// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// A simple interpreter for the Irregexp byte code.

#include "vm/regexp_interpreter.h"

#include "platform/unicode.h"
#include "vm/object.h"
#include "vm/regexp_assembler.h"
#include "vm/regexp_bytecodes.h"
#include "vm/unibrow-inl.h"
#include "vm/unibrow.h"

namespace dart {

DEFINE_FLAG(bool, trace_regexp_bytecodes, false, "trace_regexp_bytecodes");

typedef unibrow::Mapping<unibrow::Ecma262Canonicalize> Canonicalize;

template <typename Char>
static bool BackRefMatchesNoCase(Canonicalize* interp_canonicalize,
                                 intptr_t from,
                                 intptr_t current,
                                 intptr_t len,
                                 const String& subject,
                                 bool unicode);

template <>
bool BackRefMatchesNoCase<uint16_t>(Canonicalize* interp_canonicalize,
                                    intptr_t from,
                                    intptr_t current,
                                    intptr_t len,
                                    const String& subject,
                                    bool unicode) {
  Bool& ret = Bool::Handle();
  if (unicode) {
    ret = CaseInsensitiveCompareUTF16(subject.raw(), Smi::New(from),
                                      Smi::New(current), Smi::New(len));
  } else {
    ret = CaseInsensitiveCompareUCS2(subject.raw(), Smi::New(from),
                                     Smi::New(current), Smi::New(len));
  }
  return ret.value();
}

template <>
bool BackRefMatchesNoCase<uint8_t>(Canonicalize* interp_canonicalize,
                                   intptr_t from,
                                   intptr_t current,
                                   intptr_t len,
                                   const String& subject,
                                   bool unicode) {
  // For Latin1 characters the unicode flag makes no difference.
  for (int i = 0; i < len; i++) {
    unsigned int old_char = subject.CharAt(from++);
    unsigned int new_char = subject.CharAt(current++);
    if (old_char == new_char) continue;
    // Convert both characters to lower case.
    old_char |= 0x20;
    new_char |= 0x20;
    if (old_char != new_char) return false;
    // Not letters in the ASCII range and Latin-1 range.
    if (!(old_char - 'a' <= 'z' - 'a') &&
        !(old_char - 224 <= 254 - 224 && old_char != 247)) {
      return false;
    }
  }
  return true;
}

#ifdef DEBUG
static void TraceInterpreter(const uint8_t* code_base,
                             const uint8_t* pc,
                             int stack_depth,
                             int current_position,
                             uint32_t current_char,
                             int bytecode_length,
                             const char* bytecode_name) {
  if (FLAG_trace_regexp_bytecodes) {
    bool printable = (current_char < 127 && current_char >= 32);
    const char* format =
        printable
            ? "pc = %02x, sp = %d, curpos = %d, curchar = %08x (%c), bc = %s"
            : "pc = %02x, sp = %d, curpos = %d, curchar = %08x .%c., bc = %s";
    OS::PrintErr(format, pc - code_base, stack_depth, current_position,
                 current_char, printable ? current_char : '.', bytecode_name);
    for (int i = 0; i < bytecode_length; i++) {
      OS::PrintErr(", %02x", pc[i]);
    }
    OS::PrintErr(" ");
    for (int i = 1; i < bytecode_length; i++) {
      unsigned char b = pc[i];
      if (b < 127 && b >= 32) {
        OS::PrintErr("%c", b);
      } else {
        OS::PrintErr(".");
      }
    }
    OS::PrintErr("\n");
  }
}

#define BYTECODE(name)                                                         \
  case BC_##name:                                                              \
    TraceInterpreter(code_base, pc,                                            \
                     static_cast<int>(backtrack_sp - backtrack_stack_base),    \
                     current, current_char, BC_##name##_LENGTH, #name);
#else
#define BYTECODE(name) case BC_##name:
#endif

static int32_t Load32Aligned(const uint8_t* pc) {
  ASSERT((reinterpret_cast<intptr_t>(pc) & 3) == 0);
  return *reinterpret_cast<const int32_t*>(pc);
}

static int32_t Load16Aligned(const uint8_t* pc) {
  ASSERT((reinterpret_cast<intptr_t>(pc) & 1) == 0);
  return *reinterpret_cast<const uint16_t*>(pc);
}

