プログラムの stdout をリダイレクトし、バイトを ascii85 として出力する C# コンソール アプリを作成しました。そのため、C# プロセスにバイトを出力する Windows シェルの容量について心配する必要はもうありません。
現在、ffmpeg で変換された MP3 から Webresponse バイトにオンザフライでリダイレクトするために使用していますが、動作します。
Ascii85 は、100% のオーバーヘッドを表す 16 進数の文字列と比較すると、バイト空間に対して 25% のオーバーヘッドを表し、base64 はデータサイズの 33% のオーバーヘッドを表します。
Jeff Atwood の Coding Horror ブログ ( http://www.codinghorror.com/blog/2005/10/c-implementation-of-ascii85.html ) にある、Jeff Atwood の Ascii85 クラスを使用しました。
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
namespace stdshare {
class Program
{
static void Main(string[] args)
{
Int32 dur = 0;
try
{
dur = Convert.ToInt32(args[0]);
} catch { return; }
Console.InputEncoding = Encoding.UTF8;
Console.OutputEncoding = Encoding.UTF8;
using (Stream stdin = Console.OpenStandardInput())
using (Stream stdout = Console.OpenStandardOutput())
{
byte[] buffer = new byte[2048];
byte[] buffer2 = new byte[2048];
int bytes;
int pos = 0;
using (BinaryReader br = new BinaryReader(stdin))
while ((bytes = br.Read(buffer, 0, buffer.Length)) > 0)
{
buffer2 = new byte[bytes];
Buffer.BlockCopy(buffer, 0, buffer2, 0, bytes);
string s = System.Convert.ToBase64String(buffer2);
Console.WriteLine(s);
Console.Out.Flush();
stdout.Flush();
pos += 1;
if (pos > dur) break;
}
}
}
class Ascii85
{
/// <summary>
/// Prefix mark that identifies an encoded ASCII85 string, traditionally '<~'
/// </summary>
public string PrefixMark = "<~";
/// <summary>
/// Suffix mark that identifies an encoded ASCII85 string, traditionally '~>'
/// </summary>
public string SuffixMark = "~>";
/// <summary>
/// Maximum line length for encoded ASCII85 string;
/// set to zero for one unbroken line.
/// </summary>
public int LineLength = 0;
/// <summary>
/// Add the Prefix and Suffix marks when encoding, and enforce their presence for decoding
/// </summary>
public bool EnforceMarks = true;
private const int _asciiOffset = 33;
private byte[] _encodedBlock = new byte[5];
private byte[] _decodedBlock = new byte[4];
private uint _tuple = 0;
private int _linePos = 0;
private uint[] pow85 = { 85 * 85 * 85 * 85, 85 * 85 * 85, 85 * 85, 85, 1 };
/// <summary>
/// Decodes an ASCII85 encoded string into the original binary data
/// </summary>
/// <param name="s">ASCII85 encoded string</param>
/// <returns>byte array of decoded binary data</returns>
public byte[] Decode(string s)
{
if (EnforceMarks)
{
if (!s.StartsWith(PrefixMark) | !s.EndsWith(SuffixMark))
{
throw new Exception("ASCII85 encoded data should begin with '" + PrefixMark +
"' and end with '" + SuffixMark + "' Problematic string: " + s);
}
}
// strip prefix and suffix if present
if (s.StartsWith(PrefixMark))
{
s = s.Substring(PrefixMark.Length);
}
if (s.EndsWith(SuffixMark))
{
s = s.Substring(0, s.Length - SuffixMark.Length);
}
MemoryStream ms = new MemoryStream();
int count = 0;
bool processChar = false;
foreach (char c in s)
{
switch (c)
{
case 'z':
if (count != 0)
{
throw new Exception("The character 'z' is invalid inside an ASCII85 block.");
}
_decodedBlock[0] = 0;
_decodedBlock[1] = 0;
_decodedBlock[2] = 0;
_decodedBlock[3] = 0;
ms.Write(_decodedBlock, 0, _decodedBlock.Length);
processChar = false;
break;
case '\n':
case '\r':
case '\t':
case '\0':
case '\f':
case '\b':
processChar = false;
break;
default:
if (c < '!' || c > 'u')
{
throw new Exception("Bad character '" + c + "' found. ASCII85 only allows characters '!' to 'u'.");
}
processChar = true;
break;
}
if (processChar)
{
_tuple += ((uint)(c - _asciiOffset) * pow85[count]);
count++;
if (count == _encodedBlock.Length)
{
DecodeBlock();
ms.Write(_decodedBlock, 0, _decodedBlock.Length);
_tuple = 0;
count = 0;
}
}
}
// if we have some bytes left over at the end..
if (count != 0)
{
if (count == 1)
{
throw new Exception("The last block of ASCII85 data cannot be a single byte.");
}
count--;
_tuple += pow85[count];
DecodeBlock(count);
for (int i = 0; i < count; i++)
{
ms.WriteByte(_decodedBlock[i]);
}
}
return ms.ToArray();
}
/// <summary>
/// Encodes binary data into a plaintext ASCII85 format string
/// </summary>
/// <param name="ba">binary data to encode</param>
/// <returns>ASCII85 encoded string</returns>
public string Encode(byte[] ba)
{
StringBuilder sb = new StringBuilder((int)(ba.Length * (_encodedBlock.Length / _decodedBlock.Length)));
_linePos = 0;
if (EnforceMarks)
{
AppendString(sb, PrefixMark);
}
int count = 0;
_tuple = 0;
foreach (byte b in ba)
{
if (count >= _decodedBlock.Length - 1)
{
_tuple |= b;
if (_tuple == 0)
{
AppendChar(sb, 'z');
}
else
{
EncodeBlock(sb);
}
_tuple = 0;
count = 0;
}
else
{
_tuple |= (uint)(b << (24 - (count * 8)));
count++;
}
}
// if we have some bytes left over at the end..
if (count > 0)
{
EncodeBlock(count + 1, sb);
}
if (EnforceMarks)
{
AppendString(sb, SuffixMark);
}
return sb.ToString();
}
private void EncodeBlock(StringBuilder sb)
{
EncodeBlock(_encodedBlock.Length, sb);
}
private void EncodeBlock(int count, StringBuilder sb)
{
for (int i = _encodedBlock.Length - 1; i >= 0; i--)
{
_encodedBlock[i] = (byte)((_tuple % 85) + _asciiOffset);
_tuple /= 85;
}
for (int i = 0; i < count; i++)
{
char c = (char)_encodedBlock[i];
AppendChar(sb, c);
}
}
private void DecodeBlock()
{
DecodeBlock(_decodedBlock.Length);
}
private void DecodeBlock(int bytes)
{
for (int i = 0; i < bytes; i++)
{
_decodedBlock[i] = (byte)(_tuple >> 24 - (i * 8));
}
}
private void AppendString(StringBuilder sb, string s)
{
if (LineLength > 0 && (_linePos + s.Length > LineLength))
{
_linePos = 0;
sb.Append('\n');
}
else
{
_linePos += s.Length;
}
sb.Append(s);
}
private void AppendChar(StringBuilder sb, char c)
{
sb.Append(c);
_linePos++;
if (LineLength > 0 && (_linePos >= LineLength))
{
_linePos = 0;
sb.Append('\n');
}
}
}
}
}