/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.lang3;

import java.io.Serializable;
import java.lang.reflect.Array;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.TreeSet;

import org.apache.commons.lang3.exception.CloneFailedException;
import org.apache.commons.lang3.mutable.MutableInt;

/**
 * <p>Operations on {@code Object}.</p>
 *
 * <p>This class tries to handle {@code null} input gracefully.
 * An exception will generally not be thrown for a {@code null} input.
 * Each method documents its behaviour in more detail.</p>
 *
 * <p>#ThreadSafe#</p>
 * @since 1.0
 * @version $Id: ObjectUtils.java 1199894 2011-11-09 17:53:59Z ggregory $
 */
//@Immutable
public class ObjectUtils {

    /**
     * <p>Singleton used as a {@code null} placeholder where
     * {@code null} has another meaning.</p>
     *
     * <p>For example, in a {@code HashMap} the
     * {@link java.util.HashMap#get(java.lang.Object)} method returns
     * {@code null} if the {@code Map} contains {@code null} or if there
     * is no matching key. The {@code Null} placeholder can be used to
     * distinguish between these two cases.</p>
     *
     * <p>Another example is {@code Hashtable}, where {@code null}
     * cannot be stored.</p>
     *
     * <p>This instance is Serializable.</p>
     */
    public static final Null NULL = new Null();

    /**
     * <p>{@code ObjectUtils} instances should NOT be constructed in
     * standard programming. Instead, the static methods on the class should
     * be used, such as {@code ObjectUtils.defaultIfNull("a","b");}.</p>
     *
     * <p>This constructor is public to permit tools that require a JavaBean
     * instance to operate.</p>
     */
    public ObjectUtils() {
        super();
    }

    // Defaulting
    //-----------------------------------------------------------------------
    /**
     * <p>Returns a default value if the object passed is {@code null}.</p>
     *
     * <pre>
     * ObjectUtils.defaultIfNull(null, null)      = null
     * ObjectUtils.defaultIfNull(null, "")        = ""
     * ObjectUtils.defaultIfNull(null, "zz")      = "zz"
     * ObjectUtils.defaultIfNull("abc", *)        = "abc"
     * ObjectUtils.defaultIfNull(Boolean.TRUE, *) = Boolean.TRUE
     * </pre>
     *
     * @param <T> the type of the object
     * @param object  the {@code Object} to test, may be {@code null}
     * @param defaultValue  the default value to return, may be {@code null}
     * @return {@code object} if it is not {@code null}, defaultValue otherwise
     */
    public static <T> T defaultIfNull(T object, T defaultValue) {
        return object != null ? object : defaultValue;
    }

    /**
     * <p>Returns the first value in the array which is not {@code null}.
     * If all the values are {@code null} or the array is {@code null}
     * or empty then {@code null} is returned.</p>
     *
     * <pre>
     * ObjectUtils.firstNonNull(null, null)      = null
     * ObjectUtils.firstNonNull(null, "")        = ""
     * ObjectUtils.firstNonNull(null, null, "")  = ""
     * ObjectUtils.firstNonNull(null, "zz")      = "zz"
     * ObjectUtils.firstNonNull("abc", *)        = "abc"
     * ObjectUtils.firstNonNull(null, "xyz", *)  = "xyz"
     * ObjectUtils.firstNonNull(Boolean.TRUE, *) = Boolean.TRUE
     * ObjectUtils.firstNonNull()                = null
     * </pre>
     *
     * @param <T> the component type of the array
     * @param values  the values to test, may be {@code null} or empty
     * @return the first value from {@code values} which is not {@code null},
     *  or {@code null} if there are no non-null values
     * @since 3.0
     */
    public static <T> T firstNonNull(T... values) {
        if (values != null) {
            for (T val : values) {
                if (val != null) {
                    return val;
                }
            }
        }
        return null;
    }

