/*
 * Copyright (C) 2019 The Android Open Source Project
 *
 * 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.
 */

package com.android.server.connectivity.ipmemorystore;

import static android.net.shared.Inet4AddressUtils.inet4AddressToIntHTH;
import static android.net.shared.Inet4AddressUtils.intToInet4AddressHTH;

import android.annotation.NonNull;
import android.annotation.Nullable;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteCursor;
import android.database.sqlite.SQLiteCursorDriver;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteQuery;
import android.net.ipmemorystore.NetworkAttributes;
import android.net.ipmemorystore.Status;
import android.util.Log;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.ArrayList;
import java.util.List;
import java.util.StringJoiner;

/**
 * Encapsulating class for using the SQLite database backing the memory store.
 *
 * This class groups together the contracts and the SQLite helper used to
 * use the database.
 *
 * @hide
 */
public class IpMemoryStoreDatabase {
    private static final String TAG = IpMemoryStoreDatabase.class.getSimpleName();
    // A pair of NetworkAttributes objects is group-close if the confidence that they are
    // the same is above this cutoff. See NetworkAttributes and SameL3NetworkResponse.
    private static final float GROUPCLOSE_CONFIDENCE = 0.5f;

    /**
     * Contract class for the Network Attributes table.
     */
    public static class NetworkAttributesContract {
        public static final String TABLENAME = "NetworkAttributes";

        public static final String COLNAME_L2KEY = "l2Key";
        public static final String COLTYPE_L2KEY = "TEXT NOT NULL";

        public static final String COLNAME_EXPIRYDATE = "expiryDate";
        // Milliseconds since the Epoch, in true Java style
        public static final String COLTYPE_EXPIRYDATE = "BIGINT";

        public static final String COLNAME_ASSIGNEDV4ADDRESS = "assignedV4Address";
        public static final String COLTYPE_ASSIGNEDV4ADDRESS = "INTEGER";

        public static final String COLNAME_ASSIGNEDV4ADDRESSEXPIRY = "assignedV4AddressExpiry";
        // The lease expiry timestamp in uint of milliseconds
        public static final String COLTYPE_ASSIGNEDV4ADDRESSEXPIRY = "BIGINT";

        // Please note that the group hint is only a *hint*, hence its name. The client can offer
        // this information to nudge the grouping in the decision it thinks is right, but it can't
        // decide for the memory store what is the same L3 network.
        public static final String COLNAME_GROUPHINT = "groupHint";
        public static final String COLTYPE_GROUPHINT = "TEXT";

        public static final String COLNAME_DNSADDRESSES = "dnsAddresses";
        // Stored in marshalled form as is
        public static final String COLTYPE_DNSADDRESSES = "BLOB";

        public static final String COLNAME_MTU = "mtu";
        public static final String COLTYPE_MTU = "INTEGER DEFAULT -1";

        public static final String CREATE_TABLE = "CREATE TABLE IF NOT EXISTS "
                + TABLENAME                       + " ("
                + COLNAME_L2KEY                   + " " + COLTYPE_L2KEY + " PRIMARY KEY NOT NULL, "
                + COLNAME_EXPIRYDATE              + " " + COLTYPE_EXPIRYDATE              + ", "
                + COLNAME_ASSIGNEDV4ADDRESS       + " " + COLTYPE_ASSIGNEDV4ADDRESS       + ", "
                + COLNAME_ASSIGNEDV4ADDRESSEXPIRY + " " + COLTYPE_ASSIGNEDV4ADDRESSEXPIRY + ", "
                + COLNAME_GROUPHINT               + " " + COLTYPE_GROUPHINT               + ", "
                + COLNAME_DNSADDRESSES            + " " + COLTYPE_DNSADDRESSES            + ", "
                + COLNAME_MTU                     + " " + COLTYPE_MTU                     + ")";
        public static final String DROP_TABLE = "DROP TABLE IF EXISTS " + TABLENAME;
    }

