rege_dfa.c

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/*
 * DFA routines
 * This file is #included by regexec.c.
 *
 * Copyright (c) 1998, 1999 Henry Spencer.  All rights reserved.
 *
 * Development of this software was funded, in part, by Cray Research Inc.,
 * UUNET Communications Services Inc., Sun Microsystems Inc., and Scriptics
 * Corporation, none of whom are responsible for the results.  The author
 * thanks all of them.
 *
 * Redistribution and use in source and binary forms -- with or without
 * modification -- are permitted for any purpose, provided that
 * redistributions in source form retain this entire copyright notice and
 * indicate the origin and nature of any modifications.
 *
 * I'd appreciate being given credit for this package in the documentation
 * of software which uses it, but that is not a requirement.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
 * HENRY SPENCER BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
 * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
 * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * src/backend/regex/rege_dfa.c
 *
 */
 
/*
 * longest - longest-preferred matching engine
 *
 * On success, returns match endpoint address.  Returns NULL on no match.
 * Internal errors also return NULL, with v->err set.
 */
static chr *
longest(struct vars *v,
        struct dfa *d,
        chr *start,             /* where the match should start */
        chr *stop,              /* match must end at or before here */
        int *hitstopp)          /* record whether hit v->stop, if non-NULL */
{
    chr        *cp;
    chr        *realstop = (stop == v->stop) ? stop : stop + 1;
    color       co;
    struct sset *css;
    struct sset *ss;
    chr        *post;
    int         i;
    struct colormap *cm = d->cm;
 
    /* prevent "uninitialized variable" warnings */
    if (hitstopp != NULL)
        *hitstopp = 0;
 
    /* if this is a backref to a known string, just match against that */
    if (d->backno >= 0)
    {
        assert((size_t) d->backno < v->nmatch);
        if (v->pmatch[d->backno].rm_so >= 0)
        {
            cp = dfa_backref(v, d, start, start, stop, false);
            if (cp == v->stop && stop == v->stop && hitstopp != NULL)
                *hitstopp = 1;
            return cp;
        }
    }
 
    /* fast path for matchall NFAs */
    if (d->cnfa->flags & MATCHALL)
    {
        size_t      nchr = stop - start;
        size_t      maxmatchall = d->cnfa->maxmatchall;
 
        if (nchr < d->cnfa->minmatchall)
            return NULL;
        if (maxmatchall == DUPINF)
        {
            if (stop == v->stop && hitstopp != NULL)
                *hitstopp = 1;
        }
        else
        {
            if (stop == v->stop && nchr <= maxmatchall + 1 && hitstopp != NULL)
                *hitstopp = 1;
            if (nchr > maxmatchall)
                return start + maxmatchall;
        }
        return stop;
    }
 
    /* initialize */
    css = initialize(v, d, start);
    if (css == NULL)
        return NULL;
    cp = start;
 
    /* startup */
    FDEBUG(("+++ startup +++\n"));
    if (cp == v->start)
    {
        co = d->cnfa->bos[(v->eflags & REG_NOTBOL) ? 0 : 1];
        FDEBUG(("color %ld\n", (long) co));
    }
    else
    {
        co = GETCOLOR(cm, *(cp - 1));
        FDEBUG(("char %c, color %ld\n", (char) *(cp - 1), (long) co));
    }
    css = miss(v, d, css, co, cp, start);
    if (css == NULL)
        return NULL;
    css->lastseen = cp;
 
    /*
     * This is the main text-scanning loop.  It seems worth having two copies
     * to avoid the overhead of REG_FTRACE tests here, even in REG_DEBUG
     * builds, when you're not actively tracing.
     */
#ifdef REG_DEBUG
    if (v->eflags & REG_FTRACE)
    {
        while (cp < realstop)
        {
            FDEBUG(("+++ at c%d +++\n", (int) (css - d->ssets)));
            co = GETCOLOR(cm, *cp);
            FDEBUG(("char %c, color %ld\n", (char) *cp, (long) co));
            ss = css->outs[co];
            if (ss == NULL)
            {
                ss = miss(v, d, css, co, cp + 1, start);
                if (ss == NULL)
                    break;      /* NOTE BREAK OUT */
            }
            cp++;
            ss->lastseen = cp;
            css = ss;
        }
    }
    else
#endif
    {
        while (cp < realstop)
        {
            co = GETCOLOR(cm, *cp);
            ss = css->outs[co];
            if (ss == NULL)
            {
                ss = miss(v, d, css, co, cp + 1, start);
                if (ss == NULL)
                    break;      /* NOTE BREAK OUT */
            }
            cp++;
            ss->lastseen = cp;
            css = ss;
        }
    }
 
    if (ISERR())
        return NULL;
 
