edentext/docs/architecture/formulas.md

Formulas

Insert/edit a mathematical formula (editor/extensions/formula.ts, components/FormulaDialog.svelte), round-tripping to an embedded ODF formula object and to Word's OMML.

A formula is a leaf atom, so updateFormula builds its transaction with atomAttrTr (image.ts, shared with the frames): setNodeMarkup replaces a leaf outright, and without putting the node selection back the dialog deselects the formula it just edited.

One LaTeX string is the model

The formula node stores latex and a display flag — nothing else. Everything is derived:

latex ──parse──▶ AST ──▶ MathML   (screen + .odt)
                     └──▶ OMML     (.docx)
OMML  ──parse──▶ AST ──▶ latex     (.docx import)
MathML──parse──▶ AST ──▶ latex     (.odt import)

Foreign markup is parsed into the AST and re-serialized, never passed through. That is why the node view may set innerHTML — the MathML it renders is always our own output, so a foreign annotation-xml can't smuggle in HTML. It also means an imported formula is always editable; there is no read-only state to fall back to.

The price: a construct outside the AST degrades. Unknown OMML/MathML elements recurse into their m:e/child rows, so content survives even where structure doesn't — the same trade LibreOffice makes routing OMML through StarMath.

src/lib/math/: latex.ts (AST types, parser, serializer, symbol table), mathml.ts, omml.ts. AST kinds: row, txt (i/n/o/f = identifier/number/operator/upright name), frac, script, sqrt, fence, nary, acc, matrix, space.

The LaTeX subset

\frac, _/^, \sqrt[n]{}, \left…\right, the n-ary operators with limits, the accents, matrix/pmatrix/bmatrix/Bmatrix/vmatrix/cases, the upright function names, \text{} /\mathrm{}, greek + the common operators and relations, {} grouping, & / \\, and the spacing macros. Not supported, deliberately: align environments, \substack, user macros, color. An unknown macro degrades to \text{name} rather than vanishing.

Two invariants the tests pin (tests/unit/math.test.ts, tests/unit/docx-formula.test.ts):

• astToLatex(parseLatex(x)) === x for every stored source. Without it, opening an imported formula in the dialog and pressing Apply would silently rewrite it.
• A literal ^ or _ in an imported run's text is escaped on the way out (Word documents contain typed-in code like corr.^2), or re-parsing would read it as a script marker.
• Whitespace inside an OMML run is dropped (omml.ts tokenize): math layout ignores it, and keeping it breaks the invariant above.

STIX Two Math is required, not decoration

MathML is typeset by the browser natively — no KaTeX/MathJax. But Chromium needs a font with an OpenType MATH table: measured in this container with only the bundled Liberation/Carlito faces installed, ∑ and ϕ render as nothing and ( ) stay glyph-height beside a fraction. src/assets/fonts/STIXTwoMath-Regular.woff2 (OFL, the Fontsource latin subset — the MATH table and the operators survive it) is bundled and wired in global.css; editor.css puts it first in the math font stack. Word uses Cambria Math for the same job.

ODF

Export (export/odt.ts, sentinel MTH U+E012, pass applyFormulas right after applyImages): a <draw:frame text:anchor-type="as-char" draw:style-name="MthFr"> holding <draw:object xlink:href="./Formula{n}"/>, plus Formula{n}/content.xml — an ODF formula sub-document whose root is the MathML — and two manifest entries (Formula{n}/ as application/vnd.oasis.opendocument.formula, its content.xml as text/xml).

• No svg:width/svg:height on the frame, on purpose. With a size LibreOffice scales the object to fill it (a frame measured at the dialog's preview font size magnified the formula ~1.5×); without one it typesets at the natural size, matched to the body text. Verified by rendering through soffice.
• MthFr carries style:vertical-pos="middle" style:vertical-rel="text", what LibreOffice writes for its own formula frames. bottom/baseline was tried and drops the formula below the line.
• The LaTeX rides in <annotation encoding="application/x-tex"> so our own files re-import exactly. LibreOffice strips it and substitutes a StarMath annotation, so the fallback path (parse its MathML back to LaTeX) is the one that actually runs after a LO re-save — and it reproduces the source exactly (tests/lo-roundtrip.test.ts).
• display is read off the paragraph, not the object (import/odt.ts aloneInParagraph): ODF has no inline/display flag, and LibreOffice writes display="block" on every formula it re-saves, so trusting the attribute turned every inline formula into a displayed one.

DOCX

Export (export/docx.ts, sentinel MTH, post-pack pass applyFormulasDocx beside applyTextBoxesDocx): the run holding the sentinel is replaced by <m:oMath>, or by <m:oMathPara> when display — which is what centers it on its own line. The namespace is declared inline on the m: element, like the wps/a namespaces in the text-box XML.

Import: m:oMath/m:oMathPara sit beside the w:r runs in their own namespace, so convertInline picks them up before its namespaceURI !== W guard — that guard is why every formula in a Word document was silently dropped before this feature. m:oMathPara → display: true. Word stores no size for a formula, and neither do we.

A pasted formula is a picture, not a formula

Real documents mix both: one will carry OMML formulas and equations its author pasted as raster images. Those import as image nodes and offer the image toolbar's wrap options on click — in Word and LibreOffice too. Nothing short of OCR changes that, so a click that offers wrap options instead of the formula dialog is the document's doing, not a broken import.

Testing note

npm run test:lo's formula leg needs the libreoffice-math package — LibreOffice is modular and Writer alone cannot hold a formula object, so without it every formula is dropped on load without a word (measured: converting a formula-bearing .docx with a Writer-only install yields an .odt with zero formulas). The leg then fails on "both objects survive", which looks like a broken export but is the missing component. Check dpkg -l | grep libreoffice-math before believing that failure. A desktop LibreOffice ships Math; only stripped-down server/container installs don't.