Find sieve number

#include "getSieveNum.h"

namespace
{
  //pass the sieve or not
  inline bool pass(const mp::cpp_int& start, const uint32_t steps)
  {
    mp::cpp_int val = start;
    for (uint32_t i = 0; i < steps; ++i)
    {
      if (static_cast<uint32_t>(val) & 1u)
      {
        val += (val + 1) >> 1;
      }
      else
      {
        val >>= 1;
        if (val < start)
        {
          return false;
        }
      }
    }//for()
    return true;
  } //pass()
} //namespace
//Search 'steps'-step sieve number from the start
mp::cpp_int getSieveNum(const mp::cpp_int& start, const uint32_t steps, const uint32_t *refSieve, uint32_t refSieveSize, uint32_t refSieveStep)
{
  //If there's no reference sieve, use the 2-step sieve (4k+3) instead
  const uint32_t defaultSieveNum = 3u;
  if (refSieve == NULL)
  {
    refSieve = &defaultSieveNum;
    refSieveSize = 1;
    refSieveStep = 2;
  }// if(refSieve == NULL)

  //Find start's position in the reference sieve
  uint32_t r = static_cast<uint64_t>(start) & ((1ull << refSieveStep) - 1);
  uint32_t pos = 0u;
  while ((pos < refSieveSize) && (r > refSieve[pos]))
  {
    ++pos;
  } //while()
  mp::cpp_int trunc = (start >> refSieveStep) << refSieveStep; //truncated value of start, lower 'refSieveStep' bits are cleared to make room for refSieve
  const mp::cpp_int increment = mp::cpp_int(1u) << refSieveStep;
  const mp::cpp_int maxVal = mp::cpp_int(1u) << steps; //maximal value of search range, if we reaches it, start over from 0
  //Find the sieve number
  while (true)
  {
    for (; pos < refSieveSize; ++pos)
    {
      const mp::cpp_int val = trunc + refSieve[pos];
      if (pass(val, steps))
      {
        return val;
      }
    }
    pos = 0;
    trunc += increment;
    if (trunc >= maxVal)
    {
      trunc = 0;
    }
  } //while()
  return start;
} //getSieveNum()