/*
 * LMDB Abstraction Layer.
 * Copyright (C) 2023  Yury Gubich <blue@macaw.me>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef LMDBAL_CURSOR_HPP
#define LMDBAL_CURSOR_HPP

#include "cursor.h"
#include <iostream>

/**
 * \class LMDBAL::Cursor
 * \brief An object to iterate storages.
 *
 * \tparam K type of the keys in the storage that this cursor would iterate
 * \tparam V type of the values in the storage that this cursor would iterate
 *
 * Cursor allowes you to perform sequential querries, navigate from one element to another at reduced operation price.
 * For now Cursors are read only but in the future you might also be able to write to the storage with them.
 *
 * Cursors are owned by the storage, they die with the storage.
 * They also get closed if the storage is closed (if you close by the database for example)
 *
 * You can obtain an instance of this class calling LMDBAL::Storage::createCursor()
 * and destory it calling LMDBAL::Storage::destoryCursor() at any time, LMDBAL::Base doesn't necessarily need to be opened.
 *
 * You are not supposed to instantiate or destory instances of this class yourself!
 */

/**
 * \brief Creates a cursor
 *
 * \param[in] parent    a storage that created this cursor
 */
template<class K, class V>
LMDBAL::Cursor<K, V>::Cursor(Storage<K, V>* parent):
    storage(parent),
    cursor(nullptr),
    state(closed)
{}

/**
 * \brief Destroys a cursor
 *
 * If the cursor wasn't properly closed - it's going to be upon destruction
 */
template<class K, class V>
LMDBAL::Cursor<K, V>::~Cursor () {
    close();
}

/**
 * \brief A private function the storage owning this cursor will call to inform this cursor that the thansaction needs to be aborted
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::terminated () const {
    close();        //for now it's the same, but if I ever going to make writable cursor - here is where it's gonna be different
}

/**
 * \brief Opens the cursor for operations.
 *
 * This is a normal way to start the sequence of operations with the cursor.
 * This variant of the function creates a read only transaction just for this cursor
 *
 * This function should be called when the LMDBAL::Storage is already opened and before any query with this cursor!
 * It will do nothing to a cursor that was already opened (no matter what way).
 *
 * \exception LMDBAL::Closed   thrown if you try to open the cursor on a closed database
 * \exception LMDBAL::Unknown  thrown if there was a problem opening the cursor by the lmdb, or to begin a transaction
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::open () const {
    storage->ensureOpened(openCursorMethodName);
    switch (state) {
        case closed: {
            TransactionID txn = storage->beginReadOnlyTransaction();
            int result = mdb_cursor_open(txn, storage->dbi, &cursor);
            if (result != MDB_SUCCESS)
                storage->throwUnknown(result, txn);

            storage->transactionStarted(txn, true);
            state = openedPrivate;
        } break;
        default:
            break;
    }
}

/**
 * \brief Opens the cursor for operations.
 *
 * This is a normal way to start the sequence of operations with the cursor.
 * This variant of the function uses for queries a transaction you have obtained somewhere else.
 *
 * This function should be called when the LMDBAL::Storage is already opened and before any query with this cursor!
 * It will do nothing to a cursor that was already opened (no matter what way).
 *
 * \param[in] transaction   - a transaction, can be read only
 *
 * \exception LMDBAL::Closed                thrown if you try to open the cursor on a closed database
 * \exception LMDBAL::Unknown               thrown if there was a problem opening the cursor by the lmdb
 * \exception LMDBAL::TransactionTerminated thrown if the passed transaction not active, any action with it's inner ID is an error
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::open (const Transaction& transaction) const {
    storage->ensureOpened(openCursorMethodName);
    TransactionID txn = storage->extractTransactionId(transaction, openCursorMethodName);
    switch (state) {
        case closed: {
            int result = mdb_cursor_open(txn, storage->dbi, &cursor);
            if (result != MDB_SUCCESS)
                storage->throwUnknown(result);

            state = openedPublic;
        } break;
        default:
            break;
    }
}

/**
 * \brief Renews a cursor
 *
 * This function aborts current transaction if the cursor was opened with it's own transaction
 * (does not mess up if the transaction was public),
 * creates new private transaction and rebinds this cursor to it.
 *
 * Theoretically you could call this method if your public transaction was aborted (or commited)
 * but you wish to continue to keep working with your cursor.
 * Or if you just want to rebind your cursor to a new private transaction.
 *
 * This function does nothing if the cursor is closed
 *
 * \exception LMDBAL::Closed   thrown if you try to renew the cursor on a closed database
 * \exception LMDBAL::Unknown  thrown if there was a problem beginning new transaction or if there was a problem renewing the cursor by lmdb
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::renew () const {
    storage->ensureOpened(renewCursorMethodName);
    switch (state) {
        case openedPrivate: {
            TransactionID txn = mdb_cursor_txn(cursor);
            storage->abortTransaction(txn);
            storage->transactionAborted(txn);
            [[fallthrough]];
        }
        case openedPublic: {
            TransactionID txn = storage->beginReadOnlyTransaction();
            int result = mdb_cursor_renew(txn, cursor);
            if (result != MDB_SUCCESS)
                storage->throwUnknown(result, txn);

            storage->transactionStarted(txn, true);
            state = openedPrivate;
        } break;
        default:
            break;
    }
}

