save only those that
		 * are word characters.  Set "containsNonWord" if any non-word
		 * character.  (Note: it'd probably be nicer to keep the chars in
		 * pg_wchar format for now, but ISWORDCHR wants to see multibyte.)
		 */
		for (j = 0; j < charsCount; j++)
		{
			trgm_mb_char c;

			if (!convertPgWchar(chars[j], &c))
				continue;		/* ok to ignore it altogether */
			if (ISWORDCHR(c.bytes))
				colorInfo->wordChars[colorInfo->wordCharsCount++] = c;
			else
				colorInfo->containsNonWord = true;
		}

		pfree(chars);
	}
}

/*
 * Convert pg_wchar to multibyte format.
 * Returns false if the character should be ignored completely.
 */
static bool
convertPgWchar(pg_wchar c, trgm_mb_char *result)
{
	/* "s" has enough space for a multibyte character and a trailing NUL */
	char		s[MAX_MULTIBYTE_CHAR_LEN + 1];

	/*
	 * We can ignore the NUL character, since it can never appear in a PG text
	 * string.  This avoids the need for various special cases when
	 * reconstructing trigrams.
	 */
	if (c == 0)
		return false;

	/* Do the conversion, making sure the result is NUL-terminated */
	memset(s, 0, sizeof(s));
	pg_wchar2mb_with_len(&c, s, 1);

	/*
	 * In IGNORECASE mode, we can ignore uppercase characters.  We assume that
	 * the regex engine generated both uppercase and lowercase equivalents
	 * within each color, since we used the REG_ICASE option; so there's no
	 * need to process the uppercase version.
	 *
	 * XXX this code is dependent on the assumption that lowerstr() works the
	 * same as the regex engine's internal case folding machinery.  Might be
	 * wiser to expose pg_wc_tolower and test whether c == pg_wc_tolower(c).
	 * On the other hand, the trigrams in the index were created using
	 * lowerstr(), so we're probably screwed if there's any incompatibility
	 * anyway.
	 */
#ifdef IGNORECASE
	{
		char	   *lowerCased = lowerstr(s);

		if (strcmp(lowerCased, s) != 0)
		{
			pfree(lowerCased);
			return false;
		}
		pfree(lowerCased);
	}
#endif

	/* Fill result with exactly MAX_MULTIBYTE_CHAR_LEN bytes */
	strncpy(result->bytes, s, MAX_MULTIBYTE_CHAR_LEN);
	return true;
}


/*---------------------
 * Subroutines for expanding original NFA graph into a trigram graph (stage 2).
 *---------------------
 */

/*
 * Transform the graph, given a regex and extracted color information.
 *
 * We create and process a queue of expanded-graph states until all the states
 * are processed.
 *
 * This algorithm may be stopped due to resource limitation. In this case we
 * force every unprocessed branch to immediately finish with matching (this
 * can give us false positives but no false negatives) by marking all
 * unprocessed states as final.
 */
static void
transformGraph(TrgmNFA *trgmNFA)
{
	HASHCTL		hashCtl;
	TrgmStateKey initkey;
	TrgmState  *initstate;

	/* Initialize this stage's workspace in trgmNFA struct */
	trgmNFA->queue = NIL;
	trgmNFA->keysQueue = NIL;
	trgmNFA->arcsCount = 0;
	trgmNFA->overflowed = false;

	/* Create hashtable for states */
	hashCtl.keysize = sizeof(TrgmStateKey);
	hashCtl.entrysize = sizeof(TrgmState);
	hashCtl.hcxt = CurrentMemoryContext;
	hashCtl.hash = tag_hash;
	trgmNFA->states = hash_create("Trigram NFA",
								  1024,
								  &hashCtl,
								  HASH_ELEM | HASH_CONTEXT | HASH_FUNCTION);

	/* Create initial state: ambiguous prefix, NFA's initial state */
	MemSet(&initkey, 0, sizeof(initkey));
	initkey.prefix.colors[0] = COLOR_UNKNOWN;
	initkey.prefix.colors[1] = COLOR_UNKNOWN;
	initkey.nstate = pg_reg_getinitialstate(trgmNFA->regex);

	initstate = getState(trgmNFA, &initkey);
	initstate->init = true;
	trgmNFA->initState = initstate;

	/*
	 * Recursively build the expanded graph by processing queue of states
	 * (breadth-first search).  getState already put initstate in the queue.
	 */
	while (trgmNFA->queue != NIL)
	{
		TrgmState  *state = (TrgmState *) linitial(trgmNFA->queue);

		trgmNFA->queue = list_delete_first(trgmNFA->queue);

		/*
		 * If we overflowed then just mark state as final.  Otherwise do
		 * actual processing.
		 */
		if (trgmNFA->overflowed)
			state->fin = true;
		else
			processState(trgmNFA, state);

		/* Did we overflow? */
		if (trgmNFA->arcsCount > MAX_EXPANDED_ARCS ||
			hash_get_num_entries(trgmNFA->states) > MAX_EXPANDED_STATES)
			trgmNFA->overflowed = true;
	}
}

/*
 * Process one state: add enter keys and then add outgoing arcs.
 */
static void
processState(TrgmNFA *trgmNFA, TrgmState *state)
{
	/* keysQueue should be NIL already, but make sure */
	trgmNFA->keysQueue = NIL;

	/*
	 * Add state's own key, and then process all keys added to keysQueue until
	 * queue is empty.  But we can quit if the state gets marked final.
	 */
	addKey(trgmNFA, state, &state->stateKey);
	while (trgmNFA->keysQueue != NIL && !state->fin)
	{
		TrgmStateKey *key = (TrgmStateKey *) linitial(trgmNFA->keysQueue);

		trgmNFA->keysQueue = list_delete_first(trgmNFA->keysQueue);
		addKey(trgmNFA, state, key);
	}

