Add VTK Support

[daniellivingston]

In plain text, an example in pure Fortran:

module VTUWriter

    implicit none
    private

    type, public :: VTUFile
        integer :: file_unit
        char(len=500) :: filename
    end type

    public :: VTUOpenFile
end module

function VTUOpenFile(filename)

    implicit none
    type(VTUOpenFile), pointer :: vtu
    character(len=500) :: filename
    integer :: unit = 42

    allocate(f)

    vtu%unit = unit
    vtu%filename = filename

    open(unit = unit, file = filename, status = 'new')

    VTUOpenFile => vtu

end function

function VTUCloseFile(vtu)
    VTUOpenFile :: vtu
    close(vtu%unit)
end function

function VTUWriteHeader(vtu)
    implicit none

    type(VTUFile), pointer :: vtu
    write(vtu%unit, '(A)') '<VTKFile type="UnstructuredGrid" version="1.0" header_type="UInt64">'
    write(vtu%unit, '(A)') '<UnstructuredGrid>'
    write(vtu%unit, '(A)') '<Piece NumberOfPoints="', n_vertices, '" NumberOfCells="', n_elements, '">'

end function

function VTUWritePoints()
    write(vtu%unit, '(A)') '<Points>'
    write(vtu%unit, '(A)') '<DataArray type="Float32" NumberOfComponents="3" format="ascii">'

    do (i=1,n_points)
        write(vtu%unit, '(A)') points(i, 1), points(i, 2), points(i, 3)
    end do

    write(vtu%unit, '(A)') '</DataArray>'
    write(vtu%unit, '(A)') '</Points>'
end function

function VTUWriteCells()
    write(vtu%unit, '(A)') '<Cells>'
    write(vtu%unit, '(A)') '<DataArray type="Int64" Name="connectivity" format="ascii">'

    do i=1,n_cells
        do j=1,size(cells(i))
            write(vtu%unit, '(A)') cells(i, j)
        end do
    end do

    write(vtu%unit, '(A)') '</DataArray>'
    write(vtu%unit, '(A)') '</Cells>'
    ! https://github.com/polyfem/polyfem/blob/3f58d84cd0ad930b71e8c6db917fe46e8dd4e100/src/mesh/VTUWriter.cpp#L170
end function

function VTUWriteFooter()
    write(vtu%unit, '(A)') '</Piece>'
    write(vtu%unit, '(A)') '</UnstructuredGrid>'
    write(vtu%unit, '(A)') '</VTKFile>'
end function

function VTUWriteFile()
! https://github.com/polyfem/polyfem/blob/3f58d84cd0ad930b71e8c6db917fe46e8dd4e100/src/mesh/VTUWriter.cpp#L82:21
    vtu = VTUOpenFile(filename)

    VTUWriteHeader(vtu)
    VTUWritePoints(vtu, points)
    VTUWriteFooter(vtu)

    VTUCloseFile(vtu)
end function