rumdl_lib/rules/md063_heading_capitalization/

mod.rs

/// Rule MD063: Heading capitalization
///
/// See [docs/md063.md](../../docs/md063.md) for full documentation, configuration, and examples.
///
/// This rule enforces consistent capitalization styles for markdown headings.
/// It supports title case, sentence case, and all caps styles.
///
/// **Note:** This rule is disabled by default. Enable it in your configuration:
/// ```toml
/// [MD063]
/// enabled = true
/// style = "title_case"
/// ```
use crate::rule::{Fix, LintError, LintResult, LintWarning, Rule, RuleCategory, Severity};
use crate::utils::range_utils::LineIndex;
use regex::Regex;
use std::collections::HashSet;
use std::ops::Range;
use std::sync::LazyLock;

mod md063_config;
pub(super) use md063_config::{HeadingCapStyle, MD063Config};

// Regex to match inline code spans (backticks)
static INLINE_CODE_REGEX: LazyLock<Regex> = LazyLock::new(|| Regex::new(r"`+[^`]+`+").unwrap());

// Regex to match markdown links [text](url) or [text][ref]
static LINK_REGEX: LazyLock<Regex> =
    LazyLock::new(|| Regex::new(r"\[([^\]]*)\]\([^)]*\)|\[([^\]]*)\]\[[^\]]*\]").unwrap());

// Regex to match inline HTML tags commonly used in headings
// Matches paired tags: <tag>content</tag>, <tag attr="val">content</tag>
// Matches self-closing: <tag/>, <tag />
// Uses explicit list of common inline tags to avoid backreference (not supported in Rust regex)
static HTML_TAG_REGEX: LazyLock<Regex> = LazyLock::new(|| {
    // Common inline HTML tags used in documentation headings
    let tags = "kbd|abbr|code|span|sub|sup|mark|cite|dfn|var|samp|small|strong|em|b|i|u|s|q|br|wbr";
    let pattern = format!(r"<({tags})(?:\s[^>]*)?>.*?</({tags})>|<({tags})(?:\s[^>]*)?\s*/?>");
    Regex::new(&pattern).unwrap()
});

// Regex to match custom header IDs {#id}
static CUSTOM_ID_REGEX: LazyLock<Regex> = LazyLock::new(|| Regex::new(r"\s*\{#[^}]+\}\s*$").unwrap());

/// Represents a segment of heading text
#[derive(Debug, Clone)]
enum HeadingSegment {
    /// Regular text that should be capitalized
    Text(String),
    /// Inline code that should be preserved as-is
    Code(String),
    /// Link with text that may be capitalized and URL that's preserved
    Link {
        full: String,
        text_start: usize,
        text_end: usize,
    },
    /// Inline HTML tag that should be preserved as-is
    Html(String),
}

/// Rule MD063: Heading capitalization
#[derive(Clone)]
pub struct MD063HeadingCapitalization {
    config: MD063Config,
    lowercase_set: HashSet<String>,
    /// Multi-word proper names from MD044 that must survive sentence-case transformation.
    /// Populated via `from_config` when both rules are active.
    proper_names: Vec<String>,
}

impl Default for MD063HeadingCapitalization {
    fn default() -> Self {
        Self::new()
    }
}

impl MD063HeadingCapitalization {
    pub fn new() -> Self {
        let config = MD063Config::default();
        let lowercase_set = config.lowercase_words.iter().cloned().collect();
        Self {
            config,
            lowercase_set,
            proper_names: Vec::new(),
        }
    }

    pub fn from_config_struct(config: MD063Config) -> Self {
        let lowercase_set = config.lowercase_words.iter().cloned().collect();
        Self {
            config,
            lowercase_set,
            proper_names: Vec::new(),
        }
    }

    /// Match `pattern_lower` at `start` in `text` using Unicode-aware lowercasing.
    /// Returns the end byte offset in `text` when the match succeeds.
    ///
    /// This avoids converting the full `text` to lowercase and then reusing those
    /// offsets on the original string, which can panic for case-fold expansions
    /// (e.g. `İ` -> `i̇`).
    fn match_case_insensitive_at(text: &str, start: usize, pattern_lower: &str) -> Option<usize> {
        if start > text.len() || !text.is_char_boundary(start) || pattern_lower.is_empty() {
            return None;
        }

        let mut matched_bytes = 0;

        for (offset, ch) in text[start..].char_indices() {
            if matched_bytes >= pattern_lower.len() {
                break;
            }

            let lowered: String = ch.to_lowercase().collect();
            if !pattern_lower[matched_bytes..].starts_with(&lowered) {
                return None;
            }

            matched_bytes += lowered.len();

            if matched_bytes == pattern_lower.len() {
                return Some(start + offset + ch.len_utf8());
            }
        }

