Rewrite and simplify the math rendering implementation

In the process, this fixes a bug with the previous implementation where
inline math was rendered if it started or ended with either '\(' or
'\)'. The new implementation requires that inline math blocks start with
only '\(' and must have a matching '\)'

h/static/scripts/render-markdown.js

@@ -10,6 +10,12 @@ function targetBlank() {
   return [{type: 'output', filter: filter}];
 }
 
+/**
+ * Render markdown to HTML.
+ *
+ * This function does *not* sanitize the HTML in any way, that is the caller's
+ * responsibility.
+ */
 function renderMarkdown(html) {
   // see https://github.com/showdownjs/showdown#valid-options
   var converter = new showdown.Converter({
@@ -24,83 +30,48 @@ function renderMarkdown(html) {
   return converter.makeHtml(html);
 }
 
-function renderInlineMath(textToCheck, $sanitize) {
-  var re = /\\?\\\(|\\?\\\)/g;
-  var startMath = null;
-  var endMath = null;
-  var match;
-  var indexes = [];
-  while ((match = re.exec(textToCheck))) {
-    indexes.push(match.index);
-  }
-  for (var i = 0, index; i < indexes.length; i++) {
-    index = indexes[i];
-    if (startMath === null) {
-      startMath = index + 2;
-    } else {
-      endMath = index;
-    }
-    if (startMath !== null && endMath !== null) {
-      try {
-        var math = katex.renderToString(textToCheck.substring(startMath, endMath));
-        textToCheck = (
-            textToCheck.substring(0, (startMath - 2)) + math +
-            textToCheck.substring(endMath + 2)
-            );
-        startMath = null;
-        endMath = null;
-        return renderInlineMath(textToCheck);
-      } catch (error) {
-        $sanitize(textToCheck.substring(startMath, endMath));
-      }
+/**
+ * Replaces inline math between '\(' and '\)' delimiters
+ * with math rendered by KaTeX.
+ */
+function renderInlineMath(text, $sanitize) {
+  var mathStart = text.indexOf('\\(');
+  if (mathStart !== -1) {
+    var mathEnd = text.indexOf('\\)', mathStart);
+    if (mathEnd !== -1) {
+      var markdownSection = text.slice(0, mathStart);
+      var mathSection = text.slice(mathStart + 2, mathEnd);
+      var renderedMath = katex.renderToString(mathSection);
+      return markdownSection +
+             renderedMath +
+             renderInlineMath(text.slice(mathEnd + 2), $sanitize);
     }
   }
-  return textToCheck;
+  return text;
 }
 
+/**
+ * Renders mixed blocks of markdown and LaTeX to HTML.
+ *
+ * LaTeX blocks are delimited by '$$' (for blocks) or '\(' and '\)'
+ * (for inline math).
+ */
 function renderMathAndMarkdown(textToCheck, $sanitize) {
-  var re = /\$\$/g;
-
-  var startMath = 0;
-  var endMath = 0;
-
-  var indexes = (function () {
-    var match;
-    var result = [];
-    while ((match = re.exec(textToCheck))) {
-      result.push(match.index);
-    }
-    return result;
-  })();
-  indexes.push(textToCheck.length);
-
-  var parts = (function () {
-    var result = [];
-
-    /* jshint -W083 */
-    for (var i = 0, index; i < indexes.length; i++) {
-      index = indexes[i];
-
-      result.push((function () {
-        if (startMath > endMath) {
-          endMath = index + 2;
-          try {
-            // \\displaystyle tells KaTeX to render the math in display style (full sized fonts).
-            return katex.renderToString($sanitize("\\displaystyle {" + textToCheck.substring(startMath, index) + "}"));
-          } catch (error) {
-            return $sanitize(textToCheck.substring(startMath, index));
-          }
-        } else {
-          startMath = index + 2;
-          return $sanitize(renderMarkdown(renderInlineMath(textToCheck.substring(endMath, index), $sanitize)));
-        }
-      })());
+  return textToCheck.split('$$').map(function (part, index) {
+    if (index % 2 === 0) {
+      // Plain markdown block
+      return $sanitize(renderMarkdown(renderInlineMath(part, $sanitize)));
+    } else {
+      // Math block
+      try {
+        // \\displaystyle tells KaTeX to render the math in display style
+        // (full sized fonts).
+        return katex.renderToString($sanitize("\\displaystyle {" + part + "}"));
+      } catch (error) {
+        return $sanitize(part);
+      }
     }
-    /* jshint +W083 */
-    return result;
-  })();
-
-  return parts.join('');
+  }).join('');
 }
 
 module.exports = renderMathAndMarkdown;