// Copyright 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.

// Compile regular expression to Prog.
//
// Prog and Inst are defined in prog.h.
// This file's external interface is just Regexp::CompileToProg.
// The Compiler class defined in this file is private.

#include <stdint.h>
#include <string.h>
#include <unordered_map>
#include <utility>

#include "re2/pod_array.h"
#include "re2/prog.h"
#include "re2/re2.h"
#include "re2/regexp.h"
#include "re2/walker-inl.h"
#include "util/logging.h"
#include "util/utf.h"

namespace re2 {

// List of pointers to Inst* that need to be filled in (patched).
// Because the Inst* haven't been filled in yet,
// we can use the Inst* word to hold the list's "next" pointer.
// It's kind of sleazy, but it works well in practice.
// See http://swtch.com/~rsc/regexp/regexp1.html for inspiration.
//
// Because the out and out1 fields in Inst are no longer pointers,
// we can't use pointers directly here either.  Instead, head refers
// to inst_[head>>1].out (head&1 == 0) or inst_[head>>1].out1 (head&1 == 1).
// head == 0 represents the NULL list.  This is okay because instruction #0
// is always the fail instruction, which never appears on a list.
struct PatchList {
    // Returns patch list containing just p.
    static PatchList Mk(uint32_t p) { return {p, p}; }

    // Patches all the entries on l to have value p.
    // Caller must not ever use patch list again.
    static void Patch(Prog::Inst *inst0, PatchList l, uint32_t p) {
        while (l.head != 0) {
            Prog::Inst *ip = &inst0[l.head >> 1];
            if (l.head & 1) {
                l.head = ip->out1();
                ip->out1_ = p;
            } else {
                l.head = ip->out();
                ip->set_out(p);
            }
        }
    }

    // Appends two patch lists and returns result.
    static PatchList Append(Prog::Inst *inst0, PatchList l1, PatchList l2) {
        if (l1.head == 0)
            return l2;
        if (l2.head == 0)
            return l1;
        Prog::Inst *ip = &inst0[l1.tail >> 1];
        if (l1.tail & 1)
            ip->out1_ = l2.head;
        else
            ip->set_out(l2.head);
        return {l1.head, l2.tail};
    }

    uint32_t head;
    uint32_t tail; // for constant-time append
};

static const PatchList kNullPatchList = {0, 0};

// Compiled program fragment.
struct Frag {
    uint32_t begin;
    PatchList end;
    bool nullable;

    Frag() : begin(0), end(kNullPatchList), nullable(false) {}
    Frag(uint32_t begin, PatchList end, bool nullable) : begin(begin), end(end), nullable(nullable) {}
};

// Input encodings.
enum Encoding {
    kEncodingUTF8 = 1, // UTF-8 (0-10FFFF)
    kEncodingLatin1,   // Latin-1 (0-FF)
};

class Compiler : public Regexp::Walker<Frag> {
public:
    explicit Compiler();
    ~Compiler();

    // Compiles Regexp to a new Prog.
    // Caller is responsible for deleting Prog when finished with it.
    // If reversed is true, compiles for walking over the input
    // string backward (reverses all concatenations).
    static Prog *Compile(Regexp *re, bool reversed, int64_t max_mem);

    // Compiles alternation of all the re to a new Prog.
    // Each re has a match with an id equal to its index in the vector.
    static Prog *CompileSet(Regexp *re, RE2::Anchor anchor, int64_t max_mem);

    // Interface for Regexp::Walker, which helps traverse the Regexp.
    // The walk is purely post-recursive: given the machines for the
    // children, PostVisit combines them to create the machine for
    // the current node.  The child_args are Frags.
    // The Compiler traverses the Regexp parse tree, visiting
    // each node in depth-first order.  It invokes PreVisit before
    // visiting the node's children and PostVisit after visiting
    // the children.
    Frag PreVisit(Regexp *re, Frag parent_arg, bool *stop);
    Frag PostVisit(Regexp *re, Frag parent_arg, Frag pre_arg, Frag *child_args, int nchild_args);
    Frag ShortVisit(Regexp *re, Frag parent_arg);
    Frag Copy(Frag arg);

    // Given fragment a, returns a+ or a+?; a* or a*?; a? or a??
    Frag Plus(Frag a, bool nongreedy);
    Frag Star(Frag a, bool nongreedy);
    Frag Quest(Frag a, bool nongreedy);

    // Given fragment a, returns (a) capturing as \n.
    Frag Capture(Frag a, int n);

    // Given fragments a and b, returns ab; a|b
    Frag Cat(Frag a, Frag b);
    Frag Alt(Frag a, Frag b);

    // Returns a fragment that can't match anything.
    Frag NoMatch();

    // Returns a fragment that matches the empty string.
    Frag Match(int32_t id);

    // Returns a no-op fragment.
    Frag Nop();

    // Returns a fragment matching the byte range lo-hi.
    Frag ByteRange(int lo, int hi, bool foldcase);

    // Returns a fragment matching an empty-width special op.
    Frag EmptyWidth(EmptyOp op);

    // Adds n instructions to the program.
    // Returns the index of the first one.
    // Returns -1 if no more instructions are available.
    int AllocInst(int n);

    // Rune range compiler.

