🧮 Fix AMSMath environment for LaTeX (#977)

Signed-off-by: Cristian Le <cristian.le@mpsd.mpg.de>
Co-authored-by: Rowan Cockett <rowanc1@gmail.com>

.changeset/giant-books-greet.md

@@ -0,0 +1,5 @@
+---
+"myst-to-tex": patch
+---
+
+Properly handle AMS environments, and no longer wrap with equation environment.

packages/myst-to-tex/src/math.ts

@@ -1,5 +1,35 @@
 import type { Handler, ITexSerializer } from './types.js';
 
+// Top level environments in amsmath version 2.1 (and eqnarray), see:
+// http://anorien.csc.warwick.ac.uk/mirrors/CTAN/macros/latex/required/amsmath/amsldoc.pdf
+const ENVIRONMENTS = [
+  'equation',
+  'multline',
+  'gather',
+  'align',
+  'alignat',
+  'flalign',
+  'matrix',
+  'pmatrix',
+  'bmatrix',
+  'Bmatrix',
+  'vmatrix',
+  'Vmatrix',
+  'eqnarray',
+];
+
+const RE_OPEN = new RegExp(`^\\\\begin{(${ENVIRONMENTS.join('|')})([*]?)}`);
+
+function isAmsmathEnvironment(value: string): boolean {
+  const matchOpen = value.trim().match(RE_OPEN);
+  if (!matchOpen) return false;
+  const [, environment, star] = matchOpen;
+  const end = `\\end{${environment}${star}}`;
+  const matchClose = value.trim().endsWith(end);
+  if (!matchClose) return false;
+  return true;
+}
+
 function addMacrosToState(value: string, state: ITexSerializer) {
   if (!state.options.math) return;
   Object.entries(state.options.math).forEach(([k, v]) => {
@@ -47,15 +77,24 @@ const math: Handler = (node, state) => {
     state.write(node.value);
     state.write(' \\)');
   } else {
-    // TODO: AMS math
-    state.write(`\\begin{equation${enumerated === false ? '*' : ''}}\n`);
-    if (label) {
-      state.write(`\\label{${label}}`);
+    // Check if the node is an AMSMath environment, if so, render it directly
+    const isAmsMath = isAmsmathEnvironment(node.value);
+    if (isAmsMath) {
+      // TODO: labels may be stripped previously in the transform, we may need to back that out
+      state.ensureNewLine();
+      state.write(node.value);
+      state.ensureNewLine(true);
+    } else {
+      // Otherwise enclose the math environment by equation + label
+      state.write(`\\begin{equation${enumerated === false ? '*' : ''}}\n`);
+      if (label) {
+        state.write(`\\label{${label}}`);
+      }
+      state.ensureNewLine();
+      state.write(node.value);
+      state.ensureNewLine(true);
+      state.write(`\\end{equation${enumerated === false ? '*' : ''}}`);
     }
-    state.ensureNewLine();
-    state.write(node.value);
-    state.ensureNewLine(true);
-    state.write(`\\end{equation${enumerated === false ? '*' : ''}}`);
   }
   if (!state.data.isInTable) state.closeBlock(node);
 };

packages/myst-to-tex/tests/amsmath.yml

@@ -0,0 +1,34 @@
+title: myst-to-tex amsmath tests
+cases:
+  - title: gather
+    mdast:
+      type: root
+      children:
+        - type: math
+          value: |-
+            \begin{gather}
+                E=mc^2
+            \end{gather}
+    latex: |-
+      \begin{gather}
+          E=mc^2
+      \end{gather}
+  - title: alignat*
+    mdast:
+      type: root
+      children:
+        - type: math
+          value: |-
+            \begin{alignat*}{3}
+            & m   \quad && \text{módulo}            \quad && m>0\\
+            & a   \quad && \text{multiplicador}     \quad && 0<a<m\\
+            & c   \quad && \text{constante aditiva} \quad && 0\leq c<m\\
+            & x_0 \quad && \text{valor inicial}     \quad && 0\leq x_0 <m
+            \end{alignat*}
+    latex: |-
+      \begin{alignat*}{3}
+      & m   \quad && \text{módulo}            \quad && m>0\\
+      & a   \quad && \text{multiplicador}     \quad && 0<a<m\\
+      & c   \quad && \text{constante aditiva} \quad && 0\leq c<m\\
+      & x_0 \quad && \text{valor inicial}     \quad && 0\leq x_0 <m
+      \end{alignat*}