Merge pull request #19 from mathrack/hack_doconcurrent

Add some comments for the Thomas algorithm.

Author: pbartholomew08

src/thomas.f90

@@ -202,8 +202,17 @@ subroutine zthomas_12(tt, ff, fs, fw, nx, ny, nz)
 
   end subroutine zthomas_12
 
-  ! Thomas algorithm for a 1D vector
-  subroutine thomas1d(tt, ff, fs, fw, nn)
+  !
+  ! Thomas algorithm for a 1D vector (solve My = x with tri-diagonal M)
+  ! See comments in the subroutine prepare (x3d_operator_1d.f90)
+  !
+  ! tt, inout, vector x and y
+  ! ff, in, upper diagonal of the tri-diagonal matrix
+  ! fs, in, used during the forward step
+  ! fw, in, used during the backward step
+  ! nn, in, size of the vector
+  !
+  pure subroutine thomas1d(tt, ff, fs, fw, nn)
 
     implicit none
 

src/x3d_operator_1d.f90

@@ -308,6 +308,21 @@ subroutine finalize(x3dop)
 
   end subroutine finalize
 
+   !
+   ! Prepare Thomas algorithm for a tri-diagonal matrix M
+   ! See https://en.wikipedia.org/wiki/Tridiagonal_matrix_algorithm#Method
+   ! Here :
+   !     ( c(1) f(1) 0    0    0 ...
+   ! M = ( b(1) c(2) f(2) 0    0 ...
+   !     ( 0    b(2) c(3) f(3) 0 ...
+   !
+   ! b, in, lower diagonal
+   ! c, in, diagonal coefficient
+   ! f, in, upper diagonal
+   ! s, out, vector for the forward step of the Thomas algo.
+   ! w, out, vector for the backward step of the Thomas algo.
+   ! n, in, size of the problem
+   !
    subroutine prepare(b, c, f, s, w, n)
 
       use decomp_2d, only: mytype