rumdl_lib/rules/

md070_nested_code_fence.rs

use crate::rule::{LintError, LintResult, LintWarning, Rule, RuleCategory, Severity};

/// Rule MD070: Nested code fence collision detection
///
/// Detects when a fenced code block contains fence markers that would cause
/// premature closure. Suggests using longer fences to avoid this issue.
///
/// Only checks markdown-related blocks (empty language, "markdown", "md")
/// since other languages don't use fence syntax.
///
/// See [docs/md070.md](../../docs/md070.md) for full documentation.
#[derive(Clone, Default)]
pub struct MD070NestedCodeFence;

impl MD070NestedCodeFence {
    pub fn new() -> Self {
        Self
    }

    /// Check if the given language should be checked for nested fences.
    /// Only markdown-related blocks can have fence collisions.
    fn should_check_language(lang: &str) -> bool {
        lang.is_empty() || lang.eq_ignore_ascii_case("markdown") || lang.eq_ignore_ascii_case("md")
    }

    /// Find the maximum fence length of same-character fences in the content
    /// Returns (line_offset, fence_length) of the first collision, if any
    fn find_fence_collision(content: &str, fence_char: char, outer_fence_length: usize) -> Option<(usize, usize)> {
        for (line_idx, line) in content.lines().enumerate() {
            let trimmed = line.trim_start();

            // Check if line starts with the same fence character
            if trimmed.starts_with(fence_char) {
                let count = trimmed.chars().take_while(|&c| c == fence_char).count();

                // Collision if same char AND at least as long as outer fence
                if count >= outer_fence_length {
                    // Verify it looks like a fence line (only fence chars + optional language/whitespace)
                    let after_fence = &trimmed[count..];
                    // A fence line is: fence chars + optional language identifier + optional whitespace
                    // We detect collision if:
                    // - Line ends after fence chars (closing fence)
                    // - Line has alphanumeric after fence (opening fence with language)
                    // - Line has only whitespace after fence
                    if after_fence.is_empty()
                        || after_fence.trim().is_empty()
                        || after_fence
                            .chars()
                            .next()
                            .is_some_and(|c| c.is_alphabetic() || c == '{')
                    {
                        return Some((line_idx, count));
                    }
                }
            }
        }
        None
    }

    /// Find the maximum fence length needed to safely contain the content
    fn find_safe_fence_length(content: &str, fence_char: char) -> usize {
        let mut max_fence = 0;

        for line in content.lines() {
            let trimmed = line.trim_start();
            if trimmed.starts_with(fence_char) {
                let count = trimmed.chars().take_while(|&c| c == fence_char).count();
                if count >= 3 {
                    // Only count valid fence-like patterns
                    let after_fence = &trimmed[count..];
                    if after_fence.is_empty()
                        || after_fence.trim().is_empty()
                        || after_fence
                            .chars()
                            .next()
                            .is_some_and(|c| c.is_alphabetic() || c == '{')
                    {
                        max_fence = max_fence.max(count);
                    }
                }
            }
        }

        max_fence
    }

    /// Parse a fence marker from a line, returning (indent, fence_char, fence_length, info_string)
    fn parse_fence_line(line: &str) -> Option<(usize, char, usize, &str)> {
        let indent = line.len() - line.trim_start().len();
        // Per CommonMark, fence must have 0-3 spaces of indentation
        if indent > 3 {
            return None;
        }

        let trimmed = line.trim_start();

        if trimmed.starts_with("```") {
            let count = trimmed.chars().take_while(|&c| c == '`').count();
            if count >= 3 {
                let info = trimmed[count..].trim();
                return Some((indent, '`', count, info));
            }
        } else if trimmed.starts_with("~~~") {
            let count = trimmed.chars().take_while(|&c| c == '~').count();
            if count >= 3 {
                let info = trimmed[count..].trim();
                return Some((indent, '~', count, info));
            }
        }

