/*
 * Copyright (C) 2007 The Guava Authors
 *
 * 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.google.common.base;

import static com.google.common.base.Preconditions.checkNotNull;

import java.util.Arrays;

import javax.annotation.CheckReturnValue;
import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;

/**
 * Helper functions that can operate on any {@code Object}.
 *
 * <p>
 * See the Guava User Guide on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/CommonObjectUtilitiesExplained">writing
 * {@code Object} methods with {@code Objects}</a>.
 *
 * @author Laurence Gonsalves
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible
public final class Objects {
	private Objects() {
	}

	/**
	 * Determines whether two possibly-null objects are equal. Returns:
	 *
	 * <ul>
	 * <li>{@code true} if {@code a} and {@code b} are both null.
	 * <li>{@code true} if {@code a} and {@code b} are both non-null and they are
	 * equal according to {@link Object#equals(Object)}.
	 * <li>{@code false} in all other situations.
	 * </ul>
	 *
	 * <p>
	 * This assumes that any non-null objects passed to this function conform to the
	 * {@code equals()} contract.
	 */
	@CheckReturnValue
	public static boolean equal(@Nullable Object a, @Nullable Object b) {
		return a == b || (a != null && a.equals(b));
	}

	/**
	 * Generates a hash code for multiple values. The hash code is generated by
	 * calling {@link Arrays#hashCode(Object[])}. Note that array arguments to this
	 * method, with the exception of a single Object array, do not get any special
	 * handling; their hash codes are based on identity and not contents.
	 *
	 * <p>
	 * This is useful for implementing {@link Object#hashCode()}. For example, in an
	 * object that has three properties, {@code x}, {@code y}, and {@code z}, one
	 * could write:
	 * 
	 * <pre>
	 *    {@code
	 *   public int hashCode() {
	 *     return Objects.hashCode(getX(), getY(), getZ());
	 *   }}
	 * </pre>
	 *
	 * <p>
	 * <b>Warning</b>: When a single object is supplied, the returned hash code does
	 * not equal the hash code of that object.
	 */
	public static int hashCode(@Nullable Object... objects) {
		return Arrays.hashCode(objects);
	}

	/**
	 * Creates an instance of {@link ToStringHelper}.
	 *
	 * <p>
	 * This is helpful for implementing {@link Object#toString()}. Specification by
	 * example:
	 * 
	 * <pre>
	 *    {@code
	 *   // Returns "ClassName{}"
	 *   Objects.toStringHelper(this)
	 *       .toString();
	 *
	 *   // Returns "ClassName{x=1}"
	 *   Objects.toStringHelper(this)
	 *       .add("x", 1)
	 *       .toString();
	 *
	 *   // Returns "MyObject{x=1}"
	 *   Objects.toStringHelper("MyObject")
	 *       .add("x", 1)
	 *       .toString();
	 *
	 *   // Returns "ClassName{x=1, y=foo}"
	 *   Objects.toStringHelper(this)
	 *       .add("x", 1)
	 *       .add("y", "foo")
	 *       .toString();
	 *
	 *   // Returns "ClassName{x=1}"
	 *   Objects.toStringHelper(this)
	 *       .omitNullValues()
	 *       .add("x", 1)
	 *       .add("y", null)
	 *       .toString();
	 *   }}
	 * </pre>
	 *
	 * <p>
	 * Note that in GWT, class names are often obfuscated.
	 *
	 * @param self the object to generate the string for (typically {@code this}),
	 *             used only for its class name
	 * @since 2.0
	 */
	public static ToStringHelper toStringHelper(Object self) {
		return new ToStringHelper(simpleName(self.getClass()));
	}

	/**
	 * Creates an instance of {@link ToStringHelper} in the same manner as
	 * {@link Objects#toStringHelper(Object)}, but using the name of {@code clazz}
	 * instead of using an instance's {@link Object#getClass()}.
	 *
	 * <p>
	 * Note that in GWT, class names are often obfuscated.
	 *
	 * @param clazz the {@link Class} of the instance
	 * @since 7.0 (source-compatible since 2.0)
	 */
	public static ToStringHelper toStringHelper(Class<?> clazz) {
		return new ToStringHelper(simpleName(clazz));
	}