/**
 * \brief Renews a cursor
 *
 * This function aborts current transaction if the cursor was opened with it's own transaction
 * (does not mess up if the transaction was public),
 *  and rebinds this cursor to a passed new transaction.
 *
 * Theoretically you could call this method if your previous public transaction was aborted (or commited)
 * but you wish to continue to keep working with your cursor.
 * Or if you just want to rebind your cursor to another public transaction.
 *
 * This function does nothing if the cursor is closed
 *
 * \param[in]   transaction     - a transaction you wish this cursor to be bound to
 *
 * \exception LMDBAL::Closed                thrown if you try to renew the cursor on a closed database
 * \exception LMDBAL::Unknown               thrown if there was a problem renewing the cursor by lmdb
 * \exception LMDBAL::TransactionTerminated thrown if the passed transaction not active, any action with it's inner ID is an error
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::renew (const Transaction& transaction) const {
    storage->ensureOpened(renewCursorMethodName);
    TransactionID txn = storage->extractTransactionId(transaction, renewCursorMethodName);
    switch (state) {
        case openedPrivate: {
            TransactionID txn = mdb_cursor_txn(cursor);
            storage->abortTransaction(txn);
            storage->transactionAborted(txn);
            [[fallthrough]];
        }
        case openedPublic: {
            int result = mdb_cursor_renew(txn, cursor);
            if (result != MDB_SUCCESS)
                storage->throwUnknown(result);

            state = openedPublic;
        } break;
        default:
            break;
    }
}

/**
 * \brief Termiates a sequence of operations with the cursor
 *
 * This is a normal way to tell that you're done with the cursor and don't want to continue the sequence of queries.
 * The state of the cursor is lost after calling this method, some inner resorce is freed.
 *
 * If the cursor was opened with the private transaction - the owner storage will be notified of the aborted transaction.
 *
 * This function does nothing on a closed cursor.
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::close () const {
    switch (state) {
        case openedPublic: {
            mdb_cursor_close(cursor);

            state = closed;
        } break;
        case openedPrivate: {
            TransactionID txn = mdb_cursor_txn(cursor);
            mdb_cursor_close(cursor);
            storage->abortTransaction(txn);
            storage->transactionAborted(txn);

            state = closed;
        } break;
        default:
            break;
    }
}

/**
 * \brief Tells if the cursor is open
 */
template<class K, class V>
bool LMDBAL::Cursor<K, V>::opened () const {
    return state != closed;
}

/**
 * \brief Queries the first element in the storage
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \param[out]  key     a reference to an object the key of queried element is going to be assigned
 * \param[out]  value   a reference to an object the value of queried element is going to be assigned
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         thrown if there are no elements in the storage
 * \exception LMDBAL::Unknown          thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::first (K& key, V& value) const {
    operateCursorRead(key, value, MDB_FIRST, firstMethodName, firstOperationName);
}

/**
 * \brief Queries the last element in the storage
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \param[out]  key     a reference to an object the key of queried element is going to be assigned
 * \param[out]  value   a reference to an object the value of queried element is going to be assigned
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         thrown if there are no elements in the storage
 * \exception LMDBAL::Unknown          thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::last (K& key, V& value) const {
    operateCursorRead(key, value, MDB_LAST, lastMethodName, lastOperationName);
}

/**
 * \brief Queries the next element from the storage
 *
 * If there was no operation before this method positions the cursor on the first element and returns it
 * so, it's basically doing the same as LMDBAL::Cursor::first(K key, V value).
 *
 * It will also throw LMDBAL::NotFound if you call this method being on the last element
 * or if there are no elements in the database.
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \param[out]  key     a reference to an object the key of queried element is going to be assigned
 * \param[out]  value   a reference to an object the value of queried element is going to be assigned
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         thrown if the cursor already was on the last element or if there are no elements in the storage
 * \exception LMDBAL::Unknown          thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::next (K& key, V& value) const {
    operateCursorRead(key, value, MDB_NEXT, nextMethodName, nextOperationName);
}

/**
 * \brief Queries the previous element from the storage
 *
 * If there was no operation before this method positions the cursor on the last element and returns it
 * so, it's basically doing the same as LMDBAL::Cursor::last(K key, V value).
 *
 * It will also throw LMDBAL::NotFound if you call this method being on the first element
 * or if there are no elements in the database.
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \param[out]  key     a reference to an object the key of queried element is going to be assigned
 * \param[out]  value   a reference to an object the value of queried element is going to be assigned
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         thrown if the cursor already was on the first element or if there are no elements in the storage
 * \exception LMDBAL::Unknown          thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::prev (K& key, V& value) const {
    operateCursorRead(key, value, MDB_PREV, prevMethodName, prevOperationName);
}