	/*
	 * Add outgoing arcs only if state isn't final (we have no interest in
	 * outgoing arcs if we already match)
	 */
	if (!state->fin)
		addArcs(trgmNFA, state);
}

/*
 * Add the given enter key into the state's enterKeys list, and determine
 * whether this should result in any further enter keys being added.
 * If so, add those keys to keysQueue so that processState will handle them.
 *
 * If the enter key is for the NFA's final state, set state->fin = TRUE.
 * This situation means that we can reach the final state from this expanded
 * state without reading any predictable trigram, so we must consider this
 * state as an accepting one.
 *
 * The given key could be a duplicate of one already in enterKeys, or be
 * redundant with some enterKeys.  So we check that before doing anything.
 *
 * Note that we don't generate any actual arcs here.  addArcs will do that
 * later, after we have identified all the enter keys for this state.
 */
static void
addKey(TrgmNFA *trgmNFA, TrgmState *state, TrgmStateKey *key)
{
	regex_arc_t *arcs;
	TrgmStateKey destKey;
	ListCell   *cell,
			   *prev,
			   *next;
	int			i,
				arcsCount;

	/*
	 * Ensure any pad bytes in destKey are zero, since it may get used as a
	 * hashtable key by getState.
	 */
	MemSet(&destKey, 0, sizeof(destKey));

	/*
	 * Compare key to each existing enter key of the state to check for
	 * redundancy.  We can drop either old key(s) or the new key if we find
	 * redundancy.
	 */
	prev = NULL;
	cell = list_head(state->enterKeys);
	while (cell)
	{
		TrgmStateKey *existingKey = (TrgmStateKey *) lfirst(cell);

		next = lnext(cell);
		if (existingKey->nstate == key->nstate)
		{
			if (prefixContains(&existingKey->prefix, &key->prefix))
			{
				/* This old key already covers the new key. Nothing to do */
				return;
			}
			if (prefixContains(&key->prefix, &existingKey->prefix))
			{
				/*
				 * The new key covers this old key. Remove the old key, it's
				 * no longer needed once we add this key to the list.
				 */
				state->enterKeys = list_delete_cell(state->enterKeys,
													cell, prev);
			}
			else
				prev = cell;
		}
		else
			prev = cell;
		cell = next;
	}

	/* No redundancy, so add this key to the state's list */
	state->enterKeys = lappend(state->enterKeys, key);

	/* If state is now known final, mark it and we're done */
	if (key->nstate == pg_reg_getfinalstate(trgmNFA->regex))
	{
		state->fin = true;
		return;
	}

	/*
	 * Loop through all outgoing arcs of the corresponding state in the
	 * original NFA.
	 */
	arcsCount = pg_reg_getnumoutarcs(trgmNFA->regex, key->nstate);
	arcs = (regex_arc_t *) palloc(sizeof(regex_arc_t) * arcsCount);
	pg_reg_getoutarcs(trgmNFA->regex, key->nstate, arcs, arcsCount);

	for (i = 0; i < arcsCount; i++)
	{
		regex_arc_t *arc = &arcs[i];

		if (pg_reg_colorisbegin(trgmNFA->regex, arc->co))
		{
			/*
			 * Start of line/string (^).  Trigram extraction treats start of
			 * line same as start of word: double space prefix is added.
			 * Hence, make an enter key showing we can reach the arc
			 * destination with all-blank prefix.
			 */
			destKey.prefix.colors[0] = COLOR_BLANK;
			destKey.prefix.colors[1] = COLOR_BLANK;
			destKey.nstate = arc->to;

			/* Add enter key to this state */
			addKeyToQueue(trgmNFA, &destKey);
		}
		else if (pg_reg_colorisend(trgmNFA->regex, arc->co))
		{
			/*
			 * End of line/string ($).  We must consider this arc as a
			 * transition that doesn't read anything.  The reason for adding
			 * this enter key to the state is that if the arc leads to the
			 * NFA's final state, we must mark this expanded state as final.
			 */
			destKey.prefix.colors[0] = COLOR_UNKNOWN;
			destKey.prefix.colors[1] = COLOR_UNKNOWN;
			destKey.nstate = arc->to;

			/* Add enter key to this state */
			addKeyToQueue(trgmNFA, &destKey);
		}
		else
		{
			/* Regular color */
			TrgmColorInfo *colorInfo = &trgmNFA->colorInfo[arc->co];

			if (colorInfo->expandable)
			{
				if (colorInfo->containsNonWord &&
					!validArcLabel(key, COLOR_BLANK))
				{
					/*
					 * We can reach the arc destination after reading a
					 * non-word character, but the prefix is not something
					 * that addArc will accept with COLOR_BLANK, so no trigram
					 * arc can get made for this transition.  We must make an
					 * enter key to show that the arc destination is
					 * reachable.  Set it up with an all-blank prefix, since
					 * that corresponds to what the trigram extraction code
					 * will do at a word starting boundary.
					 */
					destKey.prefix.colors[0] = COLOR_BLANK;
					destKey.prefix.colors[1] = COLOR_BLANK;
					destKey.nstate = arc->to;
					addKeyToQueue(trgmNFA, &destKey);
				}

				if (colorInfo->wordCharsCount > 0 &&
					!validArcLabel(key, arc->co))
				{
					/*
					 * We can reach the arc destination after reading a word
					 * character, but the prefix is not something that addArc
					 * will accept, so no trigram arc can get made for this
					 * transition.  We must make an enter key to show that the
					 * arc destination is reachable.  The prefix for the enter
					 * key should reflect the info we have for this arc.
					 */
					destKey.prefix.colors[0] = key->prefix.colors[1];
					destKey.prefix.colors[1] = arc->co;
					destKey.nstate = arc->to;
					addKeyToQueue(trgmNFA, &destKey);
				}
			}
			else
			{
				/*
				 * Unexpandable color.  Add enter key with ambiguous prefix,
				 * showing we can reach the destination from this state, but
				 * the preceding colors will be uncertain.  (We do not set the
				 * first prefix color to key->prefix.colors[1], because a
				 * prefix of known followed by unknown is invalid.)
				 */
				destKey.prefix.colors[0] = COLOR_UNKNOWN;
				destKey.prefix.colors[1] = COLOR_UNKNOWN;
				destKey.nstate = arc->to;
				addKeyToQueue(trgmNFA, &destKey);
			}
		}
	}

