stream-cinema/CinemaLib/Webshare/BaseEncoding.cs

using System;
using System.Text;

namespace Cinema.Webshare;

/// <summary>
/// Performs generic binary-to-text encoding.
/// </summary>
class BaseEncoding : Encoding
{
  int _bitCount;
  int _bitMask;
  string _characters;
  bool _msbComesFirst;
  Dictionary<char, int> _values;

  /// <summary>
  /// Defines a binary-to-text encoding.
  /// </summary>
  /// <param name="characterSet">The characters of the encoding.</param>
  /// <param name="msbComesFirst">
  ///     <c>true</c> to begin with the most-significant bit of each byte.
  ///     Otherwise, the encoding begins with the least-significant bit.
  /// </param>
  public BaseEncoding(string characterSet, bool msbComesFirst)
      : this(characterSet, msbComesFirst, null)
  {
  }

  /// <summary>
  /// Defines a binary-to-text encoding.
  /// Additional decode characters let you add aliases, and a filter callback can be used
  /// to make decoding case-insensitive among other things.
  /// </summary>
  /// <param name="characterSet">The characters of the encoding.</param>
  /// <param name="msbComesFirst">
  ///     <c>true</c> to begin with the most-significant bit of each byte.
  ///     Otherwise, the encoding begins with the least-significant bit.
  /// </param>
  /// <param name="additionalDecodeCharacters">
  ///     A dictionary of alias characters, or <c>null</c> if no aliases are desired.
  /// </param>
  /// <param name="decodeFilterCallback">
  ///     A callback to map arbitrary characters onto the characters that can be decoded.
  /// </param>
  public BaseEncoding(string characterSet, bool msbComesFirst,
                      IDictionary<char, int> additionalDecodeCharacters)
  {
    if (characterSet == null)
      throw new ArgumentNullException();

    if (!IsPositivePowerOf2(characterSet.Length))
    {
      throw new ArgumentException("Length must be a power of 2.", "characterSet");
    }

    if (characterSet.Length > 256)
    {
      throw new ArgumentException("Character sets with over 256 characters are not supported.", "characterSet");
    }

    _bitCount = 31 - CountLeadingZeros(characterSet.Length);
    _bitMask = (1 << _bitCount) - 1;
    _characters = characterSet;
    _msbComesFirst = msbComesFirst;

    _values = additionalDecodeCharacters != null
        ? new Dictionary<char, int>(additionalDecodeCharacters)
        : new Dictionary<char, int>();
    for (int i = 0; i < characterSet.Length; i++)
    {
      char ch = characterSet[i];
      if (_values.ContainsKey(ch))
      {
        throw new ArgumentException("Duplicate characters are not supported.", "characterSet");
      }
      _values.Add(ch, (byte)i);
    }
  }

  /// <summary>
  /// Gets the value corresponding to the specified character.
  /// </summary>
  /// <param name="character">A character.</param>
  /// <returns>A value, or <c>-1</c> if the character is not part of the encoding.</returns>
  public virtual int GetValue(char character)
  {
    int value;
    return _values.TryGetValue(character, out value) ? value : -1;
  }

  /// <summary>
  /// Gets the character corresponding to the specified value.
  /// </summary>
  /// <param name="value">A value.</param>
  /// <returns>A character.</returns>
  public virtual char GetChar(int value)
  {
    return _characters[value & BitMask];
  }

  /// <summary>
  /// The bit mask for a single character in the current encoding.
  /// </summary>
  public int BitMask
  {
    get { return _bitMask; }
  }

  /// <summary>
  /// The number of bits per character in the current encoding.
  /// </summary>
  public int BitsPerCharacter
  {
    get { return _bitCount; }
  }