// A simple abstraction over the backtracking stack used by the interpreter.
// This backtracking stack does not grow automatically, but it ensures that the
// the memory held by the stack is released or remembered in a cache if the
// matching terminates.
class BacktrackStack {
 public:
  explicit BacktrackStack(Zone* zone) {
    data_ = zone->Alloc<intptr_t>(kBacktrackStackSize);
  }

  intptr_t* data() const { return data_; }

  intptr_t max_size() const { return kBacktrackStackSize; }

 private:
  static const intptr_t kBacktrackStackSize = 1 << 16;

  intptr_t* data_;

  DISALLOW_COPY_AND_ASSIGN(BacktrackStack);
};

template <typename Char>
static IrregexpInterpreter::IrregexpResult RawMatch(const uint8_t* code_base,
                                                    const String& subject,
                                                    int32_t* registers,
                                                    intptr_t current,
                                                    uint32_t current_char,
                                                    Zone* zone) {
  const uint8_t* pc = code_base;
  // BacktrackStack ensures that the memory allocated for the backtracking stack
  // is returned to the system or cached if there is no stack being cached at
  // the moment.
  BacktrackStack backtrack_stack(zone);
  intptr_t* backtrack_stack_base = backtrack_stack.data();
  intptr_t* backtrack_sp = backtrack_stack_base;
  intptr_t backtrack_stack_space = backtrack_stack.max_size();

  // TODO(zerny): Optimize as single instance. V8 has this as an
  // isolate member.
  unibrow::Mapping<unibrow::Ecma262Canonicalize> canonicalize;

  intptr_t subject_length = subject.Length();

