/*
    * 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.commons.text.io;
  
  import java.io.FilterReader;
  import java.io.IOException;
  import java.io.Reader;
  
  import org.apache.commons.text.StringSubstitutor;
  import org.apache.commons.text.TextStringBuilder;
  import org.apache.commons.text.matcher.StringMatcher;
  import org.apache.commons.text.matcher.StringMatcherFactory;
  
  /**
   * A {@link Reader} that performs string substitution on a source {@code Reader} using a {@link StringSubstitutor}.
   *
   * <p>
   * Using this Reader avoids reading a whole file into memory as a {@code String} to perform string substitution, for
   * example, when a Servlet filters a file to a client.
   * </p>
   * <p>
   * This class is not thread-safe.
   * </p>
   *
   * @since 1.9
   */
  public class StringSubstitutorReader extends FilterReader {
  
      /** The end-of-stream character marker. */
      private static final int EOS = -1;
  
      /** Our internal buffer. */
      private final TextStringBuilder buffer = new TextStringBuilder();
  
      /** End-of-Stream flag. */
      private boolean eos;
  
      /** Matches escaped variable starts. */
      private final StringMatcher prefixEscapeMatcher;
  
      /** Internal buffer for {@link #read()} method. */
      private final char[] read1CharBuffer = {0};
  
      /** The underlying StringSubstitutor. */
      private final StringSubstitutor stringSubstitutor;
  
      /** We don't always want to drain the whole buffer. */
      private int toDrain;
  
      /**
       * Constructs a new instance.
       *
       * @param reader the underlying reader containing the template text known to the given {@code StringSubstitutor}.
       * @param stringSubstitutor How to replace as we read.
       * @throws NullPointerException if {@code reader} is {@code null}.
       * @throws NullPointerException if {@code stringSubstitutor} is {@code null}.
       */
      public StringSubstitutorReader(final Reader reader, final StringSubstitutor stringSubstitutor) {
          super(reader);
          this.stringSubstitutor = new StringSubstitutor(stringSubstitutor);
          this.prefixEscapeMatcher = StringMatcherFactory.INSTANCE.charMatcher(stringSubstitutor.getEscapeChar())
              .andThen(stringSubstitutor.getVariablePrefixMatcher());
      }
  
      /**
       * Buffers the requested number of characters if available.
       */
      private int buffer(final int requestReadCount) throws IOException {
          final int actualReadCount = buffer.readFrom(super.in, requestReadCount);
          eos = actualReadCount == EOS;
          return actualReadCount;
      }
  
      /**
       * Reads a requested number of chars from the underlying reader into the buffer. On EOS, set the state is DRAINING,
       * drain, and return a drain count, otherwise, returns the actual read count.
       */
      private int bufferOrDrainOnEos(final int requestReadCount, final char[] target, final int targetIndex,
          final int targetLength) throws IOException {
          final int actualReadCount = buffer(requestReadCount);
          return drainOnEos(actualReadCount, target, targetIndex, targetLength);
      }
  
      /**
      * Drains characters from our buffer to the given {@code target}.
      */
     private int drain(final char[] target, final int targetIndex, final int targetLength) {
         final int actualLen = Math.min(buffer.length(), targetLength);
         final int drainCount = buffer.drainChars(0, actualLen, target, targetIndex);
         toDrain -= drainCount;
         if (buffer.isEmpty() || toDrain == 0) {
             // nothing or everything drained.
             toDrain = 0;
         }
         return drainCount;
     }
 
     /**
      * Drains from the buffer to the target only if we are at EOS per the input count. If input count is EOS, drain and
      * returns the drain count, otherwise return the input count. If draining, the state is set to DRAINING.
      */
     private int drainOnEos(final int readCountOrEos, final char[] target, final int targetIndex,
         final int targetLength) {
         if (readCountOrEos == EOS) {
             // At EOS, drain.
             if (buffer.isNotEmpty()) {
                 toDrain = buffer.size();
                 return drain(target, targetIndex, targetLength);
             }
             return EOS;
         }
         return readCountOrEos;
     }
 
     /**
      * Tests if our buffer matches the given string matcher at the given position in the buffer.
      */
     private boolean isBufferMatchAt(final StringMatcher stringMatcher, final int pos) {
         return stringMatcher.isMatch(buffer, pos) == stringMatcher.size();
     }
 
     /**
      * Tests if we are draining.
      */
     private boolean isDraining() {
         return toDrain > 0;
     }
 
     /**
      * Reads a single character.
      *
      * @return a character as an {@code int} or {@code -1} for end-of-stream.
      * @throws IOException If an I/O error occurs
      */
     @Override
     public int read() throws IOException {
         int count = 0;
         // ask until we get a char or EOS
         do {
             count = read(read1CharBuffer, 0, 1);
             if (count == EOS) {
                 return EOS;
             }
             // keep on buffering
         } while (count < 1);
         return read1CharBuffer[0];
     }
 