/**
 * \brief Returns current cursor element from the storage
 *
 * If there was no operation before this method throws LMDBAL::Unknown for some reason
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \param[out]  key     a reference to an object the key of queried element is going to be assigned
 * \param[out]  value   a reference to an object the value of queried element is going to be assigned
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         probably never thrown but there might be still some corner case I don't know about
 * \exception LMDBAL::Unknown          thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::current (K& key, V& value) const {
    operateCursorRead(key, value, MDB_GET_CURRENT, currentMethodName, currentOperationName);
}

/**
 * \brief Queries the first element in the storage
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \returns std::pair where first is element key and second is element value
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         thrown if there are no elements in the storage
 * \exception LMDBAL::Unknown          thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
std::pair<K, V> LMDBAL::Cursor<K, V>::first () const {
    std::pair<K, V> result;
    operateCursorRead(result.first, result.second, MDB_FIRST, firstMethodName, firstOperationName);
    return result;
}

/**
 * \brief Queries the last element in the storage
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \returns std::pair where first is element key and second is element value
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         thrown if there are no elements in the storage
 * \exception LMDBAL::Unknown          thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
std::pair<K, V> LMDBAL::Cursor<K, V>::last () const {
    std::pair<K, V> result;
    operateCursorRead(result.first, result.second, MDB_LAST, lastMethodName, lastOperationName);
    return result;
}

/**
 * \brief Queries the next element from the storage
 *
 * If there was no operation before this method positions the cursor on the first element and returns it
 * so, it's basically doing the same as LMDBAL::Cursor::first().
 *
 * It will also throw LMDBAL::NotFound if you call this method being on the last element
 * or if there are no elements in the database.
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \returns std::pair where first is element key and second is element value
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         thrown if the cursor already was on the last element or if there are no elements in the storage
 * \exception LMDBAL::Unknown          thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
std::pair<K, V> LMDBAL::Cursor<K, V>::next () const {
    std::pair<K, V> result;
    operateCursorRead(result.first, result.second, MDB_NEXT, nextMethodName, nextOperationName);
    return result;
}

/**
 * \brief Queries the previous element from the storage
 *
 * If there was no operation before this method positions the cursor on the last element and returns it
 * so, it's basically doing the same as LMDBAL::Cursor::last().
 *
 * It will also throw LMDBAL::NotFound if you call this method being on the first element
 * or if there are no elements in the database.
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \returns std::pair where first is element key and second is element value
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         thrown if the cursor already was on the first element or if there are no elements in the storage
 * \exception LMDBAL::Unknown          thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
std::pair<K, V> LMDBAL::Cursor<K, V>::prev () const {
    std::pair<K, V> result;
    operateCursorRead(result.first, result.second, MDB_PREV, prevMethodName, prevOperationName);
    return result;
}

/**
 * \brief Returns current cursor element from the storage
 *
 * If there was no operation before this method throws LMDBAL::Unknown for some reason
 *
 * Notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \returns std::pair where first is element key and second is element value
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         probably never thrown but there might be still some corner case I don't know about
 * \exception LMDBAL::Unknown          thrown if there was no positioning operation before of if there was some unexpected problem with lmdb
 */
template<class K, class V>
std::pair<K, V> LMDBAL::Cursor<K, V>::current () const {
    std::pair<K, V> result;
    operateCursorRead(result.first, result.second, MDB_GET_CURRENT, currentMethodName, currentOperationName);
    return result;
}

/**
 * \brief a private mothod that actually doing all the reading
 *
 * Queries the storage, deserializes the output, notifies the storage of the queried element calling LMDBAL::Storage::discoveredRecord()
 *
 * \param[out]  key         a reference to an object the key of queried element is going to be assigned
 * \param[out]  value       a reference to an object the value of queried element is going to be assigned
 * \param[in]   operation   LMDB cursor <a class="el" href="http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127">operation code</a>
 * \param[in]   methodName      a name of the method you called it from, just for the exception message if something goes not as expected
 * \param[in]   operationName   a name of the opeartion, just for the exception message if something goes not as expected
 *
 * \exception LMDBAL::CursorNotReady   thrown if you try to call this method on a closed cursor
 * \exception LMDBAL::NotFound         mostly thrown if the query wasn't found
 * \exception LMDBAL::Unknown          mostly thrown if there was some unexpected problem with lmdb
 */
template<class K, class V>
void LMDBAL::Cursor<K, V>::operateCursorRead(
    K& key,
    V& value,
    MDB_cursor_op operation,
    const std::string& methodName,
    const std::string& operationName
) const {
    if (state == closed)
        storage->throwCursorNotReady(methodName);

    MDB_val mdbKey, mdbValue;
    int result = mdb_cursor_get(cursor, &mdbKey, &mdbValue, operation);
    if (result != MDB_SUCCESS)
        storage->throwNotFoundOrUnknown(result, operationName);

    storage->keySerializer.deserialize(mdbKey, key);
    storage->valueSerializer.deserialize(mdbValue, value);

    if (state == openedPrivate)
        storage->discoveredRecord(key, value);
    else
        storage->discoveredRecord(key, value, mdb_cursor_txn(cursor));
}

#endif //LMDBAL_CURSOR_HPP