    // Begins a new alternation.
    void BeginRange();

    // Adds a fragment matching the rune range lo-hi.
    void AddRuneRange(Rune lo, Rune hi, bool foldcase);
    void AddRuneRangeLatin1(Rune lo, Rune hi, bool foldcase);
    void AddRuneRangeUTF8(Rune lo, Rune hi, bool foldcase);
    void Add_80_10ffff();

    // New suffix that matches the byte range lo-hi, then goes to next.
    int UncachedRuneByteSuffix(uint8_t lo, uint8_t hi, bool foldcase, int next);
    int CachedRuneByteSuffix(uint8_t lo, uint8_t hi, bool foldcase, int next);

    // Returns true iff the suffix is cached.
    bool IsCachedRuneByteSuffix(int id);

    // Adds a suffix to alternation.
    void AddSuffix(int id);

    // Adds a suffix to the trie starting from the given root node.
    // Returns zero iff allocating an instruction fails. Otherwise, returns
    // the current root node, which might be different from what was given.
    int AddSuffixRecursive(int root, int id);

    // Finds the trie node for the given suffix. Returns a Frag in order to
    // distinguish between pointing at the root node directly (end.head == 0)
    // and pointing at an Alt's out1 or out (end.head&1 == 1 or 0, respectively).
    Frag FindByteRange(int root, int id);

    // Compares two ByteRanges and returns true iff they are equal.
    bool ByteRangeEqual(int id1, int id2);

    // Returns the alternation of all the added suffixes.
    Frag EndRange();

    // Single rune.
    Frag Literal(Rune r, bool foldcase);

    void Setup(Regexp::ParseFlags flags, int64_t max_mem, RE2::Anchor anchor);
    Prog *Finish(Regexp *re);

    // Returns .* where dot = any byte
    Frag DotStar();

private:
    Prog *prog_;        // Program being built.
    bool failed_;       // Did we give up compiling?
    Encoding encoding_; // Input encoding
    bool reversed_;     // Should program run backward over text?

    PODArray<Prog::Inst> inst_;
    int ninst_;     // Number of instructions used.
    int max_ninst_; // Maximum number of instructions.

    int64_t max_mem_; // Total memory budget.

    std::unordered_map<uint64_t, int> rune_cache_;
    Frag rune_range_;

    RE2::Anchor anchor_; // anchor mode for RE2::Set

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

Compiler::Compiler() {
    prog_ = new Prog();
    failed_ = false;
    encoding_ = kEncodingUTF8;
    reversed_ = false;
    ninst_ = 0;
    max_ninst_ = 1; // make AllocInst for fail instruction okay
    max_mem_ = 0;
    int fail = AllocInst(1);
    inst_[fail].InitFail();
    max_ninst_ = 0; // Caller must change
}

Compiler::~Compiler() { delete prog_; }

int Compiler::AllocInst(int n) {
    if (failed_ || ninst_ + n > max_ninst_) {
        failed_ = true;
        return -1;
    }

    if (ninst_ + n > inst_.size()) {
        int cap = inst_.size();
        if (cap == 0)
            cap = 8;
        while (ninst_ + n > cap)
            cap *= 2;
        PODArray<Prog::Inst> inst(cap);
        if (inst_.data() != NULL)
            memmove(inst.data(), inst_.data(), ninst_ * sizeof inst_[0]);
        memset(inst.data() + ninst_, 0, (cap - ninst_) * sizeof inst_[0]);
        inst_ = std::move(inst);
    }
    int id = ninst_;
    ninst_ += n;
    return id;
}

// These routines are somewhat hard to visualize in text --
// see http://swtch.com/~rsc/regexp/regexp1.html for
// pictures explaining what is going on here.

// Returns an unmatchable fragment.
Frag Compiler::NoMatch() { return Frag(); }

// Is a an unmatchable fragment?
static bool IsNoMatch(Frag a) { return a.begin == 0; }

// Given fragments a and b, returns fragment for ab.
Frag Compiler::Cat(Frag a, Frag b) {
    if (IsNoMatch(a) || IsNoMatch(b))
        return NoMatch();

    // Elide no-op.
    Prog::Inst *begin = &inst_[a.begin];
    if (begin->opcode() == kInstNop && a.end.head == (a.begin << 1) && begin->out() == 0) {
        // in case refs to a somewhere
        PatchList::Patch(inst_.data(), a.end, b.begin);
        return b;
    }

    // To run backward over string, reverse all concatenations.
    if (reversed_) {
        PatchList::Patch(inst_.data(), b.end, a.begin);
        return Frag(b.begin, a.end, b.nullable && a.nullable);
    }

    PatchList::Patch(inst_.data(), a.end, b.begin);
    return Frag(a.begin, b.end, a.nullable && b.nullable);
}

// Given fragments for a and b, returns fragment for a|b.
Frag Compiler::Alt(Frag a, Frag b) {
    // Special case for convenience in loops.
    if (IsNoMatch(a))
        return b;
    if (IsNoMatch(b))
        return a;

    int id = AllocInst(1);
    if (id < 0)
        return NoMatch();

    inst_[id].InitAlt(a.begin, b.begin);
    return Frag(id, PatchList::Append(inst_.data(), a.end, b.end), a.nullable || b.nullable);
}

// When capturing submatches in like-Perl mode, a kOpAlt Inst
// treats out_ as the first choice, out1_ as the second.
//
// For *, +, and ?, if out_ causes another repetition,
// then the operator is greedy.  If out1_ is the repetition
// (and out_ moves forward), then the operator is non-greedy.