	pfree(arcs);
}

/*
 * Add copy of given key to keysQueue for later processing.
 */
static void
addKeyToQueue(TrgmNFA *trgmNFA, TrgmStateKey *key)
{
	TrgmStateKey *keyCopy = (TrgmStateKey *) palloc(sizeof(TrgmStateKey));

	memcpy(keyCopy, key, sizeof(TrgmStateKey));
	trgmNFA->keysQueue = lappend(trgmNFA->keysQueue, keyCopy);
}

/*
 * Add outgoing arcs from given state, whose enter keys are all now known.
 */
static void
addArcs(TrgmNFA *trgmNFA, TrgmState *state)
{
	TrgmStateKey destKey;
	ListCell   *cell;
	regex_arc_t *arcs;
	int			arcsCount,
				i;

	/*
	 * Ensure any pad bytes in destKey are zero, since it may get used as a
	 * hashtable key by getState.
	 */
	MemSet(&destKey, 0, sizeof(destKey));

	/*
	 * Iterate over enter keys associated with this expanded-graph state. This
	 * includes both the state's own stateKey, and any enter keys we added to
	 * it during addKey (which represent expanded-graph states that are not
	 * distinguishable from this one by means of trigrams).  For each such
	 * enter key, examine all the out-arcs of the key's underlying NFA state,
	 * and try to make a trigram arc leading to where the out-arc leads.
	 * (addArc will deal with whether the arc is valid or not.)
	 */
	foreach(cell, state->enterKeys)
	{
		TrgmStateKey *key = (TrgmStateKey *) lfirst(cell);

		arcsCount = pg_reg_getnumoutarcs(trgmNFA->regex, key->nstate);
		arcs = (regex_arc_t *) palloc(sizeof(regex_arc_t) * arcsCount);
		pg_reg_getoutarcs(trgmNFA->regex, key->nstate, arcs, arcsCount);

		for (i = 0; i < arcsCount; i++)
		{
			regex_arc_t *arc = &arcs[i];
			TrgmColorInfo *colorInfo = &trgmNFA->colorInfo[arc->co];

			/*
			 * Ignore non-expandable colors; addKey already handled the case.
			 *
			 * We need no special check for begin/end pseudocolors here.  We
			 * don't need to do any processing for them, and they will be
			 * marked non-expandable since the regex engine will have reported
			 * them that way.
			 */
			if (!colorInfo->expandable)
				continue;

			if (colorInfo->containsNonWord)
			{
				/*
				 * Color includes non-word character(s).
				 *
				 * Generate an arc, treating this transition as occurring on
				 * BLANK.  This allows word-ending trigrams to be manufactured
				 * if possible.
				 */
				destKey.prefix.colors[0] = key->prefix.colors[1];
				destKey.prefix.colors[1] = COLOR_BLANK;
				destKey.nstate = arc->to;

				addArc(trgmNFA, state, key, COLOR_BLANK, &destKey);
			}

			if (colorInfo->wordCharsCount > 0)
			{
				/*
				 * Color includes word character(s).
				 *
				 * Generate an arc.  Color is pushed into prefix of target
				 * state.
				 */
				destKey.prefix.colors[0] = key->prefix.colors[1];
				destKey.prefix.colors[1] = arc->co;
				destKey.nstate = arc->to;

				addArc(trgmNFA, state, key, arc->co, &destKey);
			}
		}

		pfree(arcs);
	}
}

/*
 * Generate an out-arc of the expanded graph, if it's valid and not redundant.
 *
 * state: expanded-graph state we want to add an out-arc to
 * key: provides prefix colors (key->nstate is not used)
 * co: transition color
 * destKey: identifier for destination state of expanded graph
 */
static void
addArc(TrgmNFA *trgmNFA, TrgmState *state, TrgmStateKey *key,
	   TrgmColor co, TrgmStateKey *destKey)
{
	TrgmArc    *arc;
	ListCell   *cell;

	/* Do nothing if this wouldn't be a valid arc label trigram */
	if (!validArcLabel(key, co))
		return;

	/*
	 * Check if we are going to reach key which is covered by a key which is
	 * already listed in this state.  If so arc is useless: the NFA can bypass
	 * it through a path that doesn't require any predictable trigram, so
	 * whether the arc's trigram is present or not doesn't really matter.
	 */
	foreach(cell, state->enterKeys)
	{
		TrgmStateKey *existingKey = (TrgmStateKey *) lfirst(cell);

		if (existingKey->nstate == destKey->nstate &&
			prefixContains(&existingKey->prefix, &destKey->prefix))
			return;
	}

	/* Checks were successful, add new arc */
	arc = (TrgmArc *) palloc(sizeof(TrgmArc));
	arc->target = getState(trgmNFA, destKey);
	arc->ctrgm.colors[0] = key->prefix.colors[0];
	arc->ctrgm.colors[1] = key->prefix.colors[1];
	arc->ctrgm.colors[2] = co;

	state->arcs = lappend(state->arcs, arc);
	trgmNFA->arcsCount++;
}

/*
 * Can we make a valid trigram arc label from the given prefix and arc color?
 *
 * This is split out so that tests in addKey and addArc will stay in sync.
 */
static bool
validArcLabel(TrgmStateKey *key, TrgmColor co)
{
	/*
	 * We have to know full trigram in order to add outgoing arc.  So we can't
	 * do it if prefix is ambiguous.
	 */
	if (key->prefix.colors[0] == COLOR_UNKNOWN)
		return false;