        None
    }

    /// Find the next case-insensitive match of `pattern_lower` in `text`,
    /// returning byte offsets in the ORIGINAL string.
    fn find_case_insensitive_match(text: &str, pattern_lower: &str, search_start: usize) -> Option<(usize, usize)> {
        if pattern_lower.is_empty() || search_start >= text.len() || !text.is_char_boundary(search_start) {
            return None;
        }

        for (offset, _) in text[search_start..].char_indices() {
            let start = search_start + offset;
            if let Some(end) = Self::match_case_insensitive_at(text, start, pattern_lower) {
                return Some((start, end));
            }
        }

        None
    }

    /// Build a map from word byte-position → canonical form for all proper names
    /// that appear in the heading text (case-insensitive phrase match).
    ///
    /// This is used in `apply_sentence_case` so that words belonging to a proper
    /// name phrase are never lowercased to begin with.
    fn proper_name_canonical_forms(&self, text: &str) -> std::collections::HashMap<usize, &str> {
        let mut map = std::collections::HashMap::new();

        for name in &self.proper_names {
            if name.is_empty() {
                continue;
            }
            let name_lower = name.to_lowercase();
            let canonical_words: Vec<&str> = name.split_whitespace().collect();
            if canonical_words.is_empty() {
                continue;
            }
            let mut search_start = 0;

            while search_start < text.len() {
                let Some((abs_pos, end_pos)) = Self::find_case_insensitive_match(text, &name_lower, search_start)
                else {
                    break;
                };

                // Require word boundaries
                let before_ok = abs_pos == 0 || !text[..abs_pos].chars().last().is_some_and(char::is_alphanumeric);
                let after_ok =
                    end_pos >= text.len() || !text[end_pos..].chars().next().is_some_and(char::is_alphanumeric);

                if before_ok && after_ok {
                    // Map each word in the matched region to its canonical form.
                    // We zip the words found in the text slice with the words of the
                    // canonical name so that every word gets the right casing.
                    let text_slice = &text[abs_pos..end_pos];
                    let mut word_idx = 0;
                    let mut slice_offset = 0;

                    for text_word in text_slice.split_whitespace() {
                        if let Some(w_rel) = text_slice[slice_offset..].find(text_word) {
                            let word_abs = abs_pos + slice_offset + w_rel;
                            if let Some(&canonical_word) = canonical_words.get(word_idx) {
                                map.insert(word_abs, canonical_word);
                            }
                            slice_offset += w_rel + text_word.len();
                            word_idx += 1;
                        }
                    }
                }

                // Advance by one Unicode scalar value to allow overlapping matches
                // while staying on a UTF-8 char boundary.
                search_start = abs_pos + text[abs_pos..].chars().next().map_or(1, char::len_utf8);
            }
        }

        map
    }

    /// Check if a word has internal capitals (like "iPhone", "macOS", "GitHub", "iOS")
    fn has_internal_capitals(&self, word: &str) -> bool {
        let chars: Vec<char> = word.chars().collect();
        if chars.len() < 2 {
            return false;
        }

        let first = chars[0];
        let rest = &chars[1..];
        let has_upper_in_rest = rest.iter().any(|c| c.is_uppercase());
        let has_lower_in_rest = rest.iter().any(|c| c.is_lowercase());

        // Case 1: Mixed case after first character (like "iPhone", "macOS", "GitHub", "JavaScript")
        if has_upper_in_rest && has_lower_in_rest {
            return true;
        }

        // Case 2: Lowercase first + uppercase in rest (like "iOS", "eBay")
        if first.is_lowercase() && has_upper_in_rest {
            return true;
        }

        false
    }

    /// Check if a word is an all-caps acronym (2+ consecutive uppercase letters)
    /// Examples: "API", "GPU", "HTTP2", "IO" return true
    /// Examples: "A", "iPhone", "npm" return false
    fn is_all_caps_acronym(&self, word: &str) -> bool {
        // Skip single-letter words (handled by title case rules)
        if word.len() < 2 {
            return false;
        }

        let mut consecutive_upper = 0;
        let mut max_consecutive = 0;

        for c in word.chars() {
            if c.is_uppercase() {
                consecutive_upper += 1;
                max_consecutive = max_consecutive.max(consecutive_upper);
            } else if c.is_lowercase() {
                // Any lowercase letter means not all-caps
                return false;
            } else {
                // Non-letter (number, punctuation) - reset counter but don't fail
                consecutive_upper = 0;
            }
        }

        // Must have at least 2 consecutive uppercase letters
        max_consecutive >= 2
    }