    /* shutdown */
    FDEBUG(("+++ shutdown at c%d +++\n", (int) (css - d->ssets)));
    if (cp == v->stop && stop == v->stop)
    {
        if (hitstopp != NULL)
            *hitstopp = 1;
        co = d->cnfa->eos[(v->eflags & REG_NOTEOL) ? 0 : 1];
        FDEBUG(("color %ld\n", (long) co));
        ss = miss(v, d, css, co, cp, start);
        if (ISERR())
            return NULL;
        /* special case:  match ended at eol? */
        if (ss != NULL && (ss->flags & POSTSTATE))
            return cp;
        else if (ss != NULL)
            ss->lastseen = cp;  /* to be tidy */
    }
 
    /* find last match, if any */
    post = d->lastpost;
    for (ss = d->ssets, i = d->nssused; i > 0; ss++, i--)
        if ((ss->flags & POSTSTATE) && post != ss->lastseen &&
            (post == NULL || post < ss->lastseen))
            post = ss->lastseen;
    if (post != NULL)           /* found one */
        return post - 1;
 
    return NULL;
}
 
/*
 * shortest - shortest-preferred matching engine
 *
 * On success, returns match endpoint address.  Returns NULL on no match.
 * Internal errors also return NULL, with v->err set.
 */
static chr *
shortest(struct vars *v,
         struct dfa *d,
         chr *start,            /* where the match should start */
         chr *min,              /* match must end at or after here */
         chr *max,              /* match must end at or before here */
         chr **coldp,           /* store coldstart pointer here, if non-NULL */
         int *hitstopp)         /* record whether hit v->stop, if non-NULL */
{
    chr        *cp;
    chr        *realmin = (min == v->stop) ? min : min + 1;
    chr        *realmax = (max == v->stop) ? max : max + 1;
    color       co;
    struct sset *css;
    struct sset *ss;
    struct colormap *cm = d->cm;
 
    /* prevent "uninitialized variable" warnings */
    if (coldp != NULL)
        *coldp = NULL;
    if (hitstopp != NULL)
        *hitstopp = 0;
 
    /* if this is a backref to a known string, just match against that */
    if (d->backno >= 0)
    {
        assert((size_t) d->backno < v->nmatch);
        if (v->pmatch[d->backno].rm_so >= 0)
        {
            cp = dfa_backref(v, d, start, min, max, true);
            if (cp != NULL && coldp != NULL)
                *coldp = start;
            /* there is no case where we should set *hitstopp */
            return cp;
        }
    }
 
    /* fast path for matchall NFAs */
    if (d->cnfa->flags & MATCHALL)
    {
        size_t      nchr = min - start;
 
        if (d->cnfa->maxmatchall != DUPINF &&
            nchr > d->cnfa->maxmatchall)
            return NULL;
        if ((max - start) < d->cnfa->minmatchall)
            return NULL;
        if (nchr < d->cnfa->minmatchall)
            min = start + d->cnfa->minmatchall;
        if (coldp != NULL)
            *coldp = start;
        /* there is no case where we should set *hitstopp */
        return min;
    }
 
    /* initialize */
    css = initialize(v, d, start);
    if (css == NULL)
        return NULL;
    cp = start;
 
    /* startup */
    FDEBUG(("--- startup ---\n"));
    if (cp == v->start)
    {
        co = d->cnfa->bos[(v->eflags & REG_NOTBOL) ? 0 : 1];
        FDEBUG(("color %ld\n", (long) co));
    }
    else
    {
        co = GETCOLOR(cm, *(cp - 1));
        FDEBUG(("char %c, color %ld\n", (char) *(cp - 1), (long) co));
    }
    css = miss(v, d, css, co, cp, start);
    if (css == NULL)
        return NULL;
    css->lastseen = cp;
    ss = css;
 
