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Pseudocode

width1 = getbyte()
width2 = getbyte()
width3 = getbyte()
index = 0
while (index < length) {
  if (getbits(1) == 0) {
    dist = 2 + getbits(width2)
    count = 2 + getbits(width1)
    copy count bytes from index - dist to index, add count to index
  }
  else if(getbits(1) == 0) {
    dist = 2 + (1 << width2) getbits(width3)
    count = 2 + getbits(width1)
    copy count bytes from index - dist to index, add count to index
  }
  else copy 8 bits from input to index, add 1 to index
}

// Project: decompress 
// Created: 2019-03-12

// show all errors
SetErrorMode(2)

// set window properties
SetWindowTitle( "decompress" )
SetWindowSize( 1024, 768, 0 )
SetWindowAllowResize( 1 ) // allow the user to resize the window

// set display properties
SetVirtualResolution( 1024, 768 ) // doesn't have to match the window
SetOrientationAllowed( 1, 1, 1, 1 ) // allow both portrait and landscape on mobile devices
SetSyncRate( 30, 0 ) // 30fps instead of 60 to save battery
SetScissor( 0,0,0,0 ) // use the maximum available screen space, no black borders
UseNewDefaultFonts( 1 ) // since version 2.0.22 we can use nicer default fonts


file as integer
file = OpenToRead("roads.lzs")

offset as integer
length as integer

offset = ReadWord(file) + 222 // 222 = gravity(2) + fuel(2) + oxygen(2) + Palette(72*3)
length = ReadWord(file)

output as string
output = decompress(file, offset, length)

do
    Print( "Output length=" + str(len(output)) + " Should be="+str(length))
    Sync()
loop

global	_bitPointer as integer = 8
global	_bitByte 	as integer
global	_bitBin		as string
global	_bitHex		as string

function	decompress(file as integer,  offset as integer, length as integer)
	width1	as integer
	width2	as integer
	width3	as integer
	mult	as integer
	dist	as integer
	count 	as integer
	v		as integer
	output	as string
	o		as integer

	SetFilePos(file, offset)
	
	width1 = ReadByte(file)
	width2 = ReadByte(file)
	width3 = ReadByte(file)

	mult = width2 << 1

	while len(output) < length
		v = ReadBits(file, 1)

		select	v
			case 0
				dist = ReadBits(file, width2) + 2
				count = ReadBits(file, width1) + 2
				output = output + mid(output, len(output) - dist, count)
			endcase
			case 1
				v = ReadBits(file, 1)
				select v
					case 0
						dist = ReadBits(file, width3) + 2 + mult
						count = ReadBits(file, width1) + 2
						output = output + mid(output, len(output) - dist, count)								
					endcase
					case 1
						output = output + chr(ReadBits(file, 8))
					endcase
				endselect			
			endcase
		endselect
	endwhile

endfunction output

function	ReadBits(file as integer, length as integer)
	c 	as integer
	o	as string

	repeat
		if _bitPointer > 7
			_bitByte = ReadByte(file)
			_bitHex = hex(_bitByte)
			_bitBin = right("00000000" + bin(_BitByte), 8)
			_bitPointer = 0
		endif

		o = o + Mid(_bitbin, _bitpointer + 1, 1)
		inc _bitpointer
		dec length
	until length < 1

	c = val(o, 2)
endfunction c

function	ReadWord(file as integer)
	v	as integer
	
	v = ReadByte(file) + ReadByte(file) * 256
endfunction v

 if (getbits(1) == 0) {
    dist = 2 + getbits(width2)
    count = 2 + getbits(width1)
    copy count bytes from index - dist to index, add count to index
  }
  else if(getbits(1) == 0) {
    dist = 2 + (1 << width2) getbits(width3)
    count = 2 + getbits(width1)
    copy count bytes from index - dist to index, add count to index

// Project: decompress 
// Created: 2019-03-12

// show all errors
SetErrorMode(2)

// set window properties
SetWindowTitle( "decompress" )
SetWindowSize( 1024, 768, 0 )
SetWindowAllowResize( 1 ) // allow the user to resize the window

// set display properties
SetVirtualResolution( 1024, 768 ) // doesn't have to match the window
SetOrientationAllowed( 1, 1, 1, 1 ) // allow both portrait and landscape on mobile devices
SetSyncRate( 30, 0 ) // 30fps instead of 60 to save battery
SetScissor( 0,0,0,0 ) // use the maximum available screen space, no black borders
UseNewDefaultFonts( 1 ) // since version 2.0.22 we can use nicer default fonts


file as integer
file = OpenToRead("roads.lzs")

offset as integer
length as integer

offset = ReadWord(file) + 222 // 222 = gravity(2) + fuel(2) + oxygen(2) + Palette(72*3)
length = ReadWord(file) - 222

l as integer
l = decompress(file, offset, length)

do
    Print( "Output length=" + str(l) + " Should be="+str(length))
    Sync()
loop

global	_bitPointer as integer = 9
global	_bitByte 	as integer
global	_bitBin		as string
global	_bitHex		as string
global	_bitLast	as integer

function	decompress(file as integer,  offset as integer, length as integer)
	width1	as integer
	width2	as integer
	width3	as integer
	mult	as integer
	dist	as integer
	count 	as integer
	v		as integer
	output	as string[]
	l		as integer
	d		as integer

