advent of code 2017 day 6 part 1

bank.cpp

#include <algorithm>
#include <fstream>
#include <iostream>
#include <iterator>
#include <string>
#include <unordered_set>
#include <vector>

namespace std {
template <> struct hash<std::vector<int>> {

  size_t operator()(const std::vector<int> &v) const {
    std::size_t seed = v.size();
    for (auto &i : v) {
      seed ^= i + 0x9e3779b9 + (seed << 6) + (seed >> 2);
    }
    return seed;
  }
};
} // namespace std

std::vector<int> readFromFile(const std::string &filename) {
  std::ifstream ifs{filename};
  std::vector<int> result{std::istream_iterator<int>{ifs}, {}};
  return result;
}

std::vector<int> redistribute(std::vector<int> v) {
  std::vector<int>::iterator index = std::max_element(v.begin(), v.end());
  int maximum = *index;
  int quotient = maximum / v.size();
  int remainder = maximum % v.size();
  *index = 0;
  std::for_each(v.begin(), v.end(), [quotient](auto &a) { a += quotient; });
  while (remainder > 0) {
    ++index;
    if (index == v.end()) {
      index = v.begin();
    }
    ++*index;
    --remainder;
  }
  return v;
}

int find_cycle(std::vector<int> v) {
  std::unordered_set<std::vector<int>> s;
  int step = 0;
  while (true) {
    if (s.count(v)) {
      return step;
    } else {
      s.insert(v);
      ++step;
      v = redistribute(v);
    }
  }
  return step;
}

int main() {
  std::vector<int> bank = readFromFile("text");
  std::cout << find_cycle(bank) << std::endl;
  return 0;
}

Another solution found on reddit, https://www.reddit.com/r/adventofcode/comments/7hvtoq/2017_day_6_solutions/dqu8p6a/

std::vector<int> banks{std::istream_iterator<int>{std::cin}, {}};
std::map<std::vector<int>,int> unique;
for (int count{0}; unique.emplace(banks, count).second; ++count) {
  auto max = std::max_element(banks.begin(), banks.end());
  for (int iters{std::exchange(*max, 0)}; iters--; ++*max)
    if (++max == banks.end())
      max = banks.begin();
}
std::cout << unique.size() - (part2 ? unique[banks] : 0) << '\n';

It is much cleaner than mine.