    /*
     * This is the main text-scanning loop.  It seems worth having two copies
     * to avoid the overhead of REG_FTRACE tests here, even in REG_DEBUG
     * builds, when you're not actively tracing.
     */
#ifdef REG_DEBUG
    if (v->eflags & REG_FTRACE)
    {
        while (cp < realmax)
        {
            FDEBUG(("--- at c%d ---\n", (int) (css - d->ssets)));
            co = GETCOLOR(cm, *cp);
            FDEBUG(("char %c, color %ld\n", (char) *cp, (long) co));
            ss = css->outs[co];
            if (ss == NULL)
            {
                ss = miss(v, d, css, co, cp + 1, start);
                if (ss == NULL)
                    break;      /* NOTE BREAK OUT */
            }
            cp++;
            ss->lastseen = cp;
            css = ss;
            if ((ss->flags & POSTSTATE) && cp >= realmin)
                break;          /* NOTE BREAK OUT */
        }
    }
    else
#endif
    {
        while (cp < realmax)
        {
            co = GETCOLOR(cm, *cp);
            ss = css->outs[co];
            if (ss == NULL)
            {
                ss = miss(v, d, css, co, cp + 1, start);
                if (ss == NULL)
                    break;      /* NOTE BREAK OUT */
            }
            cp++;
            ss->lastseen = cp;
            css = ss;
            if ((ss->flags & POSTSTATE) && cp >= realmin)
                break;          /* NOTE BREAK OUT */
        }
    }
 
    if (ss == NULL)
        return NULL;
 
    if (coldp != NULL)          /* report last no-progress state set, if any */
        *coldp = lastcold(v, d);
 
    if ((ss->flags & POSTSTATE) && cp > min)
    {
        assert(cp >= realmin);
        cp--;
    }
    else if (cp == v->stop && max == v->stop)
    {
        co = d->cnfa->eos[(v->eflags & REG_NOTEOL) ? 0 : 1];
        FDEBUG(("color %ld\n", (long) co));
        ss = miss(v, d, css, co, cp, start);
        /* match might have ended at eol */
        if ((ss == NULL || !(ss->flags & POSTSTATE)) && hitstopp != NULL)
            *hitstopp = 1;
    }
 
    if (ss == NULL || !(ss->flags & POSTSTATE))
        return NULL;
 
    return cp;
}
 
/*
 * matchuntil - incremental matching engine
 *
 * This is meant for use with a search-style NFA (that is, the pattern is
 * known to act as though it had a leading .*).  We determine whether a
 * match exists starting at v->start and ending at probe.  Multiple calls
 * require only O(N) time not O(N^2) so long as the probe values are
 * nondecreasing.  *lastcss and *lastcp must be initialized to NULL before
 * starting a series of calls.
 *
 * Returns 1 if a match exists, 0 if not.
 * Internal errors also return 0, with v->err set.
 */
static int
matchuntil(struct vars *v,
           struct dfa *d,
           chr *probe,          /* we want to know if a match ends here */
           struct sset **lastcss,   /* state storage across calls */
           chr **lastcp)        /* state storage across calls */
{
    chr        *cp = *lastcp;
    color       co;
    struct sset *css = *lastcss;
    struct sset *ss;
    struct colormap *cm = d->cm;
 
    /* fast path for matchall NFAs */
    if (d->cnfa->flags & MATCHALL)
    {
        size_t      nchr = probe - v->start;
 
        if (nchr < d->cnfa->minmatchall)
            return 0;
        /* maxmatchall will always be infinity, cf. makesearch() */
        assert(d->cnfa->maxmatchall == DUPINF);
        return 1;
    }
 
    /* initialize and startup, or restart, if necessary */
    if (cp == NULL || cp > probe)
    {
        cp = v->start;
        css = initialize(v, d, cp);
        if (css == NULL)
            return 0;
 
        FDEBUG((">>> startup >>>\n"));
        co = d->cnfa->bos[(v->eflags & REG_NOTBOL) ? 0 : 1];
        FDEBUG(("color %ld\n", (long) co));
 
        css = miss(v, d, css, co, cp, v->start);
        if (css == NULL)
            return 0;
        css->lastseen = cp;
    }
    else if (css == NULL)
    {
        /* we previously found that no match is possible beyond *lastcp */
        return 0;
    }
    ss = css;
 