    // Null-safe equals/hashCode
    //-----------------------------------------------------------------------
    /**
     * <p>Compares two objects for equality, where either one or both
     * objects may be {@code null}.</p>
     *
     * <pre>
     * ObjectUtils.equals(null, null)                  = true
     * ObjectUtils.equals(null, "")                    = false
     * ObjectUtils.equals("", null)                    = false
     * ObjectUtils.equals("", "")                      = true
     * ObjectUtils.equals(Boolean.TRUE, null)          = false
     * ObjectUtils.equals(Boolean.TRUE, "true")        = false
     * ObjectUtils.equals(Boolean.TRUE, Boolean.TRUE)  = true
     * ObjectUtils.equals(Boolean.TRUE, Boolean.FALSE) = false
     * </pre>
     *
     * @param object1  the first object, may be {@code null}
     * @param object2  the second object, may be {@code null}
     * @return {@code true} if the values of both objects are the same
     */
    public static boolean equals(Object object1, Object object2) {
        if (object1 == object2) {
            return true;
        }
        if (object1 == null || object2 == null) {
            return false;
        }
        return object1.equals(object2);
    }

    /**
     * <p>Compares two objects for inequality, where either one or both
     * objects may be {@code null}.</p>
     *
     * <pre>
     * ObjectUtils.notEqual(null, null)                  = false
     * ObjectUtils.notEqual(null, "")                    = true
     * ObjectUtils.notEqual("", null)                    = true
     * ObjectUtils.notEqual("", "")                      = false
     * ObjectUtils.notEqual(Boolean.TRUE, null)          = true
     * ObjectUtils.notEqual(Boolean.TRUE, "true")        = true
     * ObjectUtils.notEqual(Boolean.TRUE, Boolean.TRUE)  = false
     * ObjectUtils.notEqual(Boolean.TRUE, Boolean.FALSE) = true
     * </pre>
     *
     * @param object1  the first object, may be {@code null}
     * @param object2  the second object, may be {@code null}
     * @return {@code false} if the values of both objects are the same
     */
    public static boolean notEqual(Object object1, Object object2) {
        return ObjectUtils.equals(object1, object2) == false;
    }

    /**
     * <p>Gets the hash code of an object returning zero when the
     * object is {@code null}.</p>
     *
     * <pre>
     * ObjectUtils.hashCode(null)   = 0
     * ObjectUtils.hashCode(obj)    = obj.hashCode()
     * </pre>
     *
     * @param obj  the object to obtain the hash code of, may be {@code null}
     * @return the hash code of the object, or zero if null
     * @since 2.1
     */
    public static int hashCode(Object obj) {
        // hashCode(Object) retained for performance, as hash code is often critical
        return obj == null ? 0 : obj.hashCode();
    }

    /**
     * <p>Gets the hash code for multiple objects.</p>
     * 
     * <p>This allows a hash code to be rapidly calculated for a number of objects.
     * The hash code for a single object is the <em>not</em> same as {@link #hashCode(Object)}.
     * The hash code for multiple objects is the same as that calculated by an
     * {@code ArrayList} containing the specified objects.</p>
     *
     * <pre>
     * ObjectUtils.hashCodeMulti()                 = 1
     * ObjectUtils.hashCodeMulti((Object[]) null)  = 1
     * ObjectUtils.hashCodeMulti(a)                = 31 + a.hashCode()
     * ObjectUtils.hashCodeMulti(a,b)              = (31 + a.hashCode()) * 31 + b.hashCode()
     * ObjectUtils.hashCodeMulti(a,b,c)            = ((31 + a.hashCode()) * 31 + b.hashCode()) * 31 + c.hashCode()
     * </pre>
     *
     * @param objects  the objects to obtain the hash code of, may be {@code null}
     * @return the hash code of the objects, or zero if null
     * @since 3.0
     */
    public static int hashCodeMulti(Object... objects) {
        int hash = 1;
        if (objects != null) {
            for (Object object : objects) {
                hash = hash * 31 + ObjectUtils.hashCode(object);
            }
        }
        return hash;
    }

