src/include/fst/generic-register.h

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Copyright 2005-2010 Google, Inc.
// Author: jpr@google.com (Jake Ratkiewicz)

#ifndef FST_LIB_GENERIC_REGISTER_H_
#define FST_LIB_GENERIC_REGISTER_H_

#include <map>
#include <string>

#include <fst/compat.h>
#include <fst/types.h>

// Generic class representing a globally-stored correspondence between
// objects of KeyType and EntryType.
// KeyType must:
//  a) be such as can be stored as a key in a map<>
//  b) be concatenable with a const char* with the + operator
//     (or you must subclass and redefine LoadEntryFromSharedObject)
// EntryType must be default constructible.
//
// The third template parameter should be the type of a subclass of this class
// (think CRTP). This is to allow GetRegister() to instantiate and return
// an object of the appropriate type.

namespace fst {

template<class KeyType, class EntryType, class RegisterType>
class GenericRegister {
 public:
  typedef KeyType Key;
  typedef EntryType Entry;

  static RegisterType *GetRegister() {
    FstOnceInit(&register_init_,
                   &RegisterType::Init);

    return register_;
  }

  void SetEntry(const KeyType &key,
                const EntryType &entry) {
    MutexLock l(register_lock_);

    register_table_.insert(make_pair(key, entry));
  }

  EntryType GetEntry(const KeyType &key) const {
    const EntryType *entry = LookupEntry(key);
    if (entry) {
      return *entry;
    } else {
      return LoadEntryFromSharedObject(key);
    }
  }

  virtual ~GenericRegister() { }

 protected:
  // Override this if you want to be able to load missing definitions from
  // shared object files.
  virtual EntryType LoadEntryFromSharedObject(const KeyType &key) const {
    string so_filename = ConvertKeyToSoFilename(key);

    void *handle = dlopen(so_filename.c_str(), RTLD_LAZY);
    if (handle == 0) {
      LOG(ERROR) << "GenericRegister::GetEntry : " << dlerror();
      return EntryType();
    }

    // We assume that the DSO constructs a static object in its global
    // scope that does the registration. Thus we need only load it, not
    // call any methods.
    const EntryType *entry = this->LookupEntry(key);
    if (entry == 0) {
      LOG(ERROR) << "GenericRegister::GetEntry : "
                 << "lookup failed in shared object: " << so_filename;
      return EntryType();
    }
    return *entry;
  }

  // Override this to define how to turn a key into an SO filename.
  virtual string ConvertKeyToSoFilename(const KeyType& key) const = 0;

  virtual const EntryType *LookupEntry(
      const KeyType &key) const {
    MutexLock l(register_lock_);

    typename RegisterMapType::const_iterator it = register_table_.find(key);

    if (it != register_table_.end()) {
      return &it->second;
    } else {
      return 0;
    }
  }

 private:
  typedef map<KeyType, EntryType> RegisterMapType;

  static void Init() {
    register_lock_ = new Mutex;
    register_ = new RegisterType;
  }

  static FstOnceType register_init_;
  static Mutex *register_lock_;
  static RegisterType *register_;

  RegisterMapType register_table_;
};

template<class KeyType, class EntryType, class RegisterType>
FstOnceType GenericRegister<KeyType, EntryType,
                               RegisterType>::register_init_ = FST_ONCE_INIT;

template<class KeyType, class EntryType, class RegisterType>
Mutex *GenericRegister<KeyType, EntryType, RegisterType>::register_lock_ = 0;

template<class KeyType, class EntryType, class RegisterType>
RegisterType *GenericRegister<KeyType, EntryType, RegisterType>::register_ = 0;

//
// GENERIC REGISTRATION
//

// Generic register-er class capable of creating new register entries in the
// given RegisterType template parameter. This type must define types Key
// and Entry, and have appropriate static GetRegister() and instance
// SetEntry() functions. An easy way to accomplish this is to have RegisterType
// be the type of a subclass of GenericRegister.
template<class RegisterType>
class GenericRegisterer {
 public:
  typedef typename RegisterType::Key Key;
  typedef typename RegisterType::Entry Entry;

  GenericRegisterer(Key key, Entry entry) {
    RegisterType *reg = RegisterType::GetRegister();
    reg->SetEntry(key, entry);
  }
};

}  // namespace fst

#endif  // FST_LIB_GENERIC_REGISTER_H_