    /*
     * This is the main text-scanning loop.  It seems worth having two copies
     * to avoid the overhead of REG_FTRACE tests here, even in REG_DEBUG
     * builds, when you're not actively tracing.
     */
#ifdef REG_DEBUG
    if (v->eflags & REG_FTRACE)
    {
        while (cp < probe)
        {
            FDEBUG((">>> at c%d >>>\n", (int) (css - d->ssets)));
            co = GETCOLOR(cm, *cp);
            FDEBUG(("char %c, color %ld\n", (char) *cp, (long) co));
            ss = css->outs[co];
            if (ss == NULL)
            {
                ss = miss(v, d, css, co, cp + 1, v->start);
                if (ss == NULL)
                    break;      /* NOTE BREAK OUT */
            }
            cp++;
            ss->lastseen = cp;
            css = ss;
        }
    }
    else
#endif
    {
        while (cp < probe)
        {
            co = GETCOLOR(cm, *cp);
            ss = css->outs[co];
            if (ss == NULL)
            {
                ss = miss(v, d, css, co, cp + 1, v->start);
                if (ss == NULL)
                    break;      /* NOTE BREAK OUT */
            }
            cp++;
            ss->lastseen = cp;
            css = ss;
        }
    }
 
    *lastcss = ss;
    *lastcp = cp;
 
    if (ss == NULL)
        return 0;               /* impossible match, or internal error */
 
    /* We need to process one more chr, or the EOS symbol, to check match */
    if (cp < v->stop)
    {
        FDEBUG((">>> at c%d >>>\n", (int) (css - d->ssets)));
        co = GETCOLOR(cm, *cp);
        FDEBUG(("char %c, color %ld\n", (char) *cp, (long) co));
        ss = css->outs[co];
        if (ss == NULL)
            ss = miss(v, d, css, co, cp + 1, v->start);
    }
    else
    {
        assert(cp == v->stop);
        co = d->cnfa->eos[(v->eflags & REG_NOTEOL) ? 0 : 1];
        FDEBUG(("color %ld\n", (long) co));
        ss = miss(v, d, css, co, cp, v->start);
    }
 
    if (ss == NULL || !(ss->flags & POSTSTATE))
        return 0;
 
    return 1;
}
 
/*
 * dfa_backref - find best match length for a known backref string
 *
 * When the backref's referent is already available, we can deliver an exact
 * answer with considerably less work than running the backref node's NFA.
 *
 * Return match endpoint for longest or shortest valid repeated match,
 * or NULL if there is no valid match.
 *
 * Should be in sync with cbrdissect(), although that has the different task
 * of checking a match to a predetermined section of the string.
 */
static chr *
dfa_backref(struct vars *v,
            struct dfa *d,
            chr *start,         /* where the match should start */
            chr *min,           /* match must end at or after here */
            chr *max,           /* match must end at or before here */
            bool shortest)
{
    int         n = d->backno;
    int         backmin = d->backmin;
    int         backmax = d->backmax;
    size_t      numreps;
    size_t      minreps;
    size_t      maxreps;
    size_t      brlen;
    chr        *brstring;
    chr        *p;
 
    /* get the backreferenced string (caller should have checked this) */
    if (v->pmatch[n].rm_so == -1)
        return NULL;
    brstring = v->start + v->pmatch[n].rm_so;
    brlen = v->pmatch[n].rm_eo - v->pmatch[n].rm_so;
 
    /* special-case zero-length backreference to avoid divide by zero */
    if (brlen == 0)
    {
        /*
         * matches only a zero-length string, but any number of repetitions
         * can be considered to be present
         */
        if (min == start && backmin <= backmax)
            return start;
        return NULL;
    }
 
    /*
     * convert min and max into numbers of possible repetitions of the backref
     * string, rounding appropriately
     */
    if (min <= start)
        minreps = 0;
    else
        minreps = (min - start - 1) / brlen + 1;
    maxreps = (max - start) / brlen;
 
    /* apply bounds, then see if there is any allowed match length */
    if (minreps < backmin)
        minreps = backmin;
    if (backmax != DUPINF && maxreps > backmax)
        maxreps = backmax;
    if (maxreps < minreps)
        return NULL;
 
    /* quick exit if zero-repetitions match is valid and preferred */
    if (shortest && minreps == 0)
        return start;
 