    /**
     * Contract class for the Private Data table.
     */
    public static class PrivateDataContract {
        public static final String TABLENAME = "PrivateData";

        public static final String COLNAME_L2KEY = "l2Key";
        public static final String COLTYPE_L2KEY = "TEXT NOT NULL";

        public static final String COLNAME_CLIENT = "client";
        public static final String COLTYPE_CLIENT = "TEXT NOT NULL";

        public static final String COLNAME_DATANAME = "dataName";
        public static final String COLTYPE_DATANAME = "TEXT NOT NULL";

        public static final String COLNAME_DATA = "data";
        public static final String COLTYPE_DATA = "BLOB NOT NULL";

        public static final String CREATE_TABLE = "CREATE TABLE IF NOT EXISTS "
                + TABLENAME        + " ("
                + COLNAME_L2KEY    + " " + COLTYPE_L2KEY    + ", "
                + COLNAME_CLIENT   + " " + COLTYPE_CLIENT   + ", "
                + COLNAME_DATANAME + " " + COLTYPE_DATANAME + ", "
                + COLNAME_DATA     + " " + COLTYPE_DATA     + ", "
                + "PRIMARY KEY ("
                + COLNAME_L2KEY    + ", "
                + COLNAME_CLIENT   + ", "
                + COLNAME_DATANAME + "))";
        public static final String DROP_TABLE = "DROP TABLE IF EXISTS " + TABLENAME;
    }

    // To save memory when the DB is not used, close it after 30s of inactivity. This is
    // determined manually based on what feels right.
    private static final long IDLE_CONNECTION_TIMEOUT_MS = 30_000;

    /** The SQLite DB helper */
    public static class DbHelper extends SQLiteOpenHelper {
        // Update this whenever changing the schema.
        private static final int SCHEMA_VERSION = 4;
        private static final String DATABASE_FILENAME = "IpMemoryStore.db";
        private static final String TRIGGER_NAME = "delete_cascade_to_private";

        public DbHelper(@NonNull final Context context) {
            super(context, DATABASE_FILENAME, null, SCHEMA_VERSION);
            setIdleConnectionTimeout(IDLE_CONNECTION_TIMEOUT_MS);
        }

        /** Called when the database is created */
        @Override
        public void onCreate(@NonNull final SQLiteDatabase db) {
            db.execSQL(NetworkAttributesContract.CREATE_TABLE);
            db.execSQL(PrivateDataContract.CREATE_TABLE);
            createTrigger(db);
        }

        /** Called when the database is upgraded */
        @Override
        public void onUpgrade(@NonNull final SQLiteDatabase db, final int oldVersion,
                final int newVersion) {
            try {
                if (oldVersion < 2) {
                    // upgrade from version 1 to version 2
                    // since we starts from version 2, do nothing here
                }

                if (oldVersion < 3) {
                    // upgrade from version 2 to version 3
                    final String sqlUpgradeAddressExpiry = "alter table"
                            + " " + NetworkAttributesContract.TABLENAME + " ADD"
                            + " " + NetworkAttributesContract.COLNAME_ASSIGNEDV4ADDRESSEXPIRY
                            + " " + NetworkAttributesContract.COLTYPE_ASSIGNEDV4ADDRESSEXPIRY;
                    db.execSQL(sqlUpgradeAddressExpiry);
                }

                if (oldVersion < 4) {
                    createTrigger(db);
                }
            } catch (SQLiteException e) {
                Log.e(TAG, "Could not upgrade to the new version", e);
                // create database with new version
                db.execSQL(NetworkAttributesContract.DROP_TABLE);
                db.execSQL(PrivateDataContract.DROP_TABLE);
                onCreate(db);
            }
        }

