// Copyright 2006-2007 The RE2 Authors.  All Rights Reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Tested by search_test.cc.
//
// Prog::SearchNFA, an NFA search.
// This is an actual NFA like the theorists talk about,
// not the pseudo-NFA found in backtracking regexp implementations.
//
// IMPLEMENTATION
//
// This algorithm is a variant of one that appeared in Rob Pike's sam editor,
// which is a variant of the one described in Thompson's 1968 CACM paper.
// See http://swtch.com/~rsc/regexp/ for various history.  The main feature
// over the DFA implementation is that it tracks submatch boundaries.
//
// When the choice of submatch boundaries is ambiguous, this particular
// implementation makes the same choices that traditional backtracking
// implementations (in particular, Perl and PCRE) do.
// Note that unlike in Perl and PCRE, this algorithm *cannot* take exponential
// time in the length of the input.
//
// Like Thompson's original machine and like the DFA implementation, this
// implementation notices a match only once it is one byte past it.

#include <algorithm>
#include <deque>
#include <stdio.h>
#include <string.h>
#include <string>
#include <utility>
#include <vector>

#include "re2/pod_array.h"
#include "re2/prog.h"
#include "re2/regexp.h"
#include "re2/sparse_array.h"
#include "re2/sparse_set.h"
#include "util/logging.h"
#include "util/strutil.h"

namespace re2 {

class NFA {
public:
    NFA(Prog *prog);
    ~NFA();

    // Searches for a matching string.
    //   * If anchored is true, only considers matches starting at offset.
    //     Otherwise finds lefmost match at or after offset.
    //   * If longest is true, returns the longest match starting
    //     at the chosen start point.  Otherwise returns the so-called
    //     left-biased match, the one traditional backtracking engines
    //     (like Perl and PCRE) find.
    // Records submatch boundaries in submatch[1..nsubmatch-1].
    // Submatch[0] is the entire match.  When there is a choice in
    // which text matches each subexpression, the submatch boundaries
    // are chosen to match what a backtracking implementation would choose.
    bool Search(const StringPiece &text, const StringPiece &context, bool anchored, bool longest, StringPiece *submatch, int nsubmatch);

private:
    struct Thread {
        union {
            int ref;
            Thread *next; // when on free list
        };
        const char **capture;
    };

    // State for explicit stack in AddToThreadq.
    struct AddState {
        int id;    // Inst to process
        Thread *t; // if not null, set t0 = t before processing id
    };

    // Threadq is a list of threads.  The list is sorted by the order
    // in which Perl would explore that particular state -- the earlier
    // choices appear earlier in the list.
    typedef SparseArray<Thread *> Threadq;

    inline Thread *AllocThread();
    inline Thread *Incref(Thread *t);
    inline void Decref(Thread *t);

    // Follows all empty arrows from id0 and enqueues all the states reached.
    // Enqueues only the ByteRange instructions that match byte c.
    // context is used (with p) for evaluating empty-width specials.
    // p is the current input position, and t0 is the current thread.
    void AddToThreadq(Threadq *q, int id0, int c, const StringPiece &context, const char *p, Thread *t0);

    // Run runq on byte c, appending new states to nextq.
    // Updates matched_ and match_ as new, better matches are found.
    // context is used (with p) for evaluating empty-width specials.
    // p is the position of byte c in the input string for AddToThreadq;
    // p-1 will be used when processing Match instructions.
    // Frees all the threads on runq.
    // If there is a shortcut to the end, returns that shortcut.
    int Step(Threadq *runq, Threadq *nextq, int c, const StringPiece &context, const char *p);

    // Returns text version of capture information, for debugging.
    std::string FormatCapture(const char **capture);

    void CopyCapture(const char **dst, const char **src) { memmove(dst, src, ncapture_ * sizeof src[0]); }

    Prog *prog_;               // underlying program
    int start_;                // start instruction in program
    int ncapture_;             // number of submatches to track
    bool longest_;             // whether searching for longest match
    bool endmatch_;            // whether match must end at text.end()
    const char *btext_;        // beginning of text (for FormatSubmatch)
    const char *etext_;        // end of text (for endmatch_)
    Threadq q0_, q1_;          // pre-allocated for Search.
    PODArray<AddState> stack_; // pre-allocated for AddToThreadq
    std::deque<Thread> arena_; // thread arena
    Thread *freelist_;         // thread freelist
    const char **match_;       // best match so far
    bool matched_;             // any match so far?

