Modulef Formatted Mesh — MFM (.mfm)

The Modulef Formatted Mesh is a compact ASCII format used by FEconv (a simplified NOPO/Modulef mesh). A file holds a single element type (non-hybrid): an 8-integer header, the connectivity, per-entity reference arrays, the vertex coordinates and a per-element subdomain array.

Format name	mfm
Extensions	.mfm
Read / Write	✓ / ✓
Extra dependencies	—

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("mesh.mfm")
meshioplusplus.mfm.write("out.mfm", mesh, float_fmt=".16e")

• float_fmt (default ".16e") — coordinate format string.

File structure

nel nnod nver dim lnn lnv lne lnf   # header, 8 ints

nel/nver are the element/vertex counts; lnn/lnv/lne/lnf are the local (per-element) node/vertex/edge/face counts, which together identify the element type. After the header, the file is a flat whitespace-token stream (no line-structure requirement — Python reads it with f.read().split() and the C++ reader does the equivalent) laid out as:

1. mm — connectivity, nel × lnv integers, 1-based, element-major.
2. nrc — face reference array, nel × lnf integers — only present if dim == 3 — read but discarded.
3. nra — edge reference array, nel × lne integers — only present if dim >= 2 — discarded.
4. nrv — vertex reference array, nel × lnv integers — always present — discarded.
5. z — vertex coordinates, nver × dim floats, vertex-major.
6. nsd — per-element subdomain/material integer, nel values → cell_data["mfm:ref"].

The writer emits the same sections in the same order, with nrc/nra/nrv written as all-zero placeholders (they carry no meshio++-side data).

Cell types

The element type is recovered from (lnv, lne, lnf) plus lnn == lnv:

type	lnv	lne	lnf
line	2	1	0
triangle	3	3	1
quad	4	4	1
tetra	4	6	4
hexahedron	8	12	6
wedge	6	9	5

Linear elements only. Because MFM only stores vertex coordinates (no mid-edge node positions), second-order (P2) elements would necessarily be straight-sided if forced through this representation — meshio++ rejects them outright (requires lnn == lnv) rather than silently discarding curvature information.

Data mapping

• cell_data["mfm:ref"] — per-element subdomain/material reference (a single int array; defaults to all-ones on write if absent).

Quirks & limitations

• Single element type per file: the writer requires exactly one cell type across the whole mesh, raising WriteError for a mixed-type mesh — MFM is fundamentally single-type ("non-hybrid").
• nrc/nra/nrv (face/edge/vertex reference arrays) are read and discarded entirely — there is no meshio++-side representation for them, and they're always written as zeros.
• Requires nnod == nver (no separate "node" vs "vertex" numbering) in addition to lnn == lnv — both checks reject P2 elements.

Notes

• Fully handled by the C++ core (Python fallback only for buffers or a non-default float_fmt).
• No reference fixture exists under tests/meshes/mfm/; tests round-trip every supported linear type and explicitly verify WriteError on a mixed-type mesh.