        /** Called when the database is downgraded */
        @Override
        public void onDowngrade(@NonNull final SQLiteDatabase db, final int oldVersion,
                final int newVersion) {
            // Downgrades always nuke all data and recreate an empty table.
            db.execSQL(NetworkAttributesContract.DROP_TABLE);
            db.execSQL(PrivateDataContract.DROP_TABLE);
            db.execSQL("DROP TRIGGER " + TRIGGER_NAME);
            onCreate(db);
        }

        private void createTrigger(@NonNull final SQLiteDatabase db) {
            final String createTrigger = "CREATE TRIGGER " + TRIGGER_NAME
                    + " DELETE ON " + NetworkAttributesContract.TABLENAME
                    + " BEGIN"
                    + " DELETE FROM " + PrivateDataContract.TABLENAME + " WHERE OLD."
                    + NetworkAttributesContract.COLNAME_L2KEY
                    + "=" + PrivateDataContract.COLNAME_L2KEY
                    + "; END;";
            db.execSQL(createTrigger);
        }
    }

    @NonNull
    private static byte[] encodeAddressList(@NonNull final List<InetAddress> addresses) {
        final ByteArrayOutputStream os = new ByteArrayOutputStream();
        for (final InetAddress address : addresses) {
            final byte[] b = address.getAddress();
            os.write(b.length);
            os.write(b, 0, b.length);
        }
        return os.toByteArray();
    }

    @NonNull
    private static ArrayList<InetAddress> decodeAddressList(@NonNull final byte[] encoded) {
        final ByteArrayInputStream is = new ByteArrayInputStream(encoded);
        final ArrayList<InetAddress> addresses = new ArrayList<>();
        int d = -1;
        while ((d = is.read()) != -1) {
            final byte[] bytes = new byte[d];
            is.read(bytes, 0, d);
            try {
                addresses.add(InetAddress.getByAddress(bytes));
            } catch (UnknownHostException e) { /* Hopefully impossible */ }
        }
        return addresses;
    }

    @NonNull
    private static ContentValues toContentValues(@Nullable final NetworkAttributes attributes) {
        final ContentValues values = new ContentValues();
        if (null == attributes) return values;
        if (null != attributes.assignedV4Address) {
            values.put(NetworkAttributesContract.COLNAME_ASSIGNEDV4ADDRESS,
                    inet4AddressToIntHTH(attributes.assignedV4Address));
        }
        if (null != attributes.assignedV4AddressExpiry) {
            values.put(NetworkAttributesContract.COLNAME_ASSIGNEDV4ADDRESSEXPIRY,
                    attributes.assignedV4AddressExpiry);
        }
        if (null != attributes.groupHint) {
            values.put(NetworkAttributesContract.COLNAME_GROUPHINT, attributes.groupHint);
        }
        if (null != attributes.dnsAddresses) {
            values.put(NetworkAttributesContract.COLNAME_DNSADDRESSES,
                    encodeAddressList(attributes.dnsAddresses));
        }
        if (null != attributes.mtu) {
            values.put(NetworkAttributesContract.COLNAME_MTU, attributes.mtu);
        }
        return values;
    }

    // Convert a NetworkAttributes object to content values to store them in a table compliant
    // with the contract defined in NetworkAttributesContract.
    @NonNull
    private static ContentValues toContentValues(@NonNull final String key,
            @Nullable final NetworkAttributes attributes, final long expiry) {
        final ContentValues values = toContentValues(attributes);
        values.put(NetworkAttributesContract.COLNAME_L2KEY, key);
        values.put(NetworkAttributesContract.COLNAME_EXPIRYDATE, expiry);
        return values;
    }

    // Convert a byte array into content values to store it in a table compliant with the
    // contract defined in PrivateDataContract.
    @NonNull
    private static ContentValues toContentValues(@NonNull final String key,
            @NonNull final String clientId, @NonNull final String name,
            @NonNull final byte[] data) {
        final ContentValues values = new ContentValues();
        values.put(PrivateDataContract.COLNAME_L2KEY, key);
        values.put(PrivateDataContract.COLNAME_CLIENT, clientId);
        values.put(PrivateDataContract.COLNAME_DATANAME, name);
        values.put(PrivateDataContract.COLNAME_DATA, data);
        return values;
    }