    NFA(const NFA &) = delete;
    NFA &operator=(const NFA &) = delete;
};

NFA::NFA(Prog *prog) {
    prog_ = prog;
    start_ = prog_->start();
    ncapture_ = 0;
    longest_ = false;
    endmatch_ = false;
    btext_ = NULL;
    etext_ = NULL;
    q0_.resize(prog_->size());
    q1_.resize(prog_->size());
    // See NFA::AddToThreadq() for why this is so.
    int nstack = 2 * prog_->inst_count(kInstCapture) + prog_->inst_count(kInstEmptyWidth) + prog_->inst_count(kInstNop) + 1; // + 1 for start inst
    stack_ = PODArray<AddState>(nstack);
    freelist_ = NULL;
    match_ = NULL;
    matched_ = false;
}

NFA::~NFA() {
    delete[] match_;
    for (const Thread &t : arena_)
        delete[] t.capture;
}

NFA::Thread *NFA::AllocThread() {
    Thread *t = freelist_;
    if (t != NULL) {
        freelist_ = t->next;
        t->ref = 1;
        // We don't need to touch t->capture because
        // the caller will immediately overwrite it.
        return t;
    }
    arena_.emplace_back();
    t = &arena_.back();
    t->ref = 1;
    t->capture = new const char *[ncapture_];
    return t;
}

NFA::Thread *NFA::Incref(Thread *t) {
    DCHECK(t != NULL);
    t->ref++;
    return t;
}

void NFA::Decref(Thread *t) {
    DCHECK(t != NULL);
    t->ref--;
    if (t->ref > 0)
        return;
    DCHECK_EQ(t->ref, 0);
    t->next = freelist_;
    freelist_ = t;
}

// Follows all empty arrows from id0 and enqueues all the states reached.
// Enqueues only the ByteRange instructions that match byte c.
// context is used (with p) for evaluating empty-width specials.
// p is the current input position, and t0 is the current thread.
void NFA::AddToThreadq(Threadq *q, int id0, int c, const StringPiece &context, const char *p, Thread *t0) {
    if (id0 == 0)
        return;

    // Use stack_ to hold our stack of instructions yet to process.
    // It was preallocated as follows:
    //   two entries per Capture;
    //   one entry per EmptyWidth; and
    //   one entry per Nop.
    // This reflects the maximum number of stack pushes that each can
    // perform. (Each instruction can be processed at most once.)
    AddState *stk = stack_.data();
    int nstk = 0;

    stk[nstk++] = {id0, NULL};
    while (nstk > 0) {
        DCHECK_LE(nstk, stack_.size());
        AddState a = stk[--nstk];

    Loop:
        if (a.t != NULL) {
            // t0 was a thread that we allocated and copied in order to
            // record the capture, so we must now decref it.
            Decref(t0);
            t0 = a.t;
        }

        int id = a.id;
        if (id == 0)
            continue;
        if (q->has_index(id)) {
            continue;
        }

        // Create entry in q no matter what.  We might fill it in below,
        // or we might not.  Even if not, it is necessary to have it,
        // so that we don't revisit id0 during the recursion.
        q->set_new(id, NULL);
        Thread **tp = &q->get_existing(id);
        int j;
        Thread *t;
        Prog::Inst *ip = prog_->inst(id);
        switch (ip->opcode()) {
            default:
                LOG(DFATAL) << "unhandled " << ip->opcode() << " in AddToThreadq";
                break;

            case kInstFail:
                break;

            case kInstAltMatch:
                // Save state; will pick up at next byte.
                t = Incref(t0);
                *tp = t;

