[tor-commits] [tor/master] Move ceil call back into the geometric sampler.

[nickm at torproject.org]

commit 531df9590d006434b31cc81871b73c31ca9f896b
Author: Taylor R Campbell <campbell+tor at mumble.net>
Date:   Thu Jan 10 17:12:56 2019 +0000

    Move ceil call back into the geometric sampler.
    
    Test exactly what the geometric sampler returns, because that's what
    the downstream callers of it are going to use.
    
    While here, also assert that the geometric sampler returns a positive
    integer.  (Our geometric distribution is the one suported on {1, 2,
    3, ...} that returns the number of trials before the first success,
    not the one supported on {0, 1, 2, ...} that returns the number of
    failures before the first success.)
---
 src/lib/math/prob_distr.c  |  2 +-
 src/test/test_prob_distr.c | 12 +++++++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/lib/math/prob_distr.c b/src/lib/math/prob_distr.c
index f5e5218aa..e170d000f 100644
--- a/src/lib/math/prob_distr.c
+++ b/src/lib/math/prob_distr.c
@@ -1308,7 +1308,7 @@ sample_geometric(uint32_t s, double p0, double p)
   if (p >= 1)
     return 1;
 
-  return (-x/log1p(-p));
+  return ceil(-x/log1p(-p));
 }
 
 /*******************************************************************/
diff --git a/src/test/test_prob_distr.c b/src/test/test_prob_distr.c
index 75e7e360a..ec4e943e9 100644
--- a/src/test/test_prob_distr.c
+++ b/src/test/test_prob_distr.c
@@ -958,7 +958,17 @@ test_stochastic_geometric_impl(double p)
     size_t C[PSI_DF] = {0};
 
     for (j = 0; j < NSAMPLES; j++) {
-      double n_tmp = ceil(geometric_sample(p));
+      double n_tmp = geometric_sample(p);
+
+      /* Must be an integer.  (XXX -Wfloat-equal)  */
+      tor_assert(ceil(n_tmp) <= n_tmp && ceil(n_tmp) >= n_tmp);
+
+      /* Must be a positive integer.  */
+      tor_assert(n_tmp >= 1);
+
+      /* Probability of getting a value in the billions is negligible.  */
+      tor_assert(n_tmp <= (double)UINT_MAX);
+
       unsigned n = (unsigned) n_tmp;
 
       if (n > PSI_DF)