    // Identity ToString
    //-----------------------------------------------------------------------
    /**
     * <p>Gets the toString that would be produced by {@code Object}
     * if a class did not override toString itself. {@code null}
     * will return {@code null}.</p>
     *
     * <pre>
     * ObjectUtils.identityToString(null)         = null
     * ObjectUtils.identityToString("")           = "java.lang.String@1e23"
     * ObjectUtils.identityToString(Boolean.TRUE) = "java.lang.Boolean@7fa"
     * </pre>
     *
     * @param object  the object to create a toString for, may be
     *  {@code null}
     * @return the default toString text, or {@code null} if
     *  {@code null} passed in
     */
    public static String identityToString(Object object) {
        if (object == null) {
            return null;
        }
        StringBuffer buffer = new StringBuffer();
        identityToString(buffer, object);
        return buffer.toString();
    }

    /**
     * <p>Appends the toString that would be produced by {@code Object}
     * if a class did not override toString itself. {@code null}
     * will throw a NullPointerException for either of the two parameters. </p>
     *
     * <pre>
     * ObjectUtils.identityToString(buf, "")            = buf.append("java.lang.String@1e23"
     * ObjectUtils.identityToString(buf, Boolean.TRUE)  = buf.append("java.lang.Boolean@7fa"
     * ObjectUtils.identityToString(buf, Boolean.TRUE)  = buf.append("java.lang.Boolean@7fa")
     * </pre>
     *
     * @param buffer  the buffer to append to
     * @param object  the object to create a toString for
     * @since 2.4
     */
    public static void identityToString(StringBuffer buffer, Object object) {
        if (object == null) {
            throw new NullPointerException("Cannot get the toString of a null identity");
        }
        buffer.append(object.getClass().getName())
              .append('@')
              .append(Integer.toHexString(System.identityHashCode(object)));
    }

    // ToString
    //-----------------------------------------------------------------------
    /**
     * <p>Gets the {@code toString} of an {@code Object} returning
     * an empty string ("") if {@code null} input.</p>
     *
     * <pre>
     * ObjectUtils.toString(null)         = ""
     * ObjectUtils.toString("")           = ""
     * ObjectUtils.toString("bat")        = "bat"
     * ObjectUtils.toString(Boolean.TRUE) = "true"
     * </pre>
     *
     * @see StringUtils#defaultString(String)
     * @see String#valueOf(Object)
     * @param obj  the Object to {@code toString}, may be null
     * @return the passed in Object's toString, or nullStr if {@code null} input
     * @since 2.0
     */
    public static String toString(Object obj) {
        return obj == null ? "" : obj.toString();
    }

    /**
     * <p>Gets the {@code toString} of an {@code Object} returning
     * a specified text if {@code null} input.</p>
     *
     * <pre>
     * ObjectUtils.toString(null, null)           = null
     * ObjectUtils.toString(null, "null")         = "null"
     * ObjectUtils.toString("", "null")           = ""
     * ObjectUtils.toString("bat", "null")        = "bat"
     * ObjectUtils.toString(Boolean.TRUE, "null") = "true"
     * </pre>
     *
     * @see StringUtils#defaultString(String,String)
     * @see String#valueOf(Object)
     * @param obj  the Object to {@code toString}, may be null
     * @param nullStr  the String to return if {@code null} input, may be null
     * @return the passed in Object's toString, or nullStr if {@code null} input
     * @since 2.0
     */
    public static String toString(Object obj, String nullStr) {
        return obj == null ? nullStr : obj.toString();
    }

    // Comparable
    //-----------------------------------------------------------------------
    /**
     * <p>Null safe comparison of Comparables.</p>
     *
     * @param <T> type of the values processed by this method
     * @param values the set of comparable values, may be null
     * @return
     *  <ul>
     *   <li>If any objects are non-null and unequal, the lesser object.
     *   <li>If all objects are non-null and equal, the first.
     *   <li>If any of the comparables are null, the lesser of the non-null objects.
     *   <li>If all the comparables are null, null is returned.
     *  </ul>
     */
    public static <T extends Comparable<? super T>> T min(T... values) {
        T result = null;
        if (values != null) {
            for (T value : values) {
                if (compare(value, result, true) < 0) {
                    result = value;
                }
            }
        }
        return result;
    }

