source: CIVL/examples/mpi/collective/reduceScatterOptimized/driver.cvl

#include "mpi.h"
#include "stdlib.h"
#include "string.h"
#include  "assert.h"
#include "reduceScatter_optimized.h"

$input int NP;                                    // nprocs
$input int N;                                     // size of input data per proc
$input int NB;                                    // upper bound of the size of input data per proc
$assume(0 < N && N < NB);                          
$input double DATA[N * NP];                       // arbitrary input data, N per proc
$input int VEC[NP];                               // arbitrary recv counts
$assume($forall (int i : 0 .. NP-1) VEC[i] > 0);  

#ifdef _CHECK_MAX
#define MPIOP  MPI_MAX
#else
#define MPIOP  MPI_SUM
#endif

int main() {
  double * sendbuf, * recvbuf;
  int rank, size;
  
  MPI_Init(NULL, NULL);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &size);

  // NP is nprocs:
  $assume(NP == size);
    
  int recvcnts[size];
  int total_counts = 0;                           
  int my_oft = 0;

  // initializes recv counts
  // computes the total count for completing the assumption over the recv counts
  // computes my offset
  for (int i = 0; i < size; i++) {
    recvcnts[i] = VEC[i];
    total_counts += VEC[i];
    if (i < rank) my_oft += recvcnts[i];
  }
  // assumption over the recv count:
  $assume(total_counts == N);  

  sendbuf = (double*)malloc(sizeof(double) * N);
  recvbuf = (double*)malloc(sizeof(double) * recvcnts[rank]);
  
  memcpy(sendbuf, DATA + rank * N, sizeof(double) * N);
  reduce_scatter_double(sendbuf, recvbuf, recvcnts, MPIOP, MPI_COMM_WORLD);

  // assertions:
#ifdef _CHECK_MAX  

  for (int i = 0; i < recvcnts[rank]; i++)
    assert(recvbuf[i] >= sendbuf[my_oft + i]);

#else

  for (int i = 0; i < recvcnts[rank]; i++) {
    double expect = 0;
    
    for (int j = 0; j < size; j++) 
      expect += DATA[j * N + my_oft + i];
    assert(recvbuf[i] == expect);
  }
#endif

  free(sendbuf);
  free(recvbuf);
  MPI_Finalize();
}