// Copyright 2008 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.

// A DFA (deterministic finite automaton)-based regular expression search.
//
// The DFA search has two main parts: the construction of the automaton,
// which is represented by a graph of State structures, and the execution
// of the automaton over a given input string.
//
// The basic idea is that the State graph is constructed so that the
// execution can simply start with a state s, and then for each byte c in
// the input string, execute "s = s->next[c]", checking at each point whether
// the current s represents a matching state.
//
// The simple explanation just given does convey the essence of this code,
// but it omits the details of how the State graph gets constructed as well
// as some performance-driven optimizations to the execution of the automaton.
// All these details are explained in the comments for the code following
// the definition of class DFA.
//
// See http://swtch.com/~rsc/regexp/ for a very bare-bones equivalent.

#include <algorithm>
#include <atomic>
#include <deque>
#include <mutex>
#include <new>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#include "re2/pod_array.h"
#include "re2/prog.h"
#include "re2/re2.h"
#include "re2/sparse_set.h"
#include "re2/stringpiece.h"
#include "util/logging.h"
#include "util/mix.h"
#include "util/mutex.h"
#include "util/strutil.h"

// Silence "zero-sized array in struct/union" warning for DFA::State::next_.
#ifdef _MSC_VER
#pragma warning(disable : 4200)
#endif

namespace re2 {

// Controls whether the DFA should bail out early if the NFA would be faster.
static bool dfa_should_bail_when_slow = true;

void Prog::TESTING_ONLY_set_dfa_should_bail_when_slow(bool b) { dfa_should_bail_when_slow = b; }

// A DFA implementation of a regular expression program.
// Since this is entirely a forward declaration mandated by C++,
// some of the comments here are better understood after reading
// the comments in the sections that follow the DFA definition.
class DFA {
public:
    DFA(Prog *prog, Prog::MatchKind kind, int64_t max_mem);
    ~DFA();
    bool ok() const { return !init_failed_; }
    Prog::MatchKind kind() { return kind_; }

    // Searches for the regular expression in text, which is considered
    // as a subsection of context for the purposes of interpreting flags
    // like ^ and $ and \A and \z.
    // Returns whether a match was found.
    // If a match is found, sets *ep to the end point of the best match in text.
    // If "anchored", the match must begin at the start of text.
    // If "want_earliest_match", the match that ends first is used, not
    //   necessarily the best one.
    // If "run_forward" is true, the DFA runs from text.begin() to text.end().
    //   If it is false, the DFA runs from text.end() to text.begin(),
    //   returning the leftmost end of the match instead of the rightmost one.
    // If the DFA cannot complete the search (for example, if it is out of
    //   memory), it sets *failed and returns false.
    bool Search(const StringPiece &text,
                const StringPiece &context,
                bool anchored,
                bool want_earliest_match,
                bool run_forward,
                bool *failed,
                const char **ep,
                SparseSet *matches);

    // Builds out all states for the entire DFA.
    // If cb is not empty, it receives one callback per state built.
    // Returns the number of states built.
    // FOR TESTING OR EXPERIMENTAL PURPOSES ONLY.
    int BuildAllStates(const Prog::DFAStateCallback &cb);

    // Computes min and max for matching strings.  Won't return strings
    // bigger than maxlen.
    bool PossibleMatchRange(std::string *min, std::string *max, int maxlen);

    // These data structures are logically private, but C++ makes it too
    // difficult to mark them as such.
    class RWLocker;
    class StateSaver;
    class Workq;

    // A single DFA state.  The DFA is represented as a graph of these
    // States, linked by the next_ pointers.  If in state s and reading
    // byte c, the next state should be s->next_[c].
    struct State {
        inline bool IsMatch() const { return (flag_ & kFlagMatch) != 0; }

        int *inst_;     // Instruction pointers in the state.
        int ninst_;     // # of inst_ pointers.
        uint32_t flag_; // Empty string bitfield flags in effect on the way
                        // into this state, along with kFlagMatch if this
                        // is a matching state.

        // fixes from https://github.com/girishji/re2/commit/80b212f289c4ef75408b1510b9fc85e6cb9a447c
        std::atomic<State *> *next_; // Outgoing arrows from State,

        // one per input byte class
    };

    enum {
        kByteEndText = 256, // imaginary byte at end of text

        kFlagEmptyMask = 0xFF,  // State.flag_: bits holding kEmptyXXX flags
        kFlagMatch = 0x0100,    // State.flag_: this is a matching state
        kFlagLastWord = 0x0200, // State.flag_: last byte was a word char
        kFlagNeedShift = 16,    // needed kEmpty bits are or'ed in shifted left
    };

    struct StateHash {
        size_t operator()(const State *a) const {
            DCHECK(a != NULL);
            HashMix mix(a->flag_);
            for (int i = 0; i < a->ninst_; i++)
                mix.Mix(a->inst_[i]);
            mix.Mix(0);
            return mix.get();
        }
    };

    struct StateEqual {
        bool operator()(const State *a, const State *b) const {
            DCHECK(a != NULL);
            DCHECK(b != NULL);
            if (a == b)
                return true;
            if (a->flag_ != b->flag_)
                return false;
            if (a->ninst_ != b->ninst_)
                return false;
            for (int i = 0; i < a->ninst_; i++)
                if (a->inst_[i] != b->inst_[i])
                    return false;
            return true;
        }
    };

    typedef std::unordered_set<State *, StateHash, StateEqual> StateSet;

private:
    // Make it easier to swap in a scalable reader-writer mutex.
    using CacheMutex = Mutex;

    enum {
        // Indices into start_ for unanchored searches.
        // Add kStartAnchored for anchored searches.
        kStartBeginText = 0,        // text at beginning of context
        kStartBeginLine = 2,        // text at beginning of line
        kStartAfterWordChar = 4,    // text follows a word character
        kStartAfterNonWordChar = 6, // text follows non-word character
        kMaxStart = 8,

        kStartAnchored = 1,
    };

    // Resets the DFA State cache, flushing all saved State* information.
    // Releases and reacquires cache_mutex_ via cache_lock, so any
    // State* existing before the call are not valid after the call.
    // Use a StateSaver to preserve important states across the call.
    // cache_mutex_.r <= L < mutex_
    // After: cache_mutex_.w <= L < mutex_
    void ResetCache(RWLocker *cache_lock);

    // Looks up and returns the State corresponding to a Workq.
    // L >= mutex_
    State *WorkqToCachedState(Workq *q, Workq *mq, uint32_t flag);

    // Looks up and returns a State matching the inst, ninst, and flag.
    // L >= mutex_
    State *CachedState(int *inst, int ninst, uint32_t flag);

    // Clear the cache entirely.
    // Must hold cache_mutex_.w or be in destructor.
    void ClearCache();

    // Converts a State into a Workq: the opposite of WorkqToCachedState.
    // L >= mutex_
    void StateToWorkq(State *s, Workq *q);

    // Runs a State on a given byte, returning the next state.
    State *RunStateOnByteUnlocked(State *, int); // cache_mutex_.r <= L < mutex_
    State *RunStateOnByte(State *, int);         // L >= mutex_

    // Runs a Workq on a given byte followed by a set of empty-string flags,
    // producing a new Workq in nq.  If a match instruction is encountered,
    // sets *ismatch to true.
    // L >= mutex_
    void RunWorkqOnByte(Workq *q, Workq *nq, int c, uint32_t flag, bool *ismatch);

    // Runs a Workq on a set of empty-string flags, producing a new Workq in nq.
    // L >= mutex_
    void RunWorkqOnEmptyString(Workq *q, Workq *nq, uint32_t flag);

    // Adds the instruction id to the Workq, following empty arrows
    // according to flag.
    // L >= mutex_
    void AddToQueue(Workq *q, int id, uint32_t flag);

    // For debugging, returns a text representation of State.
    static std::string DumpState(State *state);

    // For debugging, returns a text representation of a Workq.
    static std::string DumpWorkq(Workq *q);

    // Search parameters
    struct SearchParams {
        SearchParams(const StringPiece &text, const StringPiece &context, RWLocker *cache_lock)
            : text(text), context(context), anchored(false), can_prefix_accel(false), want_earliest_match(false), run_forward(false), start(NULL),
              cache_lock(cache_lock), failed(false), ep(NULL), matches(NULL) {}

        StringPiece text;
        StringPiece context;
        bool anchored;
        bool can_prefix_accel;
        bool want_earliest_match;
        bool run_forward;
        State *start;
        RWLocker *cache_lock;
        bool failed;    // "out" parameter: whether search gave up
        const char *ep; // "out" parameter: end pointer for match
        SparseSet *matches;

    private:
        SearchParams(const SearchParams &) = delete;
        SearchParams &operator=(const SearchParams &) = delete;
    };