        None
    }

    /// Check if a line is a valid closing fence for the given opening fence
    /// Per CommonMark, closing fences can have 0-3 spaces of indentation regardless of opening fence
    fn is_closing_fence(line: &str, fence_char: char, min_length: usize) -> bool {
        let indent = line.len() - line.trim_start().len();
        // Per CommonMark spec, closing fence can have 0-3 spaces of indentation
        if indent > 3 {
            return false;
        }

        let trimmed = line.trim_start();
        if !trimmed.starts_with(fence_char) {
            return false;
        }

        let count = trimmed.chars().take_while(|&c| c == fence_char).count();
        if count < min_length {
            return false;
        }

        // Closing fence must have only whitespace after fence chars
        trimmed[count..].trim().is_empty()
    }
}

impl Rule for MD070NestedCodeFence {
    fn name(&self) -> &'static str {
        "MD070"
    }

    fn description(&self) -> &'static str {
        "Nested code fence collision - use longer fence to avoid premature closure"
    }

    fn check(&self, ctx: &crate::lint_context::LintContext) -> LintResult {
        let content = ctx.content;
        let mut warnings = Vec::new();
        let lines: Vec<&str> = content.lines().collect();

        let mut i = 0;
        while i < lines.len() {
            // Skip lines in contexts that shouldn't be processed
            if let Some(line_info) = ctx.lines.get(i)
                && (line_info.in_front_matter || line_info.in_html_comment || line_info.in_html_block)
            {
                i += 1;
                continue;
            }

            // Skip if we're already inside a code block (check previous line).
            // This handles list-indented code blocks (4+ spaces) which our rule doesn't
            // parse directly, but the context detects correctly. If the previous line
            // is in a code block, this line is either content or a closing fence for
            // that block - not a new opening fence.
            if i > 0
                && let Some(prev_line_info) = ctx.lines.get(i - 1)
                && prev_line_info.in_code_block
            {
                i += 1;
                continue;
            }

            let line = lines[i];

            // Try to parse as opening fence
            if let Some((_indent, fence_char, fence_length, info_string)) = Self::parse_fence_line(line) {
                let block_start = i;

                // Extract the language (first word of info string)
                let language = info_string.split_whitespace().next().unwrap_or("");

                // Find the closing fence
                let mut block_end = None;
                for (j, line_j) in lines.iter().enumerate().skip(i + 1) {
                    if Self::is_closing_fence(line_j, fence_char, fence_length) {
                        block_end = Some(j);
                        break;
                    }
                }

                if let Some(end_line) = block_end {
                    // We have a complete code block from block_start to end_line
                    // Check if we should analyze this block
                    if Self::should_check_language(language) {
                        // Get the content between fences
                        let block_content: String = if block_start + 1 < end_line {
                            lines[(block_start + 1)..end_line].join("\n")
                        } else {
                            String::new()
                        };

                        // Check for fence collision
                        if let Some((collision_line_offset, _collision_length)) =
                            Self::find_fence_collision(&block_content, fence_char, fence_length)
                        {
                            let safe_length = Self::find_safe_fence_length(&block_content, fence_char) + 1;
                            let suggested_fence: String = std::iter::repeat_n(fence_char, safe_length).collect();
                            let current_fence: String = std::iter::repeat_n(fence_char, fence_length).collect();

                            let collision_line_num = block_start + 1 + collision_line_offset + 1; // 1-indexed

                            // Single warning with clear message
                            // Format matches other rules: "Problem description — solution"
                            warnings.push(LintWarning {
                                rule_name: Some(self.name().to_string()),
                                message: format!(
                                    "Nested {current_fence} at line {collision_line_num} closes block prematurely — use {suggested_fence} for outer fence"
                                ),
                                line: block_start + 1,
                                column: 1,
                                end_line: end_line + 1, // Span includes both fences
                                end_column: lines[end_line].len() + 1,
                                severity: Severity::Warning,
                                fix: None, // Fix is handled by the fix() method which updates both fences
                            });
                        }
                    }