    /// Check if a word should be preserved as-is
    fn should_preserve_word(&self, word: &str) -> bool {
        // Check ignore_words list (case-sensitive exact match)
        if self.config.ignore_words.iter().any(|w| w == word) {
            return true;
        }

        // Numeric ordinals ("1st", "5th", "21st", ...) must always flow
        // through the normal title-case path so mis-cased forms like
        // "5Th" get normalised back to "5th". Skip the preserve_cased_words
        // heuristics, which would otherwise treat "5Th" as intentionally
        // mixed-case and leave it untouched.
        let is_ordinal = Self::is_numeric_ordinal(word);

        if !is_ordinal {
            // Check if word has internal capitals and preserve_cased_words is enabled
            if self.config.preserve_cased_words && self.has_internal_capitals(word) {
                return true;
            }

            // Check if word is an all-caps acronym (2+ consecutive uppercase)
            if self.config.preserve_cased_words && self.is_all_caps_acronym(word) {
                return true;
            }
        }

        // Preserve caret notation for control characters (^A, ^Z, ^@, etc.)
        if self.is_caret_notation(word) {
            return true;
        }

        false
    }

    /// Detect numeric ordinals like `1st`, `2nd`, `3rd`, `4th`, `21st`,
    /// `100th`, ignoring the case of the suffix and any trailing
    /// punctuation (e.g. `5th.`, `1st,`, `3rd!`).
    ///
    /// Such tokens have a fixed lower-case alphabetic suffix in title case
    /// — `21st Century`, never `21St Century` — and must be detected
    /// before applying the generic "capitalise first letter" rule.
    fn is_numeric_ordinal(word: &str) -> bool {
        let bytes = word.as_bytes();

        // Require at least one leading ASCII digit followed by a letter.
        let alpha_start = match bytes.iter().position(|&b| !b.is_ascii_digit()) {
            Some(pos) if pos > 0 => pos,
            _ => return false,
        };

        // Find where the alphabetic suffix ends (trailing punctuation, etc.).
        let alpha_end = bytes[alpha_start..]
            .iter()
            .position(|b| !b.is_ascii_alphabetic())
            .map_or(bytes.len(), |p| alpha_start + p);

        let suffix = &word[alpha_start..alpha_end];
        matches!(suffix.to_ascii_lowercase().as_str(), "st" | "nd" | "rd" | "th")
    }

    /// Check if a word is caret notation for control characters (e.g., ^A, ^C, ^Z)
    fn is_caret_notation(&self, word: &str) -> bool {
        let chars: Vec<char> = word.chars().collect();
        // Pattern: ^ followed by uppercase letter or @[\]^_
        if chars.len() >= 2 && chars[0] == '^' {
            let second = chars[1];
            // Control characters: ^@ (NUL) through ^_ (US), which includes ^A-^Z
            if second.is_ascii_uppercase() || "@[\\]^_".contains(second) {
                return true;
            }
        }
        false
    }

    /// Check if a word is a "lowercase word" (articles, prepositions, etc.)
    fn is_lowercase_word(&self, word: &str) -> bool {
        self.lowercase_set.contains(&word.to_lowercase())
    }

    /// Apply title case to a single word
    fn title_case_word(&self, word: &str, is_first: bool, is_last: bool) -> String {
        if word.is_empty() {
            return word.to_string();
        }

        // Preserve words in ignore list or with internal capitals
        if self.should_preserve_word(word) {
            return word.to_string();
        }

        // First and last words are always capitalized
        if is_first || is_last {
            return self.capitalize_first(word);
        }

        // Check if it's a lowercase word (articles, prepositions, etc.)
        if self.is_lowercase_word(word) {
            return Self::lowercase_preserving_composition(word);
        }

        // Regular word - capitalize first letter
        self.capitalize_first(word)
    }

    /// Apply canonical proper-name casing while preserving any trailing punctuation
    /// attached to the original whitespace token (e.g. `javascript,` -> `JavaScript,`).
    fn apply_canonical_form_to_word(word: &str, canonical: &str) -> String {
        let canonical_lower = canonical.to_lowercase();
        if canonical_lower.is_empty() {
            return canonical.to_string();
        }

        if let Some(end_pos) = Self::match_case_insensitive_at(word, 0, &canonical_lower) {
            let mut out = String::with_capacity(canonical.len() + word.len().saturating_sub(end_pos));
            out.push_str(canonical);
            out.push_str(&word[end_pos..]);
            out
        } else {
            canonical.to_string()
        }
    }

    /// Capitalize the first letter of a word, handling Unicode properly
    fn capitalize_first(&self, word: &str) -> String {
        if word.is_empty() {
            return String::new();
        }