#ifdef DEBUG
  if (FLAG_trace_regexp_bytecodes) {
    OS::PrintErr("Start irregexp bytecode interpreter\n");
  }
#endif
  while (true) {
    int32_t insn = Load32Aligned(pc);
    switch (insn & BYTECODE_MASK) {
      BYTECODE(BREAK)
      UNREACHABLE();
      return IrregexpInterpreter::RE_FAILURE;
      BYTECODE(PUSH_CP)
      if (--backtrack_stack_space < 0) {
        return IrregexpInterpreter::RE_EXCEPTION;
      }
      *backtrack_sp++ = current;
      pc += BC_PUSH_CP_LENGTH;
      break;
      BYTECODE(PUSH_BT)
      if (--backtrack_stack_space < 0) {
        return IrregexpInterpreter::RE_EXCEPTION;
      }
      *backtrack_sp++ = Load32Aligned(pc + 4);
      pc += BC_PUSH_BT_LENGTH;
      break;
      BYTECODE(PUSH_REGISTER)
      if (--backtrack_stack_space < 0) {
        return IrregexpInterpreter::RE_EXCEPTION;
      }
      *backtrack_sp++ = registers[insn >> BYTECODE_SHIFT];
      pc += BC_PUSH_REGISTER_LENGTH;
      break;
      BYTECODE(SET_REGISTER)
      registers[insn >> BYTECODE_SHIFT] = Load32Aligned(pc + 4);
      pc += BC_SET_REGISTER_LENGTH;
      break;
      BYTECODE(ADVANCE_REGISTER)
      registers[insn >> BYTECODE_SHIFT] += Load32Aligned(pc + 4);
      pc += BC_ADVANCE_REGISTER_LENGTH;
      break;
      BYTECODE(SET_REGISTER_TO_CP)
      registers[insn >> BYTECODE_SHIFT] = current + Load32Aligned(pc + 4);
      pc += BC_SET_REGISTER_TO_CP_LENGTH;
      break;
      BYTECODE(SET_CP_TO_REGISTER)
      current = registers[insn >> BYTECODE_SHIFT];
      pc += BC_SET_CP_TO_REGISTER_LENGTH;
      break;
      BYTECODE(SET_REGISTER_TO_SP)
      registers[insn >> BYTECODE_SHIFT] =
          static_cast<int>(backtrack_sp - backtrack_stack_base);
      pc += BC_SET_REGISTER_TO_SP_LENGTH;
      break;
      BYTECODE(SET_SP_TO_REGISTER)
      backtrack_sp = backtrack_stack_base + registers[insn >> BYTECODE_SHIFT];
      backtrack_stack_space =
          backtrack_stack.max_size() -
          static_cast<int>(backtrack_sp - backtrack_stack_base);
      pc += BC_SET_SP_TO_REGISTER_LENGTH;
      break;
      BYTECODE(POP_CP)
      backtrack_stack_space++;
      --backtrack_sp;
      current = *backtrack_sp;
      pc += BC_POP_CP_LENGTH;
      break;
      BYTECODE(POP_BT)
      backtrack_stack_space++;
      --backtrack_sp;
      pc = code_base + *backtrack_sp;
      break;
      BYTECODE(POP_REGISTER)
      backtrack_stack_space++;
      --backtrack_sp;
      registers[insn >> BYTECODE_SHIFT] = *backtrack_sp;
      pc += BC_POP_REGISTER_LENGTH;
      break;
      BYTECODE(FAIL)
      return IrregexpInterpreter::RE_FAILURE;
      BYTECODE(SUCCEED)
      return IrregexpInterpreter::RE_SUCCESS;
      BYTECODE(ADVANCE_CP)
      current += insn >> BYTECODE_SHIFT;
      pc += BC_ADVANCE_CP_LENGTH;
      break;
      BYTECODE(GOTO)
      pc = code_base + Load32Aligned(pc + 4);
      break;
      BYTECODE(ADVANCE_CP_AND_GOTO)
      current += insn >> BYTECODE_SHIFT;
      pc = code_base + Load32Aligned(pc + 4);
      break;
      BYTECODE(CHECK_GREEDY)
      if (current == backtrack_sp[-1]) {
        backtrack_sp--;
        backtrack_stack_space++;
        pc = code_base + Load32Aligned(pc + 4);
      } else {
        pc += BC_CHECK_GREEDY_LENGTH;
      }
      break;
      BYTECODE(LOAD_CURRENT_CHAR) {
        int pos = current + (insn >> BYTECODE_SHIFT);
        if (pos < 0 || pos >= subject_length) {
          pc = code_base + Load32Aligned(pc + 4);
        } else {
          current_char = subject.CharAt(pos);
          pc += BC_LOAD_CURRENT_CHAR_LENGTH;
        }
        break;
      }
      BYTECODE(LOAD_CURRENT_CHAR_UNCHECKED) {
        int pos = current + (insn >> BYTECODE_SHIFT);
        current_char = subject.CharAt(pos);
        pc += BC_LOAD_CURRENT_CHAR_UNCHECKED_LENGTH;
        break;
      }
      BYTECODE(LOAD_2_CURRENT_CHARS) {
        int pos = current + (insn >> BYTECODE_SHIFT);
        if (pos + 2 > subject_length) {
          pc = code_base + Load32Aligned(pc + 4);
        } else {
          Char next = subject.CharAt(pos + 1);
          current_char =