    // Before each search, the parameters to Search are analyzed by
    // AnalyzeSearch to determine the state in which to start.
    struct StartInfo {
        StartInfo() : start(NULL) {}
        std::atomic<State *> start;
    };

    // Fills in params->start and params->can_prefix_accel using
    // the other search parameters.  Returns true on success,
    // false on failure.
    // cache_mutex_.r <= L < mutex_
    bool AnalyzeSearch(SearchParams *params);
    bool AnalyzeSearchHelper(SearchParams *params, StartInfo *info, uint32_t flags);

    // The generic search loop, inlined to create specialized versions.
    // cache_mutex_.r <= L < mutex_
    // Might unlock and relock cache_mutex_ via params->cache_lock.
    template <bool can_prefix_accel, bool want_earliest_match, bool run_forward>
    inline bool InlinedSearchLoop(SearchParams *params);

    // The specialized versions of InlinedSearchLoop.  The three letters
    // at the ends of the name denote the true/false values used as the
    // last three parameters of InlinedSearchLoop.
    // cache_mutex_.r <= L < mutex_
    // Might unlock and relock cache_mutex_ via params->cache_lock.
    bool SearchFFF(SearchParams *params);
    bool SearchFFT(SearchParams *params);
    bool SearchFTF(SearchParams *params);
    bool SearchFTT(SearchParams *params);
    bool SearchTFF(SearchParams *params);
    bool SearchTFT(SearchParams *params);
    bool SearchTTF(SearchParams *params);
    bool SearchTTT(SearchParams *params);

    // The main search loop: calls an appropriate specialized version of
    // InlinedSearchLoop.
    // cache_mutex_.r <= L < mutex_
    // Might unlock and relock cache_mutex_ via params->cache_lock.
    bool FastSearchLoop(SearchParams *params);

    // Looks up bytes in bytemap_ but handles case c == kByteEndText too.
    int ByteMap(int c) {
        if (c == kByteEndText)
            return prog_->bytemap_range();
        return prog_->bytemap()[c];
    }

    // Constant after initialization.
    Prog *prog_;           // The regular expression program to run.
    Prog::MatchKind kind_; // The kind of DFA.
    bool init_failed_;     // initialization failed (out of memory)

    Mutex mutex_; // mutex_ >= cache_mutex_.r

    // Scratch areas, protected by mutex_.
    Workq *q0_; // Two pre-allocated work queues.
    Workq *q1_;
    PODArray<int> stack_; // Pre-allocated stack for AddToQueue

    // State* cache.  Many threads use and add to the cache simultaneously,
    // holding cache_mutex_ for reading and mutex_ (above) when adding.
    // If the cache fills and needs to be discarded, the discarding is done
    // while holding cache_mutex_ for writing, to avoid interrupting other
    // readers.  Any State* pointers are only valid while cache_mutex_
    // is held.
    CacheMutex cache_mutex_;
    int64_t mem_budget_;   // Total memory budget for all States.
    int64_t state_budget_; // Amount of memory remaining for new States.
    StateSet state_cache_; // All States computed so far.
    StartInfo start_[kMaxStart];

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

// Shorthand for casting to uint8_t*.
static inline const uint8_t *BytePtr(const void *v) { return reinterpret_cast<const uint8_t *>(v); }

// Work queues

// Marks separate thread groups of different priority
// in the work queue when in leftmost-longest matching mode.
// #define Mark (-1)
constexpr auto Mark = -1;

// Separates the match IDs from the instructions in inst_.
// Used only for "many match" DFA states.
// #define MatchSep (-2)
constexpr auto MatchSep = -2;

// Internally, the DFA uses a sparse array of
// program instruction pointers as a work queue.
// In leftmost longest mode, marks separate sections
// of workq that started executing at different
// locations in the string (earlier locations first).
class DFA::Workq : public SparseSet {
public:
    // Constructor: n is number of normal slots, maxmark number of mark slots.
    Workq(int n, int maxmark) : SparseSet(n + maxmark), n_(n), maxmark_(maxmark), nextmark_(n), last_was_mark_(true) {}

    bool is_mark(int i) { return i >= n_; }

    int maxmark() { return maxmark_; }

    void clear() {
        SparseSet::clear();
        nextmark_ = n_;
    }

    void mark() {
        if (last_was_mark_)
            return;
        last_was_mark_ = false;
        SparseSet::insert_new(nextmark_++);
    }

    int size() { return n_ + maxmark_; }

    void insert(int id) {
        if (contains(id))
            return;
        insert_new(id);
    }

    void insert_new(int id) {
        last_was_mark_ = false;
        SparseSet::insert_new(id);
    }

private:
    int n_;              // size excluding marks
    int maxmark_;        // maximum number of marks
    int nextmark_;       // id of next mark
    bool last_was_mark_; // last inserted was mark

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

DFA::DFA(Prog *prog, Prog::MatchKind kind, int64_t max_mem)
    : prog_(prog), kind_(kind), init_failed_(false), q0_(NULL), q1_(NULL), mem_budget_(max_mem) {
    int nmark = 0;
    if (kind_ == Prog::kLongestMatch)
        nmark = prog_->size();
    // See DFA::AddToQueue() for why this is so.
    int nstack = prog_->inst_count(kInstCapture) + prog_->inst_count(kInstEmptyWidth) + prog_->inst_count(kInstNop) + nmark + 1; // + 1 for start inst

    // Account for space needed for DFA, q0, q1, stack.
    mem_budget_ -= sizeof(DFA);
    mem_budget_ -= (prog_->size() + nmark) * (sizeof(int) + sizeof(int)) * 2; // q0, q1
    mem_budget_ -= nstack * sizeof(int);                                      // stack
    if (mem_budget_ < 0) {
        init_failed_ = true;
        return;
    }

    state_budget_ = mem_budget_;

    // Make sure there is a reasonable amount of working room left.
    // At minimum, the search requires room for two states in order
    // to limp along, restarting frequently.  We'll get better performance
    // if there is room for a larger number of states, say 20.
    // Note that a state stores list heads only, so we use the program
    // list count for the upper bound, not the program size.
    int nnext = prog_->bytemap_range() + 1; // + 1 for kByteEndText slot
    int64_t one_state = sizeof(State) + nnext * sizeof(std::atomic<State *>) + (prog_->list_count() + nmark) * sizeof(int);
    if (state_budget_ < 20 * one_state) {
        init_failed_ = true;
        return;
    }

    q0_ = new Workq(prog_->size(), nmark);
    q1_ = new Workq(prog_->size(), nmark);
    stack_ = PODArray<int>(nstack);
}

DFA::~DFA() {
    delete q0_;
    delete q1_;
    ClearCache();
}

// In the DFA state graph, s->next[c] == NULL means that the
// state has not yet been computed and needs to be.  We need
// a different special value to signal that s->next[c] is a
// state that can never lead to a match (and thus the search
// can be called off).  Hence DeadState.
#define DeadState reinterpret_cast<State *>(1)

// Signals that the rest of the string matches no matter what it is.
#define FullMatchState reinterpret_cast<State *>(2)

#define SpecialStateMax FullMatchState

// Debugging printouts

// For debugging, returns a string representation of the work queue.
std::string DFA::DumpWorkq(Workq *q) {
    std::string s;
    const char *sep = "";
    for (Workq::iterator it = q->begin(); it != q->end(); ++it) {
        if (q->is_mark(*it)) {
            s += "|";
            sep = "";
        } else {
            s += StringPrintf("%s%d", sep, *it);
            sep = ",";
        }
    }
    return s;
}

// For debugging, returns a string representation of the state.
std::string DFA::DumpState(State *state) {
    if (state == NULL)
        return "_";
    if (state == DeadState)
        return "X";
    if (state == FullMatchState)
        return "*";
    std::string s;
    const char *sep = "";
    s += StringPrintf("(%p)", state);
    for (int i = 0; i < state->ninst_; i++) {
        if (state->inst_[i] == Mark) {
            s += "|";
            sep = "";
        } else if (state->inst_[i] == MatchSep) {
            s += "||";
            sep = "";
        } else {
            s += StringPrintf("%s%d", sep, state->inst_[i]);
            sep = ",";
        }
    }
    s += StringPrintf(" flag=%#x", state->flag_);
    return s;
}

