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   /*
    * Copyright 2013 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.util.internal;
  
  import static io.netty.util.internal.ObjectUtil.checkPositive;
  import static io.netty.util.internal.ObjectUtil.checkNonEmpty;
  
  import java.util.Arrays;
  
  public final class AppendableCharSequence implements CharSequence, Appendable {
      private char[] chars;
      private int pos;
  
      public AppendableCharSequence(int length) {
          chars = new char[checkPositive(length, "length")];
      }
  
      private AppendableCharSequence(char[] chars) {
          this.chars = checkNonEmpty(chars, "chars");
          pos = chars.length;
      }
  
      public void setLength(int length) {
          if (length < 0 || length > pos) {
              throw new IllegalArgumentException("length: " + length + " (length: >= 0, <= " + pos + ')');
          }
          this.pos = length;
      }
  
      @Override
      public int length() {
          return pos;
      }
  
      @Override
      public char charAt(int index) {
          if (index > pos) {
              throw new IndexOutOfBoundsException();
          }
          return chars[index];
      }
  
      /**
       * Access a value in this {@link CharSequence}.
       * This method is considered unsafe as index values are assumed to be legitimate.
       * Only underlying array bounds checking is done.
       * @param index The index to access the underlying array at.
       * @return The value at {@code index}.
       */
      public char charAtUnsafe(int index) {
          return chars[index];
      }
  
      @Override
      public AppendableCharSequence subSequence(int start, int end) {
          if (start == end) {
              // If start and end index is the same we need to return an empty sequence to conform to the interface.
              // As our expanding logic depends on the fact that we have a char[] with length > 0 we need to construct
              // an instance for which this is true.
              return new AppendableCharSequence(Math.min(16, chars.length));
          }
          return new AppendableCharSequence(Arrays.copyOfRange(chars, start, end));
      }
  
      @Override
      public AppendableCharSequence append(char c) {
          if (pos == chars.length) {
              char[] old = chars;
              chars = new char[old.length << 1];
              System.arraycopy(old, 0, chars, 0, old.length);
          }
          chars[pos++] = c;
          return this;
      }
  
      @Override
      public AppendableCharSequence append(CharSequence csq) {
          return append(csq, 0, csq.length());
      }
  
      @Override
      public AppendableCharSequence append(CharSequence csq, int start, int end) {
          if (csq.length() < end) {
              throw new IndexOutOfBoundsException("expected: csq.length() >= ("
                      + end + "),but actual is (" + csq.length() + ")");
          }
         int length = end - start;
         if (length > chars.length - pos) {
             chars = expand(chars, pos + length, pos);
         }
         if (csq instanceof AppendableCharSequence) {
             // Optimize append operations via array copy
             AppendableCharSequence seq = (AppendableCharSequence) csq;
             char[] src = seq.chars;
             System.arraycopy(src, start, chars, pos, length);
             pos += length;
             return this;
         }
         for (int i = start; i < end; i++) {
             chars[pos++] = csq.charAt(i);
         }
 
         return this;
     }
 
     /**
      * Reset the {@link AppendableCharSequence}. Be aware this will only reset the current internal position and not
      * shrink the internal char array.
      */
     public void reset() {
         pos = 0;
     }
 
     @Override
     public String toString() {
         return new String(chars, 0, pos);
     }
 
     /**
      * Create a new {@link String} from the given start to end.
      */
     public String substring(int start, int end) {
         int length = end - start;
         if (start > pos || length > pos) {
             throw new IndexOutOfBoundsException("expected: start and length <= ("
                     + pos + ")");
         }
         return new String(chars, start, length);
     }
 
     /**
      * Create a new {@link String} from the given start to end.
      * This method is considered unsafe as index values are assumed to be legitimate.
      * Only underlying array bounds checking is done.
      */
     public String subStringUnsafe(int start, int end) {
         return new String(chars, start, end - start);
     }
 
     private static char[] expand(char[] array, int neededSpace, int size) {
         int newCapacity = array.length;
         do {
             // double capacity until it is big enough
             newCapacity <<= 1;
 
             if (newCapacity < 0) {
                 throw new IllegalStateException();
             }
 
         } while (neededSpace > newCapacity);
 
         char[] newArray = new char[newCapacity];
         System.arraycopy(array, 0, newArray, 0, size);
 
         return newArray;
     }
 }