    @Nullable
    private static NetworkAttributes readNetworkAttributesLine(@NonNull final Cursor cursor) {
        // Make sure the data hasn't expired
        final long expiry = getLong(cursor, NetworkAttributesContract.COLNAME_EXPIRYDATE, -1L);
        if (expiry < System.currentTimeMillis()) return null;

        final NetworkAttributes.Builder builder = new NetworkAttributes.Builder();
        final int assignedV4AddressInt = getInt(cursor,
                NetworkAttributesContract.COLNAME_ASSIGNEDV4ADDRESS, 0);
        final long assignedV4AddressExpiry = getLong(cursor,
                NetworkAttributesContract.COLNAME_ASSIGNEDV4ADDRESSEXPIRY, 0);
        final String groupHint = getString(cursor, NetworkAttributesContract.COLNAME_GROUPHINT);
        final byte[] dnsAddressesBlob =
                getBlob(cursor, NetworkAttributesContract.COLNAME_DNSADDRESSES);
        final int mtu = getInt(cursor, NetworkAttributesContract.COLNAME_MTU, -1);
        if (0 != assignedV4AddressInt) {
            builder.setAssignedV4Address(intToInet4AddressHTH(assignedV4AddressInt));
        }
        if (0 != assignedV4AddressExpiry) {
            builder.setAssignedV4AddressExpiry(assignedV4AddressExpiry);
        }
        builder.setGroupHint(groupHint);
        if (null != dnsAddressesBlob) {
            builder.setDnsAddresses(decodeAddressList(dnsAddressesBlob));
        }
        if (mtu >= 0) {
            builder.setMtu(mtu);
        }
        return builder.build();
    }

    private static final String[] EXPIRY_COLUMN = new String[] {
        NetworkAttributesContract.COLNAME_EXPIRYDATE
    };
    static final int EXPIRY_ERROR = -1; // Legal values for expiry are positive

    static final String SELECT_L2KEY = NetworkAttributesContract.COLNAME_L2KEY + " = ?";

    // Returns the expiry date of the specified row, or one of the error codes above if the
    // row is not found or some other error
    static long getExpiry(@NonNull final SQLiteDatabase db, @NonNull final String key) {
        final Cursor cursor = db.query(NetworkAttributesContract.TABLENAME,
                EXPIRY_COLUMN, // columns
                SELECT_L2KEY, // selection
                new String[] { key }, // selectionArgs
                null, // groupBy
                null, // having
                null // orderBy
        );
        // L2KEY is the primary key ; it should not be possible to get more than one
        // result here. 0 results means the key was not found.
        if (cursor.getCount() != 1) return EXPIRY_ERROR;
        cursor.moveToFirst();
        final long result = cursor.getLong(0); // index in the EXPIRY_COLUMN array
        cursor.close();
        return result;
    }

    static final int RELEVANCE_ERROR = -1; // Legal values for relevance are positive

    // Returns the relevance of the specified row, or one of the error codes above if the
    // row is not found or some other error
    static int getRelevance(@NonNull final SQLiteDatabase db, @NonNull final String key) {
        final long expiry = getExpiry(db, key);
        return expiry < 0 ? (int) expiry : RelevanceUtils.computeRelevanceForNow(expiry);
    }

    // If the attributes are null, this will only write the expiry.
    // Returns an int out of Status.{SUCCESS, ERROR_*}
    static int storeNetworkAttributes(@NonNull final SQLiteDatabase db, @NonNull final String key,
            final long expiry, @Nullable final NetworkAttributes attributes) {
        final ContentValues cv = toContentValues(key, attributes, expiry);
        db.beginTransaction();
        try {
            // Unfortunately SQLite does not have any way to do INSERT OR UPDATE. Options are
            // to either insert with on conflict ignore then update (like done here), or to
            // construct a custom SQL INSERT statement with nested select.
            final long resultId = db.insertWithOnConflict(NetworkAttributesContract.TABLENAME,
                    null, cv, SQLiteDatabase.CONFLICT_IGNORE);
            if (resultId < 0) {
                db.update(NetworkAttributesContract.TABLENAME, cv, SELECT_L2KEY, new String[]{key});
            }
            db.setTransactionSuccessful();
            return Status.SUCCESS;
        } catch (SQLiteException e) {
            // No space left on disk or something
            Log.e(TAG, "Could not write to the memory store", e);
        } finally {
            db.endTransaction();
        }
        return Status.ERROR_STORAGE;
    }

