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Fix sampling bug for 16-bit rand()

According to David Rowe, when rand() returns RAND_MAX (which is likely
for 16-bit output), we end up producing a click.

--- a/dnn/nnet.c

+++ b/dnn/nnet.c

@@ -392,10 +392,10 @@

         tmp[i] = tmp[i-1] + MAX16(0, norm*tmp[i] - pdf_floor);

     }

     /* Do the sampling (from the cdf). */

-    r = tmp[N-1] * ((float)rand()/RAND_MAX);

+    r = tmp[N-1] * ((rand()+.5f)/(RAND_MAX+1.f));

     for (i=0;i<N-1;i++)

     {

-        if (r < tmp[i]) return i;

+        if (r <= tmp[i]) return i;

     }

     return N-1;

 }

-- 

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