//////////////////////////////////////////////////////////////////////
//
// DFA state graph construction.
//
// The DFA state graph is a heavily-linked collection of State* structures.
// The state_cache_ is a set of all the State structures ever allocated,
// so that if the same state is reached by two different paths,
// the same State structure can be used.  This reduces allocation
// requirements and also avoids duplication of effort across the two
// identical states.
//
// A State is defined by an ordered list of instruction ids and a flag word.
//
// The choice of an ordered list of instructions differs from a typical
// textbook DFA implementation, which would use an unordered set.
// Textbook descriptions, however, only care about whether
// the DFA matches, not where it matches in the text.  To decide where the
// DFA matches, we need to mimic the behavior of the dominant backtracking
// implementations like PCRE, which try one possible regular expression
// execution, then another, then another, stopping when one of them succeeds.
// The DFA execution tries these many executions in parallel, representing
// each by an instruction id.  These pointers are ordered in the State.inst_
// list in the same order that the executions would happen in a backtracking
// search: if a match is found during execution of inst_[2], inst_[i] for i>=3
// can be discarded.
//
// Textbooks also typically do not consider context-aware empty string operators
// like ^ or $.  These are handled by the flag word, which specifies the set
// of empty-string operators that should be matched when executing at the
// current text position.  These flag bits are defined in prog.h.
// The flag word also contains two DFA-specific bits: kFlagMatch if the state
// is a matching state (one that reached a kInstMatch in the program)
// and kFlagLastWord if the last processed byte was a word character, for the
// implementation of \B and \b.
//
// The flag word also contains, shifted up 16 bits, the bits looked for by
// any kInstEmptyWidth instructions in the state.  These provide a useful
// summary indicating when new flags might be useful.
//
// The permanent representation of a State's instruction ids is just an array,
// but while a state is being analyzed, these instruction ids are represented
// as a Workq, which is an array that allows iteration in insertion order.

// NOTE(rsc): The choice of State construction determines whether the DFA
// mimics backtracking implementations (so-called leftmost first matching) or
// traditional DFA implementations (so-called leftmost longest matching as
// prescribed by POSIX).  This implementation chooses to mimic the
// backtracking implementations, because we want to replace PCRE.  To get
// POSIX behavior, the states would need to be considered not as a simple
// ordered list of instruction ids, but as a list of unordered sets of instruction
// ids.  A match by a state in one set would inhibit the running of sets
// farther down the list but not other instruction ids in the same set.  Each
// set would correspond to matches beginning at a given point in the string.
// This is implemented by separating different sets with Mark pointers.

// Looks in the State cache for a State matching q, flag.
// If one is found, returns it.  If one is not found, allocates one,
// inserts it in the cache, and returns it.
// If mq is not null, MatchSep and the match IDs in mq will be appended
// to the State.
DFA::State *DFA::WorkqToCachedState(Workq *q, Workq *mq, uint32_t flag) {
    // mutex_.AssertHeld();

    // Construct array of instruction ids for the new state.
    // Only ByteRange, EmptyWidth, and Match instructions are useful to keep:
    // those are the only operators with any effect in
    // RunWorkqOnEmptyString or RunWorkqOnByte.
    PODArray<int> inst(q->size());
    int n = 0;
    uint32_t needflags = 0; // flags needed by kInstEmptyWidth instructions
    bool sawmatch = false;  // whether queue contains guaranteed kInstMatch
    bool sawmark = false;   // whether queue contains a Mark

    for (Workq::iterator it = q->begin(); it != q->end(); ++it) {
        int id = *it;
        if (sawmatch && (kind_ == Prog::kFirstMatch || q->is_mark(id)))
            break;
        if (q->is_mark(id)) {
            if (n > 0 && inst[n - 1] != Mark) {
                sawmark = true;
                inst[n++] = Mark;
            }
            continue;
        }
        Prog::Inst *ip = prog_->inst(id);
        switch (ip->opcode()) {
            case kInstAltMatch:
                // This state will continue to a match no matter what
                // the rest of the input is.  If it is the highest priority match
                // being considered, return the special FullMatchState
                // to indicate that it's all matches from here out.
                if (kind_ != Prog::kManyMatch && (kind_ != Prog::kFirstMatch || (it == q->begin() && ip->greedy(prog_))) &&
                    (kind_ != Prog::kLongestMatch || !sawmark) && (flag & kFlagMatch)) {
                    return FullMatchState;
                }
                FALLTHROUGH_INTENDED;
            default:
                // Record iff id is the head of its list, which must
                // be the case if id-1 is the last of *its* list. :)
                if (prog_->inst(id - 1)->last())
                    inst[n++] = *it;
                if (ip->opcode() == kInstEmptyWidth)
                    needflags |= ip->empty();
                if (ip->opcode() == kInstMatch && !prog_->anchor_end())
                    sawmatch = true;
                break;
        }
    }
    DCHECK_LE(n, q->size());
    if (n > 0 && inst[n - 1] == Mark)
        n--;

    // If there are no empty-width instructions waiting to execute,
    // then the extra flag bits will not be used, so there is no
    // point in saving them.  (Discarding them reduces the number
    // of distinct states.)
    if (needflags == 0)
        flag &= kFlagMatch;

    // NOTE(rsc): The code above cannot do flag &= needflags,
    // because if the right flags were present to pass the current
    // kInstEmptyWidth instructions, new kInstEmptyWidth instructions
    // might be reached that in turn need different flags.
    // The only sure thing is that if there are no kInstEmptyWidth
    // instructions at all, no flags will be needed.
    // We could do the extra work to figure out the full set of
    // possibly needed flags by exploring past the kInstEmptyWidth
    // instructions, but the check above -- are any flags needed
    // at all? -- handles the most common case.  More fine-grained
    // analysis can only be justified by measurements showing that
    // too many redundant states are being allocated.

    // If there are no Insts in the list, it's a dead state,
    // which is useful to signal with a special pointer so that
    // the execution loop can stop early.  This is only okay
    // if the state is *not* a matching state.
    if (n == 0 && flag == 0) {
        return DeadState;
    }

    // If we're in longest match mode, the state is a sequence of
    // unordered state sets separated by Marks.  Sort each set
    // to canonicalize, to reduce the number of distinct sets stored.
    if (kind_ == Prog::kLongestMatch) {
        int *ip = inst.data();
        int *ep = ip + n;
        while (ip < ep) {
            int *markp = ip;
            while (markp < ep && *markp != Mark)
                markp++;
            std::sort(ip, markp);
            if (markp < ep)
                markp++;
            ip = markp;
        }
    }

    // If we're in many match mode, canonicalize for similar reasons:
    // we have an unordered set of states (i.e. we don't have Marks)
    // and sorting will reduce the number of distinct sets stored.
    if (kind_ == Prog::kManyMatch) {
        int *ip = inst.data();
        int *ep = ip + n;
        std::sort(ip, ep);
    }

    // Append MatchSep and the match IDs in mq if necessary.
    if (mq != NULL) {
        inst[n++] = MatchSep;
        for (Workq::iterator i = mq->begin(); i != mq->end(); ++i) {
            int id = *i;
            Prog::Inst *ip = prog_->inst(id);
            if (ip->opcode() == kInstMatch)
                inst[n++] = ip->match_id();
        }
    }

    // Save the needed empty-width flags in the top bits for use later.
    flag |= needflags << kFlagNeedShift;

    State *state = CachedState(inst.data(), n, flag);
    return state;
}

// Looks in the State cache for a State matching inst, ninst, flag.
// If one is found, returns it.  If one is not found, allocates one,
// inserts it in the cache, and returns it.
DFA::State *DFA::CachedState(int *inst, int ninst, uint32_t flag) {
    // mutex_.AssertHeld();

    // Look in the cache for a pre-existing state.
    // We have to initialise the struct like this because otherwise
    // MSVC will complain about the flexible array member. :(
    State state;
    state.inst_ = inst;
    state.ninst_ = ninst;
    state.flag_ = flag;
    StateSet::iterator it = state_cache_.find(&state);
    if (it != state_cache_.end()) {
        return *it;
    }

    // Must have enough memory for new state.
    // In addition to what we're going to allocate,
    // the state cache hash table seems to incur about 40 bytes per
    // State*, empirically.
    const int kStateCacheOverhead = 40;
    int nnext = prog_->bytemap_range() + 1; // + 1 for kByteEndText slot
    int mem = sizeof(State) + nnext * sizeof(std::atomic<State *>) + ninst * sizeof(int);
    if (mem_budget_ < mem + kStateCacheOverhead) {
        mem_budget_ = -1;
        return NULL;
    }
    mem_budget_ -= mem + kStateCacheOverhead;

    // Allocate new state along with room for next_ and inst_.
    char *space = std::allocator<char>().allocate(mem);
    State *s = new (space) State;
    s->next_ = new (space + sizeof(State)) std::atomic<State *>[nnext];
    // Work around a unfortunate bug in older versions of libstdc++.
    // (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64658)
    for (int i = 0; i < nnext; i++)
        (void)new (s->next_ + i) std::atomic<State *>(NULL);
    s->inst_ = new (s->next_ + nnext) int[ninst];
    memmove(s->inst_, inst, ninst * sizeof s->inst_[0]);
    s->ninst_ = ninst;
    s->flag_ = flag;
    // Put state in cache and return it.
    state_cache_.insert(s);
    return s;
}

