std::reduce

Use case

Sum segment sizes, calculate total pages needed.

Explanation

std::reduce sums elements (parallelizable). std::transform_reduce transforms each element first, then reduces. Both require associative (grouping does not matter) and commutative (order does not matter).

Code

#include <cstdint>
#include <iostream>
#include <numeric>
#include <vector>

int main() {
  std::vector<uint64_t> segmentSizes{0x1234, 0x2345, 0x3456};
  auto rawTotal = std::reduce(segmentSizes.begin(), segmentSizes.end());
  std::cout << "Raw total: 0x" << std::hex << rawTotal << "\n";

  // Count total 4k pages needed.
  // Adding 0xFFF (page size - 1) before dividing rounds up.
  // (0x1234 + 0xFFF) / 0x1000 = 0x2233 / 0x1000 = 2 pages.
  auto totalPages = std::transform_reduce(
      segmentSizes.begin(), segmentSizes.end(), uint64_t{0}, std::plus{},
      [](uint64_t size) { return (size + 0xFFF) / 0x1000; });

  std::cout << "Pages needed: " << std::dec << totalPages << "\n";

  return 0;
}

View on GitHub.

Output

$ ./src/c++17/build/std-reduce
Raw total: 0x69cf
Pages needed: 9