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Non-domain infrastructure code that is common to many Neo4j components.
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j 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 .
 */
package org.neo4j.util;

import java.util.Objects;

/**
 * Feature toggles are used for features that are possible to configure, but where the configuration is always fixed in
 * a production system.
 *
 * Typical use cases for feature toggles are features that are integrated into the code base for integration and
 * testing purposes, but are not ready for inclusion in the finished product yet, or features that are always on or has
 * a fixed configured value in the finished product, but can assume a different configuration or be turned off in some
 * test.
 *
 * Feature toggles are passed to the JVM through {@linkplain System#getProperty(String) system properties}, and
 * expected to be looked up from a static context, a {@code static final} fields of the class the toggle controls.
 *
 * All methods in this class returns the default value if the system property has not been assigned, or if the value of
 * the system property cannot be interpreted as a value of the expected type.
 *
 * For features that the user is ever expected to touch, feature toggles is the wrong abstraction!
 */
public final class FeatureToggles {
    /**
     * Get the value of a {@code boolean} system property.
     *
     * The absolute name of the system property is computed based on the provided class and local name.
     *
     * @param location the class that owns the flag.
     * @param name the local name of the flag.
     * @param defaultValue the default value of the flag if the system property is not assigned.
     * @return the parsed value of the system property, or the default value.
     */
    public static boolean flag(Class location, String name, boolean defaultValue) {
        return booleanProperty(name(location, name), defaultValue);
    }

    /**
     * Get the value of a {@code long} system property.
     *
     * The absolute name of the system property is computed based on the provided class and local name.
     *
     * @param location the class that owns the flag.
     * @param name the local name of the flag.
     * @param defaultValue the default value of the flag if the system property is not assigned.
     * @return the parsed value of the system property, or the default value.
     */
    public static long getLong(Class location, String name, long defaultValue) {
        return Long.getLong(name(location, name), defaultValue);
    }

    /**
     * Get the value of a {@code int} system property.
     *
     * The absolute name of the system property is computed based on the provided class and local name.
     *
     * @param location the class that owns the flag.
     * @param name the local name of the flag.
     * @param defaultValue the default value of the flag if the system property is not assigned.
     * @return the parsed value of the system property, or the default value.
     */
    public static int getInteger(Class location, String name, int defaultValue) {
        return Integer.getInteger(name(location, name), defaultValue);
    }

    /**
     * Get the value of a {@code double} system property.
     *
     * The absolute name of the system property is computed based on the provided class and local name.
     *
     * @param location the class that owns the flag.
     * @param name the local name of the flag
     * @param defaultValue the default value of the flag if the system property is not assigned.
     * @return the parsed value of the system property, or the default value.
     */
    public static double getDouble(Class location, String name, double defaultValue) {
        try {
            String propertyValue = System.getProperty(name(location, name));
            if (propertyValue != null && !propertyValue.isEmpty()) {
                return Double.parseDouble(propertyValue);
            }
        } catch (Exception e) {
            // ignored
        }
        return defaultValue;
    }

    /**
     * Get the value of a {@link String} system property.
     * 
     * The absolute name of the system property is computed based on the provided class and local name.
     *
     * @param location the class that owns the flag.
     * @param name the local name of the flag
     * @param defaultValue the default value of the flag if the system property is not assigned.
     * @return the parsed value of the system property, or the default value.
     */
    public static String getString(Class location, String name, String defaultValue) {
        String propertyValue = System.getProperty(name(location, name));
        return propertyValue == null || propertyValue.isEmpty() ? defaultValue : propertyValue;
    }

    /**
     * Get the value of a {@code enum} system property.
     *
     * The absolute name of the system property is computed based on the provided class and local name.
     *
     * @param location the class that owns the flag.
     * @param name the local name of the flag.
     * @param defaultValue the default value of the flag if the system property is not assigned.
     * @param  the enum value type.
     * @return the parsed value of the system property, or the default value.
     */
    public static > E flag(Class location, String name, E defaultValue) {
        return enumProperty(defaultValue.getDeclaringClass(), name(location, name), defaultValue);
    }

    /**
     * Set the value of a system property.
     * 

     * The name of the system property is computed based on the provided class and local name.
     *
     * @param location the class that owns the flag.
     * @param name the local name of the flag.
     * @param value the value to assign to the system property.
     */
    public static void set(Class location, String name, Object value) {
        System.setProperty(name(location, name), Objects.toString(value));
    }

    /**
     * Clear the value of a system property.
     * 
     * The name of the system property is computed based on the provided class and local name.
     *
     * @param location the class that owns the flag.
     * @param name the local name of the flag.
     */
    public static void clear(Class location, String name) {
        System.clearProperty(name(location, name));
    }

    private FeatureToggles() {}

    private static String name(Class location, String name) {
        return location.getCanonicalName() + "." + name;
    }

    private static boolean booleanProperty(String flag, boolean defaultValue) {
        return parseBoolean(System.getProperty(flag), defaultValue);
    }

    private static boolean parseBoolean(String value, boolean defaultValue) {
        return defaultValue ? !"false".equalsIgnoreCase(value) : "true".equalsIgnoreCase(value);
    }

    private static > E enumProperty(Class enumClass, String name, E defaultValue) {
        try {
            return Enum.valueOf(enumClass, System.getProperty(name, defaultValue.name()));
        } catch (IllegalArgumentException e) {
            return defaultValue;
        }
    }
}