// Clear the cache.  Must hold cache_mutex_.w or be in destructor.
void DFA::ClearCache() {
    StateSet::iterator begin = state_cache_.begin();
    StateSet::iterator end = state_cache_.end();
    while (begin != end) {
        StateSet::iterator tmp = begin;
        ++begin;
        // Deallocate the blob of memory that we allocated in DFA::CachedState().
        // We recompute mem in order to benefit from sized delete where possible.
        int ninst = (*tmp)->ninst_;
        int nnext = prog_->bytemap_range() + 1; // + 1 for kByteEndText slot
        int mem = sizeof(State) + nnext * sizeof(std::atomic<State *>) + ninst * sizeof(int);
        std::allocator<char>().deallocate(reinterpret_cast<char *>(*tmp), mem);
    }
    state_cache_.clear();
}

// Copies insts in state s to the work queue q.
void DFA::StateToWorkq(State *s, Workq *q) {
    q->clear();
    for (int i = 0; i < s->ninst_; i++) {
        if (s->inst_[i] == Mark) {
            q->mark();
        } else if (s->inst_[i] == MatchSep) {
            // Nothing after this is an instruction!
            break;
        } else {
            // Explore from the head of the list.
            AddToQueue(q, s->inst_[i], s->flag_ & kFlagEmptyMask);
        }
    }
}

// Adds ip to the work queue, following empty arrows according to flag.
void DFA::AddToQueue(Workq *q, int id, uint32_t flag) {

    // Use stack_ to hold our stack of instructions yet to process.
    // It was preallocated as follows:
    //   one entry 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.)
    // When using marks, we also added nmark == prog_->size().
    // (Otherwise, nmark == 0.)
    int *stk = stack_.data();
    int nstk = 0;

    stk[nstk++] = id;
    while (nstk > 0) {
        DCHECK_LE(nstk, stack_.size());
        id = stk[--nstk];

    Loop:
        if (id == Mark) {
            q->mark();
            continue;
        }

        if (id == 0)
            continue;

        // If ip is already on the queue, nothing to do.
        // Otherwise add it.  We don't actually keep all the
        // ones that get added, but adding all of them here
        // increases the likelihood of q->contains(id),
        // reducing the amount of duplicated work.
        if (q->contains(id))
            continue;
        q->insert_new(id);

        // Process instruction.
        Prog::Inst *ip = prog_->inst(id);
        switch (ip->opcode()) {
            default:
                LOG(DFATAL) << "unhandled opcode: " << ip->opcode();
                break;

            case kInstByteRange: // just save these on the queue
            case kInstMatch:
                if (ip->last())
                    break;
                id = id + 1;
                goto Loop;

            case kInstCapture: // DFA treats captures as no-ops.
            case kInstNop:
                if (!ip->last())
                    stk[nstk++] = id + 1;

                // If this instruction is the [00-FF]* loop at the beginning of
                // a leftmost-longest unanchored search, separate with a Mark so
                // that future threads (which will start farther to the right in
                // the input string) are lower priority than current threads.
                if (ip->opcode() == kInstNop && q->maxmark() > 0 && id == prog_->start_unanchored() && id != prog_->start())
                    stk[nstk++] = Mark;
                id = ip->out();
                goto Loop;

            case kInstAltMatch:
                DCHECK(!ip->last());
                id = id + 1;
                goto Loop;

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

                // Continue on if we have all the right flag bits.
                if (ip->empty() & ~flag)
                    break;
                id = ip->out();
                goto Loop;
        }
    }
}

// Running of work queues.  In the work queue, order matters:
// the queue is sorted in priority order.  If instruction i comes before j,
// then the instructions that i produces during the run must come before
// the ones that j produces.  In order to keep this invariant, all the
// work queue runners have to take an old queue to process and then
// also a new queue to fill in.  It's not acceptable to add to the end of
// an existing queue, because new instructions will not end up in the
// correct position.

// Runs the work queue, processing the empty strings indicated by flag.
// For example, flag == kEmptyBeginLine|kEmptyEndLine means to match
// both ^ and $.  It is important that callers pass all flags at once:
// processing both ^ and $ is not the same as first processing only ^
// and then processing only $.  Doing the two-step sequence won't match
// ^$^$^$ but processing ^ and $ simultaneously will (and is the behavior
// exhibited by existing implementations).
void DFA::RunWorkqOnEmptyString(Workq *oldq, Workq *newq, uint32_t flag) {
    newq->clear();
    for (Workq::iterator i = oldq->begin(); i != oldq->end(); ++i) {
        if (oldq->is_mark(*i))
            AddToQueue(newq, Mark, flag);
        else
            AddToQueue(newq, *i, flag);
    }
}

// Runs the work queue, processing the single byte c followed by any empty
// strings indicated by flag.  For example, c == 'a' and flag == kEmptyEndLine,
// means to match c$.  Sets the bool *ismatch to true if the end of the
// regular expression program has been reached (the regexp has matched).
void DFA::RunWorkqOnByte(Workq *oldq, Workq *newq, int c, uint32_t flag, bool *ismatch) {
    // mutex_.AssertHeld();

    newq->clear();
    for (Workq::iterator i = oldq->begin(); i != oldq->end(); ++i) {
        if (oldq->is_mark(*i)) {
            if (*ismatch)
                return;
            newq->mark();
            continue;
        }
        int id = *i;
        Prog::Inst *ip = prog_->inst(id);
        switch (ip->opcode()) {
            default:
                LOG(DFATAL) << "unhandled opcode: " << ip->opcode();
                break;

            case kInstFail:       // never succeeds
            case kInstCapture:    // already followed
            case kInstNop:        // already followed
            case kInstAltMatch:   // already followed
            case kInstEmptyWidth: // already followed
                break;

            case kInstByteRange: // can follow if c is in range
                if (!ip->Matches(c))
                    break;
                AddToQueue(newq, ip->out(), flag);
                if (ip->hint() != 0) {
                    // We have a hint, but we must cancel out the
                    // increment that will occur after the break.
                    i += ip->hint() - 1;
                } else {
                    // We have no hint, so we must find the end
                    // of the current list and then skip to it.
                    Prog::Inst *ip0 = ip;
                    while (!ip->last())
                        ++ip;
                    i += ip - ip0;
                }
                break;

            case kInstMatch:
                if (prog_->anchor_end() && c != kByteEndText && kind_ != Prog::kManyMatch)
                    break;
                *ismatch = true;
                if (kind_ == Prog::kFirstMatch) {
                    // Can stop processing work queue since we found a match.
                    return;
                }
                break;
        }
    }
}

// Processes input byte c in state, returning new state.
// Caller does not hold mutex.
DFA::State *DFA::RunStateOnByteUnlocked(State *state, int c) {
    // Keep only one RunStateOnByte going
    // even if the DFA is being run by multiple threads.
    MutexLock l(&mutex_);
    return RunStateOnByte(state, c);
}

// Processes input byte c in state, returning new state.
DFA::State *DFA::RunStateOnByte(State *state, int c) {
    // mutex_.AssertHeld();

    if (state <= SpecialStateMax) {
        if (state == FullMatchState) {
            // It is convenient for routines like PossibleMatchRange
            // if we implement RunStateOnByte for FullMatchState:
            // once you get into this state you never get out,
            // so it's pretty easy.
            return FullMatchState;
        }
        if (state == DeadState) {
            LOG(DFATAL) << "DeadState in RunStateOnByte";
            return NULL;
        }
        if (state == NULL) {
            LOG(DFATAL) << "NULL state in RunStateOnByte";
            return NULL;
        }
        LOG(DFATAL) << "Unexpected special state in RunStateOnByte";
        return NULL;
    }

    // If someone else already computed this, return it.
    State *ns = state->next_[ByteMap(c)].load(std::memory_order_relaxed);
    if (ns != NULL)
        return ns;

    // Convert state into Workq.
    StateToWorkq(state, q0_);

    // Flags marking the kinds of empty-width things (^ $ etc)
    // around this byte.  Before the byte we have the flags recorded
    // in the State structure itself.  After the byte we have
    // nothing yet (but that will change: read on).
    uint32_t needflag = state->flag_ >> kFlagNeedShift;
    uint32_t beforeflag = state->flag_ & kFlagEmptyMask;
    uint32_t oldbeforeflag = beforeflag;
    uint32_t afterflag = 0;

    if (c == '\n') {
        // Insert implicit $ and ^ around \n
        beforeflag |= kEmptyEndLine;
        afterflag |= kEmptyBeginLine;
    }

    if (c == kByteEndText) {
        // Insert implicit $ and \z before the fake "end text" byte.
        beforeflag |= kEmptyEndLine | kEmptyEndText;
    }

