Fix hangs in binomial sampling #747

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1. dhardy

   added this to the 0.7 release milestone on Mar 1, 2019
2. dhardy

   added

   X-bugType: bug reportType: bug report

   on Mar 1, 2019
3. dhardy commented on Mar 7, 2019

   

   dhardy

   on Mar 7, 2019

   Last edited by dhardy

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   Progress report:

   • after an argument with SELinux I am able to run the MIPS test locally
   • putting a limit on the loop for the small-expected-value of 100 iterations, the test succeeds on the native target but hits this limit when run on MIPS

   On MIPS, it fails with this output (from just before the loop starts and panicking when x>100):

   Sampling from Bin(20, 0.7)
   Start: u=0.9136724307867923, r=0.0000459986536544739, a=9.000000000000002, s=0.42857142857142866, q=0.7
   thread 'main' panicked at 'Too many tries! u=0.8560244207867922, r=0, x=101', src/distributions/binomial.rs:115:21
   

   Here r is wrong. Is this because of the pow method?

   Edit: using let mut r = q.powi(self.n as i32); instead appears to fix the test (no hang), so this would appear to be the case, though the problem does not appear to be with bit logic (using numeric operations instead does not help).
4. vks commented on Mar 7, 2019

   

   vks

   on Mar 7, 2019

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   Could this be due to different rounding behavior?
5. dhardy commented on Mar 7, 2019

   

   dhardy

   on Mar 7, 2019

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   I don't know. It seems quite a large error, for I think 5 FP ops.
6. dhardy

   closed this as completedin #752on Mar 7, 2019