From 9644fc5ddaf9ebceb652f64d3be81bb066d7b06c Mon Sep 17 00:00:00 2001
From: fredrikr79 <fredrikrobertsen7@gmail.com>
Date: Mon, 22 Sep 2025 19:13:17 +0200
Subject: [PATCH] ex3: task 5

---
 exercise3/wave_1d_parallel.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/exercise3/wave_1d_parallel.c b/exercise3/wave_1d_parallel.c
index ef9847c..36c5b39 100644
--- a/exercise3/wave_1d_parallel.c
+++ b/exercise3/wave_1d_parallel.c
@@ -115,7 +115,16 @@ void boundary_condition(void) {
 // Communicate the border between processes.
 void border_exchange(void) {
   // BEGIN: T5
-  ;
+  int left = (comm_size + comm_rank - 1) % comm_size;
+  int right = (comm_rank + 1) % comm_size;
+
+  MPI_Sendrecv(&U(n - 1), 1, MPI_DOUBLE, right, 0,
+               &U(-1), 1, MPI_DOUBLE, left, 0,
+               MPI_COMM_WORLD, MPI_STATUS_IGNORE);
+
+  MPI_Sendrecv(&U(0), 1, MPI_DOUBLE, left, 1,
+               &U(n), 1, MPI_DOUBLE, right, 1,
+               MPI_COMM_WORLD, MPI_STATUS_IGNORE);
   // END: T5
 }