/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF 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
 *
 *   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 org.apache.juneau.commons.io;


import java.io.*;
import java.nio.*;
import java.nio.charset.*;
import static org.apache.juneau.commons.utils.Shorts.*;

/**
 * {@link InputStream} implementation that reads a character stream from a {@link Reader}
 * and transforms it to a byte stream using a specified charset encoding. The stream
 * is transformed using a {@link CharsetEncoder} object, guaranteeing that all charset
 * encodings supported by the JRE are handled correctly. In particular for charsets such as
 * UTF-16, the implementation ensures that one and only one byte order marker
 * is produced.
 * <p>
 * Since in general it is not possible to predict the number of characters to be read from the
 * {@link Reader} to satisfy a read request on the {@link ReaderInputStream}, all reads from
 * the {@link Reader} are buffered. There is therefore no well defined correlation
 * between the current position of the {@link Reader} and that of the {@link ReaderInputStream}.
 * This also implies that in general there is no need to wrap the underlying {@link Reader}
 * in a {@link BufferedReader}.
 * <p>
 * {@link ReaderInputStream} implements the inverse transformation of {@link InputStreamReader};
 * in the following example, reading from {@code in2} would return the same byte
 * sequence as reading from {@code in} (provided that the initial byte sequence is legal
 * with respect to the charset encoding):
 * <pre>
 * InputStream in = ...
 * Charset cs = ...
 * InputStreamReader reader = new InputStreamReader(in, cs);
 * ReaderInputStream in2 = new ReaderInputStream(reader, cs);</pre>
 * {@link ReaderInputStream} implements the same transformation as {@link OutputStreamWriter},
 * except that the control flow is reversed: both classes transform a character stream
 * into a byte stream, but {@link OutputStreamWriter} pushes data to the underlying stream,
 * while {@link ReaderInputStream} pulls it from the underlying stream.
 * <p>
 * Note that while there are use cases where there is no alternative to using
 * this class, very often the need to use this class is an indication of a flaw
 * in the design of the code. This class is typically used in situations where an existing
 * API only accepts an {@link InputStream}, but where the most natural way to produce the data
 * is as a character stream, i.e. by providing a {@link Reader} instance. An example of a situation
 * where this problem may appear is when implementing the {@code javax.activation.DataSource}
 * interface from the Java Activation Framework.
 * <p>
 * Given the fact that the {@link Reader} class doesn't provide any way to predict whether the next
 * read operation will block or not, it is not possible to provide a meaningful
 * implementation of the {@link InputStream#available()} method. A call to this method
 * will always return 0. Also, this class doesn't support {@link InputStream#mark(int)}.
 * <p>
 *
 * <h5 class='section'>Notes:</h5><ul>
 *    <li class='warn'>This class is not thread safe and is typically discarded after one use.
 * </ul>
 *
 */
@SuppressWarnings({
   "java:S1192", // Duplicated literals (argument/property names) read more clearly inline than as constants
   "resource" // Intentionally not owned; caller retains responsibility for closing the underlying Reader
})
public class ReaderInputStream extends InputStream {
   private static final int DEFAULT_BUFFER_SIZE = 1024;

   private final Reader reader;
   private final CharsetEncoder encoder;

   /**
    * CharBuffer used as input for the decoder. It should be reasonably
    * large as we read data from the underlying Reader into this buffer.
    */
   private final CharBuffer encoderIn;

   /**
    * ByteBuffer used as output for the decoder. This buffer can be small
    * as it is only used to transfer data from the decoder to the
    * buffer provided by the caller.
    */
   private final ByteBuffer encoderOut;

   private CoderResult lastCoderResult;
   private boolean endOfInput;

   /**
    * Construct a new {@link ReaderInputStream} with a default input buffer size of
    * <c>1024</c> characters.
    *
    * @param reader the target {@link Reader}.  Must not be <jk>null</jk>.
    * @param charset the charset encoding.  Must not be <jk>null</jk>.
    */
   public ReaderInputStream(Reader reader, Charset charset) {
      this(reader, charset, DEFAULT_BUFFER_SIZE);
   }

   /**
    * Construct a new {@link ReaderInputStream}.
    *
    * @param reader the target {@link Reader}.  Must not be <jk>null</jk>.
    * @param charset the charset encoding.  Must not be <jk>null</jk>.
    * @param bufferSize the size of the input buffer in number of characters.  Must be positive.
    */
   public ReaderInputStream(Reader reader, Charset charset, int bufferSize) {
      // @formatter:off
      this(reqnn("reader", reader),
          reqnn("charset", charset).newEncoder()
               .onMalformedInput(CodingErrorAction.REPLACE)
               .onUnmappableCharacter(CodingErrorAction.REPLACE),
          bufferSize
      );
      // @formatter:on
   }

   /**
    * Construct a new {@link ReaderInputStream}.
    *
    * @param reader the target {@link Reader}.  Must not be <jk>null</jk>.
    * @param encoder the charset encoder.  Must not be <jk>null</jk>.
    * @since 2.1
    */
   public ReaderInputStream(Reader reader, CharsetEncoder encoder) {
      this(reader, encoder, DEFAULT_BUFFER_SIZE);
   }

