From 265bb2ac36771915e9d4c6eafb15889f67b53029 Mon Sep 17 00:00:00 2001
From: Johannes Winklehner <johannes.winklehner@gmail.com>
Date: Wed, 11 May 2016 22:00:19 +0200
Subject: [PATCH] =?UTF-8?q?Binom=5FReduce=20funktioniert=20soweit=20denk?=
 =?UTF-8?q?=20ich,=20ob=20kommutativit=C3=A4t=20passt=20bin=20ich=20nicht?=
 =?UTF-8?q?=20sicher?=
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---
 hpc_mpi/src/hpc_mpi.c | 78 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 hpc_mpi/src/hpc_mpi.c

diff --git a/hpc_mpi/src/hpc_mpi.c b/hpc_mpi/src/hpc_mpi.c
new file mode 100644
index 0000000..6c89719
--- /dev/null
+++ b/hpc_mpi/src/hpc_mpi.c
@@ -0,0 +1,78 @@
+/*
+ ============================================================================
+ Name        : hpc_mpi.c
+ Author      : 
+ Version     :
+ Copyright   : Your copyright notice
+ Description : Hello MPI World in C 
+ ============================================================================
+ */
+#include <stdio.h>
+#include <string.h>
+#include "mpi.h"
+
+int Binom_Reduce(const void *sendbuf, void *recvbuf, int count,
+		MPI_Datatype datatype, MPI_Op op, int root, MPI_Comm comm);
+
+int main(int argc, char* argv[]) {
+	int my_rank; /* rank of process */
+	int p; /* number of processes */
+
+	/* start up MPI */
+
+	MPI_Init(&argc, &argv);
+
+	/* find out process rank */
+	MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
+
+	/* find out number of processes */
+	MPI_Comm_size(MPI_COMM_WORLD, &p);
+
+	int a = my_rank;
+	int recv;
+
+	Binom_Reduce(&a, &recv, 1, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);
+
+	if (my_rank == 0) {
+		printf("%i\n", recv);
+	}
+	/* shut down MPI */
+	MPI_Finalize();
+
+	return 0;
+}
+
+int Binom_Reduce(const void *sendbuf, void *recvbuf, int count,
+		MPI_Datatype datatype, MPI_Op op, int root, MPI_Comm comm) {
+	int r, p, size;
+	MPI_Status status;
+	void *recv;
+	void *reduced;
+
+	MPI_Comm_rank(comm, &r);
+	MPI_Comm_size(comm, &p);
+	MPI_Type_size(datatype, &size);
+
+	MPI_Alloc_mem(count * size, MPI_INFO_NULL, &recv);
+	if (r != root) {
+		MPI_Alloc_mem(count * size, MPI_INFO_NULL, &reduced);
+	} else {
+		reduced = recvbuf;
+	}
+	memcpy(reduced, sendbuf, count * size);
+
+	int i = 1;
+	while ((r + i) % (2 * i) != 0 && i < p) {
+		if (r + i < p) {
+			MPI_Recv(recv, count, datatype, r + i, i, comm, &status);
+			MPI_Reduce_local(recv, reduced, count, datatype, op);
+		}
+		i <<= 1;
+	}
+
+	if (r != root) {
+		MPI_Send(reduced, count, datatype, r - i, i, comm);
+	}
+
+	return 0;
+}