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奇偶交换后的保留操作可以用更优的二路归并解决，这里偷懒直接使用qsort。
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#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
int cmpInt(const void *a, const void *b) { return *(int *)a - *(int *)b; }
int main(int argc, char **argv)
{
MPI_Init(&argc, &argv);
int comSize, comRank, n, *globalData;
MPI_Comm_size(MPI_COMM_WORLD, &comSize);
MPI_Comm_rank(MPI_COMM_WORLD, &comRank);
if (!comRank)
{
printf("Input array size:");
scanf("%d", &n);
globalData = malloc(n * sizeof(int));
printf("Then input %d number(s):", n);
for (int i = 0; i < n; ++i)
scanf("%d", &globalData[i]);
}
MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
int *sendcounts = malloc(comSize * sizeof(int)), *displs = malloc(comSize * sizeof(int));
for (int i = 0, cnt = (n + comSize - 1) / comSize;
i < comSize; ++i)
{
displs[i] = i * cnt;
sendcounts[i] = n - displs[i] < cnt ? n - displs[i] : cnt;
}
int *localData = malloc((sendcounts[comRank] + sendcounts[0]) * sizeof(int));
MPI_Scatterv(
globalData,
sendcounts,
displs,
MPI_INT,
localData,
sendcounts[comRank],
MPI_INT,
0,
MPI_COMM_WORLD);
qsort(localData, sendcounts[comRank], sizeof(int), &cmpInt);
for (int i = 0; i < comSize; ++i)
{
int partner = comRank + ((i + comRank) & 1 ? 1 : -1);
if (0 <= partner && partner < comSize)
{
MPI_Sendrecv(
localData,
sendcounts[comRank],
MPI_INT,
partner,
i,
localData + sendcounts[comRank],
sendcounts[partner],
MPI_INT,
partner,
i,
MPI_COMM_WORLD,
MPI_STATUS_IGNORE);
qsort(localData, sendcounts[comRank] + sendcounts[partner], sizeof(int), &cmpInt);
if (comRank > partner)
for (int i = 0; i < sendcounts[comRank]; ++i)
localData[i] = localData[i + sendcounts[partner]];
}
}
MPI_Gatherv(
localData,
sendcounts[comRank],
MPI_INT,
globalData,
sendcounts,
displs,
MPI_INT,
0,
MPI_COMM_WORLD);
free(localData), free(sendcounts), free(displs);
if (!comRank)
{
for (int i = 0; i < n; ++i)
printf("%d ", globalData[i]);
free(globalData);
}
MPI_Finalize();
}