    /**
     * <p>Null safe comparison of Comparables.</p>
     *
     * @param <T> type of the values processed by this method
     * @param values the set of comparable values, may be null
     * @return
     *  <ul>
     *   <li>If any objects are non-null and unequal, the greater object.
     *   <li>If all objects are non-null and equal, the first.
     *   <li>If any of the comparables are null, the greater of the non-null objects.
     *   <li>If all the comparables are null, null is returned.
     *  </ul>
     */
    public static <T extends Comparable<? super T>> T max(T... values) {
        T result = null;
        if (values != null) {
            for (T value : values) {
                if (compare(value, result, false) > 0) {
                    result = value;
                }
            }
        }
        return result;
    }

    /**
     * <p>Null safe comparison of Comparables.
     * {@code null} is assumed to be less than a non-{@code null} value.</p>
     *
     * @param <T> type of the values processed by this method
     * @param c1  the first comparable, may be null
     * @param c2  the second comparable, may be null
     * @return a negative value if c1 < c2, zero if c1 = c2
     *  and a positive value if c1 > c2
     */
    public static <T extends Comparable<? super T>> int compare(T c1, T c2) {
        return compare(c1, c2, false);
    }

    /**
     * <p>Null safe comparison of Comparables.</p>
     *
     * @param <T> type of the values processed by this method
     * @param c1  the first comparable, may be null
     * @param c2  the second comparable, may be null
     * @param nullGreater if true {@code null} is considered greater
     *  than a non-{@code null} value or if false {@code null} is
     *  considered less than a Non-{@code null} value
     * @return a negative value if c1 < c2, zero if c1 = c2
     *  and a positive value if c1 > c2
     * @see java.util.Comparator#compare(Object, Object)
     */
    public static <T extends Comparable<? super T>> int compare(T c1, T c2, boolean nullGreater) {
        if (c1 == c2) {
            return 0;
        } else if (c1 == null) {
            return nullGreater ? 1 : -1;
        } else if (c2 == null) {
            return nullGreater ? -1 : 1;
        }
        return c1.compareTo(c2);
    }

    /**
     * Find the "best guess" middle value among comparables. If there is an even
     * number of total values, the lower of the two middle values will be returned.
     * @param <T> type of values processed by this method
     * @param items to compare
     * @return T at middle position
     * @throws NullPointerException if items is {@code null}
     * @throws IllegalArgumentException if items is empty or contains {@code null} values
     * @since 3.0.1
     */
    public static <T extends Comparable<? super T>> T median(T... items) {
        Validate.notEmpty(items);
        Validate.noNullElements(items);
        TreeSet<T> sort = new TreeSet<T>();
        Collections.addAll(sort, items);
        @SuppressWarnings("unchecked") //we know all items added were T instances
        T result = (T) sort.toArray()[(sort.size() - 1) / 2];
        return result;
    }

    /**
     * Find the "best guess" middle value among comparables. If there is an even
     * number of total values, the lower of the two middle values will be returned.
     * @param <T> type of values processed by this method
     * @param comparator to use for comparisons
     * @param items to compare
     * @return T at middle position
     * @throws NullPointerException if items or comparator is {@code null}
     * @throws IllegalArgumentException if items is empty or contains {@code null} values
     * @since 3.0.1
     */
    public static <T> T median(Comparator<T> comparator, T... items) {
        Validate.notEmpty(items, "null/empty items");
        Validate.noNullElements(items);
        Validate.notNull(comparator, "null comparator");
        TreeSet<T> sort = new TreeSet<T>(comparator);
        Collections.addAll(sort, items);
        @SuppressWarnings("unchecked") //we know all items added were T instances
        T result = (T) sort.toArray()[(sort.size() - 1) / 2];
        return result;
    }