// Given a fragment for a, returns a fragment for a+ or a+? (if nongreedy)
Frag Compiler::Plus(Frag a, bool nongreedy) {
    int id = AllocInst(1);
    if (id < 0)
        return NoMatch();
    PatchList pl;
    if (nongreedy) {
        inst_[id].InitAlt(0, a.begin);
        pl = PatchList::Mk(id << 1);
    } else {
        inst_[id].InitAlt(a.begin, 0);
        pl = PatchList::Mk((id << 1) | 1);
    }
    PatchList::Patch(inst_.data(), a.end, id);
    return Frag(a.begin, pl, a.nullable);
}

// Given a fragment for a, returns a fragment for a* or a*? (if nongreedy)
Frag Compiler::Star(Frag a, bool nongreedy) {
    // When the subexpression is nullable, one Alt isn't enough to guarantee
    // correct priority ordering within the transitive closure. The simplest
    // solution is to handle it as (a+)? instead, which adds the second Alt.
    if (a.nullable)
        return Quest(Plus(a, nongreedy), nongreedy);

    int id = AllocInst(1);
    if (id < 0)
        return NoMatch();
    PatchList pl;
    if (nongreedy) {
        inst_[id].InitAlt(0, a.begin);
        pl = PatchList::Mk(id << 1);
    } else {
        inst_[id].InitAlt(a.begin, 0);
        pl = PatchList::Mk((id << 1) | 1);
    }
    PatchList::Patch(inst_.data(), a.end, id);
    return Frag(id, pl, true);
}

// Given a fragment for a, returns a fragment for a? or a?? (if nongreedy)
Frag Compiler::Quest(Frag a, bool nongreedy) {
    if (IsNoMatch(a))
        return Nop();
    int id = AllocInst(1);
    if (id < 0)
        return NoMatch();
    PatchList pl;
    if (nongreedy) {
        inst_[id].InitAlt(0, a.begin);
        pl = PatchList::Mk(id << 1);
    } else {
        inst_[id].InitAlt(a.begin, 0);
        pl = PatchList::Mk((id << 1) | 1);
    }
    return Frag(id, PatchList::Append(inst_.data(), pl, a.end), true);
}

// Returns a fragment for the byte range lo-hi.
Frag Compiler::ByteRange(int lo, int hi, bool foldcase) {
    int id = AllocInst(1);
    if (id < 0)
        return NoMatch();
    inst_[id].InitByteRange(lo, hi, foldcase, 0);
    return Frag(id, PatchList::Mk(id << 1), false);
}

// Returns a no-op fragment.  Sometimes unavoidable.
Frag Compiler::Nop() {
    int id = AllocInst(1);
    if (id < 0)
        return NoMatch();
    inst_[id].InitNop(0);
    return Frag(id, PatchList::Mk(id << 1), true);
}

// Returns a fragment that signals a match.
Frag Compiler::Match(int32_t match_id) {
    int id = AllocInst(1);
    if (id < 0)
        return NoMatch();
    inst_[id].InitMatch(match_id);
    return Frag(id, kNullPatchList, false);
}

// Returns a fragment matching a particular empty-width op (like ^ or $)
Frag Compiler::EmptyWidth(EmptyOp empty) {
    int id = AllocInst(1);
    if (id < 0)
        return NoMatch();
    inst_[id].InitEmptyWidth(empty, 0);
    return Frag(id, PatchList::Mk(id << 1), true);
}

// Given a fragment a, returns a fragment with capturing parens around a.
Frag Compiler::Capture(Frag a, int n) {
    if (IsNoMatch(a))
        return NoMatch();
    int id = AllocInst(2);
    if (id < 0)
        return NoMatch();
    inst_[id].InitCapture(2 * n, a.begin);
    inst_[id + 1].InitCapture(2 * n + 1, 0);
    PatchList::Patch(inst_.data(), a.end, id + 1);

    return Frag(id, PatchList::Mk((id + 1) << 1), a.nullable);
}

// A Rune is a name for a Unicode code point.
// Returns maximum rune encoded by UTF-8 sequence of length len.
static int MaxRune(int len) {
    int b; // number of Rune bits in len-byte UTF-8 sequence (len < UTFmax)
    if (len == 1)
        b = 7;
    else
        b = 8 - (len + 1) + 6 * (len - 1);
    return (1 << b) - 1; // maximum Rune for b bits.
}

// The rune range compiler caches common suffix fragments,
// which are very common in UTF-8 (e.g., [80-bf]).
// The fragment suffixes are identified by their start
// instructions.  NULL denotes the eventual end match.
// The Frag accumulates in rune_range_.  Caching common
// suffixes reduces the UTF-8 "." from 32 to 24 instructions,
// and it reduces the corresponding one-pass NFA from 16 nodes to 8.

void Compiler::BeginRange() {
    rune_cache_.clear();
    rune_range_.begin = 0;
    rune_range_.end = kNullPatchList;
}

int Compiler::UncachedRuneByteSuffix(uint8_t lo, uint8_t hi, bool foldcase, int next) {
    Frag f = ByteRange(lo, hi, foldcase);
    if (next != 0) {
        PatchList::Patch(inst_.data(), f.end, next);
    } else {
        rune_range_.end = PatchList::Append(inst_.data(), rune_range_.end, f.end);
    }
    return f.begin;
}

static uint64_t MakeRuneCacheKey(uint8_t lo, uint8_t hi, bool foldcase, int next) {
    return (uint64_t)next << 17 | (uint64_t)lo << 9 | (uint64_t)hi << 1 | (uint64_t)foldcase;
}

int Compiler::CachedRuneByteSuffix(uint8_t lo, uint8_t hi, bool foldcase, int next) {
    uint64_t key = MakeRuneCacheKey(lo, hi, foldcase, next);
    std::unordered_map<uint64_t, int>::const_iterator it = rune_cache_.find(key);
    if (it != rune_cache_.end())
        return it->second;
    int id = UncachedRuneByteSuffix(lo, hi, foldcase, next);
    rune_cache_[key] = id;
    return id;
}