                    // Move past this code block
                    i = end_line + 1;
                    continue;
                }
            }

            i += 1;
        }

        Ok(warnings)
    }

    fn fix(&self, ctx: &crate::lint_context::LintContext) -> Result<String, LintError> {
        let content = ctx.content;
        let mut result = String::new();
        let lines: Vec<&str> = content.lines().collect();

        let mut i = 0;
        while i < lines.len() {
            // Skip lines in contexts that shouldn't be processed
            if let Some(line_info) = ctx.lines.get(i)
                && (line_info.in_front_matter || line_info.in_html_comment || line_info.in_html_block)
            {
                result.push_str(lines[i]);
                result.push('\n');
                i += 1;
                continue;
            }

            // Skip if we're already inside a code block (check previous line)
            if i > 0
                && let Some(prev_line_info) = ctx.lines.get(i - 1)
                && prev_line_info.in_code_block
            {
                result.push_str(lines[i]);
                result.push('\n');
                i += 1;
                continue;
            }

            let line = lines[i];

            // Try to parse as opening fence
            if let Some((indent, fence_char, fence_length, info_string)) = Self::parse_fence_line(line) {
                let block_start = i;

                // Extract the language
                let language = info_string.split_whitespace().next().unwrap_or("");

                // Find the first closing fence (what CommonMark sees)
                let mut first_close = None;
                for (j, line_j) in lines.iter().enumerate().skip(i + 1) {
                    if Self::is_closing_fence(line_j, fence_char, fence_length) {
                        first_close = Some(j);
                        break;
                    }
                }

                if let Some(end_line) = first_close {
                    // Check if we should fix this block
                    if Self::should_check_language(language) {
                        // Get the content between fences
                        let block_content: String = if block_start + 1 < end_line {
                            lines[(block_start + 1)..end_line].join("\n")
                        } else {
                            String::new()
                        };

                        // Check for fence collision
                        if Self::find_fence_collision(&block_content, fence_char, fence_length).is_some() {
                            // When there's a collision, find the INTENDED closing fence
                            // This is the last matching closing fence at similar indentation
                            let mut intended_close = end_line;
                            for (j, line_j) in lines.iter().enumerate().skip(end_line + 1) {
                                if Self::is_closing_fence(line_j, fence_char, fence_length) {
                                    intended_close = j;
                                    // Don't break - we want the last one in a reasonable range
                                    // But stop if we hit another opening fence at same indent
                                } else if Self::parse_fence_line(line_j).is_some_and(|(ind, ch, _, info)| {
                                    ind <= indent && ch == fence_char && !info.is_empty()
                                }) {
                                    break; // Hit a new block, stop looking
                                }
                            }

                            // Get content between opening and intended close
                            let full_block_content: String = if block_start + 1 < intended_close {
                                lines[(block_start + 1)..intended_close].join("\n")
                            } else {
                                String::new()
                            };

                            let safe_length = Self::find_safe_fence_length(&full_block_content, fence_char) + 1;
                            let suggested_fence: String = std::iter::repeat_n(fence_char, safe_length).collect();

                            // Write fixed opening fence
                            let opening_indent = " ".repeat(indent);
                            result.push_str(&format!("{opening_indent}{suggested_fence}{info_string}\n"));

                            // Write content
                            for line_content in &lines[(block_start + 1)..intended_close] {
                                result.push_str(line_content);
                                result.push('\n');
                            }

                            // Write fixed closing fence
                            let closing_line = lines[intended_close];
                            let closing_indent = closing_line.len() - closing_line.trim_start().len();
                            let closing_indent_str = " ".repeat(closing_indent);
                            result.push_str(&format!("{closing_indent_str}{suggested_fence}\n"));

                            i = intended_close + 1;
                            continue;
                        }
                    }

                    // No collision or not a checked language - preserve as-is
                    for line_content in &lines[block_start..=end_line] {
                        result.push_str(line_content);
                        result.push('\n');
                    }
                    i = end_line + 1;
                    continue;
                }
            }