        // Find the first alphabetic character to capitalize
        let first_alpha_pos = word.find(|c: char| c.is_alphabetic());
        let Some(pos) = first_alpha_pos else {
            return word.to_string();
        };

        let prefix = &word[..pos];
        let suffix = &word[pos..];

        // Numeric ordinals ("1st", "21st", "5th", ...) keep their
        // alphabetic suffix lower-cased even at title-case positions.
        if Self::is_numeric_ordinal(word) {
            let suffix_lower = Self::lowercase_preserving_composition(suffix);
            return format!("{prefix}{suffix_lower}");
        }

        let mut chars = suffix.chars();
        let first = chars.next().unwrap();
        // Use composition-preserving uppercase to avoid decomposing
        // precomposed characters (e.g., ῷ → Ω + combining marks + Ι)
        let first_upper = Self::uppercase_preserving_composition(&first.to_string());
        let rest: String = chars.collect();
        let rest_lower = Self::lowercase_preserving_composition(&rest);
        format!("{prefix}{first_upper}{rest_lower}")
    }

    /// Lowercase a string character-by-character, preserving precomposed
    /// characters that would decompose during case conversion.
    fn lowercase_preserving_composition(s: &str) -> String {
        let mut result = String::with_capacity(s.len());
        for c in s.chars() {
            let lower: String = c.to_lowercase().collect();
            if lower.chars().count() == 1 {
                result.push_str(&lower);
            } else {
                // Lowercasing would decompose this character; keep original
                result.push(c);
            }
        }
        result
    }

    /// Uppercase a string character-by-character, preserving precomposed
    /// characters that would decompose during case conversion.
    /// For example, ῷ (U+1FF7) would decompose into Ω + combining marks + Ι
    /// via to_uppercase(); this function keeps ῷ unchanged instead.
    fn uppercase_preserving_composition(s: &str) -> String {
        let mut result = String::with_capacity(s.len());
        for c in s.chars() {
            let upper: String = c.to_uppercase().collect();
            if upper.chars().count() == 1 {
                result.push_str(&upper);
            } else {
                // Uppercasing would decompose this character; keep original
                result.push(c);
            }
        }
        result
    }

    /// Apply title case to text, using our own title-case logic.
    /// We avoid the external titlecase crate because it decomposes
    /// precomposed Unicode characters during case conversion.
    fn apply_title_case(&self, text: &str) -> String {
        let canonical_forms = self.proper_name_canonical_forms(text);

        let original_words: Vec<&str> = text.split_whitespace().collect();
        let total_words = original_words.len();

        // Pre-compute byte position of each word for canonical form lookup.
        // Use usize::MAX as sentinel for unfound words so canonical_forms.get() returns None.
        let mut word_positions: Vec<usize> = Vec::with_capacity(original_words.len());
        let mut pos = 0;
        for word in &original_words {
            if let Some(rel) = text[pos..].find(word) {
                word_positions.push(pos + rel);
                pos = pos + rel + word.len();
            } else {
                word_positions.push(usize::MAX);
            }
        }

        let result_words: Vec<String> = original_words
            .iter()
            .enumerate()
            .map(|(i, word)| {
                let after_period = i > 0 && original_words[i - 1].ends_with('.');
                let is_first = i == 0 || after_period;
                let is_last = i == total_words - 1;

                // Words that are part of an MD044 proper name use the canonical form directly.
                if let Some(&canonical) = word_positions.get(i).and_then(|&p| canonical_forms.get(&p)) {
                    return Self::apply_canonical_form_to_word(word, canonical);
                }

                // Preserve words in ignore list or with internal capitals
                if self.should_preserve_word(word) {
                    return (*word).to_string();
                }

                // Handle hyphenated words
                if word.contains('-') {
                    return self.handle_hyphenated_word(word, is_first, is_last);
                }

                self.title_case_word(word, is_first, is_last)
            })
            .collect();

        result_words.join(" ")
    }

    /// Handle hyphenated words like "self-documenting"
    fn handle_hyphenated_word(&self, word: &str, is_first: bool, is_last: bool) -> String {
        let parts: Vec<&str> = word.split('-').collect();
        let total_parts = parts.len();

        let result_parts: Vec<String> = parts
            .iter()
            .enumerate()
            .map(|(i, part)| {
                // First part of first word and last part of last word get special treatment
                let part_is_first = is_first && i == 0;
                let part_is_last = is_last && i == total_parts - 1;
                self.title_case_word(part, part_is_first, part_is_last)
            })
            .collect();

        result_parts.join("-")
    }

    /// Apply sentence case to text
    fn apply_sentence_case(&self, text: &str) -> String {
        if text.is_empty() {
            return text.to_string();
        }

        let canonical_forms = self.proper_name_canonical_forms(text);
        let mut result = String::new();
        let mut current_pos = 0;
        let mut is_first_word = true;