	SetFilePos(file, offset)
	
	width1 = ReadByte(file)
	width2 = ReadByte(file)
	width3 = ReadByte(file)

	mult = width2 << 1

	while output.length + 1 < length
		v = ReadBits(file, 1)
		
		select	v
			case 0
				dist = ReadBits(file, width2) + 2
				count = ReadBits(file, width1) + 2
				append(output, output.length - (dist - 1), count)
				inc l, count
			endcase
			case default
				v = ReadBits(file, 1)
				select v
					case 0
						dist = ReadBits(file, width3) + 2 + mult
						count = ReadBits(file, width1) + 2
						append(output, output.length - (dist - 1), count)
						inc l, count
					endcase
					case default
						output.insert(chr(ReadBits(file, 8)))
						inc l
					endcase
				endselect			
			endcase
		endselect
	endwhile

endfunction l

function	Append(f ref as string[], o as integer, l as integer)
	repeat
		f.insert(f[o])
		dec l
		inc o
	until l < 1

endfunction

function	ReadBits(file as integer, length as integer)
	c 	as integer
	o	as string
	w	as string
	i	as integer

	repeat
		if _bitPointer > 8
			_bitByte = ReadByte(file)
			_bitHex = hex(_bitByte)
			_bitbin = right("00000000" + bin(_BitByte), 8)
			_bitPointer = 1
		endif

		o = o + Mid(_bitbin, _bitpointer, 1)
		inc _bitpointer
		dec length
	until length < 1

	c = val(o, 2)
	_bitLast = c
endfunction c

function	ReadWord(file as integer)
	v	as integer
	
	v = ReadByte(file) + ReadByte(file) * 256
endfunction v

function decompress(road, finalLength) {



  var byteLength = road.length;

  var inBuf = new Uint8Array(byteLength);

  for (var i = 0; i < byteLength; i++) {

    inBuf[i] = road[i];

  };

  var input = new BitStream(inBuf);

  var outBuf = new ArrayBuffer(finalLength);



  var width1 = input.readBits(8);

  var width2 = input.readBits(8);

  var width3 = input.readBits(8);



  var index = 0;

  var dist = 0;

  var count = 0;

  var val = null;



  while (index < finalLength) {

    if (input.readBits(1) == 0) {

      dist = 2 + input.readBits(width2);

      count = 2 + input.readBits(width1);

      copyTo(outBuf, index - dist, count, outBuf, index);

      index += count;

    } else if (input.readBits(1) == 0) {

      dist = 2 + (1 << width2) + input.readBits(width3);

      count = 2 + input.readBits(width1);

      copyTo(outBuf, index - dist, count, outBuf, index);

      index += count;

    } else {

      val = input.readBits(8);

      outBuf[index] = val;

      index++;

    }

  }

  var output = new BitStream(outBuf);

  return output;

}

function copyTo(from, fromOffset, count, to, toOffset) {

  var out = "";

  for (var i = 0; i < count; i++) {

    to[toOffset + i] = from[fromOffset + i];

    out += from[fromOffset + i];

  }

}

%decompress LZSS bytes knowing resulting bytes should be length N
function dbytes = decompressLZSS(bytes, N)
 
%flip the bit order within the compressed section
bytes = [bytes(1:3); bin2dec(fliplr(dec2bin(bytes(4:end))))];
 
%save bits to temp file
ftemp = fopen('temp.lzs', 'w+');
fwrite(ftemp, bytes, 'ubit8');
frewind(ftemp)
fout = fopen('out.lzs', 'w+');
 
% bits = fread(ftemp, inf, 'ubit1');
% fprintf('%d', (bits(1:end).'))
% fprintf('\n')
% frewind(ftemp);
 
%read compression header bytes
width1 = fread(ftemp, 1, 'uint8');
width2 = fread(ftemp, 1, 'uint8');
width3 = fread(ftemp, 1, 'uint8');
 
%step through compressed bits and decompress
%stop once N bits have been written to output
while ftell(fout) < N
    b = fread(ftemp, 1, 'ubit1');
    if ~b
        %read width2 bits, add 2 -> dist
        dist = uintbe(fread(ftemp, width2, 'ubit1'))+2;
        %read width1 bits, add 2 -> count
        count = uintbe(fread(ftemp, width1, 'ubit1'))+2;
        %rewind dist bytes in output
        fseek(fout, -dist, 'eof');
        %copy count bytes from output to end
        copy = fread(fout, count, 'ubit8');
        fseek(fout, 0, 'eof');
        fwrite(fout, copy, 'ubit8');
%         fprintf('Copy from small -%d, %d bytes\n', dist, count);
    else
        b = fread(ftemp, 1, 'ubit1');
        if ~b
            %read width3 bits, add 2+1<<width2 -> dist
            dist = uintbe(fread(ftemp, width3, 'ubit1'))+bitshift(1, width2)+2;
            %read width1 bits, add 2 -> count
            count = uintbe(fread(ftemp, width1, 'ubit1'))+2;
            %rewind dist bytes in output
            fseek(fout, -dist, 'eof');
            %copy count bytes from output to end
            copy = fread(fout, count, 'ubit8');
            fseek(fout, 0, 'eof');
            fwrite(fout, copy, 'ubit8');
%             fprintf('Copy from large -%d, %d bytes\n', dist, count);
        else
            %read a byte and copy to end of output
            copy = uintbe(fread(ftemp, 8, 'ubit1'));
            fseek(fout, 0, 'eof');
            fwrite(fout, copy, 'ubit8');
%             fprintf('Read byte %d: %d\n', ftell(fout)-1, copy);
        end
    end
end
 
%rewind output and read it to get all bytes
frewind(fout);
dbytes = fread(fout, inf, 'uint8');
 
%close files and delete temps
fclose(ftemp);
fclose(fout);
delete('temp.lzs');
delete('out.lzs');
 
end
 
%convert BE bits to unsigned int
function i = uintbe(bits)
 
i = (2.^(length(bits)-1:-1:0))*bits(:);
 
end