/*
 * Copyright (C) 2013 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.format;

import javax.annotation.Nullable;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;

/**
 * Static convenience methods that serve the same purpose as Java language
 * <a href=
 * "http://docs.oracle.com/javase/7/docs/technotes/guides/language/assert.html">
 * assertions</a>, except that they are always enabled. These methods should be
 * used instead of Java assertions whenever there is a chance the check may fail
 * "in real life". Example:
 * 
 * <pre>
 * {
 * 	&#64;code
 *
 * 	Bill bill = remoteService.getLastUnpaidBill();
 *
 * 	// In case bug 12345 happens again we'd rather just die
 * 	Verify.verify(bill.status() == Status.UNPAID, "Unexpected bill status: %s", bill.status());
 * }
 * </pre>
 *
 * <h3>Comparison to alternatives</h3>
 *
 * <p>
 * <b>Note:</b> In some cases the differences explained below can be subtle.
 * When it's unclear which approach to use, <b>don't worry</b> too much about
 * it; just pick something that seems reasonable and it will be fine.
 *
 * <ul>
 * <li>If checking whether the <i>caller</i> has violated your method or
 * constructor's contract (such as by passing an invalid argument), use the
 * utilities of the {@link Preconditions} class instead.
 *
 * <li>If checking an <i>impossible</i> condition (which <i>cannot</i> happen
 * unless your own class or its <i>trusted</i> dependencies is badly broken),
 * this is what ordinary Java assertions are for. Note that assertions are not
 * enabled by default; they are essentially considered "compiled comments."
 *
 * <li>An explicit {@code if/throw} (as illustrated above) is always acceptable;
 * we still recommend using our {@link VerifyException} exception type. Throwing
 * a plain {@link RuntimeException} is frowned upon.
 *
 * <li>Use of {@link java.util.Objects#requireNonNull(Object)} is generally
 * discouraged, since {@link #verifyNotNull(Object)} and
 * {@link Preconditions#checkNotNull(Object)} perform the same function with
 * more clarity.
 * </ul>
 *
 * <h3>Warning about performance</h3>
 *
 * <p>
 * Remember that parameter values for message construction must all be computed
 * eagerly, and autoboxing and varargs array creation may happen as well, even
 * when the verification succeeds and the message ends up unneeded.
 * Performance-sensitive verification checks should continue to use usual form:
 * 
 * <pre>
 * {
 * 	&#64;code
 *
 * 	Bill bill = remoteService.getLastUnpaidBill();
 * 	if (bill.status() != Status.UNPAID) {
 * 		throw new VerifyException("Unexpected bill status: " + bill.status());
 * 	}
 * }
 * </pre>
 *
 * <h3>Only {@code %s} is supported</h3>
 *
 * <p>
 * As with {@link Preconditions} error message template strings, only the
 * {@code "%s"} specifier is supported, not the full range of
 * {@link java.util.Formatter} specifiers. However, note that if the number of
 * arguments does not match the number of occurrences of {@code "%s"} in the
 * format string, {@code Verify} will still behave as expected, and will still
 * include all argument values in the error message; the message will simply not
 * be formatted exactly as intended.
 *
 * <h3>More information</h3>
 *
 * See <a href=
 * "http://code.google.com/p/guava-libraries/wiki/ConditionalFailuresExplained">Conditional
 * failures explained</a> in the Guava User Guide for advice on when this class
 * should be used.
 *
 * @since 17.0
 */
@Beta
@GwtCompatible
public final class Verify {
	/**
	 * Ensures that {@code expression} is {@code true}, throwing a
	 * {@code VerifyException} with no message otherwise.
	 */
	public static void verify(boolean expression) {
		if (!expression) {
			throw new VerifyException();
		}
	}

	/**
	 * Ensures that {@code expression} is {@code true}, throwing a
	 * {@code VerifyException} with a custom message otherwise.
	 *
	 * @param expression           a boolean expression
	 * @param errorMessageTemplate a template for the exception message should the
	 *                             check fail. The message is formed by replacing
	 *                             each {@code %s} placeholder in the template with
	 *                             an argument. These are matched by position - the
	 *                             first {@code %s} gets
	 *                             {@code errorMessageArgs[0]}, etc. Unmatched
	 *                             arguments will be appended to the formatted
	 *                             message in square braces. Unmatched placeholders
	 *                             will be left as-is.
	 * @param errorMessageArgs     the arguments to be substituted into the message
	 *                             template. Arguments are converted to strings
	 *                             using {@link String#valueOf(Object)}.
	 * @throws VerifyException if {@code expression} is {@code false}
	 */
	public static void verify(boolean expression, @Nullable String errorMessageTemplate,
			@Nullable Object... errorMessageArgs) {
		if (!expression) {
			throw new VerifyException(format(errorMessageTemplate, errorMessageArgs));
		}
	}

	/**
	 * Ensures that {@code reference} is non-null, throwing a
	 * {@code VerifyException} with a default message otherwise.
	 *
	 * @return {@code reference}, guaranteed to be non-null, for convenience
	 */
	public static <T> T verifyNotNull(@Nullable T reference) {
		return verifyNotNull(reference, "expected a non-null reference");
	}

	/**
	 * Ensures that {@code reference} is non-null, throwing a
	 * {@code VerifyException} with a custom message otherwise.
	 *
	 * @param errorMessageTemplate a template for the exception message should the
	 *                             check fail. The message is formed by replacing
	 *                             each {@code %s} placeholder in the template with
	 *                             an argument. These are matched by position - the
	 *                             first {@code %s} gets
	 *                             {@code errorMessageArgs[0]}, etc. Unmatched
	 *                             arguments will be appended to the formatted
	 *                             message in square braces. Unmatched placeholders
	 *                             will be left as-is.
	 * @param errorMessageArgs     the arguments to be substituted into the message
	 *                             template. Arguments are converted to strings
	 *                             using {@link String#valueOf(Object)}.
	 * @return {@code reference}, guaranteed to be non-null, for convenience
	 */
	public static <T> T verifyNotNull(@Nullable T reference, @Nullable String errorMessageTemplate,
			@Nullable Object... errorMessageArgs) {
		verify(reference != null, errorMessageTemplate, errorMessageArgs);
		return reference;
	}

	// TODO(kevinb): consider <T> T verifySingleton(Iterable<T>) to take over for
	// Iterables.getOnlyElement()

	private Verify() {
	}
}