bool Compiler::IsCachedRuneByteSuffix(int id) {
    uint8_t lo = inst_[id].byte_range.lo_;
    uint8_t hi = inst_[id].byte_range.hi_;
    bool foldcase = inst_[id].foldcase() != 0;
    int next = inst_[id].out();

    uint64_t key = MakeRuneCacheKey(lo, hi, foldcase, next);
    return rune_cache_.find(key) != rune_cache_.end();
}

void Compiler::AddSuffix(int id) {
    if (failed_)
        return;

    if (rune_range_.begin == 0) {
        rune_range_.begin = id;
        return;
    }

    if (encoding_ == kEncodingUTF8) {
        // Build a trie in order to reduce fanout.
        rune_range_.begin = AddSuffixRecursive(rune_range_.begin, id);
        return;
    }

    int alt = AllocInst(1);
    if (alt < 0) {
        rune_range_.begin = 0;
        return;
    }
    inst_[alt].InitAlt(rune_range_.begin, id);
    rune_range_.begin = alt;
}

int Compiler::AddSuffixRecursive(int root, int id) {
    DCHECK(inst_[root].opcode() == kInstAlt || inst_[root].opcode() == kInstByteRange);

    Frag f = FindByteRange(root, id);
    if (IsNoMatch(f)) {
        int alt = AllocInst(1);
        if (alt < 0)
            return 0;
        inst_[alt].InitAlt(root, id);
        return alt;
    }

    int br;
    if (f.end.head == 0)
        br = root;
    else if (f.end.head & 1)
        br = inst_[f.begin].out1();
    else
        br = inst_[f.begin].out();

    if (IsCachedRuneByteSuffix(br)) {
        // We can't fiddle with cached suffixes, so make a clone of the head.
        int byterange = AllocInst(1);
        if (byterange < 0)
            return 0;
        inst_[byterange].InitByteRange(inst_[br].lo(), inst_[br].hi(), inst_[br].foldcase(), inst_[br].out());

        // Ensure that the parent points to the clone, not to the original.
        // Note that this could leave the head unreachable except via the cache.
        br = byterange;
        if (f.end.head == 0)
            root = br;
        else if (f.end.head & 1)
            inst_[f.begin].out1_ = br;
        else
            inst_[f.begin].set_out(br);
    }

    int out = inst_[id].out();
    if (!IsCachedRuneByteSuffix(id)) {
        // The head should be the instruction most recently allocated, so free it
        // instead of leaving it unreachable.
        DCHECK_EQ(id, ninst_ - 1);
        inst_[id].out_opcode_ = 0;
        inst_[id].out1_ = 0;
        ninst_--;
    }

    out = AddSuffixRecursive(inst_[br].out(), out);
    if (out == 0)
        return 0;

    inst_[br].set_out(out);
    return root;
}

bool Compiler::ByteRangeEqual(int id1, int id2) {
    return inst_[id1].lo() == inst_[id2].lo() && inst_[id1].hi() == inst_[id2].hi() && inst_[id1].foldcase() == inst_[id2].foldcase();
}

Frag Compiler::FindByteRange(int root, int id) {
    if (inst_[root].opcode() == kInstByteRange) {
        if (ByteRangeEqual(root, id))
            return Frag(root, kNullPatchList, false);
        else
            return NoMatch();
    }

    while (inst_[root].opcode() == kInstAlt) {
        int out1 = inst_[root].out1();
        if (ByteRangeEqual(out1, id))
            return Frag(root, PatchList::Mk((root << 1) | 1), false);

        // CharClass is a sorted list of ranges, so if out1 of the root Alt wasn't
        // what we're looking for, then we can stop immediately. Unfortunately, we
        // can't short-circuit the search in reverse mode.
        if (!reversed_)
            return NoMatch();

        int out = inst_[root].out();
        if (inst_[out].opcode() == kInstAlt)
            root = out;
        else if (ByteRangeEqual(out, id))
            return Frag(root, PatchList::Mk(root << 1), false);
        else
            return NoMatch();
    }

    LOG(DFATAL) << "should never happen";
    return NoMatch();
}

Frag Compiler::EndRange() { return rune_range_; }

// Converts rune range lo-hi into a fragment that recognizes
// the bytes that would make up those runes in the current
// encoding (Latin 1 or UTF-8).
// This lets the machine work byte-by-byte even when
// using multibyte encodings.

void Compiler::AddRuneRange(Rune lo, Rune hi, bool foldcase) {
    switch (encoding_) {
        default:
        case kEncodingUTF8:
            AddRuneRangeUTF8(lo, hi, foldcase);
            break;
        case kEncodingLatin1:
            AddRuneRangeLatin1(lo, hi, foldcase);
            break;
    }
}

void Compiler::AddRuneRangeLatin1(Rune lo, Rune hi, bool foldcase) {
    // Latin-1 is easy: runes *are* bytes.
    if (lo > hi || lo > 0xFF)
        return;
    if (hi > 0xFF)
        hi = 0xFF;
    AddSuffix(UncachedRuneByteSuffix(static_cast<uint8_t>(lo), static_cast<uint8_t>(hi), foldcase, 0));
}