                DCHECK(!ip->last());
                a = {id + 1, NULL};
                goto Loop;

            case kInstNop:
                if (!ip->last())
                    stk[nstk++] = {id + 1, NULL};

                // Continue on.
                a = {ip->out(), NULL};
                goto Loop;

            case kInstCapture:
                if (!ip->last())
                    stk[nstk++] = {id + 1, NULL};

                if ((j = ip->cap()) < ncapture_) {
                    // Push a dummy whose only job is to restore t0
                    // once we finish exploring this possibility.
                    stk[nstk++] = {0, t0};

                    // Record capture.
                    t = AllocThread();
                    CopyCapture(t->capture, t0->capture);
                    t->capture[j] = p;
                    t0 = t;
                }
                a = {ip->out(), NULL};
                goto Loop;

            case kInstByteRange:
                if (!ip->Matches(c))
                    goto Next;

                // Save state; will pick up at next byte.
                t = Incref(t0);
                *tp = t;

                if (ip->hint() == 0)
                    break;
                a = {id + ip->hint(), NULL};
                goto Loop;

            case kInstMatch:
                // Save state; will pick up at next byte.
                t = Incref(t0);
                *tp = t;

            Next:
                if (ip->last())
                    break;
                a = {id + 1, NULL};
                goto Loop;

            case kInstEmptyWidth:
                if (!ip->last())
                    stk[nstk++] = {id + 1, NULL};

                // Continue on if we have all the right flag bits.
                if (ip->empty() & ~Prog::EmptyFlags(context, p))
                    break;
                a = {ip->out(), NULL};
                goto Loop;
        }
    }
}

// Run runq on byte c, appending new states to nextq.
// Updates matched_ and match_ as new, better matches are found.
// context is used (with p) for evaluating empty-width specials.
// p is the position of byte c in the input string for AddToThreadq;
// p-1 will be used when processing Match instructions.
// Frees all the threads on runq.
// If there is a shortcut to the end, returns that shortcut.
int NFA::Step(Threadq *runq, Threadq *nextq, int c, const StringPiece &context, const char *p) {
    nextq->clear();

    for (Threadq::iterator i = runq->begin(); i != runq->end(); ++i) {
        Thread *t = i->value();
        if (t == NULL)
            continue;

        if (longest_) {
            // Can skip any threads started after our current best match.
            if (matched_ && match_[0] < t->capture[0]) {
                Decref(t);
                continue;
            }
        }

        int id = i->index();
        Prog::Inst *ip = prog_->inst(id);

        switch (ip->opcode()) {
            default:
                // Should only see the values handled below.
                LOG(DFATAL) << "Unhandled " << ip->opcode() << " in step";
                break;

            case kInstByteRange:
                AddToThreadq(nextq, ip->out(), c, context, p, t);
                break;

            case kInstAltMatch:
                if (i != runq->begin())
                    break;
                // The match is ours if we want it.
                if (ip->greedy(prog_) || longest_) {
                    CopyCapture(match_, t->capture);
                    matched_ = true;

                    Decref(t);
                    for (++i; i != runq->end(); ++i) {
                        if (i->value() != NULL)
                            Decref(i->value());
                    }
                    runq->clear();
                    if (ip->greedy(prog_))
                        return ip->out1();
                    return ip->out();
                }
                break;

            case kInstMatch: {
                // Avoid invoking undefined behavior (arithmetic on a null pointer)
                // by storing p instead of p-1. (What would the latter even mean?!)
                // This complements the special case in NFA::Search().
                if (p == NULL) {
                    CopyCapture(match_, t->capture);
                    match_[1] = p;
                    matched_ = true;
                    break;
                }

                if (endmatch_ && p - 1 != etext_)
                    break;

                if (longest_) {
                    // Leftmost-longest mode: save this match only if
                    // it is either farther to the left or at the same
                    // point but longer than an existing match.
                    if (!matched_ || t->capture[0] < match_[0] || (t->capture[0] == match_[0] && p - 1 > match_[1])) {
                        CopyCapture(match_, t->capture);
                        match_[1] = p - 1;
                        matched_ = true;
                    }
                } else {
                    // Leftmost-biased mode: this match is by definition
                    // better than what we've already found (see next line).
                    CopyCapture(match_, t->capture);
                    match_[1] = p - 1;
                    matched_ = true;