   /**
    * Construct a new {@link ReaderInputStream}.
    *
    * @param reader the target {@link Reader}.  Must not be <jk>null</jk>.
    * @param encoder the charset encoder.  Must not be <jk>null</jk>.
    * @param bufferSize the size of the input buffer in number of characters.  Must be positive.
    */
   public ReaderInputStream(Reader reader, CharsetEncoder encoder, int bufferSize) {
      this.reader = reqnn("reader", reader);
      this.encoder = reqnn("encoder", encoder);
      req(bufferSize > 0, "Argument 'bufferSize' must be positive.");
      this.encoderIn = CharBuffer.allocate(bufferSize);
      this.encoderIn.flip(); // Fixes Java 11 issue.
      this.encoderOut = ByteBuffer.allocate(128);
      this.encoderOut.flip(); // Fixes Java 11 issue.
   }

   /**
    * Construct a new {@link ReaderInputStream} with a default input buffer size of
    * <c>1024</c> characters.
    *
    * @param reader the target {@link Reader}.  Must not be <jk>null</jk>.
    * @param charsetName the name of the charset encoding.  Must not be <jk>null</jk>.
    */
   public ReaderInputStream(Reader reader, String charsetName) {
      this(reader, charsetName, DEFAULT_BUFFER_SIZE);
   }

   /**
    * Construct a new {@link ReaderInputStream}.
    *
    * @param reader the target {@link Reader}.  Must not be <jk>null</jk>.
    * @param charsetName the name of the charset encoding.  Must not be <jk>null</jk>.
    * @param bufferSize the size of the input buffer in number of characters.  Must be positive.
    */
   public ReaderInputStream(Reader reader, String charsetName, int bufferSize) {
      this(reader, Charset.forName(reqnn("charsetName", charsetName)), bufferSize);
   }

   /**
    * Close the stream. This method will cause the underlying {@link Reader}
    * to be closed.
    * @throws IOException if an I/O error occurs
    */
   @Override
   public void close() throws IOException {
      reader.close();
   }

   /**
    * Read a single byte.
    *
    * @return either the byte read or <code>-1</code> if the end of the stream
    *     has been reached
    * @throws IOException if an I/O error occurs
    */
   @Override
   public int read() throws IOException {
      for (;;) {
         if (encoderOut.hasRemaining()) {
            return encoderOut.get() & 0xFF;
         }
         fillBuffer();
         if (endOfInput && ! encoderOut.hasRemaining()) {
            return -1;
         }
      }
   }

   /**
    * Read the specified number of bytes into an array.
    *
    * @param b the byte array to read into.  Must not be <jk>null</jk>.
    * @return the number of bytes read or <code>-1</code>
    *     if the end of the stream has been reached
    * @throws IOException if an I/O error occurs
    */
   @Override
   public int read(byte[] b) throws IOException {
      reqnn("b", b);
      return read(b, 0, b.length);
   }

   /**
    * Read the specified number of bytes into an array.
    *
    * @param array the byte array to read into.  Must not be <jk>null</jk>.
    * @param off the offset to start reading bytes into
    * @param len the number of bytes to read
    * @return the number of bytes read or <code>-1</code>
    *     if the end of the stream has been reached
    * @throws IOException if an I/O error occurs
    */
   @Override
   public int read(byte[] array, int off, int len) throws IOException {
      reqnn("array", array);
      if (len < 0 || off < 0 || (off + len) > array.length) {
         throw new IndexOutOfBoundsException("Array Size=" + array.length + ", offset=" + off + ", length=" + len);
      }
      int read = 0;
      if (len == 0) {
         return 0; // Always return 0 if len == 0
      }
      while (len > 0) {
         if (encoderOut.hasRemaining()) {
            final int c = Math.min(encoderOut.remaining(), len);
            encoderOut.get(array, off, c);
            off += c;
            len -= c;
            read += c;
         } else {
            fillBuffer();
            if (endOfInput && ! encoderOut.hasRemaining()) {
               break;
            }
         }
      }
      return read == 0 && endOfInput ? -1 : read;
   }

   /**
    * Fills the internal char buffer from the reader.
    *
    * @throws IOException
    *        If an I/O error occurs
    */
   private void fillBuffer() throws IOException {
      if (! endOfInput && (lastCoderResult == null || lastCoderResult.isUnderflow())) {
         encoderIn.compact();
         final int position = encoderIn.position();
         // We don't use Reader#read(CharBuffer) here because it is more efficient
         // to write directly to the underlying char array (the default implementation
         // copies data to a temporary char array).
         final int c = reader.read(encoderIn.array(), position, encoderIn.remaining());
         if (c == -1) {
            endOfInput = true;
         } else {
            encoderIn.position(position + c);
         }
         encoderIn.flip();
      }
      encoderOut.compact();
      lastCoderResult = encoder.encode(encoderIn, encoderOut, endOfInput);
      encoderOut.flip();
   }
}