replaced the lambda function for calculating combined index with a macro function

fixed a bug

src/support/mo_math_utilities.cpp

@@ -17,11 +17,7 @@ void tdma_solver_vec(double *a, double *b, double *c, double *d,
     double* cp = new double[nrows * ncols];
     double* dp = new double[nrows * ncols];
 
-    // Helper function to access 2D arrays stored as 1D
-    auto idx = [&](int row, int col) { return col * nrows + row; };
-
-    // Start timing
-    auto start_time = std::chrono::high_resolution_clock::now();
+    #define IDX(row, col) ((col) * nrows + (row)) // performs better than lambda function
 
     // OpenACC Parallel Region
     #pragma acc parallel default(present) create(cp[:nrows*ncols], dp[:nrows*ncols]) async(acc_queue)
@@ -29,8 +25,8 @@ void tdma_solver_vec(double *a, double *b, double *c, double *d,
         // Initialize c-prime and d-prime
         #pragma acc loop gang(static: 1) vector
         for (int jc = startidx; jc < endidx; ++jc) {
-            cp[idx(jc, slev)] = c[idx(jc, slev)] / b[idx(jc, slev)];
-            dp[idx(jc, slev)] = d[idx(jc, slev)] / b[idx(jc, slev)];
+            cp[IDX(jc, slev)] = c[IDX(jc, slev)] / b[IDX(jc, slev)];
+            dp[IDX(jc, slev)] = d[IDX(jc, slev)] / b[IDX(jc, slev)];
         }
 
         // Solve for vectors c-prime and d-prime
@@ -38,16 +34,16 @@ void tdma_solver_vec(double *a, double *b, double *c, double *d,
         for (int i = slev + 1; i < elev; ++i) {
             #pragma acc loop gang(static: 1) vector
             for (int jc = startidx; jc < endidx; ++jc) {
-                double m = 1.0 / (b[idx(jc, i)] - cp[idx(jc, i - 1)] * a[idx(jc, i)]);
-                cp[idx(jc, i)] = c[idx(jc, i)] * m;
-                dp[idx(jc, i)] = (d[idx(jc, i)] - dp[idx(jc, i - 1)] * a[idx(jc, i)]) * m;
+                double m = 1.0 / (b[IDX(jc, i)] - cp[IDX(jc, i - 1)] * a[IDX(jc, i)]);
+                cp[IDX(jc, i)] = c[IDX(jc, i)] * m;
+                dp[IDX(jc, i)] = (d[IDX(jc, i)] - dp[IDX(jc, i - 1)] * a[IDX(jc, i)]) * m;
             }
         }
 
         // Initialize varout
         #pragma acc loop gang(static: 1) vector
         for (int jc = startidx; jc < endidx; ++jc) {
-            varout[idx(jc, elev-1)] = dp[idx(jc, elev-1)];
+            varout[IDX(jc, elev-1)] = dp[IDX(jc, elev-1)];
         }
 
         // Solve for varout from the vectors c-prime and d-prime
@@ -55,7 +51,7 @@ void tdma_solver_vec(double *a, double *b, double *c, double *d,
         for (int i = elev - 2; i >= slev; --i) {
             #pragma acc loop gang(static: 1) vector
             for (int jc = startidx; jc < endidx; ++jc) {
-                varout[idx(jc, i)] = dp[idx(jc, i)] - cp[idx(jc, i)] * varout[idx(jc, i + 1)];
+                varout[IDX(jc, i)] = dp[IDX(jc, i)] - cp[IDX(jc, i)] * varout[IDX(jc, i + 1)];
             }
         }
     }
@@ -70,6 +66,7 @@ void tdma_solver_vec(double *a, double *b, double *c, double *d,
     // Free memory at the end
     delete[] cp;
     delete[] dp;
+
     // End timing
     auto end_time = std::chrono::high_resolution_clock::now();
     std::chrono::duration<double> elapsed_time = end_time - start_time;