generate: support HTML comment block

This is only one of many possible HTML blocks [1], but probably the most
important one.

[1] https://spec.commonmark.org/0.30/#html-blocks

src/generate/generate.nim

@@ -28,14 +28,19 @@ func alterHeaders(s: string, title: string): string =
   result.add &"## {title}"
   # Demote other headers
   var inFencedCodeBlock = false
+  var inCommentBlock = false
   while i < s.len:
     result.add s[i]
     if s[i] == '\n':
-      # When inside a fenced code block, don't alter a line that begins with '#'
-      if s.continuesWith("#", i+1) and not inFencedCodeBlock:
+      # Add a '#' to a line that begins with '#', unless inside a code or HTML block.
+      if s.continuesWith("#", i+1) and not (inFencedCodeBlock or inCommentBlock):
         result.add '#'
       elif s.continuesWith("```", i+1):
         inFencedCodeBlock = not inFencedCodeBlock
+      elif s.continuesWith("<!--", i+1):
+        inCommentBlock = true
+    elif inCommentBlock and s.continuesWith("-->", i):
+      inCommentBlock = false
     inc i
   strip result
 

tests/test_generate.nim

@@ -15,7 +15,9 @@ proc testGenerate =
 
         The quick brown fox jumps over a lazy dog.
 
-        The five boxing wizards jump quickly.
+        <!--
+        # This line is not a header
+        This line is in an HTML comment block -->
 
         ### Header 3
 
@@ -44,7 +46,9 @@ proc testGenerate =
 
         The quick brown fox jumps over a lazy dog.
 
-        The five boxing wizards jump quickly.
+        <!--
+        # This line is not a header
+        This line is in an HTML comment block -->
 
         #### Header 3