daffodil-io/src/main/scala/edu/illinois/ncsa/daffodil/io/FastAsciiToUnicodeConverter.scala

package edu.illinois.ncsa.daffodil.io

import java.nio.ByteBuffer

/**
 * Highly optimized converter for Ascii to Unicode
 */
object FastAsciiToUnicodeConverter {
  
  def convert (bb : ByteBuffer) = {
    val cb = ByteBuffer.allocate(2 * bb.limit)
    val cbChar = cb.asCharBuffer()
    //
    // Go after data in the largest chunks we can (Long)
    // so as to eliminate per-byte/char bounds checks
    //
    val bbBytesOfWholeLongWords = ((bb.limit >> 3) << 3).toLong
    val numBytesTrailingFragment = bb.limit - bbBytesOfWholeLongWords
    
    val bLong = bb.asLongBuffer()
    val cbLong = cb.asLongBuffer()
    1 to bLong.limit foreach { i =>
      val bbl = bLong.get()
      val long1 : Int = (bbl >> 32).toInt & 0xFFFFFFFF
      val long2 : Int = bbl.toInt & 0xFFFFFFFF
      val cbl1 = convertLong(long1)
      val cbl2 = convertLong(long2)
      cbLong.put(cbl1)
      cbLong.put(cbl2)
    }
    
    1 to numBytesTrailingFragment.toInt foreach { j =>
      val pos = bb.limit - j
      val byte = bb.get(pos)
      val char = convertByte(byte)
      cbChar.put(pos, char)
    }
    
    cb.asCharBuffer()
  }
  
  val UnicodeReplacementCharacter = 0xFFFD.toChar
  /**
   * Convert a single byte of ascii to unicode.
   * If the MSBit is set (negative byte) then that's
   * not a legal character code so produce the unicode
   * replacement character.
   */
  @inline
  def convertByte(byte: Byte) = {
    if (byte < 0) UnicodeReplacementCharacter
    else byte.toChar
  }
  
  @inline
  def convertInt(int : Int) = {
    val i = int & 0xFF
    if (i > 127) UnicodeReplacementCharacter
    else i.toChar
  }
  
  @inline
  def convertLong(bytes : Int) : Long = {
    val int1 = bytes & 0xFF
    val int2 = (bytes >> 8) & 0xFF
    val int3 = (bytes >> 16) & 0xFF
    val int4 = (bytes >> 24) & 0xFF
    val char1 = convertInt(int1)
    val char2 = convertInt(int2)
    val char3 = convertInt(int3)
    val char4 = convertInt(int4)
    val res = (char4.toLong << 48) | (char3.toLong << 32) | (char2.toLong << 16) | char1
    res
  }
    
}