                    // Cut off the threads that can only find matches
                    // worse than the one we just found: don't run the
                    // rest of the current Threadq.
                    Decref(t);
                    for (++i; i != runq->end(); ++i) {
                        if (i->value() != NULL)
                            Decref(i->value());
                    }
                    runq->clear();
                    return 0;
                }
                break;
            }
        }
        Decref(t);
    }
    runq->clear();
    return 0;
}

std::string NFA::FormatCapture(const char **capture) {
    std::string s;
    for (int i = 0; i < ncapture_; i += 2) {
        if (capture[i] == NULL)
            s += "(?,?)";
        else if (capture[i + 1] == NULL)
            s += StringPrintf("(%td,?)", capture[i] - btext_);
        else
            s += StringPrintf("(%td,%td)", capture[i] - btext_, capture[i + 1] - btext_);
    }
    return s;
}

bool NFA::Search(const StringPiece &text, const StringPiece &const_context, bool anchored, bool longest, StringPiece *submatch, int nsubmatch) {
    if (start_ == 0)
        return false;

    StringPiece context = const_context;
    if (context.data() == NULL)
        context = text;

    // Sanity check: make sure that text lies within context.
    if (BeginPtr(text) < BeginPtr(context) || EndPtr(text) > EndPtr(context)) {
        LOG(DFATAL) << "context does not contain text";
        return false;
    }

    if (prog_->anchor_start() && BeginPtr(context) != BeginPtr(text))
        return false;
    if (prog_->anchor_end() && EndPtr(context) != EndPtr(text))
        return false;
    anchored |= prog_->anchor_start();
    if (prog_->anchor_end()) {
        longest = true;
        endmatch_ = true;
    }

    if (nsubmatch < 0) {
        LOG(DFATAL) << "Bad args: nsubmatch=" << nsubmatch;
        return false;
    }

    // Save search parameters.
    ncapture_ = 2 * nsubmatch;
    longest_ = longest;

    if (nsubmatch == 0) {
        // We need to maintain match[0], both to distinguish the
        // longest match (if longest is true) and also to tell
        // whether we've seen any matches at all.
        ncapture_ = 2;
    }

    match_ = new const char *[ncapture_];
    memset(match_, 0, ncapture_ * sizeof match_[0]);
    matched_ = false;

    // For debugging prints.
    btext_ = context.data();
    // For convenience.
    etext_ = text.data() + text.size();

    // Set up search.
    Threadq *runq = &q0_;
    Threadq *nextq = &q1_;
    runq->clear();
    nextq->clear();

    // Loop over the text, stepping the machine.
    for (const char *p = text.data();; p++) {
        // This is a no-op the first time around the loop because runq is empty.
        int id = Step(runq, nextq, p < etext_ ? p[0] & 0xFF : -1, context, p);
        DCHECK_EQ(runq->size(), 0);
        using std::swap;
        swap(nextq, runq);
        nextq->clear();
        if (id != 0) {
            // We're done: full match ahead.
            p = etext_;
            for (;;) {
                Prog::Inst *ip = prog_->inst(id);
                switch (ip->opcode()) {
                    default:
                        LOG(DFATAL) << "Unexpected opcode in short circuit: " << ip->opcode();
                        break;

                    case kInstCapture:
                        if (ip->cap() < ncapture_)
                            match_[ip->cap()] = p;
                        id = ip->out();
                        continue;

                    case kInstNop:
                        id = ip->out();
                        continue;

                    case kInstMatch:
                        match_[1] = p;
                        matched_ = true;
                        break;
                }
                break;
            }
            break;
        }

        if (p > etext_)
            break;

