// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
//
// An iterator yields a sequence of key/value pairs from a source.
// The following class defines the interface.  Multiple implementations
// are provided by this library.  In particular, iterators are provided
// to access the contents of a Table or a DB.
//
// Multiple threads can invoke const methods on an Iterator without
// external synchronization, but if any of the threads may call a
// non-const method, all threads accessing the same Iterator must use
// external synchronization.

#ifndef STORAGE_LEVELDB_INCLUDE_ITERATOR_H_
#define STORAGE_LEVELDB_INCLUDE_ITERATOR_H_

#include "export.h"
#include "slice.h"
#include "status.h"

namespace leveldb {

class LEVELDB_EXPORT Iterator {
  public:
    Iterator();

    Iterator(const Iterator &) = delete;
    Iterator &operator=(const Iterator &) = delete;

    virtual ~Iterator();

    // An iterator is either positioned at a key/value pair, or
    // not valid.  This method returns true iff the iterator is valid.
    virtual bool Valid() const = 0;

    // Position at the first key in the source.  The iterator is Valid()
    // after this call iff the source is not empty.
    virtual void SeekToFirst() = 0;

    // Position at the last key in the source.  The iterator is
    // Valid() after this call iff the source is not empty.
    virtual void SeekToLast() = 0;

    // Position at the first key in the source that is at or past target.
    // The iterator is Valid() after this call iff the source contains
    // an entry that comes at or past target.
    virtual void Seek(const Slice &target) = 0;

    // Moves to the next entry in the source.  After this call, Valid() is
    // true iff the iterator was not positioned at the last entry in the source.
    // REQUIRES: Valid()
    virtual void Next() = 0;

    // Moves to the previous entry in the source.  After this call, Valid() is
    // true iff the iterator was not positioned at the first entry in source.
    // REQUIRES: Valid()
    virtual void Prev() = 0;

    // Return the key for the current entry.  The underlying storage for
    // the returned slice is valid only until the next modification of
    // the iterator.
    // REQUIRES: Valid()
    virtual Slice key() const = 0;

    // Return the value for the current entry.  The underlying storage for
    // the returned slice is valid only until the next modification of
    // the iterator.
    // REQUIRES: Valid()
    virtual Slice value() const = 0;

    // If an error has occurred, return it.  Else return an ok status.
    virtual Status status() const = 0;

    // Clients are allowed to register function/arg1/arg2 triples that
    // will be invoked when this iterator is destroyed.
    //
    // Note that unlike all of the preceding methods, this method is
    // not abstract and therefore clients should not override it.
    using CleanupFunction = void (*)(void *arg1, void *arg2);
    void RegisterCleanup(CleanupFunction function, void *arg1, void *arg2);

  private:
    // Cleanup functions are stored in a single-linked list.
    // The list's head node is inlined in the iterator.
    struct CleanupNode {
        // True if the node is not used. Only head nodes might be unused.
        bool IsEmpty() const { return function == nullptr; }
        // Invokes the cleanup function.
        void Run() {
            assert(function != nullptr);
            (*function)(arg1, arg2);
        }

        // The head node is used if the function pointer is not null.
        CleanupFunction function;
        void *arg1;
        void *arg2;
        CleanupNode *next;
    };
    CleanupNode cleanup_head_;
};

// Return an empty iterator (yields nothing).
LEVELDB_EXPORT Iterator *NewEmptyIterator();

// Return an empty iterator with the specified status.
LEVELDB_EXPORT Iterator *NewErrorIterator(const Status &status);

}  // namespace leveldb

#endif  // STORAGE_LEVELDB_INCLUDE_ITERATOR_H_