     /**
      * Reads characters into a portion of an array.
      *
      * @param target Target buffer.
      * @param targetIndexIn Index in the target at which to start storing characters.
      * @param targetLengthIn Maximum number of characters to read.
      *
      * @return The number of characters read, or -1 on end of stream.
      * @throws IOException If an I/O error occurs
      */
     @Override
     public int read(final char[] target, final int targetIndexIn, final int targetLengthIn) throws IOException {
         // The whole thing is inefficient because we must look for a balanced suffix to match the starting prefix
         // Trying to substitute an incomplete expression can perform replacements when it should not.
         // At a high level:
         // - if draining, drain until empty or target length hit
         // - copy to target until we find a variable start
         // - buffer until a balanced suffix is read, then substitute.
         if (eos && buffer.isEmpty()) {
             return EOS;
         }
         if (targetLengthIn <= 0) {
             // short-circuit: ask nothing, give nothing
             return 0;
         }
         // drain check
         int targetIndex = targetIndexIn;
         int targetLength = targetLengthIn;
         if (isDraining()) {
             // drain as much as possible
             final int drainCount = drain(target, targetIndex, Math.min(toDrain, targetLength));
             if (drainCount == targetLength) {
                 // drained length requested, target is full, can only do more in the next invocation
                 return targetLength;
             }
             // drained less than requested, target not full.
             targetIndex += drainCount;
             targetLength -= drainCount;
         }
         // BUFFER from the underlying reader
         final int minReadLenPrefix = prefixEscapeMatcher.size();
         // READ enough to test for an [optionally escaped] variable start
         int readCount = buffer(readCount(minReadLenPrefix, 0));
         if (buffer.length() < minReadLenPrefix && targetLength < minReadLenPrefix) {
             // read less than minReadLenPrefix, no variable possible
             final int drainCount = drain(target, targetIndex, targetLength);
             targetIndex += drainCount;
             final int targetSize = targetIndex - targetIndexIn;
             return eos && targetSize <= 0 ? EOS : targetSize;
         }
         if (eos) {
             // EOS
             stringSubstitutor.replaceIn(buffer);
             toDrain = buffer.size();
             final int drainCount = drain(target, targetIndex, targetLength);
             targetIndex += drainCount;
             final int targetSize = targetIndex - targetIndexIn;
             return eos && targetSize <= 0 ? EOS : targetSize;
         }
         // PREFIX
         // buffer and drain until we find a variable start, escaped or plain.
         int balance = 0;
         final StringMatcher prefixMatcher = stringSubstitutor.getVariablePrefixMatcher();
         int pos = 0;
         while (targetLength > 0) {
             if (isBufferMatchAt(prefixMatcher, 0)) {
                 balance = 1;
                 pos = prefixMatcher.size();
                 break;
             }
             if (isBufferMatchAt(prefixEscapeMatcher, 0)) {
                 balance = 1;
                 pos = prefixEscapeMatcher.size();
                 break;
             }
             // drain first char
             final int drainCount = drain(target, targetIndex, 1);
             targetIndex += drainCount;
             targetLength -= drainCount;
             if (buffer.size() < minReadLenPrefix) {
                 readCount = bufferOrDrainOnEos(minReadLenPrefix, target, targetIndex, targetLength);
                 if (eos || isDraining()) {
                     // if draining, readCount is a drain count
                     if (readCount != EOS) {
                         targetIndex += readCount;
                         targetLength -= readCount;
                     }
                     final int actual = targetIndex - targetIndexIn;
                     return actual > 0 ? actual : EOS;
                 }
             }
         }
         // we found a variable start
         if (targetLength <= 0) {
             // no more room in target
             return targetLengthIn;
         }
         // SUFFIX
         // buffer more to find a balanced suffix
         final StringMatcher suffixMatcher = stringSubstitutor.getVariableSuffixMatcher();
         final int minReadLenSuffix = Math.max(minReadLenPrefix, suffixMatcher.size());
         readCount = buffer(readCount(minReadLenSuffix, pos));
         if (eos) {
             // EOS
             stringSubstitutor.replaceIn(buffer);
             toDrain = buffer.size();
             final int drainCount = drain(target, targetIndex, targetLength);
             return targetIndex + drainCount - targetIndexIn;
         }
         // buffer and break out when we find the end or a balanced suffix
         while (true) {
             if (isBufferMatchAt(suffixMatcher, pos)) {
                 balance--;
                 pos++;
                 if (balance == 0) {
                     break;
                 }
             } else if (isBufferMatchAt(prefixMatcher, pos)) {
                 balance++;
                 pos += prefixMatcher.size();
             } else if (isBufferMatchAt(prefixEscapeMatcher, pos)) {
                 balance++;
                 pos += prefixEscapeMatcher.size();
             } else {
                 pos++;
             }
             readCount = buffer(readCount(minReadLenSuffix, pos));
             if (readCount == EOS && pos >= buffer.size()) {
                 break;
             }
         }
         // substitute
         final int endPos = pos + 1;
         final int leftover = Math.max(0, buffer.size() - pos);
         stringSubstitutor.replaceIn(buffer, 0, Math.min(buffer.size(), endPos));
         pos = buffer.size() - leftover;
         final int drainLen = Math.min(targetLength, pos);
         // only drain up to what we've substituted
         toDrain = pos;
         drain(target, targetIndex, drainLen);
         return targetIndex - targetIndexIn + drainLen;
     }
 
     /**
      * Returns how many chars to attempt reading to have room in the buffer for {@code count} chars starting at position
      * {@code pos}.
      */
     private int readCount(final int count, final int pos) {
         final int avail = buffer.size() - pos;
         return avail >= count ? 0 : count - avail;
     }
 
 }