            // Not a fence line, preserve as-is
            result.push_str(line);
            result.push('\n');
            i += 1;
        }

        // Remove trailing newline if original didn't have one
        if !content.ends_with('\n') && result.ends_with('\n') {
            result.pop();
        }

        Ok(result)
    }

    fn category(&self) -> RuleCategory {
        RuleCategory::CodeBlock
    }

    fn should_skip(&self, ctx: &crate::lint_context::LintContext) -> bool {
        ctx.content.is_empty() || (!ctx.likely_has_code() && !ctx.has_char('~'))
    }

    fn as_any(&self) -> &dyn std::any::Any {
        self
    }

    fn from_config(_config: &crate::config::Config) -> Box<dyn Rule>
    where
        Self: Sized,
    {
        Box::new(MD070NestedCodeFence::new())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::lint_context::LintContext;

    fn run_check(content: &str) -> LintResult {
        let rule = MD070NestedCodeFence::new();
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        rule.check(&ctx)
    }

    fn run_fix(content: &str) -> Result<String, LintError> {
        let rule = MD070NestedCodeFence::new();
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        rule.fix(&ctx)
    }

    #[test]
    fn test_no_collision_simple() {
        let content = "```python\nprint('hello')\n```\n";
        let result = run_check(content).unwrap();
        assert!(result.is_empty(), "Simple code block should not trigger warning");
    }

    #[test]
    fn test_no_collision_non_doc_language() {
        // Python is not checked for nested fences
        let content = "```python\n```bash\necho hello\n```\n```\n";
        let result = run_check(content).unwrap();
        assert!(result.is_empty(), "Non-doc language should not be checked");
    }

    #[test]
    fn test_collision_markdown_language() {
        let content = "```markdown\n```python\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "Should emit single warning for collision");
        assert!(result[0].message.contains("Nested"));
        assert!(result[0].message.contains("closes block prematurely"));
        assert!(result[0].message.contains("use ````"));
    }

    #[test]
    fn test_collision_empty_language() {
        // Empty language (no language specified) is checked
        let content = "```\n```python\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "Empty language should be checked");
    }

    #[test]
    fn test_no_collision_longer_outer_fence() {
        let content = "````markdown\n```python\ncode()\n```\n````\n";
        let result = run_check(content).unwrap();
        assert!(result.is_empty(), "Longer outer fence should not trigger warning");
    }

    #[test]
    fn test_tilde_fence_ignores_backticks() {
        // Tildes and backticks don't conflict
        let content = "~~~markdown\n```python\ncode()\n```\n~~~\n";
        let result = run_check(content).unwrap();
        assert!(result.is_empty(), "Different fence types should not collide");
    }

    #[test]
    fn test_tilde_collision() {
        let content = "~~~markdown\n~~~python\ncode()\n~~~\n~~~\n";
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "Same fence type should collide");
        assert!(result[0].message.contains("~~~~"));
    }

    #[test]
    fn test_fix_increases_fence_length() {
        let content = "```markdown\n```python\ncode()\n```\n```\n";
        let fixed = run_fix(content).unwrap();
        assert!(fixed.starts_with("````markdown"), "Should increase to 4 backticks");
        assert!(
            fixed.contains("````\n") || fixed.ends_with("````"),
            "Closing should also be 4 backticks"
        );
    }

    #[test]
    fn test_fix_handles_longer_inner_fence() {
        // Inner fence has 5 backticks, so outer needs 6
        let content = "```markdown\n`````python\ncode()\n`````\n```\n";
        let fixed = run_fix(content).unwrap();
        assert!(fixed.starts_with("``````markdown"), "Should increase to 6 backticks");
    }

    #[test]
    fn test_backticks_in_code_not_fence() {
        // Template literals in JS shouldn't trigger
        let content = "```markdown\nconst x = `template`;\n```\n";
        let result = run_check(content).unwrap();
        assert!(result.is_empty(), "Inline backticks should not be detected as fences");
    }