    // The state flag kFlagLastWord says whether the last
    // byte processed was a word character.  Use that info to
    // insert empty-width (non-)word boundaries.
    bool islastword = (state->flag_ & kFlagLastWord) != 0;
    bool isword = c != kByteEndText && Prog::IsWordChar(static_cast<uint8_t>(c));
    if (isword == islastword)
        beforeflag |= kEmptyNonWordBoundary;
    else
        beforeflag |= kEmptyWordBoundary;

    // Okay, finally ready to run.
    // Only useful to rerun on empty string if there are new, useful flags.
    if (beforeflag & ~oldbeforeflag & needflag) {
        RunWorkqOnEmptyString(q0_, q1_, beforeflag);
        using std::swap;
        swap(q0_, q1_);
    }
    bool ismatch = false;
    RunWorkqOnByte(q0_, q1_, c, afterflag, &ismatch);
    using std::swap;
    swap(q0_, q1_);

    // Save afterflag along with ismatch and isword in new state.
    uint32_t flag = afterflag;
    if (ismatch)
        flag |= kFlagMatch;
    if (isword)
        flag |= kFlagLastWord;

    if (ismatch && kind_ == Prog::kManyMatch)
        ns = WorkqToCachedState(q0_, q1_, flag);
    else
        ns = WorkqToCachedState(q0_, NULL, flag);

    // Flush ns before linking to it.
    // Write barrier before updating state->next_ so that the
    // main search loop can proceed without any locking, for speed.
    // (Otherwise it would need one mutex operation per input byte.)
    state->next_[ByteMap(c)].store(ns, std::memory_order_release);
    return ns;
}

//////////////////////////////////////////////////////////////////////
// DFA cache reset.

// Reader-writer lock helper.
//
// The DFA uses a reader-writer mutex to protect the state graph itself.
// Traversing the state graph requires holding the mutex for reading,
// and discarding the state graph and starting over requires holding the
// lock for writing.  If a search needs to expand the graph but is out
// of memory, it will need to drop its read lock and then acquire the
// write lock.  Since it cannot then atomically downgrade from write lock
// to read lock, it runs the rest of the search holding the write lock.
// (This probably helps avoid repeated contention, but really the decision
// is forced by the Mutex interface.)  It's a bit complicated to keep
// track of whether the lock is held for reading or writing and thread
// that through the search, so instead we encapsulate it in the RWLocker
// and pass that around.

class DFA::RWLocker {
public:
    explicit RWLocker(CacheMutex *mu);
    ~RWLocker();

    // If the lock is only held for reading right now,
    // drop the read lock and re-acquire for writing.
    // Subsequent calls to LockForWriting are no-ops.
    // Notice that the lock is *released* temporarily.
    void LockForWriting();

private:
    CacheMutex *mu_;
    bool writing_;

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

DFA::RWLocker::RWLocker(CacheMutex *mu) : mu_(mu), writing_(false) { mu_->ReaderLock(); }

// This function is marked as NO_THREAD_SAFETY_ANALYSIS because
// the annotations don't support lock upgrade.
void DFA::RWLocker::LockForWriting() NO_THREAD_SAFETY_ANALYSIS {
    if (!writing_) {
        mu_->ReaderUnlock();
        mu_->WriterLock();
        writing_ = true;
    }
}

DFA::RWLocker::~RWLocker() {
    if (!writing_)
        mu_->ReaderUnlock();
    else
        mu_->WriterUnlock();
}

// When the DFA's State cache fills, we discard all the states in the
// cache and start over.  Many threads can be using and adding to the
// cache at the same time, so we synchronize using the cache_mutex_
// to keep from stepping on other threads.  Specifically, all the
// threads using the current cache hold cache_mutex_ for reading.
// When a thread decides to flush the cache, it drops cache_mutex_
// and then re-acquires it for writing.  That ensures there are no
// other threads accessing the cache anymore.  The rest of the search
// runs holding cache_mutex_ for writing, avoiding any contention
// with or cache pollution caused by other threads.

void DFA::ResetCache(RWLocker *cache_lock) {
    // Re-acquire the cache_mutex_ for writing (exclusive use).
    cache_lock->LockForWriting();

    hooks::GetDFAStateCacheResetHook()({
        state_budget_,
        state_cache_.size(),
    });

    // Clear the cache, reset the memory budget.
    for (int i = 0; i < kMaxStart; i++)
        start_[i].start.store(NULL, std::memory_order_relaxed);
    ClearCache();
    mem_budget_ = state_budget_;
}

// Typically, a couple States do need to be preserved across a cache
// reset, like the State at the current point in the search.
// The StateSaver class helps keep States across cache resets.
// It makes a copy of the state's guts outside the cache (before the reset)
// and then can be asked, after the reset, to recreate the State
// in the new cache.  For example, in a DFA method ("this" is a DFA):
//
//   StateSaver saver(this, s);
//   ResetCache(cache_lock);
//   s = saver.Restore();
//
// The saver should always have room in the cache to re-create the state,
// because resetting the cache locks out all other threads, and the cache
// is known to have room for at least a couple states (otherwise the DFA
// constructor fails).

class DFA::StateSaver {
public:
    explicit StateSaver(DFA *dfa, State *state);
    ~StateSaver();

    // Recreates and returns a state equivalent to the
    // original state passed to the constructor.
    // Returns NULL if the cache has filled, but
    // since the DFA guarantees to have room in the cache
    // for a couple states, should never return NULL
    // if used right after ResetCache.
    State *Restore();

private:
    DFA *dfa_;  // the DFA to use
    int *inst_; // saved info from State
    int ninst_;
    uint32_t flag_;
    bool is_special_; // whether original state was special
    State *special_;  // if is_special_, the original state

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

DFA::StateSaver::StateSaver(DFA *dfa, State *state) {
    dfa_ = dfa;
    if (state <= SpecialStateMax) {
        inst_ = NULL;
        ninst_ = 0;
        flag_ = 0;
        is_special_ = true;
        special_ = state;
        return;
    }
    is_special_ = false;
    special_ = NULL;
    flag_ = state->flag_;
    ninst_ = state->ninst_;
    inst_ = new int[ninst_];
    memmove(inst_, state->inst_, ninst_ * sizeof inst_[0]);
}

DFA::StateSaver::~StateSaver() {
    if (!is_special_)
        delete[] inst_;
}

DFA::State *DFA::StateSaver::Restore() {
    if (is_special_)
        return special_;
    MutexLock l(&dfa_->mutex_);
    State *s = dfa_->CachedState(inst_, ninst_, flag_);
    if (s == NULL)
        LOG(DFATAL) << "StateSaver failed to restore state.";
    return s;
}

//////////////////////////////////////////////////////////////////////
//
// DFA execution.
//
// The basic search loop is easy: start in a state s and then for each
// byte c in the input, s = s->next[c].
//
// This simple description omits a few efficiency-driven complications.
//
// First, the State graph is constructed incrementally: it is possible
// that s->next[c] is null, indicating that that state has not been
// fully explored.  In this case, RunStateOnByte must be invoked to
// determine the next state, which is cached in s->next[c] to save
// future effort.  An alternative reason for s->next[c] to be null is
// that the DFA has reached a so-called "dead state", in which any match
// is no longer possible.  In this case RunStateOnByte will return NULL
// and the processing of the string can stop early.
//
// Second, a 256-element pointer array for s->next_ makes each State
// quite large (2kB on 64-bit machines).  Instead, dfa->bytemap_[]
// maps from bytes to "byte classes" and then next_ only needs to have
// as many pointers as there are byte classes.  A byte class is simply a
// range of bytes that the regexp never distinguishes between.
// A regexp looking for a[abc] would have four byte ranges -- 0 to 'a'-1,
// 'a', 'b' to 'c', and 'c' to 0xFF.  The bytemap slows us a little bit
// but in exchange we typically cut the size of a State (and thus our
// memory footprint) by about 5-10x.  The comments still refer to
// s->next[c] for simplicity, but code should refer to s->next_[bytemap_[c]].
//
// Third, it is common for a DFA for an unanchored match to begin in a
// state in which only one particular byte value can take the DFA to a
// different state.  That is, s->next[c] != s for only one c.  In this
// situation, the DFA can do better than executing the simple loop.
// Instead, it can call memchr to search very quickly for the byte c.
// Whether the start state has this property is determined during a
// pre-compilation pass and the "can_prefix_accel" argument is set.
//
// Fourth, the desired behavior is to search for the leftmost-best match
// (approximately, the same one that Perl would find), which is not
// necessarily the match ending earliest in the string.  Each time a
// match is found, it must be noted, but the DFA must continue on in
// hope of finding a higher-priority match.  In some cases, the caller only
// cares whether there is any match at all, not which one is found.
// The "want_earliest_match" flag causes the search to stop at the first
// match found.
//
// Fifth, one algorithm that uses the DFA needs it to run over the
// input string backward, beginning at the end and ending at the beginning.
// Passing false for the "run_forward" flag causes the DFA to run backward.
//
// The checks for these last three cases, which in a naive implementation
// would be performed once per input byte, slow the general loop enough
// to merit specialized versions of the search loop for each of the
// eight possible settings of the three booleans.  Rather than write
// eight different functions, we write one general implementation and then
// inline it to create the specialized ones.
//
// Note that matches are delayed by one byte, to make it easier to
// accomodate match conditions depending on the next input byte (like $ and \b).
// When s->next[c]->IsMatch(), it means that there is a match ending just
// *before* byte c.