        // Use original text positions to preserve whitespace correctly
        for word in text.split_whitespace() {
            if let Some(pos) = text[current_pos..].find(word) {
                let abs_pos = current_pos + pos;

                // Preserve whitespace before this word
                result.push_str(&text[current_pos..abs_pos]);

                // Words that are part of an MD044 proper name use the canonical form
                // directly, bypassing sentence-case lowercasing entirely.
                if let Some(&canonical) = canonical_forms.get(&abs_pos) {
                    result.push_str(&Self::apply_canonical_form_to_word(word, canonical));
                    is_first_word = false;
                } else if is_first_word {
                    // Check if word should be preserved BEFORE any capitalization
                    if self.should_preserve_word(word) {
                        // Preserve ignore-words exactly as-is, even at start
                        result.push_str(word);
                    } else {
                        // First word: capitalize first letter, lowercase rest
                        let mut chars = word.chars();
                        if let Some(first) = chars.next() {
                            result.push_str(&Self::uppercase_preserving_composition(&first.to_string()));
                            let rest: String = chars.collect();
                            result.push_str(&Self::lowercase_preserving_composition(&rest));
                        }
                    }
                    is_first_word = false;
                } else {
                    // Non-first words: preserve if needed, otherwise lowercase
                    if self.should_preserve_word(word) {
                        result.push_str(word);
                    } else {
                        result.push_str(&Self::lowercase_preserving_composition(word));
                    }
                }

                current_pos = abs_pos + word.len();
            }
        }

        // Preserve any trailing whitespace
        if current_pos < text.len() {
            result.push_str(&text[current_pos..]);
        }

        result
    }

    /// Apply all caps to text (preserve whitespace)
    fn apply_all_caps(&self, text: &str) -> String {
        if text.is_empty() {
            return text.to_string();
        }

        let canonical_forms = self.proper_name_canonical_forms(text);
        let mut result = String::new();
        let mut current_pos = 0;

        // Use original text positions to preserve whitespace correctly
        for word in text.split_whitespace() {
            if let Some(pos) = text[current_pos..].find(word) {
                let abs_pos = current_pos + pos;

                // Preserve whitespace before this word
                result.push_str(&text[current_pos..abs_pos]);

                // Words that are part of an MD044 proper name use the canonical form directly.
                // This prevents oscillation with MD044 when all-caps style is active.
                if let Some(&canonical) = canonical_forms.get(&abs_pos) {
                    result.push_str(&Self::apply_canonical_form_to_word(word, canonical));
                } else if self.should_preserve_word(word) {
                    result.push_str(word);
                } else {
                    result.push_str(&Self::uppercase_preserving_composition(word));
                }

                current_pos = abs_pos + word.len();
            }
        }

        // Preserve any trailing whitespace
        if current_pos < text.len() {
            result.push_str(&text[current_pos..]);
        }

        result
    }

    /// Parse heading text into segments
    fn parse_segments(&self, text: &str) -> Vec<HeadingSegment> {
        let mut segments = Vec::new();
        let mut last_end = 0;

        // Collect all special regions (code and links)
        let mut special_regions: Vec<(usize, usize, HeadingSegment)> = Vec::new();

        // Find inline code spans
        for mat in INLINE_CODE_REGEX.find_iter(text) {
            special_regions.push((mat.start(), mat.end(), HeadingSegment::Code(mat.as_str().to_string())));
        }

        // Find links
        for caps in LINK_REGEX.captures_iter(text) {
            let full_match = caps.get(0).unwrap();
            let text_match = caps.get(1).or_else(|| caps.get(2));

            if let Some(text_m) = text_match {
                special_regions.push((
                    full_match.start(),
                    full_match.end(),
                    HeadingSegment::Link {
                        full: full_match.as_str().to_string(),
                        text_start: text_m.start() - full_match.start(),
                        text_end: text_m.end() - full_match.start(),
                    },
                ));
            }
        }

        // Find inline HTML tags
        for mat in HTML_TAG_REGEX.find_iter(text) {
            special_regions.push((mat.start(), mat.end(), HeadingSegment::Html(mat.as_str().to_string())));
        }

        // Sort by start position
        special_regions.sort_by_key(|(start, _, _)| *start);

        // Remove overlapping regions (code takes precedence)
        let mut filtered_regions: Vec<(usize, usize, HeadingSegment)> = Vec::new();
        for region in special_regions {
            let overlaps = filtered_regions.iter().any(|(s, e, _)| region.0 < *e && region.1 > *s);
            if !overlaps {
                filtered_regions.push(region);
            }
        }

