From ef92ae27b26106ace5518a9cf0293dd45404f6ab Mon Sep 17 00:00:00 2001
From: Chris Campbell <ccampbell@climateinteractive.org>
Date: Tue, 8 Apr 2025 13:16:55 -0700
Subject: [PATCH] fix: include H2 in list of automatically subscripted
 chemicals

---
 .../projects/sample-guide/content/appendix.md         |  3 ++-
 packages/docs-builder/src/gen-html.spec.ts            |  2 ++
 packages/docs-builder/src/gen-html.ts                 | 11 ++++++++---
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/examples/sample-docs/projects/sample-guide/content/appendix.md b/examples/sample-docs/projects/sample-guide/content/appendix.md
index 0b829db..96abe29 100644
--- a/examples/sample-docs/projects/sample-guide/content/appendix.md
+++ b/examples/sample-docs/projects/sample-guide/content/appendix.md
@@ -8,6 +8,7 @@ The following chemical formulas should be subscripted automatically:
 * CO2
 * CF4
 * CH4
+* H2
 * H2O
 * N2O
 * NF3
@@ -15,4 +16,4 @@ The following chemical formulas should be subscripted automatically:
 * O3
 * SF6
 
-Those formulas should not be subscripted when they appear in URLs, for example [CO2](https://www.climateinteractive.org/CO2).
\ No newline at end of file
+Those formulas should not be subscripted when they appear in URLs, for example [CO2](https://www.climateinteractive.org/CO2).
diff --git a/packages/docs-builder/src/gen-html.spec.ts b/packages/docs-builder/src/gen-html.spec.ts
index 208ce2a..db47d9a 100644
--- a/packages/docs-builder/src/gen-html.spec.ts
+++ b/packages/docs-builder/src/gen-html.spec.ts
@@ -107,9 +107,11 @@ To fix, ensure there are no spaces between link text and link url/reference, for
 
 describe('subscriptify', () => {
   it('should convert chemical formulas', () => {
+    expect(subscriptify('CO2')).toBe('CO<sub>2</sub>')
     expect(subscriptify('This is -CO2-')).toBe('This is -CO<sub>2</sub>-')
     expect(subscriptify('This is -CF4-')).toBe('This is -CF<sub>4</sub>-')
     expect(subscriptify('This is -CH4-')).toBe('This is -CH<sub>4</sub>-')
+    expect(subscriptify('This is -H2-')).toBe('This is -H<sub>2</sub>-')
     expect(subscriptify('This is -H2O-')).toBe('This is -H<sub>2</sub>O-')
     expect(subscriptify('This is -N2O-')).toBe('This is -N<sub>2</sub>O-')
     expect(subscriptify('This is -NF3-')).toBe('This is -NF<sub>3</sub>-')
diff --git a/packages/docs-builder/src/gen-html.ts b/packages/docs-builder/src/gen-html.ts
index d80d85d..a6099e1 100644
--- a/packages/docs-builder/src/gen-html.ts
+++ b/packages/docs-builder/src/gen-html.ts
@@ -528,7 +528,7 @@ const subscriptMap = new Map([
   ['CO2', 'CO<sub>2</sub>'],
   ['CF4', 'CF<sub>4</sub>'],
   ['CH4', 'CH<sub>4</sub>'],
-  ['H2O', 'H<sub>2</sub>O'],
+  ['H2', 'H<sub>2</sub>'],
   ['N2O', 'N<sub>2</sub>O'],
   ['NF3', 'NF<sub>3</sub>'],
   ['O2', 'O<sub>2</sub>'],
@@ -607,6 +607,7 @@ export function convertMarkdownToHtml(context: Context, md: string): string {
  *   CO2
  *   CF4
  *   CH4
+ *   H2
  *   H2O
  *   N2O
  *   NF3
@@ -614,12 +615,16 @@ export function convertMarkdownToHtml(context: Context, md: string): string {
  *   O3
  *   SF6
  *
+ * Note that we only convert in the case where the chemical name is either at the
+ * beginning of a line or not preceded by a letter or digit, so that we convert
+ * things like "H2" or "non-H2" but not "CH3CH2CH3".
+ *
  * @param s The input string.
  * @return A new string containing subscripted chemical names.
  */
 export function subscriptify(s: string): string {
-  return s.replace(/(CO2|CF4|CH4|H2O|N2O|NF3|O2|O3|SF6)/g, (_m, m1) => {
-    return subscriptMap.get(m1)
+  return s.replace(/(^|[^a-zA-Z0-9])(CO2|CF4|CH4|H2|N2O|NF3|O2|O3|SF6)/g, (_m, m1, m2) => {
+    return `${m1 || ''}${subscriptMap.get(m2)}`
   })
 }