  /// <summary>
  /// <c>true</c> if the encoding begins with the most-significant bit of each byte.
  /// Otherwise, the encoding begins with the least-significant bit.
  /// </summary>
  public bool MsbComesFirst
  {
    get { return _msbComesFirst; }
  }

  public override int GetMaxCharCount(int byteCount)
  {
    if (byteCount < 0)
      throw new ArgumentOutOfRangeException();

    return checked((byteCount * 8 + BitsPerCharacter - 1) / BitsPerCharacter);
  }

  /// <inheritdoc />
  public override int GetChars(byte[] bytes, int byteIndex, int byteCount, char[] chars, int charIndex)
  {
    int charCount = GetCharCount(bytes, byteIndex, byteCount);

    return GetChars(bytes, byteIndex, byteCount, chars, charIndex, charCount);
  }

  /// <summary>
  /// Converts bytes from their binary representation to a text representation.
  /// </summary>
  /// <param name="bytes">An input array of bytes.</param>
  /// <param name="byteIndex">The index of the first byte.</param>
  /// <param name="byteCount">The number of bytes to read.</param>
  /// <param name="chars">An output array of characters.</param>
  /// <param name="charIndex">The index of the first character.</param>
  /// <param name="charCount">The number of characters to write.</param>
  /// <returns>The number of characters written.</returns>
  public int GetChars(byte[] bytes, int byteIndex, int byteCount, char[] chars, int charIndex, int charCount)
  {
    if (charIndex < 0 || charCount < 0 || chars.Length - charIndex < charCount
    || byteIndex < 0 || byteCount < 0 || bytes.Length - byteIndex < byteCount)
      throw new ArgumentOutOfRangeException();

    int byteEnd = checked(byteIndex + byteCount);

    int bitStartOffset = 0;
    for (int i = 0; i < charCount; i++)
    {
      byte value;

      byte thisByte = byteIndex + 0 < byteEnd ? bytes[byteIndex + 0] : (byte)0;
      byte nextByte = byteIndex + 1 < byteEnd ? bytes[byteIndex + 1] : (byte)0;

      int bitEndOffset = bitStartOffset + BitsPerCharacter;
      if (MsbComesFirst)
      {
        value = ShiftRight(thisByte, 8 - bitStartOffset - BitsPerCharacter);
        if (bitEndOffset > 8)
        {
          value |= ShiftRight(nextByte, 16 - bitStartOffset - BitsPerCharacter);
        }
      }
      else
      {
        value = ShiftRight(thisByte, bitStartOffset);
        if (bitEndOffset > 8)
        {
          value |= ShiftRight(nextByte, bitStartOffset - 8);
        }
      }

      bitStartOffset = bitEndOffset;
      if (bitStartOffset >= 8)
      {
        bitStartOffset -= 8; byteIndex++;
      }

      chars[i] = GetChar(value);
    }

    return charCount;
  }

  /// <inheritdoc />
  public override int GetCharCount(byte[] bytes, int index, int count)
  {
    if (index < 0 || count < 0 || bytes.Length - index < count)
      throw new ArgumentOutOfRangeException();

    return GetMaxCharCount(count);
  }

  public override int GetByteCount(char[] chars, int index, int count)
  {
    throw new NotSupportedException();
  }

  public override int GetBytes(char[] chars, int charIndex, int charCount, byte[] bytes, int byteIndex)
  {
    throw new NotSupportedException();
  }

  public override int GetMaxByteCount(int charCount)
  {
    throw new NotSupportedException();
  }

  private static bool IsPositivePowerOf2(int value)
  {
    return 0 < value && 0 == (value & (value - 1));
  }

  private static int CountLeadingZeros(int value)
  {
    int count;
    for (count = 0; count < 32 && 0 == (value & (0x80000000 >> count)); count++) ;
    return count;
  }

  private static byte ShiftLeft(byte value, int bits)
  {
    return (byte)(bits > 0 ? value << bits : value >> (-bits));
  }

  private static byte ShiftRight(byte value, int bits)
  {
    return ShiftLeft(value, -bits);
  }
}