              subject.CharAt(pos) | (next << (kBitsPerByte * sizeof(Char)));
          pc += BC_LOAD_2_CURRENT_CHARS_LENGTH;
        }
        break;
      }
      BYTECODE(LOAD_2_CURRENT_CHARS_UNCHECKED) {
        int pos = current + (insn >> BYTECODE_SHIFT);
        Char next = subject.CharAt(pos + 1);
        current_char =
            subject.CharAt(pos) | (next << (kBitsPerByte * sizeof(Char)));
        pc += BC_LOAD_2_CURRENT_CHARS_UNCHECKED_LENGTH;
        break;
      }
      BYTECODE(LOAD_4_CURRENT_CHARS) {
        ASSERT(sizeof(Char) == 1);
        int pos = current + (insn >> BYTECODE_SHIFT);
        if (pos + 4 > subject_length) {
          pc = code_base + Load32Aligned(pc + 4);
        } else {
          Char next1 = subject.CharAt(pos + 1);
          Char next2 = subject.CharAt(pos + 2);
          Char next3 = subject.CharAt(pos + 3);
          current_char = (subject.CharAt(pos) | (next1 << 8) | (next2 << 16) |
                          (next3 << 24));
          pc += BC_LOAD_4_CURRENT_CHARS_LENGTH;
        }
        break;
      }
      BYTECODE(LOAD_4_CURRENT_CHARS_UNCHECKED) {
        ASSERT(sizeof(Char) == 1);
        int pos = current + (insn >> BYTECODE_SHIFT);
        Char next1 = subject.CharAt(pos + 1);
        Char next2 = subject.CharAt(pos + 2);
        Char next3 = subject.CharAt(pos + 3);
        current_char = (subject.CharAt(pos) | (next1 << 8) | (next2 << 16) |
                        (next3 << 24));
        pc += BC_LOAD_4_CURRENT_CHARS_UNCHECKED_LENGTH;
        break;
      }
      BYTECODE(CHECK_4_CHARS) {
        uint32_t c = Load32Aligned(pc + 4);
        if (c == current_char) {
          pc = code_base + Load32Aligned(pc + 8);
        } else {
          pc += BC_CHECK_4_CHARS_LENGTH;
        }
        break;
      }
      BYTECODE(CHECK_CHAR) {
        uint32_t c = (insn >> BYTECODE_SHIFT);
        if (c == current_char) {
          pc = code_base + Load32Aligned(pc + 4);
        } else {
          pc += BC_CHECK_CHAR_LENGTH;
        }
        break;
      }
      BYTECODE(CHECK_NOT_4_CHARS) {
        uint32_t c = Load32Aligned(pc + 4);
        if (c != current_char) {
          pc = code_base + Load32Aligned(pc + 8);
        } else {
          pc += BC_CHECK_NOT_4_CHARS_LENGTH;
        }
        break;
      }
      BYTECODE(CHECK_NOT_CHAR) {
        uint32_t c = (insn >> BYTECODE_SHIFT);
        if (c != current_char) {
          pc = code_base + Load32Aligned(pc + 4);
        } else {
          pc += BC_CHECK_NOT_CHAR_LENGTH;
        }
        break;
      }
      BYTECODE(AND_CHECK_4_CHARS) {
        uint32_t c = Load32Aligned(pc + 4);
        if (c == (current_char & Load32Aligned(pc + 8))) {
          pc = code_base + Load32Aligned(pc + 12);
        } else {
          pc += BC_AND_CHECK_4_CHARS_LENGTH;
        }
        break;
      }
      BYTECODE(AND_CHECK_CHAR) {
        uint32_t c = (insn >> BYTECODE_SHIFT);
        if (c == (current_char & Load32Aligned(pc + 4))) {
          pc = code_base + Load32Aligned(pc + 8);
        } else {
          pc += BC_AND_CHECK_CHAR_LENGTH;
        }
        break;
      }
      BYTECODE(AND_CHECK_NOT_4_CHARS) {
        uint32_t c = Load32Aligned(pc + 4);
        if (c != (current_char & Load32Aligned(pc + 8))) {
          pc = code_base + Load32Aligned(pc + 12);
        } else {
          pc += BC_AND_CHECK_NOT_4_CHARS_LENGTH;
        }
        break;
      }
      BYTECODE(AND_CHECK_NOT_CHAR) {
        uint32_t c = (insn >> BYTECODE_SHIFT);
        if (c != (current_char & Load32Aligned(pc + 4))) {
          pc = code_base + Load32Aligned(pc + 8);
        } else {
          pc += BC_AND_CHECK_NOT_CHAR_LENGTH;
        }
        break;
      }
      BYTECODE(MINUS_AND_CHECK_NOT_CHAR) {
        uint32_t c = (insn >> BYTECODE_SHIFT);
        uint32_t minus = Load16Aligned(pc + 4);
        uint32_t mask = Load16Aligned(pc + 6);
        if (c != ((current_char - minus) & mask)) {
          pc = code_base + Load32Aligned(pc + 8);
        } else {
          pc += BC_MINUS_AND_CHECK_NOT_CHAR_LENGTH;