	/* If key->prefix.colors[0] isn't unknown, its second color isn't either */
	Assert(key->prefix.colors[1] != COLOR_UNKNOWN);
	/* And we should not be called with an unknown arc color anytime */
	Assert(co != COLOR_UNKNOWN);

	/*
	 * We don't bother with making arcs representing three non-word
	 * characters, since that's useless for trigram extraction.
	 */
	if (key->prefix.colors[0] == COLOR_BLANK &&
		key->prefix.colors[1] == COLOR_BLANK &&
		co == COLOR_BLANK)
		return false;

	/*
	 * We also reject nonblank-blank-anything.  The nonblank-blank-nonblank
	 * case doesn't correspond to any trigram the trigram extraction code
	 * would make.  The nonblank-blank-blank case is also not possible with
	 * RPADDING = 1.  (Note that in many cases we'd fail to generate such a
	 * trigram even if it were valid, for example processing "foo bar" will
	 * not result in considering the trigram "o  ".  So if you want to support
	 * RPADDING = 2, there's more to do than just twiddle this test.)
	 */
	if (key->prefix.colors[0] != COLOR_BLANK &&
		key->prefix.colors[1] == COLOR_BLANK)
		return false;

	/*
	 * Other combinations involving blank are valid, in particular we assume
	 * blank-blank-nonblank is valid, which presumes that LPADDING is 2.
	 *
	 * Note: Using again the example "foo bar", we will not consider the
	 * trigram "  b", though this trigram would be found by the trigram
	 * extraction code.  Since we will find " ba", it doesn't seem worth
	 * trying to hack the algorithm to generate the additional trigram.
	 */

	/* arc label is valid */
	return true;
}

/*
 * Get state of expanded graph for given state key,
 * and queue the state for processing if it didn't already exist.
 */
static TrgmState *
getState(TrgmNFA *trgmNFA, TrgmStateKey *key)
{
	TrgmState  *state;
	bool		found;

	state = (TrgmState *) hash_search(trgmNFA->states, key, HASH_ENTER,
									  &found);
	if (!found)
	{
		/* New state: initialize and queue it */
		state->arcs = NIL;
		state->enterKeys = NIL;
		state->init = false;
		state->fin = false;
		state->parent = NULL;
		state->children = NIL;
		state->number = -1;

		trgmNFA->queue = lappend(trgmNFA->queue, state);
	}
	return state;
}

/*
 * Check if prefix1 "contains" prefix2.
 *
 * "contains" means that any exact prefix (with no ambiguity) that satisfies
 * prefix2 also satisfies prefix1.
 */
static bool
prefixContains(TrgmPrefix *prefix1, TrgmPrefix *prefix2)
{
	if (prefix1->colors[1] == COLOR_UNKNOWN)
	{
		/* Fully ambiguous prefix contains everything */
		return true;
	}
	else if (prefix1->colors[0] == COLOR_UNKNOWN)
	{
		/*
		 * Prefix with only first unknown color contains every prefix with
		 * same second color.
		 */
		if (prefix1->colors[1] == prefix2->colors[1])
			return true;
		else
			return false;
	}
	else
	{
		/* Exact prefix contains only the exact same prefix */
		if (prefix1->colors[0] == prefix2->colors[0] &&
			prefix1->colors[1] == prefix2->colors[1])
			return true;
		else
			return false;
	}
}


/*---------------------
 * Subroutines for expanding color trigrams into regular trigrams (stage 3).
 *---------------------
 */

/*
 * Get vector of all color trigrams in graph and select which of them
 * to expand into simple trigrams.
 *
 * Returns TRUE if OK, FALSE if exhausted resource limits.
 */
static bool
selectColorTrigrams(TrgmNFA *trgmNFA)
{
	HASH_SEQ_STATUS scan_status;
	int			arcsCount = trgmNFA->arcsCount,
				i;
	TrgmState  *state;
	ColorTrgmInfo *colorTrgms;
	int64		totalTrgmCount;
	float4		totalTrgmPenalty;
	int			number;

	/* Collect color trigrams from all arcs */
	colorTrgms = (ColorTrgmInfo *) palloc(sizeof(ColorTrgmInfo) * arcsCount);
	trgmNFA->colorTrgms = colorTrgms;

	i = 0;
	hash_seq_init(&scan_status, trgmNFA->states);
	while ((state = (TrgmState *) hash_seq_search(&scan_status)) != NULL)
	{
		ListCell   *cell;

		foreach(cell, state->arcs)
		{
			TrgmArc    *arc = (TrgmArc *) lfirst(cell);
			TrgmArcInfo *arcInfo = (TrgmArcInfo *) palloc(sizeof(TrgmArcInfo));

			arcInfo->source = state;
			arcInfo->target = arc->target;
			colorTrgms[i].arcs = list_make1(arcInfo);
			colorTrgms[i].expanded = true;
			colorTrgms[i].ctrgm = arc->ctrgm;
			i++;
		}
	}
	Assert(i == arcsCount);

	/* Remove duplicates, merging their arcs lists */
	if (arcsCount >= 2)
	{
		ColorTrgmInfo *p1,
				   *p2;

		/* Sort trigrams to ease duplicate detection */
		qsort(colorTrgms, arcsCount, sizeof(ColorTrgmInfo), colorTrgmInfoCmp);

		/* p1 is probe point, p2 is last known non-duplicate. */
		p2 = colorTrgms;
		for (p1 = colorTrgms + 1; p1 < colorTrgms + arcsCount; p1++)
		{
			if (colorTrgmInfoCmp(p1, p2) > 0)
			{
				p2++;
				*p2 = *p1;
			}
			else
			{
				p2->arcs = list_concat(p2->arcs, p1->arcs);
			}
		}
		trgmNFA->colorTrgmsCount = (p2 - colorTrgms) + 1;
	}
	else
	{
		trgmNFA->colorTrgmsCount = arcsCount;
	}