    /* okay, compare the actual string contents */
    p = start;
    numreps = 0;
    while (numreps < maxreps)
    {
        if ((*v->g->compare) (brstring, p, brlen) != 0)
            break;
        p += brlen;
        numreps++;
        if (shortest && numreps >= minreps)
            break;
    }
 
    if (numreps >= minreps)
        return p;
    return NULL;
}
 
/*
 * lastcold - determine last point at which no progress had been made
 */
static chr *                    /* endpoint, or NULL */
lastcold(struct vars *v,
         struct dfa *d)
{
    struct sset *ss;
    chr        *nopr;
    int         i;
 
    nopr = d->lastnopr;
    if (nopr == NULL)
        nopr = v->start;
    for (ss = d->ssets, i = d->nssused; i > 0; ss++, i--)
        if ((ss->flags & NOPROGRESS) && nopr < ss->lastseen)
            nopr = ss->lastseen;
    return nopr;
}
 
/*
 * newdfa - set up a fresh DFA
 *
 * Returns NULL (and sets v->err) on failure.
 */
static struct dfa *
newdfa(struct vars *v,
       struct cnfa *cnfa,
       struct colormap *cm,
       struct smalldfa *sml)    /* preallocated space, may be NULL */
{
    struct dfa *d;
    size_t      nss = cnfa->nstates * 2;
    int         wordsper = (cnfa->nstates + UBITS - 1) / UBITS;
    bool        ismalloced = false;
 
    assert(cnfa != NULL && cnfa->nstates != 0);
 
    if (nss <= FEWSTATES && cnfa->ncolors <= FEWCOLORS)
    {
        assert(wordsper == 1);
        if (sml == NULL)
        {
            sml = (struct smalldfa *) MALLOC(sizeof(struct smalldfa));
            if (sml == NULL)
            {
                ERR(REG_ESPACE);
                return NULL;
            }
            ismalloced = true;
        }
        d = &sml->dfa;
        d->ssets = sml->ssets;
        d->statesarea = sml->statesarea;
        d->work = &d->statesarea[nss];
        d->outsarea = sml->outsarea;
        d->incarea = sml->incarea;
        d->ismalloced = ismalloced;
        d->arraysmalloced = false;  /* not separately allocated, anyway */
    }
    else
    {
        /*
         * Restrict the ranges of nstates and ncolors enough that the arrays
         * we allocate here have no more than INT_MAX members.  This protects
         * not only the allocation calculations just below, but later indexing
         * into these arrays.
         */
        if (wordsper >= INT_MAX / (nss + WORK) ||
            cnfa->ncolors >= INT_MAX / nss)
        {
            ERR(REG_ETOOBIG);
            return NULL;
        }
        d = (struct dfa *) MALLOC(sizeof(struct dfa));
        if (d == NULL)
        {
            ERR(REG_ESPACE);
            return NULL;
        }
        d->ssets = MALLOC_ARRAY(struct sset, nss);
        d->statesarea = MALLOC_ARRAY(unsigned, (nss + WORK) * wordsper);
        d->work = &d->statesarea[nss * wordsper];
        d->outsarea = MALLOC_ARRAY(struct sset *, nss * cnfa->ncolors);
        d->incarea = MALLOC_ARRAY(struct arcp, nss * cnfa->ncolors);
        d->ismalloced = true;
        d->arraysmalloced = true;
        /* now freedfa() will behave sanely */
        if (d->ssets == NULL || d->statesarea == NULL ||
            d->outsarea == NULL || d->incarea == NULL)
        {
            freedfa(d);
            ERR(REG_ESPACE);
            return NULL;
        }
    }
 
    d->nssets = (v->eflags & REG_SMALL) ? 7 : nss;
    d->nssused = 0;
    d->nstates = cnfa->nstates;
    d->ncolors = cnfa->ncolors;
    d->wordsper = wordsper;
    d->cnfa = cnfa;
    d->cm = cm;
    d->lastpost = NULL;
    d->lastnopr = NULL;
    d->search = d->ssets;
    d->backno = -1;             /* may be set by caller */
    d->backmin = d->backmax = 0;
 
    /* initialization of sset fields is done as needed */
 
    return d;
}
 
/*
 * freedfa - free a DFA
 */
static void
freedfa(struct dfa *d)
{
    if (d->arraysmalloced)
    {
        if (d->ssets != NULL)
            FREE(d->ssets);
        if (d->statesarea != NULL)
            FREE(d->statesarea);
        if (d->outsarea != NULL)
            FREE(d->outsarea);
        if (d->incarea != NULL)
            FREE(d->incarea);
    }
 
    if (d->ismalloced)
        FREE(d);
}
 
/*
 * hash - construct a hash code for a bitvector
 *
 * There are probably better ways, but they're more expensive.
 */
static unsigned
hash(unsigned *uv,
     int n)
{
    int         i;
    unsigned    h;
 
    h = 0;
    for (i = 0; i < n; i++)
        h ^= uv[i];
    return h;
}
 
/*
 * initialize - hand-craft a cache entry for startup, otherwise get ready
 */
static struct sset *
initialize(struct vars *v,
           struct dfa *d,
           chr *start)
{
    struct sset *ss;
    int         i;
 