// The generic search loop.  Searches text for a match, returning
// the pointer to the end of the chosen match, or NULL if no match.
// The bools are equal to the same-named variables in params, but
// making them function arguments lets the inliner specialize
// this function to each combination (see two paragraphs above).
template <bool can_prefix_accel, bool want_earliest_match, bool run_forward>
inline bool DFA::InlinedSearchLoop(SearchParams *params) {
    State *start = params->start;
    const uint8_t *bp = BytePtr(params->text.data());                       // start of text
    const uint8_t *p = bp;                                                  // text scanning point
    const uint8_t *ep = BytePtr(params->text.data() + params->text.size()); // end of text
    const uint8_t *resetp = NULL;                                           // p at last cache reset
    if (!run_forward) {
        using std::swap;
        swap(p, ep);
    }

    const uint8_t *bytemap = prog_->bytemap();
    const uint8_t *lastmatch = NULL; // most recent matching position in text
    bool matched = false;

    State *s = start;

    if (s->IsMatch()) {
        matched = true;
        lastmatch = p;
        if (params->matches != NULL && kind_ == Prog::kManyMatch) {
            for (int i = s->ninst_ - 1; i >= 0; i--) {
                int id = s->inst_[i];
                if (id == MatchSep)
                    break;
                params->matches->insert(id);
            }
        }
        if (want_earliest_match) {
            params->ep = reinterpret_cast<const char *>(lastmatch);
            return true;
        }
    }

    while (p != ep) {

        if (can_prefix_accel && s == start) {
            // In start state, only way out is to find the prefix,
            // so we use prefix accel (e.g. memchr) to skip ahead.
            // If not found, we can skip to the end of the string.
            p = BytePtr(prog_->PrefixAccel(p, ep - p));
            if (p == NULL) {
                p = ep;
                break;
            }
        }

        int c;
        if (run_forward)
            c = *p++;
        else
            c = *--p;

        // Note that multiple threads might be consulting
        // s->next_[bytemap[c]] simultaneously.
        // RunStateOnByte takes care of the appropriate locking,
        // including a memory barrier so that the unlocked access
        // (sometimes known as "double-checked locking") is safe.
        // The alternative would be either one DFA per thread
        // or one mutex operation per input byte.
        //
        // ns == DeadState means the state is known to be dead
        // (no more matches are possible).
        // ns == NULL means the state has not yet been computed
        // (need to call RunStateOnByteUnlocked).
        // RunStateOnByte returns ns == NULL if it is out of memory.
        // ns == FullMatchState means the rest of the string matches.
        //
        // Okay to use bytemap[] not ByteMap() here, because
        // c is known to be an actual byte and not kByteEndText.

        State *ns = s->next_[bytemap[c]].load(std::memory_order_acquire);
        if (ns == NULL) {
            ns = RunStateOnByteUnlocked(s, c);
            if (ns == NULL) {
                // After we reset the cache, we hold cache_mutex exclusively,
                // so if resetp != NULL, it means we filled the DFA state
                // cache with this search alone (without any other threads).
                // Benchmarks show that doing a state computation on every
                // byte runs at about 0.2 MB/s, while the NFA (nfa.cc) can do the
                // same at about 2 MB/s.  Unless we're processing an average
                // of 10 bytes per state computation, fail so that RE2 can
                // fall back to the NFA.  However, RE2::Set cannot fall back,
                // so we just have to keep on keeping on in that case.
                if (dfa_should_bail_when_slow && resetp != NULL && static_cast<size_t>(p - resetp) < 10 * state_cache_.size() &&
                    kind_ != Prog::kManyMatch) {
                    params->failed = true;
                    return false;
                }
                resetp = p;

                // Prepare to save start and s across the reset.
                StateSaver save_start(this, start);
                StateSaver save_s(this, s);

                // Discard all the States in the cache.
                ResetCache(params->cache_lock);

                // Restore start and s so we can continue.
                if ((start = save_start.Restore()) == NULL || (s = save_s.Restore()) == NULL) {
                    // Restore already did LOG(DFATAL).
                    params->failed = true;
                    return false;
                }
                ns = RunStateOnByteUnlocked(s, c);
                if (ns == NULL) {
                    LOG(DFATAL) << "RunStateOnByteUnlocked failed after ResetCache";
                    params->failed = true;
                    return false;
                }
            }
        }
        if (ns <= SpecialStateMax) {
            if (ns == DeadState) {
                params->ep = reinterpret_cast<const char *>(lastmatch);
                return matched;
            }
            // FullMatchState
            params->ep = reinterpret_cast<const char *>(ep);
            return true;
        }

        s = ns;
        if (s->IsMatch()) {
            matched = true;
            // The DFA notices the match one byte late,
            // so adjust p before using it in the match.
            if (run_forward)
                lastmatch = p - 1;
            else
                lastmatch = p + 1;
            if (params->matches != NULL && kind_ == Prog::kManyMatch) {
                for (int i = s->ninst_ - 1; i >= 0; i--) {
                    int id = s->inst_[i];
                    if (id == MatchSep)
                        break;
                    params->matches->insert(id);
                }
            }
            if (want_earliest_match) {
                params->ep = reinterpret_cast<const char *>(lastmatch);
                return true;
            }
        }
    }

    // Process one more byte to see if it triggers a match.
    // (Remember, matches are delayed one byte.)

    int lastbyte;
    if (run_forward) {
        if (EndPtr(params->text) == EndPtr(params->context))
            lastbyte = kByteEndText;
        else
            lastbyte = EndPtr(params->text)[0] & 0xFF;
    } else {
        if (BeginPtr(params->text) == BeginPtr(params->context))
            lastbyte = kByteEndText;
        else
            lastbyte = BeginPtr(params->text)[-1] & 0xFF;
    }

    State *ns = s->next_[ByteMap(lastbyte)].load(std::memory_order_acquire);
    if (ns == NULL) {
        ns = RunStateOnByteUnlocked(s, lastbyte);
        if (ns == NULL) {
            StateSaver save_s(this, s);
            ResetCache(params->cache_lock);
            if ((s = save_s.Restore()) == NULL) {
                params->failed = true;
                return false;
            }
            ns = RunStateOnByteUnlocked(s, lastbyte);
            if (ns == NULL) {
                LOG(DFATAL) << "RunStateOnByteUnlocked failed after Reset";
                params->failed = true;
                return false;
            }
        }
    }
    if (ns <= SpecialStateMax) {
        if (ns == DeadState) {
            params->ep = reinterpret_cast<const char *>(lastmatch);
            return matched;
        }
        // FullMatchState
        params->ep = reinterpret_cast<const char *>(ep);
        return true;
    }

    s = ns;
    if (s->IsMatch()) {
        matched = true;
        lastmatch = p;
        if (params->matches != NULL && kind_ == Prog::kManyMatch) {
            for (int i = s->ninst_ - 1; i >= 0; i--) {
                int id = s->inst_[i];
                if (id == MatchSep)
                    break;
                params->matches->insert(id);
            }
        }
    }

    params->ep = reinterpret_cast<const char *>(lastmatch);
    return matched;
}

// Inline specializations of the general loop.
bool DFA::SearchFFF(SearchParams *params) { return InlinedSearchLoop<false, false, false>(params); }
bool DFA::SearchFFT(SearchParams *params) { return InlinedSearchLoop<false, false, true>(params); }
bool DFA::SearchFTF(SearchParams *params) { return InlinedSearchLoop<false, true, false>(params); }
bool DFA::SearchFTT(SearchParams *params) { return InlinedSearchLoop<false, true, true>(params); }
bool DFA::SearchTFF(SearchParams *params) { return InlinedSearchLoop<true, false, false>(params); }
bool DFA::SearchTFT(SearchParams *params) { return InlinedSearchLoop<true, false, true>(params); }
bool DFA::SearchTTF(SearchParams *params) { return InlinedSearchLoop<true, true, false>(params); }
bool DFA::SearchTTT(SearchParams *params) { return InlinedSearchLoop<true, true, true>(params); }