    #[test]
    fn test_preserves_info_string() {
        let content = "```markdown {.highlight}\n```python\ncode()\n```\n```\n";
        let fixed = run_fix(content).unwrap();
        assert!(
            fixed.contains("````markdown {.highlight}"),
            "Should preserve info string attributes"
        );
    }

    #[test]
    fn test_md_language_alias() {
        let content = "```md\n```python\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "md should be recognized as markdown");
    }

    #[test]
    fn test_real_world_docs_case() {
        // This is the actual pattern from docs/md031.md that triggered the PR
        let content = r#"```markdown
1. First item

   ```python
   code_in_list()
   ```

1. Second item

```
"#;
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "Should emit single warning for nested fence issue");
        assert!(result[0].message.contains("line 4")); // The nested ``` is on line 4

        let fixed = run_fix(content).unwrap();
        assert!(fixed.starts_with("````markdown"), "Should fix with longer fence");
    }

    #[test]
    fn test_empty_code_block() {
        let content = "```markdown\n```\n";
        let result = run_check(content).unwrap();
        assert!(result.is_empty(), "Empty code block should not trigger");
    }

    #[test]
    fn test_multiple_code_blocks() {
        // The markdown block has a collision (inner ```python closes it prematurely).
        // The orphan closing fence (line 9) is NOT treated as a new opening fence
        // because the context correctly detects it as part of the markdown block.
        let content = r#"```python
safe code
```

```markdown
```python
collision
```
```

```javascript
also safe
```
"#;
        let result = run_check(content).unwrap();
        // Only 1 warning for the markdown block collision.
        // The orphan fence is correctly ignored (not parsed as new opening fence).
        assert_eq!(result.len(), 1, "Should emit single warning for collision");
        assert!(result[0].message.contains("line 6")); // The nested ```python is on line 6
    }

    #[test]
    fn test_single_collision_properly_closed() {
        // When the outer fence is properly longer, only the intended block triggers
        let content = r#"```python
safe code
```

````markdown
```python
collision
```
````

```javascript
also safe
```
"#;
        let result = run_check(content).unwrap();
        assert!(result.is_empty(), "Properly fenced blocks should not trigger");
    }

    #[test]
    fn test_indented_code_block_in_list() {
        let content = r#"- List item
  ```markdown
  ```python
  nested
  ```
  ```
"#;
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "Should detect collision in indented block");
        assert!(result[0].message.contains("````"));
    }

    #[test]
    fn test_no_false_positive_list_indented_block() {
        // 4-space indented code blocks in list context (GFM extension) should not
        // cause false positives. The closing fence with 3-space indent should not
        // be parsed as a new opening fence.
        let content = r#"1. List item with code:

    ```json
    {"key": "value"}
    ```

2. Another item

   ```python
   code()
   ```
"#;
        let result = run_check(content).unwrap();
        // No collision - these are separate, well-formed code blocks
        assert!(
            result.is_empty(),
            "List-indented code blocks should not trigger false positives"
        );
    }

    // ==================== Comprehensive Edge Case Tests ====================

    #[test]
    fn test_case_insensitive_language() {
        // MARKDOWN, Markdown, MD should all be checked
        for lang in ["MARKDOWN", "Markdown", "MD", "Md", "mD"] {
            let content = format!("```{lang}\n```python\ncode()\n```\n```\n");
            let result = run_check(&content).unwrap();
            assert_eq!(result.len(), 1, "{lang} should be recognized as markdown");
        }
    }

    #[test]
    fn test_unclosed_outer_fence() {
        // If outer fence is never closed, no collision can be detected
        let content = "```markdown\n```python\ncode()\n```\n";
        let result = run_check(content).unwrap();
        // The outer fence finds ```python as its closing fence (premature close)
        // Then ```\n at the end becomes orphan - but context would handle this
        assert!(result.len() <= 1, "Unclosed fence should not cause issues");
    }

    #[test]
    fn test_deeply_nested_fences() {
        // Multiple levels of nesting require progressively longer fences
        let content = r#"```markdown
````markdown
```python
code()
```
````
```
"#;
        let result = run_check(content).unwrap();
        // The outer ``` sees ```` as collision (4 >= 3)
        assert_eq!(result.len(), 1, "Deep nesting should trigger warning");
        assert!(result[0].message.contains("`````")); // Needs 5 to be safe
    }