    // Returns an int out of Status.{SUCCESS, ERROR_*}
    static int storeBlob(@NonNull final SQLiteDatabase db, @NonNull final String key,
            @NonNull final String clientId, @NonNull final String name,
            @NonNull final byte[] data) {
        final long res = db.insertWithOnConflict(PrivateDataContract.TABLENAME, null,
                toContentValues(key, clientId, name, data), SQLiteDatabase.CONFLICT_REPLACE);
        return (res == -1) ? Status.ERROR_STORAGE : Status.SUCCESS;
    }

    @Nullable
    static NetworkAttributes retrieveNetworkAttributes(@NonNull final SQLiteDatabase db,
            @NonNull final String key) {
        final Cursor cursor = db.query(NetworkAttributesContract.TABLENAME,
                null, // columns, null means everything
                NetworkAttributesContract.COLNAME_L2KEY + " = ?", // selection
                new String[] { key }, // selectionArgs
                null, // groupBy
                null, // having
                null); // orderBy
        // L2KEY is the primary key ; it should not be possible to get more than one
        // result here. 0 results means the key was not found.
        if (cursor.getCount() != 1) return null;
        cursor.moveToFirst();
        final NetworkAttributes attributes = readNetworkAttributesLine(cursor);
        cursor.close();
        return attributes;
    }

    private static final String[] DATA_COLUMN = new String[] {
            PrivateDataContract.COLNAME_DATA
    };
    @Nullable
    static byte[] retrieveBlob(@NonNull final SQLiteDatabase db, @NonNull final String key,
            @NonNull final String clientId, @NonNull final String name) {
        final Cursor cursor = db.query(PrivateDataContract.TABLENAME,
                DATA_COLUMN, // columns
                PrivateDataContract.COLNAME_L2KEY + " = ? AND " // selection
                + PrivateDataContract.COLNAME_CLIENT + " = ? AND "
                + PrivateDataContract.COLNAME_DATANAME + " = ?",
                new String[] { key, clientId, name }, // selectionArgs
                null, // groupBy
                null, // having
                null); // orderBy
        // The query above is querying by (composite) primary key, so it should not be possible to
        // get more than one result here. 0 results means the key was not found.
        if (cursor.getCount() != 1) return null;
        cursor.moveToFirst();
        final byte[] result = cursor.getBlob(0); // index in the DATA_COLUMN array
        cursor.close();
        return result;
    }