	/**
	 * Creates an instance of {@link ToStringHelper} in the same manner as
	 * {@link Objects#toStringHelper(Object)}, but using {@code className} instead
	 * of using an instance's {@link Object#getClass()}.
	 *
	 * @param className the name of the instance type
	 * @since 7.0 (source-compatible since 2.0)
	 */
	public static ToStringHelper toStringHelper(String className) {
		return new ToStringHelper(className);
	}

	/**
	 * {@link Class#getSimpleName()} is not GWT compatible yet, so we provide our
	 * own implementation.
	 */
	private static String simpleName(Class<?> clazz) {
		String name = clazz.getName();

		// the nth anonymous class has a class name ending in "Outer$n"
		// and local inner classes have names ending in "Outer.$1Inner"
		name = name.replaceAll("\\$[0-9]+", "\\$");

		// we want the name of the inner class all by its lonesome
		int start = name.lastIndexOf('$');

		// if this isn't an inner class, just find the start of the
		// top level class name.
		if (start == -1) {
			start = name.lastIndexOf('.');
		}
		return name.substring(start + 1);
	}

	/**
	 * Returns the first of two given parameters that is not {@code null}, if either
	 * is, or otherwise throws a {@link NullPointerException}.
	 *
	 * <p>
	 * <b>Note:</b> if {@code first} is represented as an {@link Optional}, this can
	 * be accomplished with {@linkplain Optional#or(Object) first.or(second)}. That
	 * approach also allows for lazy evaluation of the fallback instance, using
	 * {@linkplain Optional#or(Supplier) first.or(Supplier)}.
	 *
	 * @return {@code first} if {@code first} is not {@code null}, or {@code second}
	 *         if {@code first} is {@code null} and {@code second} is not
	 *         {@code null}
	 * @throws NullPointerException if both {@code first} and {@code second} were
	 *                              {@code null}
	 * @since 3.0
	 */
	public static <T> T firstNonNull(@Nullable T first, @Nullable T second) {
		return first != null ? first : checkNotNull(second);
	}

	/**
	 * Support class for {@link Objects#toStringHelper}.
	 *
	 * @author Jason Lee
	 * @since 2.0
	 */
	public static final class ToStringHelper {
		private final String className;
		private ValueHolder holderHead = new ValueHolder();
		private ValueHolder holderTail = holderHead;
		private boolean omitNullValues = false;

		/**
		 * Use {@link Objects#toStringHelper(Object)} to create an instance.
		 */
		private ToStringHelper(String className) {
			this.className = checkNotNull(className);
		}

		/**
		 * Configures the {@link ToStringHelper} so {@link #toString()} will ignore
		 * properties with null value. The order of calling this method, relative to the
		 * {@code add()}/{@code addValue()} methods, is not significant.
		 *
		 * @since 12.0
		 */
		public ToStringHelper omitNullValues() {
			omitNullValues = true;
			return this;
		}

		/**
		 * Adds a name/value pair to the formatted output in {@code name=value} format.
		 * If {@code value} is {@code null}, the string {@code "null"} is used, unless
		 * {@link #omitNullValues()} is called, in which case this name/value pair will
		 * not be added.
		 */
		public ToStringHelper add(String name, @Nullable Object value) {
			return addHolder(name, value);
		}

		/**
		 * Adds a name/value pair to the formatted output in {@code name=value} format.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper add(String name, boolean value) {
			return addHolder(name, String.valueOf(value));
		}

		/**
		 * Adds a name/value pair to the formatted output in {@code name=value} format.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper add(String name, char value) {
			return addHolder(name, String.valueOf(value));
		}

		/**
		 * Adds a name/value pair to the formatted output in {@code name=value} format.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper add(String name, double value) {
			return addHolder(name, String.valueOf(value));
		}

		/**
		 * Adds a name/value pair to the formatted output in {@code name=value} format.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper add(String name, float value) {
			return addHolder(name, String.valueOf(value));
		}

