Line data    Source code

       1             : #pragma once
       2             : 
       3             : // This component provides sampling-related miscellanea.  It's used by both
       4             : // `TraceSampler` and `SpanSampler`.
       5             : 
       6             : #include <cstdint>
       7             : #include <limits>
       8             : 
       9             : #include "rate.h"
      10             : 
      11             : namespace datadog {
      12             : namespace tracing {
      13             : 
      14             : // Return a hash value for the specified `value`.  `value` is one of the
      15             : // following:
      16             : //
      17             : // - a 64-bit span ID
      18             : // - a 64-bit trace ID
      19             : // - the lower 64 bits of a 128-bit trace ID
      20             : //
      21             : // The resulting hash value is compared with an upper bound provided by
      22             : // `max_id_from_rate` (below) to determine whether the span/trace associated
      23             : // with `value` is eligible for keeping on statistical grounds.
      24       84833 : inline std::uint64_t knuth_hash(std::uint64_t value) {
      25       84833 :   return value * UINT64_C(1111111111111111111);
      26             : }
      27             : 
      28       84833 : inline std::uint64_t max_id_from_rate(Rate rate) {
      29             :   // `double(std::numeric_limits<uint64_t>::max())` is slightly larger than the
      30             :   // largest `uint64_t`, but consider it a fun fact that the largest `double`
      31             :   // less than 1.0 (i.e. the "previous value" to 1.0), when multiplied by the
      32             :   // max `uint64_t`, results in a number not greater than the max `uint64_t`.
      33             :   // So, the only special case to consider is 1.0.
      34       84833 :   if (rate == 1.0) {
      35       22814 :     return std::numeric_limits<uint64_t>::max();
      36             :   }
      37             : 
      38       62019 :   return rate * static_cast<double>(std::numeric_limits<std::uint64_t>::max());
      39             : }
      40             : 
      41             : }  // namespace tracing
      42             : }  // namespace datadog