/*
 * 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.daffodil.io

import java.io.InputStream
import java.io.InputStreamReader
import java.nio.charset.Charset
import java.nio.charset.StandardCharsets
import java.util.Scanner
import java.util.regex.Pattern

import org.apache.daffodil.lib.exceptions.Assert

/**
 * Can be used with any InputStream to restrict what is
 * read from it to stop before a particular regex match.
 *
 * The regex must have a finite maximum length match string.
 *
 * This can be used to forcibly stop consumption of data from a stream at
 * a length obtained from a delimiter that is described using a regex.
 *
 * The delimiter matching the regex is consumed from the underlying stream (if found), and
 * the underlying stream is left positioned at the byte after the regex match
 * string.
 *
 * IMPORTANT: The delimiter regex cannot contain any Capturing Groups!
 * Use (?: ... ) which is non-capturing, instead of regular ( ... ).
 * For example: this regex matches CRLF not followed by tab or space:
 * {{{
 *    """\r\n(?!(?:\t|\ ))"""
 * }}}
 * Notice use of the ?: to avoid a capture group around the alternatives of tab or space.
 *
 * Thread safety: This is inherently stateful - so not thread safe to use
 * this object from more than one thread.
 */
class RegexLimitingInputStream(
  inputStream: InputStream,
  regexForDelimiter: String,
  maximumLengthDelimiterExample: String,
  charset: Charset,
  targetChunkSize: Int = 32 * 1024 // TODO: should this be smaller? Limited by a tunable?
) extends InputStream {

  Assert.usage(targetChunkSize >= 1)
  Assert.usage(maximumLengthDelimiterExample.nonEmpty)

  private val in = inputStream

  /**
   * This only works because we lower the whole matching process to using iso-8859-1
   * which is equivalent to raw bytes. That way when we read a char to get a match
   * we know it consumes exactly one byte.
   *
   * This trick may be useful for dealing with DFDL's rawBytes feature.
   * In principle, a delimiter in DFDL can be a mixture of characters and
   * raw bytes. To match these, you have to lower the character parts to iso_8859_1
   * in the way done here, and then combine with the raw bytes to make a string
   * (of bytes) that can be matched pretending the data is iso_8859_1 data, when
   * it really isn't.
   *
   * Example of this might be if a UTF_8 string was delimited by bytes that
   * are illegal in UTF-8's encoding scheme. E.g., UTF-data delimited by say
   * bytes 00 and FF. So this could be %#x00;%#rFF; The 00, or NUL is a legal
   * UTF-8 code point. The FF is not.
   */
  private val maxDelimiterIn8859 =
    new String(maximumLengthDelimiterExample.getBytes(charset), StandardCharsets.ISO_8859_1)

  private val maxDelimiterLength =
    math
      .ceil(
        maxDelimiterIn8859.length *
          charset.newEncoder().maxBytesPerChar()
      )
      .toInt

  private val chunkSize = math.max(targetChunkSize, maxDelimiterLength + 1)

  /**
   * This regex matches a chunk from zero to chunksize followed by boundaryMark, or
   * anything from zero to chunksize.
   * Group 1 is the chunk matched with group 2 containing the boundaryMark.
   * Group 3 is the chunk matched if boundaryMark is not found.
   */
  private val regex =
    """([\s\S]{0,""" + chunkSize + """}?)(?=(""" + regexForDelimiter + """))|([\s\S]{0,""" + chunkSize + """})"""
  private val pattern = Pattern.compile(regex)

  /**
   * The regex can match at most the chunkSize + the maxBoundaryMarkLength in size.
   */
  private val lookAheadMax = chunkSize + maxDelimiterLength

  private lazy val charsIter = {
    val cks = chunks
    val streamChars = cks.flatten
    val iter = streamChars.iterator
    iter
  }

  override def read(): Int = {
    if (!charsIter.hasNext) -1
    else charsIter.next().toInt
  }

  override def available(): Int =
    if (charsIter.hasNext) 1 else 0

  override def close(): Unit = {
    // do nothing
  }

  private var noMoreChunks = false

  /**
   * This lazy stream stuff might look like a lot of overhead, but
   * consider that the overhead is once per chunk, so honestly the
   * regex match is of more concern.
   */
  private def chunks: Stream[String] = {
    if (noMoreChunks) Stream()
    else {
      in.mark(lookAheadMax)
      //
      // Unfortunately, we have to reconstruct these objects because they otherwise
      // might pre-cache underlying data from the input and we need them to
      // start from a precise byte location when the next scan begins.
      //
      val rdr = new InputStreamReader(in, StandardCharsets.ISO_8859_1)
      val scanner = new Scanner(rdr)
      val beforeMatchString = scanner.findWithinHorizon(pattern, 0)
      val delimMatchLength = checkScan(beforeMatchString, scanner)
      //
      // the trick is that the length of the beforeMatchString could be shorter than the
      // number of characters (aka bytes) pulled from the input stream because in
      // scanning, the scanner or reader might buffer up extra decoded characters that
      // strictly speaking aren't needed to obtain the match.
      //
      // So we reset back to the start, and advance exactly that number of bytes.
      //
      // This only works because we have lowered everything to iso-8859-1 here.
      // Decoding errors, and the complexities they create over how big the string
      // is, vs. how many bytes were consumed... those can't happen with iso-8859-1.
      //
      // We need to position our stream exactly after the characters that are terminated by the match
      // being found.
      in.reset()
      in.skip(
        beforeMatchString.length + delimMatchLength
      ) // advance exactly the right number of bytes (past the regex match)
      if (delimMatchLength > 0)
        noMoreChunks = true
      if (beforeMatchString.isEmpty)
        Stream()
      else
        // lazy list construction. chunks will not be recursively evaluated unless demanded
        beforeMatchString #:: chunks
    }
  }

  /**
   * Thorough checking that we understand the behavior of our regex and
   * scanner.
   */
  private def checkScan(matchString: String, scanner: Scanner) = {

    Assert.invariant(matchString ne null); // worst case it matches 0 length and we get ""
    //
    // Just do some error checking to be absolutely sure we understand
    // how the scanner works
    //
    val matcher = scanner.`match`()
    val beforeDelimMatch = matcher.end(1) // maybe avoids allocating the string
    val delimMatch = matcher.end(2)
    val noDelimMatch = matcher.end(3)
    val isFound = (beforeDelimMatch > -1)
    val delimMatchLength =
      if (isFound) {
        Assert.invariant(delimMatch > -1)
        Assert.invariant(noDelimMatch == -1)
        matcher.end(2) - matcher.start(2)
      } else {
        Assert.invariant(delimMatch == -1)
        Assert.invariant(noDelimMatch > -1)
        0
      }
    delimMatchLength
  }

}