		/**
		 * Adds a name/value pair to the formatted output in {@code name=value} format.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper add(String name, int value) {
			return addHolder(name, String.valueOf(value));
		}

		/**
		 * Adds a name/value pair to the formatted output in {@code name=value} format.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper add(String name, long value) {
			return addHolder(name, String.valueOf(value));
		}

		/**
		 * Adds an unnamed value to the formatted output.
		 *
		 * <p>
		 * It is strongly encouraged to use {@link #add(String, Object)} instead and
		 * give value a readable name.
		 */
		public ToStringHelper addValue(@Nullable Object value) {
			return addHolder(value);
		}

		/**
		 * Adds an unnamed value to the formatted output.
		 *
		 * <p>
		 * It is strongly encouraged to use {@link #add(String, boolean)} instead and
		 * give value a readable name.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper addValue(boolean value) {
			return addHolder(String.valueOf(value));
		}

		/**
		 * Adds an unnamed value to the formatted output.
		 *
		 * <p>
		 * It is strongly encouraged to use {@link #add(String, char)} instead and give
		 * value a readable name.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper addValue(char value) {
			return addHolder(String.valueOf(value));
		}

		/**
		 * Adds an unnamed value to the formatted output.
		 *
		 * <p>
		 * It is strongly encouraged to use {@link #add(String, double)} instead and
		 * give value a readable name.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper addValue(double value) {
			return addHolder(String.valueOf(value));
		}

		/**
		 * Adds an unnamed value to the formatted output.
		 *
		 * <p>
		 * It is strongly encouraged to use {@link #add(String, float)} instead and give
		 * value a readable name.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper addValue(float value) {
			return addHolder(String.valueOf(value));
		}

		/**
		 * Adds an unnamed value to the formatted output.
		 *
		 * <p>
		 * It is strongly encouraged to use {@link #add(String, int)} instead and give
		 * value a readable name.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper addValue(int value) {
			return addHolder(String.valueOf(value));
		}

		/**
		 * Adds an unnamed value to the formatted output.
		 *
		 * <p>
		 * It is strongly encouraged to use {@link #add(String, long)} instead and give
		 * value a readable name.
		 *
		 * @since 11.0 (source-compatible since 2.0)
		 */
		public ToStringHelper addValue(long value) {
			return addHolder(String.valueOf(value));
		}

		/**
		 * Returns a string in the format specified by
		 * {@link Objects#toStringHelper(Object)}.
		 *
		 * <p>
		 * After calling this method, you can keep adding more properties to later call
		 * toString() again and get a more complete representation of the same object;
		 * but properties cannot be removed, so this only allows limited reuse of the
		 * helper instance. The helper allows duplication of properties (multiple
		 * name/value pairs with the same name can be added).
		 */
		@Override
		public String toString() {
			// create a copy to keep it consistent in case value changes
			boolean omitNullValuesSnapshot = omitNullValues;
			String nextSeparator = "";
			StringBuilder builder = new StringBuilder(32).append(className).append('{');
			for (ValueHolder valueHolder = holderHead.next; valueHolder != null; valueHolder = valueHolder.next) {
				if (!omitNullValuesSnapshot || valueHolder.value != null) {
					builder.append(nextSeparator);
					nextSeparator = ", ";

					if (valueHolder.name != null) {
						builder.append(valueHolder.name).append('=');
					}
					builder.append(valueHolder.value);
				}
			}
			return builder.append('}').toString();
		}

		private ValueHolder addHolder() {
			ValueHolder valueHolder = new ValueHolder();
			holderTail = holderTail.next = valueHolder;
			return valueHolder;
		}

		private ToStringHelper addHolder(@Nullable Object value) {
			ValueHolder valueHolder = addHolder();
			valueHolder.value = value;
			return this;
		}

		private ToStringHelper addHolder(String name, @Nullable Object value) {
			ValueHolder valueHolder = addHolder();
			valueHolder.value = value;
			valueHolder.name = checkNotNull(name);
			return this;
		}

		private static final class ValueHolder {
			String name;
			Object value;
			ValueHolder next;
		}
	}
}