void Compiler::Add_80_10ffff() {
    // The 80-10FFFF (Runeself-Runemax) rune range occurs frequently enough
    // (for example, for /./ and /[^a-z]/) that it is worth simplifying: by
    // permitting overlong encodings in E0 and F0 sequences and code points
    // over 10FFFF in F4 sequences, the size of the bytecode and the number
    // of equivalence classes are reduced significantly.
    int id;
    if (reversed_) {
        // Prefix factoring matters, but we don't have to handle it here
        // because the rune range trie logic takes care of that already.
        id = UncachedRuneByteSuffix(0xC2, 0xDF, false, 0);
        id = UncachedRuneByteSuffix(0x80, 0xBF, false, id);
        AddSuffix(id);

        id = UncachedRuneByteSuffix(0xE0, 0xEF, false, 0);
        id = UncachedRuneByteSuffix(0x80, 0xBF, false, id);
        id = UncachedRuneByteSuffix(0x80, 0xBF, false, id);
        AddSuffix(id);

        id = UncachedRuneByteSuffix(0xF0, 0xF4, false, 0);
        id = UncachedRuneByteSuffix(0x80, 0xBF, false, id);
        id = UncachedRuneByteSuffix(0x80, 0xBF, false, id);
        id = UncachedRuneByteSuffix(0x80, 0xBF, false, id);
        AddSuffix(id);
    } else {
        // Suffix factoring matters - and we do have to handle it here.
        int cont1 = UncachedRuneByteSuffix(0x80, 0xBF, false, 0);
        id = UncachedRuneByteSuffix(0xC2, 0xDF, false, cont1);
        AddSuffix(id);

        int cont2 = UncachedRuneByteSuffix(0x80, 0xBF, false, cont1);
        id = UncachedRuneByteSuffix(0xE0, 0xEF, false, cont2);
        AddSuffix(id);

        int cont3 = UncachedRuneByteSuffix(0x80, 0xBF, false, cont2);
        id = UncachedRuneByteSuffix(0xF0, 0xF4, false, cont3);
        AddSuffix(id);
    }
}

void Compiler::AddRuneRangeUTF8(Rune lo, Rune hi, bool foldcase) {
    if (lo > hi)
        return;

    // Pick off 80-10FFFF as a common special case.
    if (lo == 0x80 && hi == 0x10ffff) {
        Add_80_10ffff();
        return;
    }

    // Split range into same-length sized ranges.
    for (int i = 1; i < UTFmax; i++) {
        Rune max = MaxRune(i);
        if (lo <= max && max < hi) {
            AddRuneRangeUTF8(lo, max, foldcase);
            AddRuneRangeUTF8(max + 1, hi, foldcase);
            return;
        }
    }

    // ASCII range is always a special case.
    if (hi < Runeself) {
        AddSuffix(UncachedRuneByteSuffix(static_cast<uint8_t>(lo), static_cast<uint8_t>(hi), foldcase, 0));
        return;
    }

    // Split range into sections that agree on leading bytes.
    for (int i = 1; i < UTFmax; i++) {
        uint32_t m = (1 << (6 * i)) - 1; // last i bytes of a UTF-8 sequence
        if ((lo & ~m) != (hi & ~m)) {
            if ((lo & m) != 0) {
                AddRuneRangeUTF8(lo, lo | m, foldcase);
                AddRuneRangeUTF8((lo | m) + 1, hi, foldcase);
                return;
            }
            if ((hi & m) != m) {
                AddRuneRangeUTF8(lo, (hi & ~m) - 1, foldcase);
                AddRuneRangeUTF8(hi & ~m, hi, foldcase);
                return;
            }
        }
    }

    // Finally.  Generate byte matching equivalent for lo-hi.
    uint8_t ulo[UTFmax], uhi[UTFmax];
    int n = runetochar(reinterpret_cast<char *>(ulo), &lo);
    int m = runetochar(reinterpret_cast<char *>(uhi), &hi);
    (void)m; // USED(m)
    DCHECK_EQ(n, m);

    // The logic below encodes this thinking:
    //
    // 1. When we have built the whole suffix, we know that it cannot
    // possibly be a suffix of anything longer: in forward mode, nothing
    // else can occur before the leading byte; in reverse mode, nothing
    // else can occur after the last continuation byte or else the leading
    // byte would have to change. Thus, there is no benefit to caching
    // the first byte of the suffix whereas there is a cost involved in
    // cloning it if it begins a common prefix, which is fairly likely.
    //
    // 2. Conversely, the last byte of the suffix cannot possibly be a
    // prefix of anything because next == 0, so we will never want to
    // clone it, but it is fairly likely to be a common suffix. Perhaps
    // more so in reverse mode than in forward mode because the former is
    // "converging" towards lower entropy, but caching is still worthwhile
    // for the latter in cases such as 80-BF.
    //
    // 3. Handling the bytes between the first and the last is less
    // straightforward and, again, the approach depends on whether we are
    // "converging" towards lower entropy: in forward mode, a single byte
    // is unlikely to be part of a common suffix whereas a byte range
    // is more likely so; in reverse mode, a byte range is unlikely to
    // be part of a common suffix whereas a single byte is more likely
    // so. The same benefit versus cost argument applies here.
    int id = 0;
    if (reversed_) {
        for (int i = 0; i < n; i++) {
            // In reverse UTF-8 mode: cache the leading byte; don't cache the last
            // continuation byte; cache anything else iff it's a single byte (XX-XX).
            if (i == 0 || (ulo[i] == uhi[i] && i != n - 1))
                id = CachedRuneByteSuffix(ulo[i], uhi[i], false, id);
            else
                id = UncachedRuneByteSuffix(ulo[i], uhi[i], false, id);
        }
    } else {
        for (int i = n - 1; i >= 0; i--) {
            // In forward UTF-8 mode: don't cache the leading byte; cache the last
            // continuation byte; cache anything else iff it's a byte range (XX-YY).
            if (i == n - 1 || (ulo[i] < uhi[i] && i != 0))
                id = CachedRuneByteSuffix(ulo[i], uhi[i], false, id);
            else
                id = UncachedRuneByteSuffix(ulo[i], uhi[i], false, id);
        }
    }
    AddSuffix(id);
}