    /**
     * The following is a horrible hack that is necessary because the Android SQLite API does not
     * have a way to query a binary blob. This, almost certainly, is an overlook.
     *
     * The Android SQLite API has two family of methods : one for query that returns data, and
     * one for more general SQL statements that can execute any statement but may not return
     * anything. All the query methods, however, take only String[] for the arguments.
     *
     * In principle it is simple to write a function that will encode the binary blob in the
     * way SQLite expects it. However, because the API forces the argument to be coerced into a
     * String, the SQLiteQuery object generated by the default query methods will bind all
     * arguments as Strings and SQL will *sanitize* them. This works okay for numeric types,
     * but the format for blobs is x'<hex string>'. Note the presence of quotes, which will
     * be sanitized, changing the contents of the field, and the query will fail to match the
     * blob.
     *
     * As far as I can tell, there are two possible ways around this problem. The first one
     * is to put the data in the query string and eschew it being an argument. This would
     * require doing the sanitizing by hand. The other is to call bindBlob directly on the
     * generated SQLiteQuery object, which not only is a lot less dangerous than rolling out
     * sanitizing, but also will do the right thing if the underlying format ever changes.
     *
     * But none of the methods that take an SQLiteQuery object can return data ; this *must*
     * be called with SQLiteDatabase#query. This object is not accessible from outside.
     * However, there is a #query version that accepts a CursorFactory and this is pretty
     * straightforward to implement as all the arguments are coming in and the SQLiteCursor
     * class is public API.
     * With this, it's possible to intercept the SQLiteQuery object, and assuming the args
     * are available, to bind them directly and work around the API's oblivious coercion into
     * Strings.
     *
     * This is really sad, but I don't see another way of having this work than this or the
     * hand-rolled sanitizing, and this is the lesser evil.
     */
    private static class CustomCursorFactory implements SQLiteDatabase.CursorFactory {
        @NonNull
        private final ArrayList<Object> mArgs;
        CustomCursorFactory(@NonNull final ArrayList<Object> args) {
            mArgs = args;
        }
        @Override
        public Cursor newCursor(final SQLiteDatabase db, final SQLiteCursorDriver masterQuery,
                final String editTable,
                final SQLiteQuery query) {
            int index = 1; // bind is 1-indexed
            for (final Object arg : mArgs) {
                if (arg instanceof String) {
                    query.bindString(index++, (String) arg);
                } else if (arg instanceof Long) {
                    query.bindLong(index++, (Long) arg);
                } else if (arg instanceof Integer) {
                    query.bindLong(index++, Long.valueOf((Integer) arg));
                } else if (arg instanceof byte[]) {
                    query.bindBlob(index++, (byte[]) arg);
                } else {
                    throw new IllegalStateException("Unsupported type CustomCursorFactory "
                            + arg.getClass().toString());
                }
            }
            return new SQLiteCursor(masterQuery, editTable, query);
        }
    }

    // Returns the l2key of the closest match, if and only if it matches
    // closely enough (as determined by group-closeness).
    @Nullable
    static String findClosestAttributes(@NonNull final SQLiteDatabase db,
            @NonNull final NetworkAttributes attr) {
        if (attr.isEmpty()) return null;
        final ContentValues values = toContentValues(attr);

        // Build the selection and args. To cut down on the number of lines to search, limit
        // the search to those with at least one argument equals to the requested attributes.
        // This works only because null attributes match only will not result in group-closeness.
        final StringJoiner sj = new StringJoiner(" OR ");
        final ArrayList<Object> args = new ArrayList<>();
        args.add(System.currentTimeMillis());
        for (final String field : values.keySet()) {
            sj.add(field + " = ?");
            args.add(values.get(field));
        }

        final String selection = NetworkAttributesContract.COLNAME_EXPIRYDATE + " > ? AND ("
                + sj.toString() + ")";
        final Cursor cursor = db.queryWithFactory(new CustomCursorFactory(args),
                false, // distinct
                NetworkAttributesContract.TABLENAME,
                null, // columns, null means everything
                selection, // selection
                null, // selectionArgs, horrendously passed to the cursor factory instead
                null, // groupBy
                null, // having
                null, // orderBy
                null); // limit
        if (cursor.getCount() <= 0) return null;
        cursor.moveToFirst();
        String bestKey = null;
        float bestMatchConfidence = GROUPCLOSE_CONFIDENCE; // Never return a match worse than this.
        while (!cursor.isAfterLast()) {
            final NetworkAttributes read = readNetworkAttributesLine(cursor);
            final float confidence = read.getNetworkGroupSamenessConfidence(attr);
            if (confidence > bestMatchConfidence) {
                bestKey = getString(cursor, NetworkAttributesContract.COLNAME_L2KEY);
                bestMatchConfidence = confidence;
            }
            cursor.moveToNext();
        }
        cursor.close();
        return bestKey;
    }

    // Drops all records that are expired. Relevance has decayed to zero of these records. Returns
    // an int out of Status.{SUCCESS, ERROR_*}
    static int dropAllExpiredRecords(@NonNull final SQLiteDatabase db) {
        db.beginTransaction();
        try {
            // Deletes NetworkAttributes that have expired.
            db.delete(NetworkAttributesContract.TABLENAME,
                    NetworkAttributesContract.COLNAME_EXPIRYDATE + " < ?",
                    new String[]{Long.toString(System.currentTimeMillis())});
            db.setTransactionSuccessful();
        } catch (SQLiteException e) {
            Log.e(TAG, "Could not delete data from memory store", e);
            return Status.ERROR_STORAGE;
        } finally {
            db.endTransaction();
        }

