fix: guard buffer trimming against capture underflow (#432)

src/ReverseMarkdown.Test/ConverterTests.cs

@@ -848,7 +848,10 @@ private static IEnumerable<CaseData> ApplyTagFilter(IEnumerable<CaseData> cases)
         private static string LoadExpected(CaseData testCase)
         {
             if (!string.IsNullOrWhiteSpace(testCase.Expected)) {
-                return testCase.Expected;
+                // Inline expectations are authored with "\n"; the converter emits the platform
+                // line ending, so normalize before comparing (file-based expectations get this
+                // from the checkout).
+                return testCase.Expected.ReplaceLineEndings();
             }
 
             if (string.IsNullOrWhiteSpace(testCase.ExpectedFile)) {

src/ReverseMarkdown.Test/TestData/cases.json

@@ -1168,6 +1168,26 @@
       "ListBulletChar": "*"
     }
   },
+  {
+    "id": "Bug432_AdjacentEmptyBlockquotes",
+    "html": "<div>\n    <blockquote>\n        <p></p>\n    </blockquote>\n    <blockquote>\n        <p></p>\n    </blockquote>\n</div>",
+    "expected": ">\n\n>"
+  },
+  {
+    "id": "Bug432_EmptyBlockquoteBetweenParagraphs",
+    "html": "<p>before</p><blockquote><p></p></blockquote><p>after</p>",
+    "expected": "before\n\n>\n\nafter"
+  },
+  {
+    "id": "Bug432_EmptyParagraphInNestedBlockquote",
+    "html": "<blockquote><p>a</p><blockquote><p></p></blockquote></blockquote>",
+    "expected": "> a\n>\n> >"
+  },
+  {
+    "id": "Bug432_AdjacentEmptyParagraphListItems",
+    "html": "<ul><li><p></p></li><li><p></p></li></ul>",
+    "expected": "- \n\n- "
+  },
   {
     "id": "SlackFlavored_Bold",
     "html": "<b>test</b> | <strong>test</strong>",

src/ReverseMarkdown/Writers/MarkdownWriterBase.cs

@@ -539,7 +539,7 @@
             Buffer.Append("](").Append(EncodeLinkDestination(node.Url));
             if (!string.IsNullOrEmpty(node.Title))
             {
                 Buffer.Append(" \"").Append(EncodeTitle(node.Title)).Append('"');
             }
 
             Buffer.Append(')');
@@ -560,7 +560,7 @@
             Buffer.Append("![").Append(alt).Append("](").Append(EncodeLinkDestination(node.Url));
             if (!string.IsNullOrEmpty(node.Title))
             {
                 Buffer.Append(" \"").Append(EncodeTitle(node.Title)).Append('"');
             }
 
             Buffer.Append(')');
@@ -814,7 +814,10 @@
 
         private protected void TrimTrailingSpaces()
         {
-            while (Buffer.Length > 0 && (Buffer[^1] == ' ' || Buffer[^1] == '\n'))
+            // Never trim below the current capture's start: whitespace before it (such as the
+            // blank line separating sibling blocks) belongs to the enclosing render, not to
+            // this node, and removing it would leave the capture with a negative length.
+            while (Buffer.Length > _captureFloor && (Buffer[^1] == ' ' || Buffer[^1] == '\n'))
             {
                 Buffer.Length--;
             }
@@ -900,12 +903,26 @@
             return sb.ToString();
         }
 
+        // Offset in Buffer where the innermost active Capture began; text below it is off limits
+        // to trimming while that capture renders. Zero when no capture is in progress.
+        private int _captureFloor;
+
         /// <summary>Render via <paramref name="render"/> and return the produced text without
         /// leaving it in the buffer — used for post-processing (e.g. blockquote line prefixes).</summary>
         protected string Capture(System.Action render)
         {
             var start = Buffer.Length;
-            render();
+            var outerFloor = _captureFloor;
+            _captureFloor = start;
+            try
+            {
+                render();
+            }
+            finally
+            {
+                _captureFloor = outerFloor;
+            }
+
             var text = Buffer.ToString(start, Buffer.Length - start);
             Buffer.Length = start;
             return text;