dbcsr_gershgorin_norm Function

   FUNCTION dbcsr_gershgorin_norm(matrix) RESULT(norm)
      !! compute a norm of a dbcsr matrix

      TYPE(dbcsr_type), INTENT(INOUT)                    :: matrix
         !! the matrix
      REAL(KIND=real_8)                                  :: norm

      CHARACTER(len=*), PARAMETER :: routineN = 'dbcsr_gershgorin_norm'

      COMPLEX(KIND=real_4), DIMENSION(:, :), POINTER     :: data_c
      COMPLEX(KIND=real_8), DIMENSION(:, :), POINTER     :: data_z
      INTEGER                                            :: blk, col, col_offset, handle, i, j, nc, &
                                                            nr, row, row_offset
      LOGICAL                                            :: any_sym, tr
      REAL(KIND=real_4), DIMENSION(:, :), POINTER        :: data_r
      REAL(KIND=real_8), DIMENSION(:, :), POINTER        :: data_d
      REAL(real_8), ALLOCATABLE, DIMENSION(:)            :: buff_d
      TYPE(dbcsr_iterator)                               :: iter

      CALL timeset(routineN, handle)

      nr = dbcsr_nfullrows_total(matrix)
      nc = dbcsr_nfullcols_total(matrix)

      any_sym = dbcsr_get_matrix_type(matrix) .EQ. dbcsr_type_symmetric .OR. &
                dbcsr_get_matrix_type(matrix) .EQ. dbcsr_type_antisymmetric

      IF (nr .NE. nc) &
         DBCSR_ABORT("not a square matrix")

      norm = 0.0_dp
      ALLOCATE (buff_d(nr))
      buff_d = 0.0_dp
      CALL dbcsr_iterator_start(iter, matrix)
      DO WHILE (dbcsr_iterator_blocks_left(iter))
         SELECT CASE (dbcsr_get_data_type(matrix))
         CASE (dbcsr_type_real_4)
            CALL dbcsr_iterator_next_block(iter, row, col, data_r, tr, blk, &
                                           row_offset=row_offset, col_offset=col_offset)
            DO j = 1, SIZE(data_r, 2)
            DO i = 1, SIZE(data_r, 1)
               buff_d(row_offset + i - 1) = buff_d(row_offset + i - 1) + ABS(data_r(i, j))
               IF (any_sym .AND. row .NE. col) &
                  buff_d(col_offset + j - 1) = buff_d(col_offset + j - 1) + ABS(data_r(i, j))
            END DO
            END DO
         CASE (dbcsr_type_real_8)
            CALL dbcsr_iterator_next_block(iter, row, col, data_d, tr, blk, &
                                           row_offset=row_offset, col_offset=col_offset)
            DO j = 1, SIZE(data_d, 2)
            DO i = 1, SIZE(data_d, 1)
               buff_d(row_offset + i - 1) = buff_d(row_offset + i - 1) + ABS(data_d(i, j))
               IF (any_sym .AND. row .NE. col) &
                  buff_d(col_offset + j - 1) = buff_d(col_offset + j - 1) + ABS(data_d(i, j))
            END DO
            END DO
         CASE (dbcsr_type_complex_4)
            CALL dbcsr_iterator_next_block(iter, row, col, data_c, tr, blk, &
                                           row_offset=row_offset, col_offset=col_offset)
            DO j = 1, SIZE(data_c, 2)
            DO i = 1, SIZE(data_c, 1)
               buff_d(row_offset + i - 1) = buff_d(row_offset + i - 1) + ABS(data_c(i, j))
               IF (any_sym .AND. row .NE. col) &
                  DBCSR_ABORT("Only nonsymmetric matrix so far")
               !     buff_d(col_offset+j-1) = buff_d(col_offset+j-1) + ABS(data_c(i,j))
            END DO
            END DO
         CASE (dbcsr_type_complex_8)
            CALL dbcsr_iterator_next_block(iter, row, col, data_z, tr, blk, &
                                           row_offset=row_offset, col_offset=col_offset)
            DO j = 1, SIZE(data_z, 2)
            DO i = 1, SIZE(data_z, 1)
               buff_d(row_offset + i - 1) = buff_d(row_offset + i - 1) + ABS(data_z(i, j))
               IF (any_sym .AND. row .NE. col) &
                  DBCSR_ABORT("Only nonsymmetric matrix so far")
               !     buff_d(col_offset+j-1) = buff_d(col_offset+j-1) + ABS(data_z(i,j))
            END DO
            END DO
         CASE DEFAULT
            DBCSR_ABORT("Wrong data type")
         END SELECT
      END DO
      CALL dbcsr_iterator_stop(iter)
      CALL mp_sum(buff_d, dbcsr_mp_group(dbcsr_distribution_mp(matrix%dist)))
      norm = MAXVAL(buff_d)
      DEALLOCATE (buff_d)

      CALL timestop(handle)

   END FUNCTION dbcsr_gershgorin_norm