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UnpackBytes in Swift

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Hi,

As part of an ongoing process to improve my coding skills, I decided to
learn Apple's new programming language Swift.

Naturally, the best approach in programming is to learn by doing a project.
Since the people who are interested in Apple II graphics programming are in
this newsgroup, I thought it might be appropriate to post the following code
snippet (written in Swift, of course) here for comments on possible bugs:


	// Ref: http://kpreid.livejournal.com/4319.html &
	// of course, Andy Fadden's CiderPress source
	// The parameter packedData is the array of packed bytes
	func unpackBytes(packedData:[UInt8]) -> [UInt8]
	{
	   enum PackedFormat : UInt8
	   {
	      case AllDifferent = 0, RepeatNextByte, RepeatNext4Bytes,
	      Repeat4ofNextByte
	   }
	   var unpackedData = [UInt8]()
	   var srcLen = packedData.count
	   var index = 0		// index into array of packed bytes
	   while (srcLen > 0)
	   {
	      let flag = packedData[index++]
	      let type = (flag & 0xc0) >> 6

	      var count = (flag & 0x3f) + 1
	      var rldBuf = [UInt8]()	// Run Len Decoder buffer
	      --srcLen
	      if let whichFormat = PackedFormat(rawValue:type)
	      {
		  switch(whichFormat)
		  {
		   case .AllDifferent:
		      for var i = 0; i < Int(count); ++i
		      {
		         rldBuf.append(packedData[index++])
		         --srcLen
		      }
		   case .RepeatNextByte:
		      let repeatedVal = packedData[index++]
		      --srcLen
	              for var i = 0; i < Int(count); ++i
		      {
		         rldBuf.append(repeatedVal)
		      }
		   case .RepeatNext4Bytes:
		      srcLen -= 4
		      var fourBytes = [UInt8](count:4, repeatedValue:0)
		      fourBytes[0] = packedData[index++]
	              fourBytes[1] = packedData[index++]
	              fourBytes[2] = packedData[index++]
	              fourBytes[3] = packedData[index++]
		      for var i = 0; i < Int(count); ++i
		      {
		         rldBuf += fourBytes
		      }
		   case .Repeat4ofNextByte:
		      let repeatedVal = packedData[index++]
		      --srcLen
		      var fourBytes = [UInt8](count:4, repeatedValue:0)
		      fourBytes[0] = repeatedVal
		      fourBytes[1] = repeatedVal
		      fourBytes[2] = repeatedVal
		      fourBytes[3] = repeatedVal
		      for var i = 0; i < Int(count); ++i
		      {
		         rldBuf += fourBytes
		      }
		   }
		}
		unpackedData += rldBuf
	   }
	   return unpackedData
	}

Sorry, if the formatting does not come out good.

The function had been tested by passing packed scanlines from files with
file/aux types 0xc0/0x0002 and entire files with file/aux types 0xc0/0x0001

As an exercise, I whip up a simple program written entirely in Swift that
will load and display the following Apple IIgs 320x200 graphic files of the
following format.

0xc0/0x0001, packed IIgs super-hires
0xc1/0x0000, unpacked  IIgs super-hires
0xc0/0x0002, Apple Preferred format

There is partial support for APF files. Only MAIN block is decoded; the rest
of the blocks are ignored

Currently program execution is slow; before a 320x200 graphic is displayed
it must be decoded. 

Here's a screen capture of the window of the program.

http://www.macgui.com/gallery/showphoto.php?pic_id=3493

A word of thanks to the person(s) who made available the graphics shown.

Regards.

Mark

-- 
Mark Lim PM

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UnpackBytes in Swift Mark P M Lim <markpmlim@yahoo.com.sg> - 2015-02-26 14:05 +0000

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