	/*
	 * Count number of simple trigrams generated by each color trigram, and
	 * also compute a penalty value, which is the number of simple trigrams
	 * times a multiplier that depends on its whitespace content.
	 *
	 * Note: per-color-trigram counts cannot overflow an int so long as
	 * COLOR_COUNT_LIMIT is not more than the cube root of INT_MAX, ie about
	 * 1290.  However, the grand total totalTrgmCount might conceivably
	 * overflow an int, so we use int64 for that within this routine.  Also,
	 * penalties are calculated in float4 arithmetic to avoid any overflow
	 * worries.
	 */
	totalTrgmCount = 0;
	totalTrgmPenalty = 0.0f;
	for (i = 0; i < trgmNFA->colorTrgmsCount; i++)
	{
		ColorTrgmInfo *trgmInfo = &colorTrgms[i];
		int			j,
					count = 1,
					typeIndex = 0;

		for (j = 0; j < 3; j++)
		{
			TrgmColor	c = trgmInfo->ctrgm.colors[j];

			typeIndex *= 2;
			if (c == COLOR_BLANK)
				typeIndex++;
			else
				count *= trgmNFA->colorInfo[c].wordCharsCount;
		}
		trgmInfo->count = count;
		totalTrgmCount += count;
		trgmInfo->penalty = penalties[typeIndex] * (float4) count;
		totalTrgmPenalty += trgmInfo->penalty;
	}

	/* Sort color trigrams in descending order of their penalties */
	qsort(colorTrgms, trgmNFA->colorTrgmsCount, sizeof(ColorTrgmInfo),
		  colorTrgmInfoPenaltyCmp);

	/*
	 * Remove color trigrams from the graph so long as total penalty of color
	 * trigrams exceeds WISH_TRGM_PENALTY.  (If we fail to get down to
	 * WISH_TRGM_PENALTY, it's OK so long as total count is no more than
	 * MAX_TRGM_COUNT.)  We prefer to remove color trigrams with higher
	 * penalty, since those are the most promising for reducing the total
	 * penalty.  When removing a color trigram we have to merge states
	 * connected by arcs labeled with that trigram.  It's necessary to not
	 * merge initial and final states, because our graph becomes useless if
	 * that happens; so we cannot always remove the trigram we'd prefer to.
	 */
	for (i = 0; i < trgmNFA->colorTrgmsCount; i++)
	{
		ColorTrgmInfo *trgmInfo = &colorTrgms[i];
		bool		canRemove = true;
		ListCell   *cell;

		/* Done if we've reached the target */
		if (totalTrgmPenalty <= WISH_TRGM_PENALTY)
			break;

		/*
		 * Does any arc of this color trigram connect initial and final
		 * states?	If so we can't remove it.
		 */
		foreach(cell, trgmInfo->arcs)
		{
			TrgmArcInfo *arcInfo = (TrgmArcInfo *) lfirst(cell);
			TrgmState  *source = arcInfo->source,
					   *target = arcInfo->target;

			/* examine parent states, if any merging has already happened */
			while (source->parent)
				source = source->parent;
			while (target->parent)
				target = target->parent;

			if ((source->init || target->init) &&
				(source->fin || target->fin))
			{
				canRemove = false;
				break;
			}
		}
		if (!canRemove)
			continue;

		/* OK, merge states linked by each arc labeled by the trigram */
		foreach(cell, trgmInfo->arcs)
		{
			TrgmArcInfo *arcInfo = (TrgmArcInfo *) lfirst(cell);
			TrgmState  *source = arcInfo->source,
					   *target = arcInfo->target;

			while (source->parent)
				source = source->parent;
			while (target->parent)
				target = target->parent;
			if (source != target)
				mergeStates(source, target);
		}

		/* Mark trigram unexpanded, and update totals */
		trgmInfo->expanded = false;
		totalTrgmCount -= trgmInfo->count;
		totalTrgmPenalty -= trgmInfo->penalty;
	}

	/* Did we succeed in fitting into MAX_TRGM_COUNT? */
	if (totalTrgmCount > MAX_TRGM_COUNT)
		return false;

	trgmNFA->totalTrgmCount = (int) totalTrgmCount;

	/*
	 * Sort color trigrams by colors (will be useful for bsearch in packGraph)
	 * and enumerate the color trigrams that are expanded.
	 */
	number = 0;
	qsort(colorTrgms, trgmNFA->colorTrgmsCount, sizeof(ColorTrgmInfo),
		  colorTrgmInfoCmp);
	for (i = 0; i < trgmNFA->colorTrgmsCount; i++)
	{
		if (colorTrgms[i].expanded)
		{
			colorTrgms[i].number = number;
			number++;
		}
	}

	return true;
}

/*
 * Expand selected color trigrams into regular trigrams.
 *
 * Returns the TRGM array to be passed to the index machinery.
 * The array must be allocated in rcontext.
 */
static TRGM *
expandColorTrigrams(TrgmNFA *trgmNFA, MemoryContext rcontext)
{
	TRGM	   *trg;
	trgm	   *p;
	int			i;
	TrgmColorInfo blankColor;
	trgm_mb_char blankChar;

	/* Set up "blank" color structure containing a single zero character */
	memset(blankChar.bytes, 0, sizeof(blankChar.bytes));
	blankColor.wordCharsCount = 1;
	blankColor.wordChars = &blankChar;

	/* Construct the trgm array */
	trg = (TRGM *)
		MemoryContextAllocZero(rcontext,
							   TRGMHDRSIZE +
							   trgmNFA->totalTrgmCount * sizeof(trgm));
	trg->flag = ARRKEY;
	SET_VARSIZE(trg, CALCGTSIZE(ARRKEY, trgmNFA->totalTrgmCount));
	p = GETARR(trg);
	for (i = 0; i < trgmNFA->colorTrgmsCount; i++)
	{
		ColorTrgmInfo *colorTrgm = &trgmNFA->colorTrgms[i];
		TrgmColorInfo *c[3];
		trgm_mb_char s[3];
		int			j,
					i1,
					i2,
					i3;

