/*
    * 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 com.google.common.annotations.GwtCompatible;
  import com.google.common.annotations.VisibleForTesting;
  
  import java.util.Formatter;
  
  import javax.annotation.Nullable;
  
  /**
   * 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
     */
    @Nullable
    public static 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));
   }
 }