// For performance, calls the appropriate specialized version
// of InlinedSearchLoop.
bool DFA::FastSearchLoop(SearchParams *params) {
    // Because the methods are private, the Searches array
    // cannot be declared at top level.
    static bool (DFA::*Searches[])(SearchParams *) = {
        &DFA::SearchFFF,
        &DFA::SearchFFT,
        &DFA::SearchFTF,
        &DFA::SearchFTT,
        &DFA::SearchTFF,
        &DFA::SearchTFT,
        &DFA::SearchTTF,
        &DFA::SearchTTT,
    };

    int index = 4 * params->can_prefix_accel + 2 * params->want_earliest_match + 1 * params->run_forward;
    return (this->*Searches[index])(params);
}

// The discussion of DFA execution above ignored the question of how
// to determine the initial state for the search loop.  There are two
// factors that influence the choice of start state.
//
// The first factor is whether the search is anchored or not.
// The regexp program (Prog*) itself has
// two different entry points: one for anchored searches and one for
// unanchored searches.  (The unanchored version starts with a leading ".*?"
// and then jumps to the anchored one.)
//
// The second factor is where text appears in the larger context, which
// determines which empty-string operators can be matched at the beginning
// of execution.  If text is at the very beginning of context, \A and ^ match.
// Otherwise if text is at the beginning of a line, then ^ matches.
// Otherwise it matters whether the character before text is a word character
// or a non-word character.
//
// The two cases (unanchored vs not) and four cases (empty-string flags)
// combine to make the eight cases recorded in the DFA's begin_text_[2],
// begin_line_[2], after_wordchar_[2], and after_nonwordchar_[2] cached
// StartInfos.  The start state for each is filled in the first time it
// is used for an actual search.

// Examines text, context, and anchored to determine the right start
// state for the DFA search loop.  Fills in params and returns true on success.
// Returns false on failure.
bool DFA::AnalyzeSearch(SearchParams *params) {
    const StringPiece &text = params->text;
    const StringPiece &context = params->context;

    // 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";
        params->start = DeadState;
        return true;
    }

    // Determine correct search type.
    int start;
    uint32_t flags;
    if (params->run_forward) {
        if (BeginPtr(text) == BeginPtr(context)) {
            start = kStartBeginText;
            flags = kEmptyBeginText | kEmptyBeginLine;
        } else if (BeginPtr(text)[-1] == '\n') {
            start = kStartBeginLine;
            flags = kEmptyBeginLine;
        } else if (Prog::IsWordChar(BeginPtr(text)[-1] & 0xFF)) {
            start = kStartAfterWordChar;
            flags = kFlagLastWord;
        } else {
            start = kStartAfterNonWordChar;
            flags = 0;
        }
    } else {
        if (EndPtr(text) == EndPtr(context)) {
            start = kStartBeginText;
            flags = kEmptyBeginText | kEmptyBeginLine;
        } else if (EndPtr(text)[0] == '\n') {
            start = kStartBeginLine;
            flags = kEmptyBeginLine;
        } else if (Prog::IsWordChar(EndPtr(text)[0] & 0xFF)) {
            start = kStartAfterWordChar;
            flags = kFlagLastWord;
        } else {
            start = kStartAfterNonWordChar;
            flags = 0;
        }
    }
    if (params->anchored)
        start |= kStartAnchored;
    StartInfo *info = &start_[start];

    // Try once without cache_lock for writing.
    // Try again after resetting the cache
    // (ResetCache will relock cache_lock for writing).
    if (!AnalyzeSearchHelper(params, info, flags)) {
        ResetCache(params->cache_lock);
        if (!AnalyzeSearchHelper(params, info, flags)) {
            params->failed = true;
            LOG(DFATAL) << "Failed to analyze start state.";
            return false;
        }
    }

    params->start = info->start.load(std::memory_order_acquire);

    // Even if we could prefix accel, we cannot do so when anchored and,
    // less obviously, we cannot do so when we are going to need flags.
    // This trick works only when there is a single byte that leads to a
    // different state!
    if (prog_->can_prefix_accel() && !params->anchored && params->start > SpecialStateMax && params->start->flag_ >> kFlagNeedShift == 0)
        params->can_prefix_accel = true;

    return true;
}

// Fills in info if needed.  Returns true on success, false on failure.
bool DFA::AnalyzeSearchHelper(SearchParams *params, StartInfo *info, uint32_t flags) {
    // Quick check.
    State *start = info->start.load(std::memory_order_acquire);
    if (start != NULL)
        return true;

    MutexLock l(&mutex_);
    start = info->start.load(std::memory_order_relaxed);
    if (start != NULL)
        return true;

    q0_->clear();
    AddToQueue(q0_, params->anchored ? prog_->start() : prog_->start_unanchored(), flags);
    start = WorkqToCachedState(q0_, NULL, flags);
    if (start == NULL)
        return false;

    // Synchronize with "quick check" above.
    info->start.store(start, std::memory_order_release);
    return true;
}

// The actual DFA search: calls AnalyzeSearch and then FastSearchLoop.
bool DFA::Search(const StringPiece &text,
                 const StringPiece &context,
                 bool anchored,
                 bool want_earliest_match,
                 bool run_forward,
                 bool *failed,
                 const char **epp,
                 SparseSet *matches) {
    *epp = NULL;
    if (!ok()) {
        *failed = true;
        return false;
    }
    *failed = false;

    RWLocker l(&cache_mutex_);
    SearchParams params(text, context, &l);
    params.anchored = anchored;
    params.want_earliest_match = want_earliest_match;
    params.run_forward = run_forward;
    params.matches = matches;

    if (!AnalyzeSearch(&params)) {
        *failed = true;
        return false;
    }
    if (params.start == DeadState)
        return false;
    if (params.start == FullMatchState) {
        if (run_forward == want_earliest_match)
            *epp = text.data();
        else
            *epp = text.data() + text.size();
        return true;
    }
    bool ret = FastSearchLoop(&params);
    if (params.failed) {
        *failed = true;
        return false;
    }
    *epp = params.ep;
    return ret;
}

DFA *Prog::GetDFA(MatchKind kind) {
    // For a forward DFA, half the memory goes to each DFA.
    // However, if it is a "many match" DFA, then there is
    // no counterpart with which the memory must be shared.
    //
    // For a reverse DFA, all the memory goes to the
    // "longest match" DFA, because RE2 never does reverse
    // "first match" searches.
    if (kind == kFirstMatch) {
        std::call_once(dfa_first_once_, [](Prog *prog) { prog->dfa_first_ = new DFA(prog, kFirstMatch, prog->dfa_mem_ / 2); }, this);
        return dfa_first_;
    } else if (kind == kManyMatch) {
        std::call_once(dfa_first_once_, [](Prog *prog) { prog->dfa_first_ = new DFA(prog, kManyMatch, prog->dfa_mem_); }, this);
        return dfa_first_;
    } else {
        std::call_once(
            dfa_longest_once_,
            [](Prog *prog) {
                if (!prog->reversed_)
                    prog->dfa_longest_ = new DFA(prog, kLongestMatch, prog->dfa_mem_ / 2);
                else
                    prog->dfa_longest_ = new DFA(prog, kLongestMatch, prog->dfa_mem_);
            },
            this);
        return dfa_longest_;
    }
}

void Prog::DeleteDFA(DFA *dfa) { delete dfa; }

// Executes the regexp program to search in text,
// which itself is inside the larger context.  (As a convenience,
// passing a NULL context is equivalent to passing text.)
// Returns true if a match is found, false if not.
// If a match is found, fills in match0->end() to point at the end of the match
// and sets match0->begin() to text.begin(), since the DFA can't track
// where the match actually began.
//
// This is the only external interface (class DFA only exists in this file).
//
bool Prog::SearchDFA(const StringPiece &text,
                     const StringPiece &const_context,
                     Anchor anchor,
                     MatchKind kind,
                     StringPiece *match0,
                     bool *failed,
                     SparseSet *matches) {
    *failed = false;

    StringPiece context = const_context;
    if (context.data() == NULL)
        context = text;
    bool caret = anchor_start();
    bool dollar = anchor_end();
    if (reversed_) {
        using std::swap;
        swap(caret, dollar);
    }
    if (caret && BeginPtr(context) != BeginPtr(text))
        return false;
    if (dollar && EndPtr(context) != EndPtr(text))
        return false;