    // Mode
    //-----------------------------------------------------------------------
    /**
     * Find the most frequently occurring item.
     * 
     * @param <T> type of values processed by this method
     * @param items to check
     * @return most populous T, {@code null} if non-unique or no items supplied
     * @since 3.0.1
     */
    public static <T> T mode(T... items) {
        if (ArrayUtils.isNotEmpty(items)) {
            HashMap<T, MutableInt> occurrences = new HashMap<T, MutableInt>(items.length);
            for (T t : items) {
                MutableInt count = occurrences.get(t);
                if (count == null) {
                    occurrences.put(t, new MutableInt(1));
                } else {
                    count.increment();
                }
            }
            T result = null;
            int max = 0;
            for (Map.Entry<T, MutableInt> e : occurrences.entrySet()) {
                int cmp = e.getValue().intValue();
                if (cmp == max) {
                    result = null;
                } else if (cmp > max) {
                    max = cmp;
                    result = e.getKey();
                }
            }
            return result;
        }
        return null;
    }

    // cloning
    //-----------------------------------------------------------------------
    /**
     * <p>Clone an object.</p>
     *
     * @param <T> the type of the object
     * @param obj  the object to clone, null returns null
     * @return the clone if the object implements {@link Cloneable} otherwise {@code null}
     * @throws CloneFailedException if the object is cloneable and the clone operation fails
     * @since 3.0
     */
    public static <T> T clone(final T obj) {
        if (obj instanceof Cloneable) {
            final Object result;
            if (obj.getClass().isArray()) {
                final Class<?> componentType = obj.getClass().getComponentType();
                if (!componentType.isPrimitive()) {
                    result = ((Object[]) obj).clone();
                } else {
                    int length = Array.getLength(obj);
                    result = Array.newInstance(componentType, length);
                    while (length-- > 0) {
                        Array.set(result, length, Array.get(obj, length));
                    }
                }
            } else {
                try {
                    final Method clone = obj.getClass().getMethod("clone");
                    result = clone.invoke(obj);
                } catch (final NoSuchMethodException e) {
                    throw new CloneFailedException("Cloneable type "
                        + obj.getClass().getName()
                        + " has no clone method", e);
                } catch (final IllegalAccessException e) {
                    throw new CloneFailedException("Cannot clone Cloneable type "
                        + obj.getClass().getName(), e);
                } catch (final InvocationTargetException e) {
                    throw new CloneFailedException("Exception cloning Cloneable type "
                        + obj.getClass().getName(), e.getCause());
                }
            }
            @SuppressWarnings("unchecked")
            final T checked = (T) result;
            return checked;
        }

        return null;
    }

    /**
     * <p>Clone an object if possible.</p>
     *
     * <p>This method is similar to {@link #clone(Object)}, but will return the provided
     * instance as the return value instead of {@code null} if the instance
     * is not cloneable. This is more convenient if the caller uses different
     * implementations (e.g. of a service) and some of the implementations do not allow concurrent
     * processing or have state. In such cases the implementation can simply provide a proper
     * clone implementation and the caller's code does not have to change.</p>
     *
     * @param <T> the type of the object
     * @param obj  the object to clone, null returns null
     * @return the clone if the object implements {@link Cloneable} otherwise the object itself
     * @throws CloneFailedException if the object is cloneable and the clone operation fails
     * @since 3.0
     */
    public static <T> T cloneIfPossible(final T obj) {
        final T clone = clone(obj);
        return clone == null ? obj : clone;
    }

    // Null
    //-----------------------------------------------------------------------
    /**
     * <p>Class used as a null placeholder where {@code null}
     * has another meaning.</p>
     *
     * <p>For example, in a {@code HashMap} the
     * {@link java.util.HashMap#get(java.lang.Object)} method returns
     * {@code null} if the {@code Map} contains {@code null} or if there is
     * no matching key. The {@code Null} placeholder can be used to distinguish
     * between these two cases.</p>
     *
     * <p>Another example is {@code Hashtable}, where {@code null}
     * cannot be stored.</p>
     */
    public static class Null implements Serializable {
        /**
         * Required for serialization support. Declare serialization compatibility with Commons Lang 1.0
         *
         * @see java.io.Serializable
         */
        private static final long serialVersionUID = 7092611880189329093L;

        /**
         * Restricted constructor - singleton.
         */
        Null() {
            super();
        }

        /**
         * <p>Ensure singleton.</p>
         *
         * @return the singleton value
         */
        private Object readResolve() {
            return ObjectUtils.NULL;
        }
    }

}