        // Build segments
        for (start, end, segment) in filtered_regions {
            // Add text before this special region
            if start > last_end {
                let text_segment = &text[last_end..start];
                if !text_segment.is_empty() {
                    segments.push(HeadingSegment::Text(text_segment.to_string()));
                }
            }
            segments.push(segment);
            last_end = end;
        }

        // Add remaining text
        if last_end < text.len() {
            let remaining = &text[last_end..];
            if !remaining.is_empty() {
                segments.push(HeadingSegment::Text(remaining.to_string()));
            }
        }

        // If no segments were found, treat the whole thing as text
        if segments.is_empty() && !text.is_empty() {
            segments.push(HeadingSegment::Text(text.to_string()));
        }

        segments
    }

    /// Apply capitalization to heading text
    fn apply_capitalization(&self, text: &str) -> String {
        // Strip custom ID if present and re-add later
        let (main_text, custom_id) = if let Some(mat) = CUSTOM_ID_REGEX.find(text) {
            (&text[..mat.start()], Some(mat.as_str()))
        } else {
            (text, None)
        };

        // Parse into segments
        let segments = self.parse_segments(main_text);

        // Count text segments to determine first/last word context
        let text_segments: Vec<usize> = segments
            .iter()
            .enumerate()
            .filter_map(|(i, s)| matches!(s, HeadingSegment::Text(_)).then_some(i))
            .collect();

        // Determine if the first segment overall is a text segment
        // For sentence case: if heading starts with code/link, the first text segment
        // should NOT capitalize its first word (the heading already has a "first element")
        let first_segment_is_text = segments.first().is_some_and(|s| matches!(s, HeadingSegment::Text(_)));

        // Determine if the last segment overall is a text segment
        // If the last segment is Code or Link, then the last text segment should NOT
        // treat its last word as the heading's last word (for lowercase-words respect)
        let last_segment_is_text = segments.last().is_some_and(|s| matches!(s, HeadingSegment::Text(_)));

        // Apply capitalization to each segment
        let mut result_parts: Vec<String> = Vec::new();

        for (i, segment) in segments.iter().enumerate() {
            match segment {
                HeadingSegment::Text(t) => {
                    let is_first_text = text_segments.first() == Some(&i);
                    // A text segment is "last" only if it's the last text segment AND
                    // the last segment overall is also text. If there's Code/Link after,
                    // the last word should respect lowercase-words.
                    let is_last_text = text_segments.last() == Some(&i) && last_segment_is_text;

                    let capitalized = match self.config.style {
                        HeadingCapStyle::TitleCase => self.apply_title_case_segment(t, is_first_text, is_last_text),
                        HeadingCapStyle::SentenceCase => {
                            // For sentence case, only capitalize first word if:
                            // 1. This is the first text segment, AND
                            // 2. The heading actually starts with text (not code/link)
                            if is_first_text && first_segment_is_text {
                                self.apply_sentence_case(t)
                            } else {
                                // Non-first segments OR heading starts with code/link
                                self.apply_sentence_case_non_first(t)
                            }
                        }
                        HeadingCapStyle::AllCaps => self.apply_all_caps(t),
                    };
                    result_parts.push(capitalized);
                }
                HeadingSegment::Code(c) => {
                    result_parts.push(c.clone());
                }
                HeadingSegment::Link {
                    full,
                    text_start,
                    text_end,
                } => {
                    // Apply capitalization to link text only
                    let link_text = &full[*text_start..*text_end];
                    let capitalized_text = match self.config.style {
                        HeadingCapStyle::TitleCase => self.apply_title_case(link_text),
                        // For sentence case, apply same preservation logic as non-first text
                        // This preserves acronyms (API), brand names (iPhone), etc.
                        HeadingCapStyle::SentenceCase => self.apply_sentence_case_non_first(link_text),
                        HeadingCapStyle::AllCaps => self.apply_all_caps(link_text),
                    };

                    let mut new_link = String::new();
                    new_link.push_str(&full[..*text_start]);
                    new_link.push_str(&capitalized_text);
                    new_link.push_str(&full[*text_end..]);
                    result_parts.push(new_link);
                }
                HeadingSegment::Html(h) => {
                    // Preserve HTML tags as-is (like code)
                    result_parts.push(h.clone());
                }
            }
        }

        let mut result = result_parts.join("");