    // Handle full match by running an anchored longest match
    // and then checking if it covers all of text.
    bool anchored = anchor == kAnchored || anchor_start() || kind == kFullMatch;
    bool endmatch = false;
    if (kind == kManyMatch) {
        // This is split out in order to avoid clobbering kind.
    } else if (kind == kFullMatch || anchor_end()) {
        endmatch = true;
        kind = kLongestMatch;
    }

    // If the caller doesn't care where the match is (just whether one exists),
    // then we can stop at the very first match we find, the so-called
    // "earliest match".
    bool want_earliest_match = false;
    if (kind == kManyMatch) {
        // This is split out in order to avoid clobbering kind.
        if (matches == NULL) {
            want_earliest_match = true;
        }
    } else if (match0 == NULL && !endmatch) {
        want_earliest_match = true;
        kind = kLongestMatch;
    }

    DFA *dfa = GetDFA(kind);
    const char *ep;
    bool matched = dfa->Search(text, context, anchored, want_earliest_match, !reversed_, failed, &ep, matches);
    if (*failed) {
        hooks::GetDFASearchFailureHook()({
            // Nothing yet...
        });
        return false;
    }
    if (!matched)
        return false;
    if (endmatch && ep != (reversed_ ? text.data() : text.data() + text.size()))
        return false;

    // If caller cares, record the boundary of the match.
    // We only know where it ends, so use the boundary of text
    // as the beginning.
    if (match0) {
        if (reversed_)
            *match0 = StringPiece(ep, static_cast<size_t>(text.data() + text.size() - ep));
        else
            *match0 = StringPiece(text.data(), static_cast<size_t>(ep - text.data()));
    }
    return true;
}

// Build out all states in DFA.  Returns number of states.
int DFA::BuildAllStates(const Prog::DFAStateCallback &cb) {
    if (!ok())
        return 0;

    // Pick out start state for unanchored search
    // at beginning of text.
    RWLocker l(&cache_mutex_);
    SearchParams params(StringPiece(), StringPiece(), &l);
    params.anchored = false;
    if (!AnalyzeSearch(&params) || params.start == NULL || params.start == DeadState)
        return 0;

    // Add start state to work queue.
    // Note that any State* that we handle here must point into the cache,
    // so we can simply depend on pointer-as-a-number hashing and equality.
    std::unordered_map<State *, int> m;
    std::deque<State *> q;
    m.emplace(params.start, static_cast<int>(m.size()));
    q.push_back(params.start);

    // Compute the input bytes needed to cover all of the next pointers.
    int nnext = prog_->bytemap_range() + 1; // + 1 for kByteEndText slot
    std::vector<int> input(nnext);
    for (int c = 0; c < 256; c++) {
        int b = prog_->bytemap()[c];
        while (c < 256 - 1 && prog_->bytemap()[c + 1] == b)
            c++;
        input[b] = c;
    }
    input[prog_->bytemap_range()] = kByteEndText;

    // Scratch space for the output.
    std::vector<int> output(nnext);

    // Flood to expand every state.
    bool oom = false;
    while (!q.empty()) {
        State *s = q.front();
        q.pop_front();
        for (int c : input) {
            State *ns = RunStateOnByteUnlocked(s, c);
            if (ns == NULL) {
                oom = true;
                break;
            }
            if (ns == DeadState) {
                output[ByteMap(c)] = -1;
                continue;
            }
            if (m.find(ns) == m.end()) {
                m.emplace(ns, static_cast<int>(m.size()));
                q.push_back(ns);
            }
            output[ByteMap(c)] = m[ns];
        }
        if (cb)
            cb(oom ? NULL : output.data(), s == FullMatchState || s->IsMatch());
        if (oom)
            break;
    }

    return static_cast<int>(m.size());
}

// Build out all states in DFA for kind.  Returns number of states.
int Prog::BuildEntireDFA(MatchKind kind, const DFAStateCallback &cb) { return GetDFA(kind)->BuildAllStates(cb); }

// Computes min and max for matching string.
// Won't return strings bigger than maxlen.
bool DFA::PossibleMatchRange(std::string *min, std::string *max, int maxlen) {
    if (!ok())
        return false;

    // NOTE: if future users of PossibleMatchRange want more precision when
    // presented with infinitely repeated elements, consider making this a
    // parameter to PossibleMatchRange.
    static int kMaxEltRepetitions = 0;

    // Keep track of the number of times we've visited states previously. We only
    // revisit a given state if it's part of a repeated group, so if the value
    // portion of the map tuple exceeds kMaxEltRepetitions we bail out and set
    // |*max| to |PrefixSuccessor(*max)|.
    //
    // Also note that previously_visited_states[UnseenStatePtr] will, in the STL
    // tradition, implicitly insert a '0' value at first use. We take advantage
    // of that property below.
    std::unordered_map<State *, int> previously_visited_states;

    // Pick out start state for anchored search at beginning of text.
    RWLocker l(&cache_mutex_);
    SearchParams params(StringPiece(), StringPiece(), &l);
    params.anchored = true;
    if (!AnalyzeSearch(&params))
        return false;
    if (params.start == DeadState) { // No matching strings
        *min = "";
        *max = "";
        return true;
    }
    if (params.start == FullMatchState) // Every string matches: no max
        return false;

    // The DFA is essentially a big graph rooted at params.start,
    // and paths in the graph correspond to accepted strings.
    // Each node in the graph has potentially 256+1 arrows
    // coming out, one for each byte plus the magic end of
    // text character kByteEndText.

    // To find the smallest possible prefix of an accepted
    // string, we just walk the graph preferring to follow
    // arrows with the lowest bytes possible.  To find the
    // largest possible prefix, we follow the largest bytes
    // possible.

    // The test for whether there is an arrow from s on byte j is
    //    ns = RunStateOnByteUnlocked(s, j);
    //    if (ns == NULL)
    //      return false;
    //    if (ns != DeadState && ns->ninst > 0)
    // The RunStateOnByteUnlocked call asks the DFA to build out the graph.
    // It returns NULL only if the DFA has run out of memory,
    // in which case we can't be sure of anything.
    // The second check sees whether there was graph built
    // and whether it is interesting graph.  Nodes might have
    // ns->ninst == 0 if they exist only to represent the fact
    // that a match was found on the previous byte.

    // Build minimum prefix.
    State *s = params.start;
    min->clear();
    MutexLock lock(&mutex_);
    for (int i = 0; i < maxlen; i++) {
        if (previously_visited_states[s] > kMaxEltRepetitions)
            break;
        previously_visited_states[s]++;

        // Stop if min is a match.
        State *ns = RunStateOnByte(s, kByteEndText);
        if (ns == NULL) // DFA out of memory
            return false;
        if (ns != DeadState && (ns == FullMatchState || ns->IsMatch()))
            break;

        // Try to extend the string with low bytes.
        bool extended = false;
        for (int j = 0; j < 256; j++) {
            ns = RunStateOnByte(s, j);
            if (ns == NULL) // DFA out of memory
                return false;
            if (ns == FullMatchState || (ns > SpecialStateMax && ns->ninst_ > 0)) {
                extended = true;
                min->append(1, static_cast<char>(j));
                s = ns;
                break;
            }
        }
        if (!extended)
            break;
    }

    // Build maximum prefix.
    previously_visited_states.clear();
    s = params.start;
    max->clear();
    for (int i = 0; i < maxlen; i++) {
        if (previously_visited_states[s] > kMaxEltRepetitions)
            break;
        previously_visited_states[s] += 1;

        // Try to extend the string with high bytes.
        bool extended = false;
        for (int j = 255; j >= 0; j--) {
            State *ns = RunStateOnByte(s, j);
            if (ns == NULL)
                return false;
            if (ns == FullMatchState || (ns > SpecialStateMax && ns->ninst_ > 0)) {
                extended = true;
                max->append(1, static_cast<char>(j));
                s = ns;
                break;
            }
        }
        if (!extended) {
            // Done, no need for PrefixSuccessor.
            return true;
        }
    }

    // Stopped while still adding to *max - round aaaaaaaaaa... to aaaa...b
    PrefixSuccessor(max);

    // If there are no bytes left, we have no way to say "there is no maximum
    // string".  We could make the interface more complicated and be able to
    // return "there is no maximum but here is a minimum", but that seems like
    // overkill -- the most common no-max case is all possible strings, so not
    // telling the caller that the empty string is the minimum match isn't a
    // great loss.
    if (max->empty())
        return false;

    return true;
}

// PossibleMatchRange for a Prog.
bool Prog::PossibleMatchRange(std::string *min, std::string *max, int maxlen) {
    // Have to use dfa_longest_ to get all strings for full matches.
    // For example, (a|aa) never matches aa in first-match mode.
    return GetDFA(kLongestMatch)->PossibleMatchRange(min, max, maxlen);
}

} // namespace re2