fix: support 4 backticks

Make it possible to use more than 3 backticks (or ~)
for code markers. This change also ensure it is possible
to use a codeblock with 4 markers, which itself contains
code blocks with 3 markers.

Basically, remember what the start was, and consider the
code block end only when the same marker is found.

src/commonMain/kotlin/ch/derlin/bitdowntoc/BitGenerator.kt

@@ -64,11 +64,12 @@ object BitGenerator {
 
         while (iter.hasNext()) {
             val line = iter.next()
-            when {
-                line.isCodeStart("```") -> iter.consumeCode("```")
-                line.isCodeStart("~~~") -> iter.consumeCode("~~~")
-                commenter.isAnchor(line) -> iter.remove()
-                else -> line.parseHeader(toc)?.let {
+            val codeMarker = listOf('`', '~').firstNotNullOfOrNull { line.getCodeStart(it) }
+
+            if (!codeMarker.isNullOrBlank()) iter.consumeCode(codeMarker)
+            else if (commenter.isAnchor(line)) iter.remove()
+            else {
+                line.parseHeader(toc)?.let {
                     if (params.generateAnchors) {
                         iter.set(commenter.toAnchor(it.link))
                         iter.add(line)
@@ -104,14 +105,17 @@ object BitGenerator {
         throw MissingTocEndException()
     }
 
-    private fun String.isCodeStart(codeMarker: String) =
-        codeStartTrimmer.replace(this, "").startsWith(codeMarker)
+    private fun String.getCodeStart(char: Char): String? =
+        codeStartTrimmer.replace(this, "")
+            // We expect at least 3 for code start (```), but 4 is also supported.
+            // See https://github.com/derlin/bitdowntoc/issues/65
+            .takeIf { it.startsWith("$char".repeat(3)) }
+            ?.let { trimmedLine -> trimmedLine.takeWhile { it == char } }
 
 
     private fun Iterator<String>.consumeCode(codeMarker: String) {
         while (this.hasNext() && !this.next().trim().startsWith(codeMarker));
     }
 
     private fun Iterable<String>.asText() = this.joinToString(NL)
-
 }

src/commonTest/kotlin/ch.derlin.bitdowntoc/GenerateTest.kt

@@ -137,6 +137,31 @@ class GenerateTest {
                   ```
             """.trimIndent()
         )
+
+        assertDoesNotChangeToc(
+            """
+            ````
+            # Markdown
+            - ```
+              code
+              ```
+
+            ~~~
+            code
+            ~~~
+            ````
+            """.trimIndent()
+        )
+
+        assertFailsWith<AssertionError> {
+            assertDoesNotChangeToc(
+                """
+            ````
+            code
+            ```
+            """.trimIndent()
+            )
+        }
     }
 
     @Test