Fix floating point issue on some compilers

1 files changed, 6 insertions, 0 deletions

diff --git a/tree.c b/tree.c
index f9ac812..98abe48 100644
--- a/tree.c
+++ b/tree.c
@@ -83,6 +83,12 @@ void CalculateEntropy(const size_t* count, size_t n, double* bitlengths) {
     its count is 1.*/
     if (count[i] == 0) bitlengths[i] = log2sum;
     else bitlengths[i] = log2sum - log(count[i]) * kInvLog2;
+    /* Depending on compiler and architecture, the above subtraction of two
+    floating point numbers may give a negative result very close to zero
+    instead of zero (e.g. -5.973954e-17 with gcc 4.1.2 on Ubuntu 11.4). Clamp
+    it to zero. These floating point imprecisions do not affect the cost model
+    significantly so this is ok. */
+    if (bitlengths[i] < 0 && bitlengths[i] > -1e-5) bitlengths[i] = 0;
     assert(bitlengths[i] >= 0);
   }
 }