programming challenge – C++, sort integers using knowledge of entire vector

I am solving the “Sort” problem on Kattis.

Mirko is a great code breaker. He knows any cipher in the world can be broken by frequency analysis. He has completely the wrong idea what frequency analysis is, however.
He intercepted an enemy message. The message consists of N
numbers, smaller than or equal to C.
Mirko belives freqency analysis consists of sorting this sequence so that more frequent numbers appear before less frequent ones.
Formally, the sequence must be sorted so that given any two numbers X
and Y, X appears before Y if the number of times X appears in the original sequence is larger than the number of time Y does. If the number of appearances is equal, the number whose value appears sooner in the input should appear sooner in the sorted sequence.
Help Mirko by creating a “frequency sorter”.
Input
First line of input contains two integers, N (1≤N≤1000), the length of the message, and C (1≤C≤1000000000), the number from the task description above.
The next line contains N positive integers smaller than or equal to C, the message itself.

Basically, the problem is as follows. Let xs be a nonempty vector of positive integers. There are only few integers in this vector, but they have a big range. (The maximum value c is given in the problem, but my code does not use the information.) Sort the integers according to the following criteria.

  1. For any two elements x and y of xs, if x occurs more often than y, then x appears first; if y appears more often, y appears first.
  2. If x and y appear equally often, then x occurs first if the very first occurrence of x is earlier than that of y.

I use a comparison sort (provided by the C++ runtime) with a smart comparator. This comparator knows the frequency and the index of the first appearance of every element. This information is not inherent to the integers. Rather, it depends entirely on their location within the vector. This contextual information is generated when a comparator is created for a given vector. Upon application on elements x and y, it returns true if x must appear before y.

I have used custom comparators before, but never have I used anything that contains state. In the disassembly with -Os I see many copy and move constructors called under sort(vector<unsigned> &). The code passes all tests, and it’s not slow.

But I wonder why the disassembly reveals so many copy and move calls, and whether this pattern of using heavy comparators is discouraged in C++. If this looks like a known pattern, I want to know its name. I appreciate general comments and insights.

#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <iostream>

typedef std::vector<unsigned> vector;

/// Comparison based on knowledge of the entire vector
struct compare {
    std::multiset<unsigned> bag;
    std::map<unsigned, size_t> indices;

    /// Extract frequency and initial index of every element.
    explicit compare(vector const &xs) {
        for (size_t i = 0u; i < xs.size(); ++i) {
            unsigned const x = xs(i);
            bag.insert(x);
            if (!indices.count(x)) {
                indices(x) = i;
            }
        }
    }

    /// True if `x` must go before `y`.
    ((nodiscard)) bool operator()(unsigned x, unsigned y) const {
        return bag.count(x) > bag.count(y)
               || (bag.count(x) == bag.count(y) && indices.at(x) < indices.at(y));
    }
};

static void sort(vector &v) {
    compare c(v);
    std::sort(v.begin(), v.end(), c);
}

int main() {
    vector v;
    {
        // Get `n` unsigned integers from console.
        // Unused: `c` (upper bound for integers)
        unsigned n, c;
        std::cin >> n >> c;
        v.reserve(n);
        while (n--) {
            unsigned x;
            std::cin >> x;
            v.push_back(x);
        }
    }
    // Sort according to the problem description
    sort(v);
    // Print all
    for (unsigned const x : v) {
        std::cout << x << ' ';
    }
    return 0;
}

Knowledge Base | Web Hosting Talk

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