		/* Ignore any unexpanded trigrams ... */
		if (!colorTrgm->expanded)
			continue;

		/* Get colors, substituting the dummy struct for COLOR_BLANK */
		for (j = 0; j < 3; j++)
		{
			if (colorTrgm->ctrgm.colors[j] != COLOR_BLANK)
				c[j] = &trgmNFA->colorInfo[colorTrgm->ctrgm.colors[j]];
			else
				c[j] = &blankColor;
		}

		/* Iterate over all possible combinations of colors' characters */
		for (i1 = 0; i1 < c[0]->wordCharsCount; i1++)
		{
			s[0] = c[0]->wordChars[i1];
			for (i2 = 0; i2 < c[1]->wordCharsCount; i2++)
			{
				s[1] = c[1]->wordChars[i2];
				for (i3 = 0; i3 < c[2]->wordCharsCount; i3++)
				{
					s[2] = c[2]->wordChars[i3];
					fillTrgm(p, s);
					p++;
				}
			}
		}
	}

	return trg;
}

/*
 * Convert trigram into trgm datatype.
 */
static void
fillTrgm(trgm *ptrgm, trgm_mb_char s[3])
{
	char		str[3 * MAX_MULTIBYTE_CHAR_LEN],
			   *p;
	int			i,
				j;

	/* Write multibyte string into "str" (we don't need null termination) */
	p = str;

	for (i = 0; i < 3; i++)
	{
		if (s[i].bytes[0] != 0)
		{
			for (j = 0; j < MAX_MULTIBYTE_CHAR_LEN && s[i].bytes[j]; j++)
				*p++ = s[i].bytes[j];
		}
		else
		{
			/* Emit a space in place of COLOR_BLANK */
			*p++ = ' ';
		}
	}

	/* Convert "str" to a standard trigram (possibly hashing it) */
	compact_trigram(ptrgm, str, p - str);
}

/*
 * Merge two states of graph.
 */
static void
mergeStates(TrgmState *state1, TrgmState *state2)
{
	ListCell   *cell;

	Assert(state1 != state2);
	Assert(!state1->parent);
	Assert(!state2->parent);

	/* state1 absorbs state2's init/fin flags */
	state1->init |= state2->init;
	state1->fin |= state2->fin;

	/* state2, and all its children, become children of state1 */
	foreach(cell, state2->children)
	{
		TrgmState  *state = (TrgmState *) lfirst(cell);

		state->parent = state1;
	}
	state2->parent = state1;
	state1->children = list_concat(state1->children, state2->children);
	state1->children = lappend(state1->children, state2);
	state2->children = NIL;
}

/*
 * Compare function for sorting of color trigrams by their colors.
 */
static int
colorTrgmInfoCmp(const void *p1, const void *p2)
{
	const ColorTrgmInfo *c1 = (const ColorTrgmInfo *) p1;
	const ColorTrgmInfo *c2 = (const ColorTrgmInfo *) p2;

	return memcmp(&c1->ctrgm, &c2->ctrgm, sizeof(ColorTrgm));
}

/*
 * Compare function for sorting color trigrams in descending order of
 * their penalty fields.
 */
static int
colorTrgmInfoPenaltyCmp(const void *p1, const void *p2)
{
	float4		penalty1 = ((const ColorTrgmInfo *) p1)->penalty;
	float4		penalty2 = ((const ColorTrgmInfo *) p2)->penalty;

	if (penalty1 < penalty2)
		return 1;
	else if (penalty1 == penalty2)
		return 0;
	else
		return -1;
}


/*---------------------
 * Subroutines for packing the graph into final representation (stage 4).
 *---------------------
 */

/*
 * Pack expanded graph into final representation.
 *
 * The result data must be allocated in rcontext.
 */
static TrgmPackedGraph *
packGraph(TrgmNFA *trgmNFA, MemoryContext rcontext)
{
	int			number = 2,
				arcIndex,
				arcsCount;
	HASH_SEQ_STATUS scan_status;
	TrgmState  *state;
	TrgmPackArcInfo *arcs,
			   *p1,
			   *p2;
	TrgmPackedArc *packedArcs;
	TrgmPackedGraph *result;
	int			i,
				j;

	/* Enumerate surviving states, giving init and fin reserved numbers */
	hash_seq_init(&scan_status, trgmNFA->states);
	while ((state = (TrgmState *) hash_seq_search(&scan_status)) != NULL)
	{
		while (state->parent)
			state = state->parent;

		if (state->number < 0)
		{
			if (state->init)
				state->number = 0;
			else if (state->fin)
				state->number = 1;
			else
			{
				state->number = number;
				number++;
			}
		}
	}

	/* Collect array of all arcs */
	arcs = (TrgmPackArcInfo *)
		palloc(sizeof(TrgmPackArcInfo) * trgmNFA->arcsCount);
	arcIndex = 0;
	hash_seq_init(&scan_status, trgmNFA->states);
	while ((state = (TrgmState *) hash_seq_search(&scan_status)) != NULL)
	{
		TrgmState  *source = state;
		ListCell   *cell;

		while (source->parent)
			source = source->parent;

		foreach(cell, state->arcs)
		{
			TrgmArc    *arc = (TrgmArc *) lfirst(cell);
			TrgmState  *target = arc->target;

			while (target->parent)
				target = target->parent;

			if (source->number != target->number)
			{
				ColorTrgmInfo *ctrgm;

				ctrgm = (ColorTrgmInfo *) bsearch(&arc->ctrgm,
												  trgmNFA->colorTrgms,
												  trgmNFA->colorTrgmsCount,
												  sizeof(ColorTrgmInfo),
												  colorTrgmInfoCmp);
				Assert(ctrgm != NULL);
				Assert(ctrgm->expanded);

				arcs[arcIndex].sourceState = source->number;
				arcs[arcIndex].targetState = target->number;
				arcs[arcIndex].colorTrgm = ctrgm->number;
				arcIndex++;
			}
		}
	}