        // Execute vacuuming here if above operation has no exception. If above operation got
        // exception, vacuuming can be ignored for reducing unnecessary consumption.
        try {
            db.execSQL("VACUUM");
        } catch (SQLiteException e) {
            // Do nothing.
        }
        return Status.SUCCESS;
    }

    // Drops number of records that start from the lowest expiryDate. Returns an int out of
    // Status.{SUCCESS, ERROR_*}
    static int dropNumberOfRecords(@NonNull final SQLiteDatabase db, int number) {
        if (number <= 0) {
            return Status.ERROR_ILLEGAL_ARGUMENT;
        }

        // Queries number of NetworkAttributes that start from the lowest expiryDate.
        final Cursor cursor = db.query(NetworkAttributesContract.TABLENAME,
                new String[] {NetworkAttributesContract.COLNAME_EXPIRYDATE}, // columns
                null, // selection
                null, // selectionArgs
                null, // groupBy
                null, // having
                NetworkAttributesContract.COLNAME_EXPIRYDATE, // orderBy
                Integer.toString(number)); // limit
        if (cursor == null || cursor.getCount() <= 0) return Status.ERROR_GENERIC;
        cursor.moveToLast();

        //Get the expiryDate from last record.
        final long expiryDate = getLong(cursor, NetworkAttributesContract.COLNAME_EXPIRYDATE, 0);
        cursor.close();

        db.beginTransaction();
        try {
            // Deletes NetworkAttributes that expiryDate are lower than given value.
            db.delete(NetworkAttributesContract.TABLENAME,
                    NetworkAttributesContract.COLNAME_EXPIRYDATE + " <= ?",
                    new String[]{Long.toString(expiryDate)});
            db.setTransactionSuccessful();
        } catch (SQLiteException e) {
            Log.e(TAG, "Could not delete data from memory store", e);
            return Status.ERROR_STORAGE;
        } finally {
            db.endTransaction();
        }

        // Execute vacuuming here if above operation has no exception. If above operation got
        // exception, vacuuming can be ignored for reducing unnecessary consumption.
        try {
            db.execSQL("VACUUM");
        } catch (SQLiteException e) {
            // Do nothing.
        }
        return Status.SUCCESS;
    }

    static int getTotalRecordNumber(@NonNull final SQLiteDatabase db) {
        // Query the total number of NetworkAttributes
        final Cursor cursor = db.query(NetworkAttributesContract.TABLENAME,
                new String[] {"COUNT(*)"}, // columns
                null, // selection
                null, // selectionArgs
                null, // groupBy
                null, // having
                null); // orderBy
        cursor.moveToFirst();
        return cursor == null ? 0 : cursor.getInt(0);
    }

    // Helper methods
    private static String getString(final Cursor cursor, final String columnName) {
        final int columnIndex = cursor.getColumnIndex(columnName);
        return (columnIndex >= 0) ? cursor.getString(columnIndex) : null;
    }
    private static byte[] getBlob(final Cursor cursor, final String columnName) {
        final int columnIndex = cursor.getColumnIndex(columnName);
        return (columnIndex >= 0) ? cursor.getBlob(columnIndex) : null;
    }
    private static int getInt(final Cursor cursor, final String columnName,
            final int defaultValue) {
        final int columnIndex = cursor.getColumnIndex(columnName);
        return (columnIndex >= 0) ? cursor.getInt(columnIndex) : defaultValue;
    }
    private static long getLong(final Cursor cursor, final String columnName,
            final long defaultValue) {
        final int columnIndex = cursor.getColumnIndex(columnName);
        return (columnIndex >= 0) ? cursor.getLong(columnIndex) : defaultValue;
    }
}