    #[test]
    fn test_very_long_fences() {
        // 10 backtick fences should work correctly
        let content = "``````````markdown\n```python\ncode()\n```\n``````````\n";
        let result = run_check(content).unwrap();
        assert!(result.is_empty(), "Very long outer fence should not trigger warning");
    }

    #[test]
    fn test_blockquote_with_fence() {
        // Fences inside blockquotes (CommonMark allows this)
        let content = "> ```markdown\n> ```python\n> code()\n> ```\n> ```\n";
        let result = run_check(content).unwrap();
        // Blockquote prefixes are part of the line, so parsing may differ
        // This documents current behavior
        assert!(result.is_empty() || result.len() == 1);
    }

    #[test]
    fn test_fence_with_attributes() {
        // Info string with attributes like {.class #id}
        let content = "```markdown {.highlight #example}\n```python\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        assert_eq!(
            result.len(),
            1,
            "Attributes in info string should not prevent detection"
        );

        let fixed = run_fix(content).unwrap();
        assert!(
            fixed.contains("````markdown {.highlight #example}"),
            "Attributes should be preserved in fix"
        );
    }

    #[test]
    fn test_trailing_whitespace_in_info_string() {
        let content = "```markdown   \n```python\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "Trailing whitespace should not affect detection");
    }

    #[test]
    fn test_only_closing_fence_pattern() {
        // Content that has only closing fence patterns (no language)
        let content = "```markdown\nsome text\n```\nmore text\n```\n";
        let result = run_check(content).unwrap();
        // The first ``` closes, second ``` is outside
        assert!(result.is_empty(), "Properly closed block should not trigger");
    }

    #[test]
    fn test_fence_at_end_of_file_no_newline() {
        let content = "```markdown\n```python\ncode()\n```\n```";
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "Should detect collision even without trailing newline");

        let fixed = run_fix(content).unwrap();
        assert!(!fixed.ends_with('\n'), "Should preserve lack of trailing newline");
    }

    #[test]
    fn test_empty_lines_between_fences() {
        let content = "```markdown\n\n\n```python\n\ncode()\n\n```\n\n```\n";
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "Empty lines should not affect collision detection");
    }

    #[test]
    fn test_tab_indented_opening_fence() {
        // Tab at start of line - CommonMark says tab = 4 spaces for indentation.
        // A 4-space indented fence is NOT a valid fenced code block per CommonMark
        // (only 0-3 spaces allowed). However, our implementation counts characters,
        // treating tab as 1 character. This means tab-indented fences ARE parsed.
        // This is intentional: consistent with other rules in rumdl and matches
        // common editor behavior where tab = 1 indent level.
        let content = "\t```markdown\n```python\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        // With tab treated as 1 char (< 3), this IS parsed as a fence and triggers collision
        assert_eq!(result.len(), 1, "Tab-indented fence is parsed (tab = 1 char)");
    }

    #[test]
    fn test_mixed_fence_types_no_collision() {
        // Backticks outer, tildes inner - should never collide
        let content = "```markdown\n~~~python\ncode()\n~~~\n```\n";
        let result = run_check(content).unwrap();
        assert!(result.is_empty(), "Different fence chars should not collide");

        // Tildes outer, backticks inner
        let content2 = "~~~markdown\n```python\ncode()\n```\n~~~\n";
        let result2 = run_check(content2).unwrap();
        assert!(result2.is_empty(), "Different fence chars should not collide");
    }

