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RunLength

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RunLength

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21 May 2013 (Updated )

RunLength coding as fast MEX and M-code

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Description

RUNLENGTH - Run-length coding

Run-length encoding splits a vector into one vector, which contains the
elements without neighboring repetitions, and a second vector, which
contains the number of repetitions.
This can reduce the memory for storing the data or allow to analyze sequences.

Encoding: [B, N, BI] = RunLength(X)
Decoding: X = RunLength(B, N)
INPUT / OUTPUT:
  X: Full input signal, row or column vector.
      Types: (U)INT8/16/32/64, SINGLE, DOUBLE, LOGICAL, CHAR.
  B: Compressed data, neighboring elements with the same value are removed.
      B and X have the same types.
  N: Number of repetitions of the elements of B in X as DOUBLE or UINT8 row vector.
  BI: Indices of elements in B in X as DOUBLE row vector.

RunLength(X, 'byte') replies N as UINT8 vector.

You can find a lot of RLE tools in the FileExchange already. This C-Mex is
about 5 times faster than good vectorized M-versions.
The M-file RunLength_M contains vectorized and loop M-code for education.

EXAMPLES:
Encode and decode:
  [b, n] = RunLength([8, 9, 9, 10, 10, 10, 11])
  x = RunLength(b, n)
  % b = [8,9,10,11], n = [1,2,3,1], x = [8,9,9,10,10,10,11]
Limit counter to 255:
  [b, n] = RunLength(ones(1, 257), 'byte')
  % b = [1, 1], n = uint8([255, 2])
LOGICAL input:
  [b, n] = RunLength([true(257, 1); false])
  % b = [true; false], n = [257, 1]
Find the longest sequence:
  x = floor(rand(1, 1e6) * 2);
  [b, n, bi] = RunLength(x);
  [longestRun, index] = max(n);
  longestPos = bi(index);

The C-code is compiled automatically the first time RunLength is called.
See "RunLength_ReadMe.txt" for more details.

The unit-test uTest_RunLength tests validity and speed.

Tested: Matlab 6.5, 7.7, 7.8, 7.13, WinXP/32, Win7/64
        Compiler: LCC3.8, BCC5.5, OWC1.8, MSVC2008/2010
Does not compile under LCC2.4 shipped with Matlab/32!
Assumed Compatibility: higher Matlab versions, Linux, MacOS.

MATLAB release MATLAB 7.13 (R2011b)
Other requirements C-compiler, or download pre-compiled file from http://www.n-simon.de/mex
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Comments and Ratings (4)
14 Dec 2013 Jan Simon

Thanks for the comments, Oleg. I'm going to enhance InstallMex to let a dialog box appear to let the user decide, if this is worth a warning or an error.

The 2nd input N was thought to be created by a former call to RunLength with 1 input argument. But there is actually no good reason to restrict the types of the 2nd input. It is only inconvenient to care about all different types of inputs inside the C-Mex: The only efficient approach is to create a subfunction for each type, because an internal conversion must consider e.g. overflows of UINT64 data when converted to DOUBLE. So I decided to let the user decide if "RunLength(B, double(N))" is sufficient. But for (U)INT16/32 this problem cannot occur and three extra lines of code are sufficient. Therefore I will add this in the next version.

01 Dec 2013 Oleg Komarov

An additional comment, is there any reason why the second input cannot be uint16, uint32 or single?

01 Dec 2013 Oleg Komarov

I have been using Urs' milestone http://www.mathworks.co.uk/matlabcentral/fileexchange/6436-rude-a-pedestrian-run-length-decoder-encoder for years now and I am pleasantly making the switch to this superb contribution.

RunLength decoding/encoding as fast as it gets!

Also, thanks to InstallMex that is bundled with Jan's submissions, it's a no brainer.

InstallMex errors if a binary is on the some Matlab's path already. However, sometimes I just want to compile for a local project. Maybe the error could be switched to a warning?

17 Sep 2013 Jan Simon

Sorry, the first submission missed the file RunLength.inc . A new version is submitted already. Please feel free to contact me, if the compilation of a file does not work.

Updates
17 Sep 2013

In the former submission the file RunLength.inc was missing. ReadMe added.

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