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README

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+KISS FFT - A power-of-two Fast Fourier Transform based up on the principle, 
+"Keep It Simple, Stupid."
+
+    There are many great fft libraries already around.  Kiss FFT is not trying
+to be better than any of them.  It only attempts to be a reasonably efficient, 
+moderately useful FFT that can use fixed or floating data types and can be 
+incorporated into someone's C program in a few minutes with trivial licensing.
+
+USAGE:
+
+    The basic usage is:
+
+        void * cfg = kiss_fft_alloc( nfft ,inverse_fft );
+        while ...
+            ... 
+            kiss_fft( cfg , cx_buf_in_out );
+            ...
+        free(cfg);
+
+    Declarations are in "kiss_fft.h", along with a brief description of the 
+two functions you'll need to use. Code definitions are in kiss_fft.c, along 
+with sample usage code.
+
+
+    The code can be easily recompiled to work with 16bit fixed point data,
+or various floating point types.  The default is float.
+
+
+BACKGROUND:
+
+    I started coding this because I couldn't find a fixed point FFT that didn't 
+use assembly code.  I started with floating point numbers so I could get the 
+theory straight before working on fixed point issues.  In the end, I had a 
+little bit of code that could be recompiled easily to do ffts with short, float,
+or double (other types should be easy too).  
+
+    Once I got my FFT working, I wanted to get some performance numbers against 
+a well respected and highly optimized fft library.  I don't want to criticize 
+this great library, so let's call it FFT_BRANDX.  
+During this process, I learned:
+
+    1. FFT_BRANDX has 500 times as many lines of code as Kiss 
+        (and that's just the C code).
+    2. It took me an embarrassingly long time to get FFT_BRANDX working.
+    3. FFT_BRANDX is almost 3 times faster than Kiss
+
+    It is wonderful that free, highly optimized libraries like FFT_BRANDX exist.
+But such libraries carry a huge burden of complexity necessary to extract every 
+last bit of performance.  
+
+    Sometimes simpler is better, even if it's not better.
+
+PERFORMANCE:
+    (on Athlon XP 2100+, with gcc 2.96, optimization O3, float data type)
+
+    Kiss performed 1000 1024-pt ffts in 136 ms. This translates to 7.5 Msamples/s.
+    Just for comparison, it took md5sum 160 ms to process the same amount of data
+
+DO NOT:
+    ... use Kiss if you need the absolute fastest fft in the world
+    ... use Kiss if you need mixed radix FFTs 
+    ... ask me to add features that will bloat the code
+
+UNDER THE HOOD:
+
+    Kiss uses a complex-only, frequency decimation, radix 2, in-place FFT. Bit reversed 
+addressing is corrected as the last step in the transform.  No scaling is done.
+
+LICENSE:
+    BSD, see COPYING for details.
+
+TODO:
+    Make the fixed point scaling and bit shifts more easily configurable.
+    Document/revisit the input/output fft scaling
+    See if the fixed point code can be optimized a little without adding complexity.
+
+AUTHOR:
+    Mark Borgerding
+    mark@borgerding.net