// Should not be called.
Frag Compiler::Copy(Frag arg) {
    // We're using WalkExponential; there should be no copying.
    failed_ = true;
    LOG(DFATAL) << "Compiler::Copy called!";
    return NoMatch();
}

// Visits a node quickly; called once WalkExponential has
// decided to cut this walk short.
Frag Compiler::ShortVisit(Regexp *re, Frag) {
    failed_ = true;
    return NoMatch();
}

// Called before traversing a node's children during the walk.
Frag Compiler::PreVisit(Regexp *re, Frag, bool *stop) {
    // Cut off walk if we've already failed.
    if (failed_)
        *stop = true;

    return Frag(); // not used by caller
}

Frag Compiler::Literal(Rune r, bool foldcase) {
    switch (encoding_) {
        default:
            return Frag();

        case kEncodingLatin1:
            return ByteRange(r, r, foldcase);

        case kEncodingUTF8: {
            if (r < Runeself) // Make common case fast.
                return ByteRange(r, r, foldcase);
            uint8_t buf[UTFmax];
            int n = runetochar(reinterpret_cast<char *>(buf), &r);
            Frag f = ByteRange((uint8_t)buf[0], buf[0], false);
            for (int i = 1; i < n; i++)
                f = Cat(f, ByteRange((uint8_t)buf[i], buf[i], false));
            return f;
        }
    }
}

// Called after traversing the node's children during the walk.
// Given their frags, build and return the frag for this re.
Frag Compiler::PostVisit(Regexp *re, Frag, Frag, Frag *child_frags, int nchild_frags) {
    // If a child failed, don't bother going forward, especially
    // since the child_frags might contain Frags with NULLs in them.
    if (failed_)
        return NoMatch();

    // Given the child fragments, return the fragment for this node.
    switch (re->op()) {
        case kRegexpRepeat:
            // Should not see; code at bottom of function will print error
            break;

        case kRegexpNoMatch:
            return NoMatch();

        case kRegexpEmptyMatch:
            return Nop();

        case kRegexpHaveMatch: {
            Frag f = Match(re->match_id());
            if (anchor_ == RE2::ANCHOR_BOTH) {
                // Append \z or else the subexpression will effectively be unanchored.
                // Complemented by the UNANCHORED case in CompileSet().
                f = Cat(EmptyWidth(kEmptyEndText), f);
            }
            return f;
        }

        case kRegexpConcat: {
            Frag f = child_frags[0];
            for (int i = 1; i < nchild_frags; i++)
                f = Cat(f, child_frags[i]);
            return f;
        }

        case kRegexpAlternate: {
            Frag f = child_frags[0];
            for (int i = 1; i < nchild_frags; i++)
                f = Alt(f, child_frags[i]);
            return f;
        }

        case kRegexpStar:
            return Star(child_frags[0], (re->parse_flags() & Regexp::NonGreedy) != 0);

        case kRegexpPlus:
            return Plus(child_frags[0], (re->parse_flags() & Regexp::NonGreedy) != 0);

        case kRegexpQuest:
            return Quest(child_frags[0], (re->parse_flags() & Regexp::NonGreedy) != 0);

        case kRegexpLiteral:
            return Literal(re->rune(), (re->parse_flags() & Regexp::FoldCase) != 0);

        case kRegexpLiteralString: {
            // Concatenation of literals.
            if (re->nrunes() == 0)
                return Nop();
            Frag f;
            for (int i = 0; i < re->nrunes(); i++) {
                Frag f1 = Literal(re->runes()[i], (re->parse_flags() & Regexp::FoldCase) != 0);
                if (i == 0)
                    f = f1;
                else
                    f = Cat(f, f1);
            }
            return f;
        }

        case kRegexpAnyChar:
            BeginRange();
            AddRuneRange(0, Runemax, false);
            return EndRange();

        case kRegexpAnyByte:
            return ByteRange(0x00, 0xFF, false);

        case kRegexpCharClass: {
            CharClass *cc = re->cc();
            if (cc->empty()) {
                // This can't happen.
                failed_ = true;
                LOG(DFATAL) << "No ranges in char class";
                return NoMatch();
            }

            // ASCII case-folding optimization: if the char class
            // behaves the same on A-Z as it does on a-z,
            // discard any ranges wholly contained in A-Z
            // and mark the other ranges as foldascii.
            // This reduces the size of a program for
            // (?i)abc from 3 insts per letter to 1 per letter.
            bool foldascii = cc->FoldsASCII();

            // Character class is just a big OR of the different
            // character ranges in the class.
            BeginRange();
            for (CharClass::iterator i = cc->begin(); i != cc->end(); ++i) {
                // ASCII case-folding optimization (see above).
                if (foldascii && 'A' <= i->lo && i->hi <= 'Z')
                    continue;

                // If this range contains all of A-Za-z or none of it,
                // the fold flag is unnecessary; don't bother.
                bool fold = foldascii;
                if ((i->lo <= 'A' && 'z' <= i->hi) || i->hi < 'A' || 'z' < i->lo || ('Z' < i->lo && i->hi < 'a'))
                    fold = false;