    #[test]
    fn test_frontmatter_not_confused_with_fence() {
        // YAML frontmatter uses --- which shouldn't be confused with fences
        let content = "---\ntitle: Test\n---\n\n```markdown\n```python\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1, "Should detect collision after frontmatter");
    }

    #[test]
    fn test_html_comment_with_fence_inside() {
        // Fences inside HTML comments should be ignored
        let content = "<!-- ```markdown\n```python\ncode()\n``` -->\n\n```markdown\nreal content\n```\n";
        let result = run_check(content).unwrap();
        // The fences inside HTML comment should be skipped
        assert!(result.is_empty(), "Fences in HTML comments should be ignored");
    }

    #[test]
    fn test_consecutive_code_blocks() {
        // Multiple consecutive markdown blocks, each with collision
        let content = r#"```markdown
```python
a()
```
```

```markdown
```ruby
b()
```
```
"#;
        let result = run_check(content).unwrap();
        // Each markdown block has its own collision
        assert!(!result.is_empty(), "Should detect collision in first block");
    }

    #[test]
    fn test_numeric_info_string() {
        // Numbers after fence - some parsers treat this differently
        let content = "```123\n```456\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        // "123" is not "markdown" or "md", so should not check
        assert!(result.is_empty(), "Numeric info string is not markdown");
    }

    #[test]
    fn test_collision_at_exact_length() {
        // An empty ``` is the closing fence, not a collision.
        // For a collision, the inner fence must have content that looks like an opening fence.
        let content = "```markdown\n```python\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        assert_eq!(
            result.len(),
            1,
            "Same-length fence with language should trigger collision"
        );

        // Inner fence one shorter than outer - not a collision
        let content2 = "````markdown\n```python\ncode()\n```\n````\n";
        let result2 = run_check(content2).unwrap();
        assert!(result2.is_empty(), "Shorter inner fence should not collide");

        // Empty markdown block followed by another fence - not a collision
        let content3 = "```markdown\n```\n";
        let result3 = run_check(content3).unwrap();
        assert!(result3.is_empty(), "Empty closing fence is not a collision");
    }

    #[test]
    fn test_fix_preserves_content_exactly() {
        // Fix should not modify the content between fences
        let content = "```markdown\n```python\n  indented\n\ttabbed\nspecial: !@#$%\n```\n```\n";
        let fixed = run_fix(content).unwrap();
        assert!(fixed.contains("  indented"), "Indentation should be preserved");
        assert!(fixed.contains("\ttabbed"), "Tabs should be preserved");
        assert!(fixed.contains("special: !@#$%"), "Special chars should be preserved");
    }

    #[test]
    fn test_warning_line_numbers_accurate() {
        let content = "# Title\n\nParagraph\n\n```markdown\n```python\ncode()\n```\n```\n";
        let result = run_check(content).unwrap();
        assert_eq!(result.len(), 1);
        assert_eq!(result[0].line, 5, "Warning should be on opening fence line");
        assert!(result[0].message.contains("line 6"), "Collision line should be line 6");
    }

    #[test]
    fn test_should_skip_optimization() {
        let rule = MD070NestedCodeFence::new();

        // No code-like content
        let ctx1 = LintContext::new("Just plain text", crate::config::MarkdownFlavor::Standard, None);
        assert!(
            rule.should_skip(&ctx1),
            "Should skip content without backticks or tildes"
        );

        // Has backticks
        let ctx2 = LintContext::new("Has `code`", crate::config::MarkdownFlavor::Standard, None);
        assert!(!rule.should_skip(&ctx2), "Should not skip content with backticks");

        // Has tildes
        let ctx3 = LintContext::new("Has ~~~", crate::config::MarkdownFlavor::Standard, None);
        assert!(!rule.should_skip(&ctx3), "Should not skip content with tildes");

        // Empty
        let ctx4 = LintContext::new("", crate::config::MarkdownFlavor::Standard, None);
        assert!(rule.should_skip(&ctx4), "Should skip empty content");
    }
}