	/* Sort arcs to ease duplicate detection */
	qsort(arcs, arcIndex, sizeof(TrgmPackArcInfo), packArcInfoCmp);

	/* We could have duplicates because states were merged. Remove them. */
	/* p1 is probe point, p2 is last known non-duplicate. */
	p2 = arcs;
	for (p1 = arcs + 1; p1 < arcs + arcIndex; p1++)
	{
		if (packArcInfoCmp(p1, p2) > 0)
		{
			p2++;
			*p2 = *p1;
		}
	}
	arcsCount = (p2 - arcs) + 1;

	/* Create packed representation */
	result = (TrgmPackedGraph *)
		MemoryContextAlloc(rcontext, sizeof(TrgmPackedGraph));

	/* Pack color trigrams information */
	result->colorTrigramsCount = 0;
	for (i = 0; i < trgmNFA->colorTrgmsCount; i++)
	{
		if (trgmNFA->colorTrgms[i].expanded)
			result->colorTrigramsCount++;
	}
	result->colorTrigramGroups = (int *)
		MemoryContextAlloc(rcontext, sizeof(int) * result->colorTrigramsCount);
	j = 0;
	for (i = 0; i < trgmNFA->colorTrgmsCount; i++)
	{
		if (trgmNFA->colorTrgms[i].expanded)
		{
			result->colorTrigramGroups[j] = trgmNFA->colorTrgms[i].count;
			j++;
		}
	}

	/* Pack states and arcs information */
	result->statesCount = number;
	result->states = (TrgmPackedState *)
		MemoryContextAlloc(rcontext, number * sizeof(TrgmPackedState));
	packedArcs = (TrgmPackedArc *)
		MemoryContextAlloc(rcontext, arcsCount * sizeof(TrgmPackedArc));
	j = 0;
	for (i = 0; i < number; i++)
	{
		int			cnt = 0;

		result->states[i].arcs = &packedArcs[j];
		while (j < arcsCount && arcs[j].sourceState == i)
		{
			packedArcs[j].targetState = arcs[j].targetState;
			packedArcs[j].colorTrgm = arcs[j].colorTrgm;
			cnt++;
			j++;
		}
		result->states[i].arcsCount = cnt;
	}

	/* Allocate working memory for trigramsMatchGraph() */
	result->colorTrigramsActive = (bool *)
		MemoryContextAlloc(rcontext, sizeof(bool) * result->colorTrigramsCount);
	result->statesActive = (bool *)
		MemoryContextAlloc(rcontext, sizeof(bool) * result->statesCount);
	result->statesQueue = (int *)
		MemoryContextAlloc(rcontext, sizeof(int) * result->statesCount);

	return result;
}

/*
 * Comparison function for sorting TrgmPackArcInfos.
 *
 * Compares arcs in following order: sourceState, colorTrgm, targetState.
 */
static int
packArcInfoCmp(const void *a1, const void *a2)
{
	const TrgmPackArcInfo *p1 = (const TrgmPackArcInfo *) a1;
	const TrgmPackArcInfo *p2 = (const TrgmPackArcInfo *) a2;

	if (p1->sourceState < p2->sourceState)
		return -1;
	if (p1->sourceState > p2->sourceState)
		return 1;
	if (p1->colorTrgm < p2->colorTrgm)
		return -1;
	if (p1->colorTrgm > p2->colorTrgm)
		return 1;
	if (p1->targetState < p2->targetState)
		return -1;
	if (p1->targetState > p2->targetState)
		return 1;
	return 0;
}


/*---------------------
 * Debugging functions
 *
 * These are designed to emit GraphViz files.
 *---------------------
 */

#ifdef TRGM_REGEXP_DEBUG

/*
 * Print initial NFA, in regexp library's representation
 */
static void
printSourceNFA(regex_t *regex, TrgmColorInfo *colors, int ncolors)
{
	StringInfoData buf;
	int			nstates = pg_reg_getnumstates(regex);
	int			state;
	int			i;

	initStringInfo(&buf);

	appendStringInfoString(&buf, "\ndigraph sourceNFA {\n");

	for (state = 0; state < nstates; state++)
	{
		regex_arc_t *arcs;
		int			i,
					arcsCount;

		appendStringInfo(&buf, "s%d", state);
		if (pg_reg_getfinalstate(regex) == state)
			appendStringInfoString(&buf, " [shape = doublecircle]");
		appendStringInfoString(&buf, ";\n");

		arcsCount = pg_reg_getnumoutarcs(regex, state);
		arcs = (regex_arc_t *) palloc(sizeof(regex_arc_t) * arcsCount);
		pg_reg_getoutarcs(regex, state, arcs, arcsCount);

		for (i = 0; i < arcsCount; i++)
		{
			appendStringInfo(&buf, "  s%d -> s%d [label = \"%d\"];\n",
							 state, arcs[i].to, arcs[i].co);
		}

		pfree(arcs);
	}

	appendStringInfoString(&buf, " node [shape = point ]; initial;\n");
	appendStringInfo(&buf, " initial -> s%d;\n",
					 pg_reg_getinitialstate(regex));

