Tune next iterator distance

include/primesieve/config.hpp

@@ -2,7 +2,7 @@
 /// @file   config.hpp
 /// @brief  primesieve compile time constants.
 ///
-/// Copyright (C) 2018 Kim Walisch, <kim.walisch@gmail.com>
+/// Copyright (C) 2019 Kim Walisch, <kim.walisch@gmail.com>
 ///
 /// This file is distributed under the BSD License. See the COPYING
 /// file in the top level directory.
@@ -37,13 +37,13 @@ enum {
   ///
   MAX_ALLOC_BYTES = (1 << 20) * 16,
 
-  /// primesieve::iterator caches at least MIN_CACHE_ITERATOR
+  /// iterator::prev_prime() caches at least MIN_CACHE_ITERATOR
   /// bytes of primes. Larger is usually faster but also
   /// requires more memory.
   ///
   MIN_CACHE_ITERATOR = (1 << 20) * 8,
 
-  /// primesieve::iterator maximum cache size in bytes, used
+  /// iterator::prev_prime() maximum cache size in bytes, used
   /// if pi(sqrt(n)) * 8 bytes > MAX_CACHE_ITERATOR.
   ///
   MAX_CACHE_ITERATOR = (1 << 20) * 1024

src/IteratorHelper.cpp

@@ -31,41 +31,40 @@ uint64_t getNextDist(uint64_t n, uint64_t dist)
   x = sqrt(x) / log(log(x));
 
   uint64_t minDist = (uint64_t) x;
-  uint64_t limit = numeric_limits<uint64_t>::max() / 4;
-  dist = max(dist, minDist);
+  uint64_t maxDist = numeric_limits<uint64_t>::max() / 4;
+  uint64_t maxCachedPrime = PrimeGenerator::maxCachedPrime();
 
-  if (dist < limit)
-    dist *= 4;
+  dist = max(dist, minDist);
+  dist = max(dist, maxCachedPrime);
+  dist *= 4;
+  dist = min(dist, maxDist);
 
   return dist;
 }
 
-uint64_t getPrevDist(uint64_t n, uint64_t* dist)
+uint64_t getPrevDist(uint64_t n, uint64_t dist)
 {
   double x = (double) n;
   x = max(x, 10.0);
+  double logx = ceil(log(x));
 
-  double minDist = config::MIN_CACHE_ITERATOR;
-  double maxDist = config::MAX_CACHE_ITERATOR;
-  double logx = log(x);
-
-  minDist *= logx;
-  maxDist *= logx;
-
+  uint64_t minDist = config::MIN_CACHE_ITERATOR;
+  uint64_t maxDist = config::MAX_CACHE_ITERATOR;
   minDist /= sizeof(uint64_t);
   maxDist /= sizeof(uint64_t);
+  minDist *= (uint64_t) logx;
+  maxDist *= (uint64_t) logx;
 
-  if (*dist < minDist)
-  {
-    minDist = (double) *dist;
-    *dist *= 4;
-  }
+  uint64_t maxCachedPrime = PrimeGenerator::maxCachedPrime();
+  dist = max(dist, maxCachedPrime);
+  dist *= 4;
 
-  double defaultDist = sqrt(x) * 2;
-  double newDist = max(minDist, defaultDist);
-  newDist = min(newDist, maxDist);
+  uint64_t defaultDist = (uint64_t) (sqrt(x) * 2);
+  dist = min(dist, minDist);
+  dist = max(dist, defaultDist);
+  dist = min(dist, maxDist);
 
-  return (uint64_t) newDist;
+  return dist;
 }
 
 bool useStopHint(uint64_t start,
@@ -120,8 +119,8 @@ void IteratorHelper::prev(uint64_t* start,
                           uint64_t* dist)
 {
   *stop = checkedSub(*start, 1);
-  uint64_t prevDist = getPrevDist(*stop, dist);
-  *start = checkedSub(*stop, prevDist);
+  *dist = getPrevDist(*stop, *dist);
+  *start = checkedSub(*stop, *dist);
 
   if (useStopHint(*start, *stop, stopHint))
     *start = checkedSub(stopHint, maxPrimeGap(stopHint));

src/iterator-c.cpp

@@ -2,7 +2,7 @@
 /// @file   iterator-c.cpp
 /// @brief  C port of primesieve::iterator.
 ///
-/// Copyright (C) 2018 Kim Walisch, <kim.walisch@gmail.com>
+/// Copyright (C) 2019 Kim Walisch, <kim.walisch@gmail.com>
 ///
 /// This file is distributed under the BSD License. See the COPYING
 /// file in the top level directory.
@@ -52,7 +52,7 @@ void primesieve_init(primesieve_iterator* it)
   it->stop_hint = get_max_stop();
   it->i = 0;
   it->last_idx = 0;
-  it->dist = PrimeGenerator::maxCachedPrime();
+  it->dist = 0;
   it->vector = new vector<uint64_t>;
   it->primeGenerator = nullptr;
   it->is_error = false;
@@ -67,7 +67,7 @@ void primesieve_skipto(primesieve_iterator* it,
   it->stop_hint = stop_hint;
   it->i = 0;
   it->last_idx = 0;
-  it->dist = PrimeGenerator::maxCachedPrime();
+  it->dist = 0;
   auto& primes = getPrimes(it);
   primes.clear();
   clearPrimeGenerator(it);

src/iterator.cpp

@@ -1,7 +1,7 @@
 ///
 /// @file  iterator.cpp
 ///
-/// Copyright (C) 2018 Kim Walisch, <kim.walisch@gmail.com>
+/// Copyright (C) 2019 Kim Walisch, <kim.walisch@gmail.com>
 ///
 /// This file is distributed under the BSD License. See the COPYING
 /// file in the top level directory.
@@ -47,7 +47,7 @@ void iterator::skipto(uint64_t start,
   stop_hint_ = stop_hint;
   i_ = 0;
   last_idx_ = 0;
-  dist_ = PrimeGenerator::maxCachedPrime();
+  dist_ = 0;
   clear(primeGenerator_);
   primes_.clear();
 }