    /* is previous one still there? */
    if (d->nssused > 0 && (d->ssets[0].flags & STARTER))
        ss = &d->ssets[0];
    else
    {                           /* no, must (re)build it */
        ss = getvacant(v, d, start, start);
        if (ss == NULL)
            return NULL;
        for (i = 0; i < d->wordsper; i++)
            ss->states[i] = 0;
        BSET(ss->states, d->cnfa->pre);
        ss->hash = HASH(ss->states, d->wordsper);
        assert(d->cnfa->pre != d->cnfa->post);
        ss->flags = STARTER | LOCKED | NOPROGRESS;
        /* lastseen dealt with below */
    }
 
    for (i = 0; i < d->nssused; i++)
        d->ssets[i].lastseen = NULL;
    ss->lastseen = start;       /* maybe untrue, but harmless */
    d->lastpost = NULL;
    d->lastnopr = NULL;
    return ss;
}
 
/*
 * miss - handle a stateset cache miss
 *
 * css is the current stateset, co is the color of the current input character,
 * cp points to the character after that (which is where we may need to test
 * LACONs).  start does not affect matching behavior but is needed for pickss'
 * heuristics about which stateset cache entry to replace.
 *
 * Ordinarily, returns the address of the next stateset (the one that is
 * valid after consuming the input character).  Returns NULL if no valid
 * NFA states remain, ie we have a certain match failure.
 * Internal errors also return NULL, with v->err set.
 */
static struct sset *
miss(struct vars *v,
     struct dfa *d,
     struct sset *css,
     color co,
     chr *cp,                   /* next chr */
     chr *start)                /* where the attempt got started */
{
    struct cnfa *cnfa = d->cnfa;
    int         i;
    unsigned    h;
    struct carc *ca;
    struct sset *p;
    int         ispseudocolor;
    int         ispost;
    int         noprogress;
    int         gotstate;
    int         dolacons;
    int         sawlacons;
 
    /* for convenience, we can be called even if it might not be a miss */
    if (css->outs[co] != NULL)
    {
        FDEBUG(("hit\n"));
        return css->outs[co];
    }
    FDEBUG(("miss\n"));
 
    /*
     * Checking for operation cancel in the inner text search loop seems
     * unduly expensive.  As a compromise, check during cache misses.
     */
    INTERRUPT(v->re);
 
    /*
     * What set of states would we end up in after consuming the co character?
     * We first consider PLAIN arcs that consume the character, and then look
     * to see what LACON arcs could be traversed after consuming it.
     */
    for (i = 0; i < d->wordsper; i++)
        d->work[i] = 0;         /* build new stateset bitmap in d->work */
    ispseudocolor = d->cm->cd[co].flags & PSEUDO;
    ispost = 0;
    noprogress = 1;
    gotstate = 0;
    for (i = 0; i < d->nstates; i++)
        if (ISBSET(css->states, i))
            for (ca = cnfa->states[i]; ca->co != COLORLESS; ca++)
                if (ca->co == co ||
                    (ca->co == RAINBOW && !ispseudocolor))
                {
                    BSET(d->work, ca->to);
                    gotstate = 1;
                    if (ca->to == cnfa->post)
                        ispost = 1;
                    if (!(cnfa->stflags[ca->to] & CNFA_NOPROGRESS))
                        noprogress = 0;
                    FDEBUG(("%d -> %d\n", i, ca->to));
                }
    if (!gotstate)
        return NULL;            /* character cannot reach any new state */
    dolacons = (cnfa->flags & HASLACONS);
    sawlacons = 0;
    /* outer loop handles transitive closure of reachable-by-LACON states */
    while (dolacons)
    {
        dolacons = 0;
        for (i = 0; i < d->nstates; i++)
            if (ISBSET(d->work, i))
                for (ca = cnfa->states[i]; ca->co != COLORLESS; ca++)
                {
                    if (ca->co < cnfa->ncolors)
                        continue;   /* not a LACON arc */
                    if (ISBSET(d->work, ca->to))
                        continue;   /* arc would be a no-op anyway */
                    sawlacons = 1;  /* this LACON affects our result */
                    if (!lacon(v, cnfa, cp, ca->co))
                    {
                        if (ISERR())
                            return NULL;
                        continue;   /* LACON arc cannot be traversed */
                    }
                    if (ISERR())
                        return NULL;
                    BSET(d->work, ca->to);
                    dolacons = 1;
                    if (ca->to == cnfa->post)
                        ispost = 1;
                    if (!(cnfa->stflags[ca->to] & CNFA_NOPROGRESS))
                        noprogress = 0;
                    FDEBUG(("%d :> %d\n", i, ca->to));
                }
    }
    h = HASH(d->work, d->wordsper);
 