        }
        break;
      }
      BYTECODE(CHECK_CHAR_IN_RANGE) {
        uint32_t from = Load16Aligned(pc + 4);
        uint32_t to = Load16Aligned(pc + 6);
        if (from <= current_char && current_char <= to) {
          pc = code_base + Load32Aligned(pc + 8);
        } else {
          pc += BC_CHECK_CHAR_IN_RANGE_LENGTH;
        }
        break;
      }
      BYTECODE(CHECK_CHAR_NOT_IN_RANGE) {
        uint32_t from = Load16Aligned(pc + 4);
        uint32_t to = Load16Aligned(pc + 6);
        if (from > current_char || current_char > to) {
          pc = code_base + Load32Aligned(pc + 8);
        } else {
          pc += BC_CHECK_CHAR_NOT_IN_RANGE_LENGTH;
        }
        break;
      }
      BYTECODE(CHECK_BIT_IN_TABLE) {
        int mask = RegExpMacroAssembler::kTableMask;
        uint8_t b = pc[8 + ((current_char & mask) >> kBitsPerByteLog2)];
        int bit = (current_char & (kBitsPerByte - 1));
        if ((b & (1 << bit)) != 0) {
          pc = code_base + Load32Aligned(pc + 4);
        } else {
          pc += BC_CHECK_BIT_IN_TABLE_LENGTH;
        }
        break;
      }
      BYTECODE(CHECK_LT) {
        uint32_t limit = (insn >> BYTECODE_SHIFT);
        if (current_char < limit) {
          pc = code_base + Load32Aligned(pc + 4);
        } else {
          pc += BC_CHECK_LT_LENGTH;
        }
        break;
      }
      BYTECODE(CHECK_GT) {
        uint32_t limit = (insn >> BYTECODE_SHIFT);
        if (current_char > limit) {
          pc = code_base + Load32Aligned(pc + 4);
        } else {
          pc += BC_CHECK_GT_LENGTH;
        }
        break;
      }
      BYTECODE(CHECK_REGISTER_LT)
      if (registers[insn >> BYTECODE_SHIFT] < Load32Aligned(pc + 4)) {
        pc = code_base + Load32Aligned(pc + 8);
      } else {
        pc += BC_CHECK_REGISTER_LT_LENGTH;
      }
      break;
      BYTECODE(CHECK_REGISTER_GE)
      if (registers[insn >> BYTECODE_SHIFT] >= Load32Aligned(pc + 4)) {
        pc = code_base + Load32Aligned(pc + 8);
      } else {
        pc += BC_CHECK_REGISTER_GE_LENGTH;
      }
      break;
      BYTECODE(CHECK_REGISTER_EQ_POS)
      if (registers[insn >> BYTECODE_SHIFT] == current) {
        pc = code_base + Load32Aligned(pc + 4);
      } else {
        pc += BC_CHECK_REGISTER_EQ_POS_LENGTH;
      }
      break;
      BYTECODE(CHECK_NOT_REGS_EQUAL)
      if (registers[insn >> BYTECODE_SHIFT] ==
          registers[Load32Aligned(pc + 4)]) {
        pc += BC_CHECK_NOT_REGS_EQUAL_LENGTH;
      } else {
        pc = code_base + Load32Aligned(pc + 8);
      }
      break;
      BYTECODE(CHECK_NOT_BACK_REF) {
        int from = registers[insn >> BYTECODE_SHIFT];
        int len = registers[(insn >> BYTECODE_SHIFT) + 1] - from;
        if (from < 0 || len <= 0) {
          pc += BC_CHECK_NOT_BACK_REF_LENGTH;
          break;
        }
        if (current + len > subject_length) {
          pc = code_base + Load32Aligned(pc + 4);
          break;
        } else {
          int i;
          for (i = 0; i < len; i++) {
            if (subject.CharAt(from + i) != subject.CharAt(current + i)) {
              pc = code_base + Load32Aligned(pc + 4);
              break;
            }
          }
          if (i < len) break;
          current += len;
        }
        pc += BC_CHECK_NOT_BACK_REF_LENGTH;
        break;
      }
      BYTECODE(CHECK_NOT_BACK_REF_NO_CASE_UNICODE)
      FALL_THROUGH;
      BYTECODE(CHECK_NOT_BACK_REF_NO_CASE) {
        const bool unicode =
            (insn & BYTECODE_MASK) == BC_CHECK_NOT_BACK_REF_NO_CASE_UNICODE;
        int from = registers[insn >> BYTECODE_SHIFT];
        int len = registers[(insn >> BYTECODE_SHIFT) + 1] - from;
        if (from < 0 || len <= 0) {
          pc += BC_CHECK_NOT_BACK_REF_NO_CASE_LENGTH;
          break;
        }
        if (current + len > subject_length) {
          pc = code_base + Load32Aligned(pc + 4);
          break;
        } else {
          if (BackRefMatchesNoCase<Char>(&canonicalize, from, current, len,
                                         subject, unicode)) {