        // Re-add custom ID if present
        if let Some(id) = custom_id {
            result.push_str(id);
        }

        result
    }

    /// Apply title case to a text segment with first/last awareness
    fn apply_title_case_segment(&self, text: &str, is_first_segment: bool, is_last_segment: bool) -> String {
        let canonical_forms = self.proper_name_canonical_forms(text);
        let words: Vec<&str> = text.split_whitespace().collect();
        let total_words = words.len();

        if total_words == 0 {
            return text.to_string();
        }

        // Pre-compute byte position of each word so we can look up canonical forms.
        // Use usize::MAX as sentinel for unfound words so canonical_forms.get() returns None.
        let mut word_positions: Vec<usize> = Vec::with_capacity(words.len());
        let mut pos = 0;
        for word in &words {
            if let Some(rel) = text[pos..].find(word) {
                word_positions.push(pos + rel);
                pos = pos + rel + word.len();
            } else {
                word_positions.push(usize::MAX);
            }
        }

        let result_words: Vec<String> = words
            .iter()
            .enumerate()
            .map(|(i, word)| {
                let after_period = i > 0 && words[i - 1].ends_with('.');
                let is_first = (is_first_segment && i == 0) || after_period;
                let is_last = is_last_segment && i == total_words - 1;

                // Words that are part of an MD044 proper name use the canonical form directly.
                if let Some(&canonical) = word_positions.get(i).and_then(|&p| canonical_forms.get(&p)) {
                    return Self::apply_canonical_form_to_word(word, canonical);
                }

                // Handle hyphenated words
                if word.contains('-') {
                    return self.handle_hyphenated_word(word, is_first, is_last);
                }

                self.title_case_word(word, is_first, is_last)
            })
            .collect();

        // Preserve original spacing
        let mut result = String::new();
        let mut word_iter = result_words.iter();
        let mut in_word = false;

        for c in text.chars() {
            if c.is_whitespace() {
                if in_word {
                    in_word = false;
                }
                result.push(c);
            } else if !in_word {
                if let Some(word) = word_iter.next() {
                    result.push_str(word);
                }
                in_word = true;
            }
        }

        result
    }

    /// Apply sentence case to non-first segments (just lowercase, preserve whitespace)
    fn apply_sentence_case_non_first(&self, text: &str) -> String {
        if text.is_empty() {
            return text.to_string();
        }

        let canonical_forms = self.proper_name_canonical_forms(text);
        let mut result = String::new();
        let mut current_pos = 0;

        // Iterate over words in the original text so byte positions are consistent
        // with the positions in canonical_forms (built from the same text).
        for word in text.split_whitespace() {
            if let Some(pos) = text[current_pos..].find(word) {
                let abs_pos = current_pos + pos;

                // Preserve whitespace before this word
                result.push_str(&text[current_pos..abs_pos]);

                // Words that are part of an MD044 proper name use the canonical form directly.
                if let Some(&canonical) = canonical_forms.get(&abs_pos) {
                    result.push_str(&Self::apply_canonical_form_to_word(word, canonical));
                } else if self.should_preserve_word(word) {
                    result.push_str(word);
                } else {
                    result.push_str(&Self::lowercase_preserving_composition(word));
                }

                current_pos = abs_pos + word.len();
            }
        }

        // Preserve any trailing whitespace
        if current_pos < text.len() {
            result.push_str(&text[current_pos..]);
        }

        result
    }

    /// Get byte range for a line
    fn get_line_byte_range(&self, content: &str, line_num: usize, line_index: &LineIndex) -> Range<usize> {
        let start_pos = line_index.get_line_start_byte(line_num).unwrap_or(content.len());
        let line = content.lines().nth(line_num - 1).unwrap_or("");
        Range {
            start: start_pos,
            end: start_pos + line.len(),
        }
    }

    /// Fix an ATX heading line
    fn fix_atx_heading(&self, _line: &str, heading: &crate::lint_context::HeadingInfo) -> String {
        // Parse the line to preserve structure
        let indent = " ".repeat(heading.marker_column);
        let hashes = "#".repeat(heading.level as usize);

        // Apply capitalization to the text
        let fixed_text = self.apply_capitalization(&heading.raw_text);

        // Reconstruct with closing sequence if present
        let closing = &heading.closing_sequence;
        if heading.has_closing_sequence {
            format!("{indent}{hashes} {fixed_text} {closing}")
        } else {
            format!("{indent}{hashes} {fixed_text}")
        }
    }

    /// Fix a Setext heading line
    fn fix_setext_heading(&self, line: &str, heading: &crate::lint_context::HeadingInfo) -> String {
        // Apply capitalization to the text
        let fixed_text = self.apply_capitalization(&heading.raw_text);