    /* Is this stateset already in the cache? */
    for (p = d->ssets, i = d->nssused; i > 0; p++, i--)
        if (HIT(h, d->work, p, d->wordsper))
        {
            FDEBUG(("cached c%d\n", (int) (p - d->ssets)));
            break;              /* NOTE BREAK OUT */
        }
    if (i == 0)
    {                           /* nope, need a new cache entry */
        p = getvacant(v, d, cp, start);
        if (p == NULL)
            return NULL;
        assert(p != css);
        for (i = 0; i < d->wordsper; i++)
            p->states[i] = d->work[i];
        p->hash = h;
        p->flags = (ispost) ? POSTSTATE : 0;
        if (noprogress)
            p->flags |= NOPROGRESS;
        /* lastseen to be dealt with by caller */
    }
 
    /*
     * Link new stateset to old, unless a LACON affected the result, in which
     * case we don't create the link.  That forces future transitions across
     * this same arc (same prior stateset and character color) to come through
     * miss() again, so that we can recheck the LACON(s), which might or might
     * not pass since context will be different.
     */
    if (!sawlacons)
    {
        FDEBUG(("c%d[%d]->c%d\n",
                (int) (css - d->ssets), co, (int) (p - d->ssets)));
        css->outs[co] = p;
        css->inchain[co] = p->ins;
        p->ins.ss = css;
        p->ins.co = co;
    }
    return p;
}
 
/*
 * lacon - lookaround-constraint checker for miss()
 */
static int                      /* predicate:  constraint satisfied? */
lacon(struct vars *v,
      struct cnfa *pcnfa,       /* parent cnfa */
      chr *cp,
      color co)                 /* "color" of the lookaround constraint */
{
    int         n;
    struct subre *sub;
    struct dfa *d;
    chr        *end;
    int         satisfied;
 
    /* Since this is recursive, it could be driven to stack overflow */
    if (STACK_TOO_DEEP(v->re))
    {
        ERR(REG_ETOOBIG);
        return 0;
    }
 
    n = co - pcnfa->ncolors;
    assert(n > 0 && n < v->g->nlacons && v->g->lacons != NULL);
    FDEBUG(("=== testing lacon %d\n", n));
    sub = &v->g->lacons[n];
    d = getladfa(v, n);
    if (d == NULL)
        return 0;
    if (LATYPE_IS_AHEAD(sub->latype))
    {
        /* used to use longest() here, but shortest() could be much cheaper */
        end = shortest(v, d, cp, cp, v->stop,
                       (chr **) NULL, (int *) NULL);
        satisfied = LATYPE_IS_POS(sub->latype) ? (end != NULL) : (end == NULL);
    }
    else
    {
        /*
         * To avoid doing O(N^2) work when repeatedly testing a lookbehind
         * constraint in an N-character string, we use matchuntil() which can
         * cache the DFA state across calls.  We only need to restart if the
         * probe point decreases, which is not common.  The NFA we're using is
         * a search NFA, so it doesn't mind scanning over stuff before the
         * nominal match.
         */
        satisfied = matchuntil(v, d, cp, &v->lblastcss[n], &v->lblastcp[n]);
        if (!LATYPE_IS_POS(sub->latype))
            satisfied = !satisfied;
    }
    FDEBUG(("=== lacon %d satisfied %d\n", n, satisfied));
    return satisfied;
}
 
