From 25ef327985d8ddf78786a816f9d4dbb8adb6aee1 Mon Sep 17 00:00:00 2001
From: "lisa.pizzo" <lisa.pizzo@noreply.imsc.uni-graz.at>
Date: Tue, 16 Dec 2025 11:04:56 +0100
Subject: [PATCH] Added a new parallelized function

---
 Sheet5/Ex5_Second_Attempt/jacsolve.cpp | 145 +++++++++++++++++++++++++
 Sheet5/Ex5_Second_Attempt/jacsolve.h   |  18 +++
 2 files changed, 163 insertions(+)
 create mode 100644 Sheet5/Ex5_Second_Attempt/jacsolve.cpp
 create mode 100644 Sheet5/Ex5_Second_Attempt/jacsolve.h

diff --git a/Sheet5/Ex5_Second_Attempt/jacsolve.cpp b/Sheet5/Ex5_Second_Attempt/jacsolve.cpp
new file mode 100644
index 0000000..7aff121
--- /dev/null
+++ b/Sheet5/Ex5_Second_Attempt/jacsolve.cpp
@@ -0,0 +1,145 @@
+#include "vdop.h"
+#include "getmatrix.h"
+#include "jacsolve.h"
+
+#include <cassert>
+#include <cmath>
+#include <iostream>
+#include <vector>
+#include <omp.h>
+using namespace std;
+
+// #####################################################################
+// 	const int neigh[], const int color,
+// 	const MPI::Intracomm& icomm,
+void JacobiSolve(CRS_Matrix const &SK, vector<double> const &f, vector<double> &u)
+{
+    const double omega   = 1.0;
+    const int    maxiter = 1000;
+    const double tol  = 1e-5,                // tolerance
+                 tol2 = tol * tol;           // tolerance^2
+
+    int nrows = SK.Nrows();                  // number of rows == number of columns
+    assert( nrows == static_cast<int>(f.size()) && f.size() == u.size() );
+
+    cout << endl << " Start Jacobi solver for " << nrows << " d.o.f.s"  << endl;
+    //  Choose initial guess
+    for (int k = 0; k < nrows; ++k)
+    {
+        u[k] = 0.0;                          //  u := 0
+    }
+
+    vector<double> dd(nrows);                // matrix diagonal
+    vector<double>  r(nrows);                // residual
+    vector<double>  w(nrows);                // correction
+
+    SK.GetDiag(dd);                          //  dd := diag(K)
+    ////DebugVector(dd);{int ijk; cin >> ijk;}
+
+    //  Initial sweep
+    SK.Defect(r, f, u);                      //  r := f - K*u
+
+    vddiv(w, r, dd);                         //  w := D^{-1}*r
+    double sigma0 = dscapr(w, r);            // s0 := <w,r>
+
+    // Iteration sweeps
+    int iter  = 0;
+    double sigma = sigma0;
+    while ( sigma > tol2 * sigma0 && maxiter > iter)
+    {
+        ++iter;
+        vdaxpy(u, u, omega, w );             //  u := u + om*w
+        SK.Defect(r, f, u);                  //  r := f - K*u
+        vddiv(w, r, dd);                     //  w := D^{-1}*r
+        sigma = dscapr(w, r);                // s0 := <w,r>
+//      	cout << "Iteration " << iter << " : " << sqrt(sigma/sigma0) << endl;
+    }
+    cout << "aver. Jacobi rate :  " << exp(log(sqrt(sigma / sigma0)) / iter) << "  (" << iter << " iter)" << endl;
+    cout << "final error: " << sqrt(sigma / sigma0) << " (rel)   " << sqrt(sigma) << " (abs)\n";
+
+    return;
+}
+
+
+void JacobiSolve_omp(CRS_Matrix const &SK,
+                     vector<double> const &f,
+                     vector<double> &u)
+{
+    const double omega   = 1.0;
+    const int    maxiter = 1000;
+    const double tol  = 1e-5;
+    const double tol2 = tol * tol;
+
+    const int nrows = SK.Nrows();
+    assert(nrows == static_cast<int>(f.size()) && f.size() == u.size());
+
+    cout << "\n Start Jacobi solver (OpenMP) for "
+         << nrows << " d.o.f.s\n";
+
+    // --------------------------------------------------
+    // Initial guess
+    // --------------------------------------------------
+    #pragma omp parallel for
+    for (int i = 0; i < nrows; ++i)
+        u[i] = 0.0;
+
+    vector<double> dd(nrows);
+    vector<double> r(nrows);
+    vector<double> w(nrows);
+
+    SK.GetDiag(dd);
+
+    // r = f - K*u
+    SK.Defect(r, f, u);
+
+    // w = D^{-1} r
+    #pragma omp parallel for
+    for (int i = 0; i < nrows; ++i)
+        w[i] = r[i] / dd[i];
+
+    // sigma0 = <w, r>
+    double sigma0 = 0.0;
+    #pragma omp parallel for reduction(+:sigma0)
+    for (int i = 0; i < nrows; ++i)
+        sigma0 += w[i] * r[i];
+
+    double sigma = sigma0;
+    int iter = 0;
+
+    // --------------------------------------------------
+    // Jacobi iteration
+    // --------------------------------------------------
+    while (sigma > tol2 * sigma0 && iter < maxiter)
+    {
+        ++iter;
+
+        // u = u + omega * w
+        #pragma omp parallel for
+        for (int i = 0; i < nrows; ++i)
+            u[i] += omega * w[i];
+
+        // r = f - K*u
+        SK.Defect(r, f, u);
+
+        // w = D^{-1} r
+        #pragma omp parallel for
+        for (int i = 0; i < nrows; ++i)
+            w[i] = r[i] / dd[i];
+
+        // sigma = <w, r>
+        sigma = 0.0;
+        #pragma omp parallel for reduction(+:sigma)
+        for (int i = 0; i < nrows; ++i)
+            sigma += w[i] * r[i];
+    }
+
+    cout << "aver. Jacobi rate :  "
+         << exp(log(sqrt(sigma / sigma0)) / iter)
+         << "  (" << iter << " iter)\n";
+
+    cout << "final error: "
+         << sqrt(sigma / sigma0) << " (rel)   "
+         << sqrt(sigma) << " (abs)\n";
+}
+
+
diff --git a/Sheet5/Ex5_Second_Attempt/jacsolve.h b/Sheet5/Ex5_Second_Attempt/jacsolve.h
new file mode 100644
index 0000000..3487aa0
--- /dev/null
+++ b/Sheet5/Ex5_Second_Attempt/jacsolve.h
@@ -0,0 +1,18 @@
+#ifndef JACSOLVE_FILE
+#define JACSOLVE_FILE
+#include "getmatrix.h"
+#include <vector>
+
+
+/**
+ * Solves linear system of equations  K @p u = @p f  via the Jacobi iteration.
+ * We use a distributed symmetric  CSR matrix @p SK and initial guess of the
+ * solution is set to 0.
+ * @param[in] SK	CSR matrix
+ * @param[in] f		distributed local vector storing the right hand side
+ * @param[out] u	accumulated local vector storing the solution.
+*/
+void JacobiSolve(CRS_Matrix const &SK, std::vector<double> const &f, std::vector<double> &u);
+void JacobiSolve_omp(CRS_Matrix const &SK, std::vector<double> const &f, std::vector<double> &u);
+
+#endif