/*
 * Copyright (C) 2010 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

import java.util.Formatter;

import javax.annotation.Nullable;

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

/**
 * Static utility methods pertaining to {@code String} or {@code CharSequence}
 * instances.
 *
 * @author Kevin Bourrillion
 * @since 3.0
 */
@GwtCompatible
public final class Strings {
	private Strings() {
	}

	/**
	 * Returns the given string if it is non-null; the empty string otherwise.
	 *
	 * @param string the string to test and possibly return
	 * @return {@code string} itself if it is non-null; {@code ""} if it is null
	 */
	public static String nullToEmpty(@Nullable String string) {
		return (string == null) ? "" : string;
	}

	/**
	 * Returns the given string if it is nonempty; {@code null} otherwise.
	 *
	 * @param string the string to test and possibly return
	 * @return {@code string} itself if it is nonempty; {@code null} if it is empty
	 *         or null
	 */
	public static @Nullable String emptyToNull(@Nullable String string) {
		return isNullOrEmpty(string) ? null : string;
	}

	/**
	 * Returns {@code true} if the given string is null or is the empty string.
	 *
	 * <p>
	 * Consider normalizing your string references with {@link #nullToEmpty}. If you
	 * do, you can use {@link String#isEmpty()} instead of this method, and you
	 * won't need special null-safe forms of methods like {@link String#toUpperCase}
	 * either. Or, if you'd like to normalize "in the other direction," converting
	 * empty strings to {@code null}, you can use {@link #emptyToNull}.
	 *
	 * @param string a string reference to check
	 * @return {@code true} if the string is null or is the empty string
	 */
	public static boolean isNullOrEmpty(@Nullable String string) {
		return string == null || string.length() == 0; // string.isEmpty() in Java 6
	}

	/**
	 * Returns a string, of length at least {@code minLength}, consisting of
	 * {@code string} prepended with as many copies of {@code padChar} as are
	 * necessary to reach that length. For example,
	 *
	 * <ul>
	 * <li>{@code padStart("7", 3, '0')} returns {@code "007"}
	 * <li>{@code padStart("2010", 3, '0')} returns {@code "2010"}
	 * </ul>
	 *
	 * <p>
	 * See {@link Formatter} for a richer set of formatting capabilities.
	 *
	 * @param string    the string which should appear at the end of the result
	 * @param minLength the minimum length the resulting string must have. Can be
	 *                  zero or negative, in which case the input string is always
	 *                  returned.
	 * @param padChar   the character to insert at the beginning of the result until
	 *                  the minimum length is reached
	 * @return the padded string
	 */
	public static String padStart(String string, int minLength, char padChar) {
		checkNotNull(string); // eager for GWT.
		if (string.length() >= minLength) {
			return string;
		}
		StringBuilder sb = new StringBuilder(minLength);
		for (int i = string.length(); i < minLength; i++) {
			sb.append(padChar);
		}
		sb.append(string);
		return sb.toString();
	}

	/**
	 * Returns a string, of length at least {@code minLength}, consisting of
	 * {@code string} appended with as many copies of {@code padChar} as are
	 * necessary to reach that length. For example,
	 *
	 * <ul>
	 * <li>{@code padEnd("4.", 5, '0')} returns {@code "4.000"}
	 * <li>{@code padEnd("2010", 3, '!')} returns {@code "2010"}
	 * </ul>
	 *
	 * <p>
	 * See {@link Formatter} for a richer set of formatting capabilities.
	 *
	 * @param string    the string which should appear at the beginning of the
	 *                  result
	 * @param minLength the minimum length the resulting string must have. Can be
	 *                  zero or negative, in which case the input string is always
	 *                  returned.
	 * @param padChar   the character to append to the end of the result until the
	 *                  minimum length is reached
	 * @return the padded string
	 */
	public static String padEnd(String string, int minLength, char padChar) {
		checkNotNull(string); // eager for GWT.
		if (string.length() >= minLength) {
			return string;
		}
		StringBuilder sb = new StringBuilder(minLength);
		sb.append(string);
		for (int i = string.length(); i < minLength; i++) {
			sb.append(padChar);
		}
		return sb.toString();
	}

	/**
	 * Returns a string consisting of a specific number of concatenated copies of an
	 * input string. For example, {@code repeat("hey", 3)} returns the string
	 * {@code "heyheyhey"}.
	 *
	 * @param string any non-null string
	 * @param count  the number of times to repeat it; a nonnegative integer
	 * @return a string containing {@code string} repeated {@code count} times (the
	 *         empty string if {@code count} is zero)
	 * @throws IllegalArgumentException if {@code count} is negative
	 */
	public static String repeat(String string, int count) {
		checkNotNull(string); // eager for GWT.

		if (count <= 1) {
			checkArgument(count >= 0, "invalid count: %s", count);
			return (count == 0) ? "" : string;
		}

		// IF YOU MODIFY THE CODE HERE, you must update StringsRepeatBenchmark
		final int len = string.length();
		final long longSize = (long) len * (long) count;
		final int size = (int) longSize;
		if (size != longSize) {
			throw new ArrayIndexOutOfBoundsException("Required array size too large: " + longSize);
		}

		final char[] array = new char[size];
		string.getChars(0, len, array, 0);
		int n;
		for (n = len; n < size - n; n <<= 1) {
			System.arraycopy(array, 0, array, n, n);
		}
		System.arraycopy(array, 0, array, n, size - n);
		return new String(array);
	}

	/**
	 * Returns the longest string {@code prefix} such that
	 * {@code a.toString().startsWith(prefix) && b.toString().startsWith(prefix)},
	 * taking care not to split surrogate pairs. If {@code a} and {@code b} have no
	 * common prefix, returns the empty string.
	 *
	 * @since 11.0
	 */
	public static String commonPrefix(CharSequence a, CharSequence b) {
		checkNotNull(a);
		checkNotNull(b);

		int maxPrefixLength = Math.min(a.length(), b.length());
		int p = 0;
		while (p < maxPrefixLength && a.charAt(p) == b.charAt(p)) {
			p++;
		}
		if (validSurrogatePairAt(a, p - 1) || validSurrogatePairAt(b, p - 1)) {
			p--;
		}
		return a.subSequence(0, p).toString();
	}

	/**
	 * Returns the longest string {@code suffix} such that
	 * {@code a.toString().endsWith(suffix) && b.toString().endsWith(suffix)},
	 * taking care not to split surrogate pairs. If {@code a} and {@code b} have no
	 * common suffix, returns the empty string.
	 *
	 * @since 11.0
	 */
	public static String commonSuffix(CharSequence a, CharSequence b) {
		checkNotNull(a);
		checkNotNull(b);

		int maxSuffixLength = Math.min(a.length(), b.length());
		int s = 0;
		while (s < maxSuffixLength && a.charAt(a.length() - s - 1) == b.charAt(b.length() - s - 1)) {
			s++;
		}
		if (validSurrogatePairAt(a, a.length() - s - 1) || validSurrogatePairAt(b, b.length() - s - 1)) {
			s--;
		}
		return a.subSequence(a.length() - s, a.length()).toString();
	}

	/**
	 * True when a valid surrogate pair starts at the given {@code index} in the
	 * given {@code string}. Out-of-range indexes return false.
	 */
	@VisibleForTesting
	static boolean validSurrogatePairAt(CharSequence string, int index) {
		return index >= 0 && index <= (string.length() - 2) && Character.isHighSurrogate(string.charAt(index))
				&& Character.isLowSurrogate(string.charAt(index + 1));
	}
}