	/* Print colors */
	appendStringInfoString(&buf, " { rank = sink;\n");
	appendStringInfoString(&buf, "  Colors [shape = none, margin=0, label=<\n");

	for (i = 0; i < ncolors; i++)
	{
		TrgmColorInfo *color = &colors[i];
		int			j;

		appendStringInfo(&buf, "<br/>Color %d: ", i);
		if (color->expandable)
		{
			for (j = 0; j < color->wordCharsCount; j++)
			{
				char		s[MAX_MULTIBYTE_CHAR_LEN + 1];

				memcpy(s, color->wordChars[j].bytes, MAX_MULTIBYTE_CHAR_LEN);
				s[MAX_MULTIBYTE_CHAR_LEN] = '\0';
				appendStringInfoString(&buf, s);
			}
		}
		else
			appendStringInfoString(&buf, "not expandable");
		appendStringInfoChar(&buf, '\n');
	}

	appendStringInfoString(&buf, "  >];\n");
	appendStringInfoString(&buf, " }\n");
	appendStringInfoString(&buf, "}\n");

	{
		/* dot -Tpng -o /tmp/source.png < /tmp/source.dot */
		FILE	   *fp = fopen("/tmp/source.dot", "w");

		fprintf(fp, "%s", buf.data);
		fclose(fp);
	}

	pfree(buf.data);
}

/*
 * Print expanded graph.
 */
static void
printTrgmNFA(TrgmNFA *trgmNFA)
{
	StringInfoData buf;
	HASH_SEQ_STATUS scan_status;
	TrgmState  *state;
	TrgmState  *initstate = NULL;

	initStringInfo(&buf);

	appendStringInfoString(&buf, "\ndigraph transformedNFA {\n");

	hash_seq_init(&scan_status, trgmNFA->states);
	while ((state = (TrgmState *) hash_seq_search(&scan_status)) != NULL)
	{
		ListCell   *cell;

		appendStringInfo(&buf, "s%p", (void *) state);
		if (state->fin)
			appendStringInfoString(&buf, " [shape = doublecircle]");
		if (state->init)
			initstate = state;
		appendStringInfo(&buf, " [label = \"%d\"]", state->stateKey.nstate);
		appendStringInfoString(&buf, ";\n");

		foreach(cell, state->arcs)
		{
			TrgmArc    *arc = (TrgmArc *) lfirst(cell);

			appendStringInfo(&buf, "  s%p -> s%p [label = \"",
							 (void *) state, (void *) arc->target);
			printTrgmColor(&buf, arc->ctrgm.colors[0]);
			appendStringInfoChar(&buf, ' ');
			printTrgmColor(&buf, arc->ctrgm.colors[1]);
			appendStringInfoChar(&buf, ' ');
			printTrgmColor(&buf, arc->ctrgm.colors[2]);
			appendStringInfoString(&buf, "\"];\n");
		}
	}

	if (initstate)
	{
		appendStringInfoString(&buf, " node [shape = point ]; initial;\n");
		appendStringInfo(&buf, " initial -> s%p;\n", (void *) initstate);
	}

	appendStringInfoString(&buf, "}\n");

	{
		/* dot -Tpng -o /tmp/transformed.png < /tmp/transformed.dot */
		FILE	   *fp = fopen("/tmp/transformed.dot", "w");

		fprintf(fp, "%s", buf.data);
		fclose(fp);
	}

	pfree(buf.data);
}

/*
 * Print a TrgmColor readably.
 */
static void
printTrgmColor(StringInfo buf, TrgmColor co)
{
	if (co == COLOR_UNKNOWN)
		appendStringInfoChar(buf, 'u');
	else if (co == COLOR_BLANK)
		appendStringInfoChar(buf, 'b');
	else
		appendStringInfo(buf, "%d", (int) co);
}

/*
 * Print final packed representation of trigram-based expanded graph.
 */
static void
printTrgmPackedGraph(TrgmPackedGraph *packedGraph, TRGM *trigrams)
{
	StringInfoData buf;
	trgm	   *p;
	int			i;

	initStringInfo(&buf);

	appendStringInfoString(&buf, "\ndigraph packedGraph {\n");

	for (i = 0; i < packedGraph->statesCount; i++)
	{
		TrgmPackedState *state = &packedGraph->states[i];
		int			j;

		appendStringInfo(&buf, " s%d", i);
		if (i == 1)
			appendStringInfoString(&buf, " [shape = doublecircle]");

		appendStringInfo(&buf, " [label = <s%d>];\n", i);

		for (j = 0; j < state->arcsCount; j++)
		{
			TrgmPackedArc *arc = &state->arcs[j];

			appendStringInfo(&buf, "  s%d -> s%d [label = \"trigram %d\"];\n",
							 i, arc->targetState, arc->colorTrgm);
		}
	}

	appendStringInfoString(&buf, " node [shape = point ]; initial;\n");
	appendStringInfo(&buf, " initial -> s%d;\n", 0);

	/* Print trigrams */
	appendStringInfoString(&buf, " { rank = sink;\n");
	appendStringInfoString(&buf, "  Trigrams [shape = none, margin=0, label=<\n");

	p = GETARR(trigrams);
	for (i = 0; i < packedGraph->colorTrigramsCount; i++)
	{
		int			count = packedGraph->colorTrigramGroups[i];
		int			j;

		appendStringInfo(&buf, "<br/>Trigram %d: ", i);

		for (j = 0; j < count; j++)
		{
			if (j > 0)
				appendStringInfoString(&buf, ", ");

			/*
			 * XXX This representation is nice only for all-ASCII trigrams.
			 */
			appendStringInfo(&buf, "\"%c%c%c\"", (*p)[0], (*p)[1], (*p)[2]);
			p++;
		}
	}

	appendStringInfoString(&buf, "  >];\n");
	appendStringInfoString(&buf, " }\n");
	appendStringInfoString(&buf, "}\n");

	{
		/* dot -Tpng -o /tmp/packed.png < /tmp/packed.dot */
		FILE	   *fp = fopen("/tmp/packed.dot", "w");

		fprintf(fp, "%s", buf.data);
		fclose(fp);
	}

	pfree(buf.data);
}

#endif   /* TRGM_REGEXP_DEBUG */