                AddRuneRange(i->lo, i->hi, fold);
            }
            return EndRange();
        }

        case kRegexpCapture:
            // If this is a non-capturing parenthesis -- (?:foo) --
            // just use the inner expression.
            if (re->cap() < 0)
                return child_frags[0];
            return Capture(child_frags[0], re->cap());

        case kRegexpBeginLine:
            return EmptyWidth(reversed_ ? kEmptyEndLine : kEmptyBeginLine);

        case kRegexpEndLine:
            return EmptyWidth(reversed_ ? kEmptyBeginLine : kEmptyEndLine);

        case kRegexpBeginText:
            return EmptyWidth(reversed_ ? kEmptyEndText : kEmptyBeginText);

        case kRegexpEndText:
            return EmptyWidth(reversed_ ? kEmptyBeginText : kEmptyEndText);

        case kRegexpWordBoundary:
            return EmptyWidth(kEmptyWordBoundary);

        case kRegexpNoWordBoundary:
            return EmptyWidth(kEmptyNonWordBoundary);
    }
    failed_ = true;
    LOG(DFATAL) << "Missing case in Compiler: " << re->op();
    return NoMatch();
}

// Is this regexp required to start at the beginning of the text?
// Only approximate; can return false for complicated regexps like (\Aa|\Ab),
// but handles (\A(a|b)).  Could use the Walker to write a more exact one.
static bool IsAnchorStart(Regexp **pre, int depth) {
    Regexp *re = *pre;
    Regexp *sub;
    // The depth limit makes sure that we don't overflow
    // the stack on a deeply nested regexp.  As the comment
    // above says, IsAnchorStart is conservative, so returning
    // a false negative is okay.  The exact limit is somewhat arbitrary.
    if (re == NULL || depth >= 4)
        return false;
    switch (re->op()) {
        default:
            break;
        case kRegexpConcat:
            if (re->nsub() > 0) {
                sub = re->sub()[0]->Incref();
                if (IsAnchorStart(&sub, depth + 1)) {
                    PODArray<Regexp *> subcopy(re->nsub());
                    subcopy[0] = sub; // already have reference
                    for (int i = 1; i < re->nsub(); i++)
                        subcopy[i] = re->sub()[i]->Incref();
                    *pre = Regexp::Concat(subcopy.data(), re->nsub(), re->parse_flags());
                    re->Decref();
                    return true;
                }
                sub->Decref();
            }
            break;
        case kRegexpCapture:
            sub = re->sub()[0]->Incref();
            if (IsAnchorStart(&sub, depth + 1)) {
                *pre = Regexp::Capture(sub, re->parse_flags(), re->cap());
                re->Decref();
                return true;
            }
            sub->Decref();
            break;
        case kRegexpBeginText:
            *pre = Regexp::LiteralString(NULL, 0, re->parse_flags());
            re->Decref();
            return true;
    }
    return false;
}

// Is this regexp required to start at the end of the text?
// Only approximate; can return false for complicated regexps like (a\z|b\z),
// but handles ((a|b)\z).  Could use the Walker to write a more exact one.
static bool IsAnchorEnd(Regexp **pre, int depth) {
    Regexp *re = *pre;
    Regexp *sub;
    // The depth limit makes sure that we don't overflow
    // the stack on a deeply nested regexp.  As the comment
    // above says, IsAnchorEnd is conservative, so returning
    // a false negative is okay.  The exact limit is somewhat arbitrary.
    if (re == NULL || depth >= 4)
        return false;
    switch (re->op()) {
        default:
            break;
        case kRegexpConcat:
            if (re->nsub() > 0) {
                sub = re->sub()[re->nsub() - 1]->Incref();
                if (IsAnchorEnd(&sub, depth + 1)) {
                    PODArray<Regexp *> subcopy(re->nsub());
                    subcopy[re->nsub() - 1] = sub; // already have reference
                    for (int i = 0; i < re->nsub() - 1; i++)
                        subcopy[i] = re->sub()[i]->Incref();
                    *pre = Regexp::Concat(subcopy.data(), re->nsub(), re->parse_flags());
                    re->Decref();
                    return true;
                }
                sub->Decref();
            }
            break;
        case kRegexpCapture:
            sub = re->sub()[0]->Incref();
            if (IsAnchorEnd(&sub, depth + 1)) {
                *pre = Regexp::Capture(sub, re->parse_flags(), re->cap());
                re->Decref();
                return true;
            }
            sub->Decref();
            break;
        case kRegexpEndText:
            *pre = Regexp::LiteralString(NULL, 0, re->parse_flags());
            re->Decref();
            return true;
    }
    return false;
}

void Compiler::Setup(Regexp::ParseFlags flags, int64_t max_mem, RE2::Anchor anchor) {
    if (flags & Regexp::Latin1)
        encoding_ = kEncodingLatin1;
    max_mem_ = max_mem;
    if (max_mem <= 0) {
        max_ninst_ = 100000; // more than enough
    } else if (static_cast<size_t>(max_mem) <= sizeof(Prog)) {
        // No room for anything.
        max_ninst_ = 0;
    } else {
        int64_t m = (max_mem - sizeof(Prog)) / sizeof(Prog::Inst);
        // Limit instruction count so that inst->id() fits nicely in an int.
        // SparseArray also assumes that the indices (inst->id()) are ints.
        // The call to WalkExponential uses 2*max_ninst_ below,
        // and other places in the code use 2 or 3 * prog->size().
        // Limiting to 2^24 should avoid overflow in those places.
        // (The point of allowing more than 32 bits of memory is to
        // have plenty of room for the DFA states, not to use it up
        // on the program.)
        if (m >= 1 << 24)
            m = 1 << 24;
        // Inst imposes its own limit (currently bigger than 2^24 but be safe).
        if (m > Prog::Inst::kMaxInst)
            m = Prog::Inst::kMaxInst;
        max_ninst_ = static_cast<int>(m);
    }
    anchor_ = anchor;
}