/*
 * getvacant - get a vacant state set
 *
 * This routine clears out the inarcs and outarcs, but does not otherwise
 * clear the innards of the state set -- that's up to the caller.
 */
static struct sset *
getvacant(struct vars *v,
          struct dfa *d,
          chr *cp,
          chr *start)
{
    int         i;
    struct sset *ss;
    struct sset *p;
    struct arcp ap;
    color       co;
 
    ss = pickss(v, d, cp, start);
    if (ss == NULL)
        return NULL;
    assert(!(ss->flags & LOCKED));
 
    /* clear out its inarcs, including self-referential ones */
    ap = ss->ins;
    while ((p = ap.ss) != NULL)
    {
        co = ap.co;
        FDEBUG(("zapping c%d's %ld outarc\n", (int) (p - d->ssets), (long) co));
        p->outs[co] = NULL;
        ap = p->inchain[co];
        p->inchain[co].ss = NULL;   /* paranoia */
    }
    ss->ins.ss = NULL;
 
    /* take it off the inarc chains of the ssets reached by its outarcs */
    for (i = 0; i < d->ncolors; i++)
    {
        p = ss->outs[i];
        assert(p != ss);        /* not self-referential */
        if (p == NULL)
            continue;           /* NOTE CONTINUE */
        FDEBUG(("del outarc %d from c%d's in chn\n", i, (int) (p - d->ssets)));
        if (p->ins.ss == ss && p->ins.co == i)
            p->ins = ss->inchain[i];
        else
        {
            struct arcp lastap = {NULL, 0};
 
            assert(p->ins.ss != NULL);
            for (ap = p->ins; ap.ss != NULL &&
                 !(ap.ss == ss && ap.co == i);
                 ap = ap.ss->inchain[ap.co])
                lastap = ap;
            assert(ap.ss != NULL);
            lastap.ss->inchain[lastap.co] = ss->inchain[i];
        }
        ss->outs[i] = NULL;
        ss->inchain[i].ss = NULL;
    }
 
    /* if ss was a success state, may need to remember location */
    if ((ss->flags & POSTSTATE) && ss->lastseen != d->lastpost &&
        (d->lastpost == NULL || d->lastpost < ss->lastseen))
        d->lastpost = ss->lastseen;
 
    /* likewise for a no-progress state */
    if ((ss->flags & NOPROGRESS) && ss->lastseen != d->lastnopr &&
        (d->lastnopr == NULL || d->lastnopr < ss->lastseen))
        d->lastnopr = ss->lastseen;
 
    return ss;
}
 
/*
 * pickss - pick the next stateset to be used
 */
static struct sset *
pickss(struct vars *v,
       struct dfa *d,
       chr *cp,
       chr *start)
{
    int         i;
    struct sset *ss;
    struct sset *end;
    chr        *ancient;
 
    /* shortcut for cases where cache isn't full */
    if (d->nssused < d->nssets)
    {
        i = d->nssused;
        d->nssused++;
        ss = &d->ssets[i];
        FDEBUG(("new c%d\n", i));
        /* set up innards */
        ss->states = &d->statesarea[i * d->wordsper];
        ss->flags = 0;
        ss->ins.ss = NULL;
        ss->ins.co = WHITE;     /* give it some value */
        ss->outs = &d->outsarea[i * d->ncolors];
        ss->inchain = &d->incarea[i * d->ncolors];
        for (i = 0; i < d->ncolors; i++)
        {
            ss->outs[i] = NULL;
            ss->inchain[i].ss = NULL;
        }
        return ss;
    }
 
    /* look for oldest, or old enough anyway */
    if (cp - start > d->nssets * 2 / 3) /* oldest 33% are expendable */
        ancient = cp - d->nssets * 2 / 3;
    else
        ancient = start;
    for (ss = d->search, end = &d->ssets[d->nssets]; ss < end; ss++)
        if ((ss->lastseen == NULL || ss->lastseen < ancient) &&
            !(ss->flags & LOCKED))
        {
            d->search = ss + 1;
            FDEBUG(("replacing c%d\n", (int) (ss - d->ssets)));
            return ss;
        }
    for (ss = d->ssets, end = d->search; ss < end; ss++)
        if ((ss->lastseen == NULL || ss->lastseen < ancient) &&
            !(ss->flags & LOCKED))
        {
            d->search = ss + 1;
            FDEBUG(("replacing c%d\n", (int) (ss - d->ssets)));
            return ss;
        }
 
    /* nobody's old enough?!? -- something's really wrong */
    FDEBUG(("cannot find victim to replace!\n"));
    ERR(REG_ASSERT);
    return NULL;
}