[3-quell-the-fracus] [Bug Fix] - Ensure that users hand typing tex in…

…to Numeric Inputs on Desktop do not cause an infinite loop. (#2182)

## Summary:
Third time's the charm on this one. This ticket is part of the Numeric Input project. 

Numeric Input has different experiences on Desktop versus Mobile: 

1. Desktop allows users to enter basic math commands using their regular keyboard. Desktop does not display / visually support TeX, but CAN parse it as an answer. 
2. Mobile allows users to enter basic math commands using a keypad that crafts TeX to display the output using MathQuill  / Math Input. 

This diverged experience resulted in the possibility to hit an infinite loop on Desktop if the user tries to hand type `\frac` or `\dfrac` TeX commands , as the parser was unable to locate the next symbols to parse. This has likely been a bug since inception, but has become far more noticeable as we're now parsing answers on the fly to provide AI support. As a result, the answers are constantly being evaluated and would hit the infinite loop as soon as the user started typing the expressions.

Issue: LEMS-198

## Test plan:
- Run tests
- New tex wrangler test

Author: SonicScrewdriver

Reviewers: mark-fitzgerald

Required Reviewers:

Approved By: mark-fitzgerald

Checks: ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ⏹️  [cancelled] Publish npm snapshot (ubuntu-latest, 20.x), ⏹️  [cancelled] Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ⏹️  [cancelled] Cypress (ubuntu-latest, 20.x), ⏹️  [cancelled] Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ⏹️  [cancelled] Check builds for changes in size (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x)

Pull Request URL: #2182

.changeset/pink-buttons-clap.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus-score": patch
+---
+
+Bugfix to ensure users cannot create infinite loop with incomplete tex in Numeric Input

packages/perseus-score/src/util/tex-wrangler.test.ts

@@ -78,3 +78,22 @@ describe("parseTex", () => {
         assertParsed(undefined, "");
     });
 });
+describe("parseTex", () => {
+    it("should replace single fractions", () => {
+        assertParsed("\\dfrac{3}{4}", "3 / 4");
+    });
+
+    it("should remove blackslash-escapes for percent signs", () => {
+        assertParsed("3\\%", "3%");
+        assertParsed("3.5\\%", "3.5%");
+        assertParsed("\\dfrac{3\\%}{4}", "3% / 4");
+    });
+
+    it("should not throw error when input is undefined", () => {
+        assertParsed(undefined, "");
+    });
+
+    it("should not cause an infinite loop if provided incomplete tex commands", () => {
+        assertParsed("\\frac", "/");
+    });
+});

packages/perseus-score/src/util/tex-wrangler.ts

@@ -42,11 +42,23 @@ function parseNextExpression(
     // Find the first '{' and grab subsequent TeX
     // Ex) tex: '{3}{7}', and we want the '3'
     const openBracketIndex = tex.indexOf("{", currentIndex);
+
+    // If there is no open bracket, set the endpoint to the end of the string
+    // and the expression to an empty string. This helps ensure we don't
+    // get stuck in an infinite loop when users handtype TeX.
+    if (openBracketIndex === -1) {
+        return {
+            endpoint: tex.length,
+            expression: "",
+        };
+    }
+
     const nextExpIndex = openBracketIndex + 1;
 
     // Truncate to only contain remaining TeX
     const endpoint = findEndpoint(tex, nextExpIndex);
     const expressionTeX = tex.substring(nextExpIndex, endpoint);
+
     const parsedExp = walkTex(expressionTeX, handler);
 
     return {