// Compiles re, returning program.
// Caller is responsible for deleting prog_.
// If reversed is true, compiles a program that expects
// to run over the input string backward (reverses all concatenations).
// The reversed flag is also recorded in the returned program.
Prog *Compiler::Compile(Regexp *re, bool reversed, int64_t max_mem) {
    Compiler c;
    c.Setup(re->parse_flags(), max_mem, RE2::UNANCHORED /* unused */);
    c.reversed_ = reversed;

    // Simplify to remove things like counted repetitions
    // and character classes like \d.
    Regexp *sre = re->Simplify();
    if (sre == NULL)
        return NULL;

    // Record whether prog is anchored, removing the anchors.
    // (They get in the way of other optimizations.)
    bool is_anchor_start = IsAnchorStart(&sre, 0);
    bool is_anchor_end = IsAnchorEnd(&sre, 0);

    // Generate fragment for entire regexp.
    Frag all = c.WalkExponential(sre, Frag(), 2 * c.max_ninst_);
    sre->Decref();
    if (c.failed_)
        return NULL;

    // Success!  Finish by putting Match node at end, and record start.
    // Turn off c.reversed_ (if it is set) to force the remaining concatenations
    // to behave normally.
    c.reversed_ = false;
    all = c.Cat(all, c.Match(0));

    c.prog_->set_reversed(reversed);
    if (c.prog_->reversed()) {
        c.prog_->set_anchor_start(is_anchor_end);
        c.prog_->set_anchor_end(is_anchor_start);
    } else {
        c.prog_->set_anchor_start(is_anchor_start);
        c.prog_->set_anchor_end(is_anchor_end);
    }

    c.prog_->set_start(all.begin);
    if (!c.prog_->anchor_start()) {
        // Also create unanchored version, which starts with a .*? loop.
        all = c.Cat(c.DotStar(), all);
    }
    c.prog_->set_start_unanchored(all.begin);

    // Hand ownership of prog_ to caller.
    return c.Finish(re);
}

Prog *Compiler::Finish(Regexp *re) {
    if (failed_)
        return NULL;

    if (prog_->start() == 0 && prog_->start_unanchored() == 0) {
        // No possible matches; keep Fail instruction only.
        ninst_ = 1;
    }

    // Hand off the array to Prog.
    prog_->inst_ = std::move(inst_);
    prog_->size_ = ninst_;

    prog_->Optimize();
    prog_->Flatten();
    prog_->ComputeByteMap();

    if (!prog_->reversed()) {
        std::string prefix;
        bool prefix_foldcase;
        if (re->RequiredPrefixForAccel(&prefix, &prefix_foldcase))
            prog_->ConfigurePrefixAccel(prefix, prefix_foldcase);
    }

    // Record remaining memory for DFA.
    if (max_mem_ <= 0) {
        prog_->set_dfa_mem(1 << 20);
    } else {
        int64_t m = max_mem_ - sizeof(Prog);
        m -= prog_->size_ * sizeof(Prog::Inst); // account for inst_
        if (prog_->CanBitState())
            m -= prog_->size_ * sizeof(uint16_t); // account for list_heads_
        if (m < 0)
            m = 0;
        prog_->set_dfa_mem(m);
    }

    Prog *p = prog_;
    prog_ = NULL;
    return p;
}

// Converts Regexp to Prog.
Prog *Regexp::CompileToProg(int64_t max_mem) { return Compiler::Compile(this, false, max_mem); }

Prog *Regexp::CompileToReverseProg(int64_t max_mem) { return Compiler::Compile(this, true, max_mem); }

Frag Compiler::DotStar() { return Star(ByteRange(0x00, 0xff, false), true); }

// Compiles RE set to Prog.
Prog *Compiler::CompileSet(Regexp *re, RE2::Anchor anchor, int64_t max_mem) {
    Compiler c;
    c.Setup(re->parse_flags(), max_mem, anchor);

    Regexp *sre = re->Simplify();
    if (sre == NULL)
        return NULL;

    Frag all = c.WalkExponential(sre, Frag(), 2 * c.max_ninst_);
    sre->Decref();
    if (c.failed_)
        return NULL;

    c.prog_->set_anchor_start(true);
    c.prog_->set_anchor_end(true);

    if (anchor == RE2::UNANCHORED) {
        // Prepend .* or else the expression will effectively be anchored.
        // Complemented by the ANCHOR_BOTH case in PostVisit().
        all = c.Cat(c.DotStar(), all);
    }
    c.prog_->set_start(all.begin);
    c.prog_->set_start_unanchored(all.begin);

    Prog *prog = c.Finish(re);
    if (prog == NULL)
        return NULL;

    // Make sure DFA has enough memory to operate,
    // since we're not going to fall back to the NFA.
    bool dfa_failed = false;
    StringPiece sp = "hello, world";
    prog->SearchDFA(sp, sp, Prog::kAnchored, Prog::kManyMatch, NULL, &dfa_failed, NULL);
    if (dfa_failed) {
        delete prog;
        return NULL;
    }

    return prog;
}

Prog *Prog::CompileSet(Regexp *re, RE2::Anchor anchor, int64_t max_mem) { return Compiler::CompileSet(re, anchor, max_mem); }

} // namespace re2
