react-native/ReactCommon/cxxreact/JSBigString.h

// Copyright (c) Facebook, Inc. and its affiliates.

// This source code is licensed under the MIT license found in the
// LICENSE file in the root directory of this source tree.

#pragma once

#include <folly/Exception.h>

#ifndef RN_EXPORT
#ifdef _MSC_VER
#define RN_EXPORT
#else
#define RN_EXPORT __attribute__((visibility("default")))
#endif
#endif

namespace facebook {
namespace react {

// JSExecutor functions sometimes take large strings, on the order of
// megabytes.  Copying these can be expensive.  Introducing a
// move-only, non-CopyConstructible type will let the compiler ensure
// that no copies occur.  folly::MoveWrapper should be used when a
// large string needs to be curried into a std::function<>, which must
// by CopyConstructible.

class JSBigString {
 public:
  JSBigString() = default;

  // Not copyable
  JSBigString(const JSBigString &) = delete;
  JSBigString &operator=(const JSBigString &) = delete;

  virtual ~JSBigString() {}

  virtual bool isAscii() const = 0;

  // This needs to be a \0 terminated string
  virtual const char *c_str() const = 0;

  // Length of the c_str without the NULL byte.
  virtual size_t size() const = 0;
};

// Concrete JSBigString implementation which holds a std::string
// instance.
class JSBigStdString : public JSBigString {
 public:
  JSBigStdString(std::string str, bool isAscii = false)
      : m_isAscii(isAscii), m_str(std::move(str)) {}

  bool isAscii() const override {
    return m_isAscii;
  }

  const char *c_str() const override {
    return m_str.c_str();
  }

  size_t size() const override {
    return m_str.size();
  }

 private:
  bool m_isAscii;
  std::string m_str;
};

// Concrete JSBigString implementation which holds a heap-allocated
// buffer, and provides an accessor for writing to it.  This can be
// used to construct a JSBigString in place, such as by reading from a
// file.
class RN_EXPORT JSBigBufferString : public JSBigString {
 public:
  JSBigBufferString(size_t size) : m_data(new char[size + 1]), m_size(size) {
    // Guarantee nul-termination.  The caller is responsible for
    // filling in the rest of m_data.
    m_data[m_size] = '\0';
  }

  ~JSBigBufferString() {
    delete[] m_data;
  }

  bool isAscii() const override {
    return true;
  }

  const char *c_str() const override {
    return m_data;
  }

  size_t size() const override {
    return m_size;
  }

  char *data() {
    return m_data;
  }

 private:
  char *m_data;
  size_t m_size;
};

// JSBigString interface implemented by a file-backed mmap region.
class RN_EXPORT JSBigFileString : public JSBigString {
 public:
  JSBigFileString(int fd, size_t size, off_t offset = 0);
  ~JSBigFileString();

  bool isAscii() const override {
    return true;
  }

  const char *c_str() const override;

  size_t size() const override;
  int fd() const;

  static std::unique_ptr<const JSBigFileString> fromPath(
      const std::string &sourceURL);

 private:
  int m_fd; // The file descriptor being mmaped
  size_t m_size; // The size of the mmaped region
  mutable off_t m_pageOff; // The offset in the mmaped region to the data.
  off_t m_mapOff; // The offset in the file to the mmaped region.
  mutable const char *m_data; // Pointer to the mmaped region.
};

} // namespace react
} // namespace facebook