/*
VIM: let b:cf5build="clang -std=c++20 -O2 -pthread -lstdc++ -I. {SRC} -o {OUT}"
VIM: let b:cf5run="{OUT}"
*/
#include <iostream>
#include <exception>
#include <chrono>
#include <string>
#include <vector>
#include <unordered_map>
#include <unordered_set>
#include <functional>

/*
https://app.codility.com/programmers/lessons/99-future_training/array_inversion_count/

An array A consisting of N integers is given. An inversion is a pair of indexes (P, Q) such that P < Q and A[Q] < A[P].

Write a function:

    int solution(vector<int> &A);

that computes the number of inversions in A, or returns −1 if it exceeds 1,000,000,000.

For example, in the following array:
 A[0] = -1 A[1] = 6 A[2] = 3
 A[3] =  4 A[4] = 7 A[5] = 4

there are four inversions:
   (1,2)  (1,3)  (1,5)  (4,5)

so the function should return 4.

Write an efficient algorithm for the following assumptions:

        N is an integer within the range [0..100,000];
        each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].

*/

int solution(const std::vector<int>& v)
{
    std::vector<size_t> iv(v.size(),0);
    for (size_t i=0; i<v.size(); ++i){
        iv[i] = i;
    }
    std::stable_sort(iv.begin(), iv.end(), [&v](size_t op1, size_t op2){
        return v[op1] < v[op2];
    });

    size_t count = 0;
    for (size_t i=0; i < iv.size(); ++i){
        if ( i < iv[i]){
            count += iv[i] - i;
        }
    }

    return count;
}

int main ( void )
{try{
    auto begin = std::chrono::high_resolution_clock::now();

    std::cout << solution({-1, 6, 3, 4, 7, 4}) << std::endl;

    std::cout << solution({0, 0, 0, 1, 1, 1}) << std::endl;
    std::cout << solution({0, 1, 0, 1, 1, 1}) << std::endl;
    std::cout << solution({0}) << std::endl;
    std::cout << solution({}) << std::endl;


    auto end = std::chrono::high_resolution_clock::now();
    std::chrono::duration<double> seconds = end - begin;
    std::cout << "Time: " << seconds.count() << std::endl;
    return 0;
}
catch ( const std::exception& e )
{
    std::cerr << std::endl
            << "std::exception(\"" << e.what() << "\")." << std::endl;
    return 2;
}
catch ( ... )
{
    std::cerr  << std::endl
            << "unknown exception." << std::endl;
    return 1;
}}