            current += len;
            pc += BC_CHECK_NOT_BACK_REF_NO_CASE_LENGTH;
          } else {
            pc = code_base + Load32Aligned(pc + 4);
          }
        }
        break;
      }
      BYTECODE(CHECK_NOT_BACK_REF_BACKWARD) {
        const int from = registers[insn >> BYTECODE_SHIFT];
        const int len = registers[(insn >> BYTECODE_SHIFT) + 1] - from;
        if (from < 0 || len <= 0) {
          pc += BC_CHECK_NOT_BACK_REF_BACKWARD_LENGTH;
          break;
        }
        if ((current - len) < 0) {
          pc = code_base + Load32Aligned(pc + 4);
          break;
        } else {
          // When looking behind, the string to match (if it is there) lies
          // before the current position, so we will check the [len] characters
          // before the current position, excluding the current position itself.
          const int start = current - len;
          int i;
          for (i = 0; i < len; i++) {
            if (subject.CharAt(from + i) != subject.CharAt(start + i)) {
              pc = code_base + Load32Aligned(pc + 4);
              break;
            }
          }
          if (i < len) break;
          current -= len;
        }
        pc += BC_CHECK_NOT_BACK_REF_BACKWARD_LENGTH;
        break;
      }
      BYTECODE(CHECK_NOT_BACK_REF_NO_CASE_UNICODE_BACKWARD)
      FALL_THROUGH;
      BYTECODE(CHECK_NOT_BACK_REF_NO_CASE_BACKWARD) {
        bool unicode = (insn & BYTECODE_MASK) ==
                       BC_CHECK_NOT_BACK_REF_NO_CASE_UNICODE_BACKWARD;
        int from = registers[insn >> BYTECODE_SHIFT];
        int len = registers[(insn >> BYTECODE_SHIFT) + 1] - from;
        if (from < 0 || len <= 0) {
          pc += BC_CHECK_NOT_BACK_REF_NO_CASE_BACKWARD_LENGTH;
          break;
        }
        if (current < len) {
          pc = code_base + Load32Aligned(pc + 4);
          break;
        } else {
          if (BackRefMatchesNoCase<Char>(&canonicalize, from, current - len,
                                         len, subject, unicode)) {
            current -= len;
            pc += BC_CHECK_NOT_BACK_REF_NO_CASE_BACKWARD_LENGTH;
          } else {
            pc = code_base + Load32Aligned(pc + 4);
          }
        }
        break;
      }
      BYTECODE(CHECK_AT_START)
      if (current == 0) {
        pc = code_base + Load32Aligned(pc + 4);
      } else {
        pc += BC_CHECK_AT_START_LENGTH;
      }
      break;
      BYTECODE(CHECK_NOT_AT_START) {
        const int32_t cp_offset = insn >> BYTECODE_SHIFT;
        if (current + cp_offset == 0) {
          pc += BC_CHECK_NOT_AT_START_LENGTH;
        } else {
          pc = code_base + Load32Aligned(pc + 4);
        }
        break;
      }
      BYTECODE(SET_CURRENT_POSITION_FROM_END) {
        int by = static_cast<uint32_t>(insn) >> BYTECODE_SHIFT;
        if (subject_length - current > by) {
          current = subject_length - by;
          current_char = subject.CharAt(current - 1);
        }
        pc += BC_SET_CURRENT_POSITION_FROM_END_LENGTH;
        break;
      }
      default:
        UNREACHABLE();
        break;
    }
  }
}

IrregexpInterpreter::IrregexpResult IrregexpInterpreter::Match(
    const TypedData& bytecode,
    const String& subject,
    int32_t* registers,
    intptr_t start_position,
    Zone* zone) {
  NoSafepointScope no_safepoint;
  const uint8_t* code_base = reinterpret_cast<uint8_t*>(bytecode.DataAddr(0));

  uint16_t previous_char = '\n';
  if (start_position != 0) {
    previous_char = subject.CharAt(start_position - 1);
  }

  if (subject.IsOneByteString() || subject.IsExternalOneByteString()) {
    return RawMatch<uint8_t>(code_base, subject, registers, start_position,
                             previous_char, zone);
  } else if (subject.IsTwoByteString() || subject.IsExternalTwoByteString()) {
    return RawMatch<uint16_t>(code_base, subject, registers, start_position,
                              previous_char, zone);
  } else {
    UNREACHABLE();
    return IrregexpInterpreter::RE_FAILURE;
  }
}

}  // namespace dart