Partition Monte Carlo range by thread

Author: WrathfulSpatula

FindAFactor/_find_a_factor.cpp

@@ -812,8 +812,10 @@ struct Factorizer {
     return 1U;
   }
 
-  BigInteger monteCarlo(rngType& gen) {
+  BigInteger monteCarlo(const size_t& CpuCount, const size_t& cpu, rngType& gen) {
     // This function enters only once per thread.
+    const BigInteger threadRange = (batchRange + CpuCount - 1) / CpuCount;
+    const BigInteger threadOffset = batchOffset + threadRange * cpu;
     size_t batchPower = 0U;
 
     // This is the outer reseeding loop.
@@ -822,7 +824,10 @@ struct Factorizer {
       BigInteger perfectSquare = 1U;
       std::vector<size_t> fv(primes.size(), 0);
       while (perfectSquare < toFactor) {
-        BigInteger n = forwardFn(((batchOffset + (dis(gen) % batchRange)) * wheelEntryCount) + (dis(gen) % wheelEntryCount));
+        const BigInteger bIndex = threadOffset + (dis(gen) % threadRange);
+        const BigInteger halfBIndex = batchOffset + (bIndex >> 1U) + 1U;
+        const BigInteger bNum = (bIndex & 1U) ? batchTotal - halfBIndex : halfBIndex;
+        BigInteger n = forwardFn((bNum * wheelEntryCount) + (dis(gen) % wheelEntryCount));
         const std::vector<size_t> pfv = factorizationVector(&n);
         if (!pfv.size()) {
           continue;
@@ -1136,9 +1141,9 @@ std::string find_a_factor(std::string toFactorStr, size_t method, size_t nodeCou
       gen.emplace_back(rng());
     }
     for (unsigned cpu = 0U; cpu < CpuCount; ++cpu) {
-      futures.push_back(std::async(std::launch::async, [&worker, cpu, &gen] {
+      futures.push_back(std::async(std::launch::async, [&worker, &CpuCount, cpu, &gen] {
         // This is as "embarrissingly parallel" as it gets.
-        return worker.monteCarlo(gen[cpu]);
+        return worker.monteCarlo(CpuCount, cpu, gen[cpu]);
       }));
     }
   } else {

pyproject.toml

@@ -8,7 +8,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "FindAFactor"
-version = "4.7.4"
+version = "4.7.5"
 requires-python = ">=3.8"
 description = "Find any nontrivial factor of a number"
 readme = {file = "README.txt", content-type = "text/markdown"}

setup.py

@@ -40,7 +40,7 @@ def build_extension(self, ext):
 
 setup(
     name='FindAFactor',
-    version='4.7.4',
+    version='4.7.5',
     author='Dan Strano',
     author_email='stranoj@gmail.com',
     description='Find any nontrivial factor of a number',