        // Start a new thread if there have not been any matches.
        // (No point in starting a new thread if there have been
        // matches, since it would be to the right of the match
        // we already found.)
        if (!matched_ && (!anchored || p == text.data())) {
            // Try to use prefix accel (e.g. memchr) to skip ahead.
            // The search must be unanchored and there must be zero
            // possible matches already.
            if (!anchored && runq->size() == 0 && p < etext_ && prog_->can_prefix_accel()) {
                p = reinterpret_cast<const char *>(prog_->PrefixAccel(p, etext_ - p));
                if (p == NULL)
                    p = etext_;
            }

            Thread *t = AllocThread();
            CopyCapture(t->capture, match_);
            t->capture[0] = p;
            AddToThreadq(runq, start_, p < etext_ ? p[0] & 0xFF : -1, context, p, t);
            Decref(t);
        }

        // If all the threads have died, stop early.
        if (runq->size() == 0) {
            break;
        }

        // Avoid invoking undefined behavior (arithmetic on a null pointer)
        // by simply not continuing the loop.
        // This complements the special case in NFA::Step().
        if (p == NULL) {
            (void)Step(runq, nextq, -1, context, p);
            DCHECK_EQ(runq->size(), 0);
            using std::swap;
            swap(nextq, runq);
            nextq->clear();
            break;
        }
    }

    for (Threadq::iterator i = runq->begin(); i != runq->end(); ++i) {
        if (i->value() != NULL)
            Decref(i->value());
    }

    if (matched_) {
        for (int i = 0; i < nsubmatch; i++)
            submatch[i] = StringPiece(match_[2 * i], static_cast<size_t>(match_[2 * i + 1] - match_[2 * i]));
        return true;
    }
    return false;
}

bool Prog::SearchNFA(const StringPiece &text, const StringPiece &context, Anchor anchor, MatchKind kind, StringPiece *match, int nmatch) {

    NFA nfa(this);
    StringPiece sp;
    if (kind == kFullMatch) {
        anchor = kAnchored;
        if (nmatch == 0) {
            match = &sp;
            nmatch = 1;
        }
    }
    if (!nfa.Search(text, context, anchor == kAnchored, kind != kFirstMatch, match, nmatch))
        return false;
    if (kind == kFullMatch && EndPtr(match[0]) != EndPtr(text))
        return false;
    return true;
}

// For each instruction i in the program reachable from the start, compute the
// number of instructions reachable from i by following only empty transitions
// and record that count as fanout[i].
//
// fanout holds the results and is also the work queue for the outer iteration.
// reachable holds the reached nodes for the inner iteration.
void Prog::Fanout(SparseArray<int> *fanout) {
    DCHECK_EQ(fanout->max_size(), size());
    SparseSet reachable(size());
    fanout->clear();
    fanout->set_new(start(), 0);
    for (SparseArray<int>::iterator i = fanout->begin(); i != fanout->end(); ++i) {
        int *count = &i->value();
        reachable.clear();
        reachable.insert(i->index());
        for (SparseSet::iterator j = reachable.begin(); j != reachable.end(); ++j) {
            int id = *j;
            Prog::Inst *ip = inst(id);
            switch (ip->opcode()) {
                default:
                    LOG(DFATAL) << "unhandled " << ip->opcode() << " in Prog::Fanout()";
                    break;

                case kInstByteRange:
                    if (!ip->last())
                        reachable.insert(id + 1);

                    (*count)++;
                    if (!fanout->has_index(ip->out())) {
                        fanout->set_new(ip->out(), 0);
                    }
                    break;

                case kInstAltMatch:
                    DCHECK(!ip->last());
                    reachable.insert(id + 1);
                    break;

                case kInstCapture:
                case kInstEmptyWidth:
                case kInstNop:
                    if (!ip->last())
                        reachable.insert(id + 1);

                    reachable.insert(ip->out());
                    break;

                case kInstMatch:
                    if (!ip->last())
                        reachable.insert(id + 1);
                    break;

                case kInstFail:
                    break;
            }
        }
    }
}

} // namespace re2