        // Preserve leading whitespace from original line
        let leading_ws: String = line.chars().take_while(|c| c.is_whitespace()).collect();

        format!("{leading_ws}{fixed_text}")
    }
}

impl Rule for MD063HeadingCapitalization {
    fn name(&self) -> &'static str {
        "MD063"
    }

    fn description(&self) -> &'static str {
        "Heading capitalization"
    }

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

    fn should_skip(&self, ctx: &crate::lint_context::LintContext) -> bool {
        !ctx.likely_has_headings() || !ctx.lines.iter().any(|line| line.heading.is_some())
    }

    fn check(&self, ctx: &crate::lint_context::LintContext) -> LintResult {
        let content = ctx.content;

        if content.is_empty() {
            return Ok(Vec::new());
        }

        let mut warnings = Vec::new();
        let line_index = &ctx.line_index;

        for (line_num, line_info) in ctx.lines.iter().enumerate() {
            if let Some(heading) = &line_info.heading {
                // Check level filter
                if heading.level < self.config.min_level || heading.level > self.config.max_level {
                    continue;
                }

                // Skip headings in code blocks (indented headings)
                if line_info.visual_indent >= 4 && matches!(heading.style, crate::lint_context::HeadingStyle::ATX) {
                    continue;
                }

                // Skip invalid headings (e.g., `#tag` which lacks required space after #)
                if !heading.is_valid {
                    continue;
                }

                // Apply capitalization and compare
                let original_text = &heading.raw_text;
                let fixed_text = self.apply_capitalization(original_text);

                if original_text != &fixed_text {
                    let line = line_info.content(ctx.content);
                    let style_name = match self.config.style {
                        HeadingCapStyle::TitleCase => "title case",
                        HeadingCapStyle::SentenceCase => "sentence case",
                        HeadingCapStyle::AllCaps => "ALL CAPS",
                    };

                    warnings.push(LintWarning {
                        rule_name: Some(self.name().to_string()),
                        line: line_num + 1,
                        column: heading.content_column + 1,
                        end_line: line_num + 1,
                        end_column: heading.content_column + 1 + original_text.len(),
                        message: format!("Heading should use {style_name}: '{original_text}' -> '{fixed_text}'"),
                        severity: Severity::Warning,
                        fix: Some(Fix::new(
                            self.get_line_byte_range(content, line_num + 1, line_index),
                            match heading.style {
                                crate::lint_context::HeadingStyle::ATX => self.fix_atx_heading(line, heading),
                                _ => self.fix_setext_heading(line, heading),
                            },
                        )),
                    });
                }
            }
        }

        Ok(warnings)
    }

    fn fix(&self, ctx: &crate::lint_context::LintContext) -> Result<String, LintError> {
        let content = ctx.content;

        if content.is_empty() {
            return Ok(content.to_string());
        }

        let lines = ctx.raw_lines();
        let mut fixed_lines: Vec<String> = lines.iter().map(|&s| s.to_string()).collect();

        for (line_num, line_info) in ctx.lines.iter().enumerate() {
            // Skip lines where the rule is disabled via inline config
            if ctx.is_rule_disabled(self.name(), line_num + 1) {
                continue;
            }

            if let Some(heading) = &line_info.heading {
                // Check level filter
                if heading.level < self.config.min_level || heading.level > self.config.max_level {
                    continue;
                }

                // Skip headings in code blocks
                if line_info.visual_indent >= 4 && matches!(heading.style, crate::lint_context::HeadingStyle::ATX) {
                    continue;
                }

                // Skip invalid headings (e.g., `#tag` which lacks required space after #)
                if !heading.is_valid {
                    continue;
                }

                let original_text = &heading.raw_text;
                let fixed_text = self.apply_capitalization(original_text);

                if original_text != &fixed_text {
                    let line = line_info.content(ctx.content);
                    fixed_lines[line_num] = match heading.style {
                        crate::lint_context::HeadingStyle::ATX => self.fix_atx_heading(line, heading),
                        _ => self.fix_setext_heading(line, heading),
                    };
                }
            }
        }

        // Reconstruct content preserving line endings
        let mut result = String::with_capacity(content.len());
        for (i, line) in fixed_lines.iter().enumerate() {
            result.push_str(line);
            if i < fixed_lines.len() - 1 || content.ends_with('\n') {
                result.push('\n');
            }
        }

        Ok(result)
    }

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

    fn default_config_section(&self) -> Option<(String, toml::Value)> {
        let json_value = serde_json::to_value(&self.config).ok()?;
        Some((
            self.name().to_string(),
            crate::rule_config_serde::json_to_toml_value(&json_value)?,
        ))
    }

    fn from_config(config: &crate::config::Config) -> Box<dyn Rule>
    where
        Self: Sized,
    {
        let rule_config = crate::rule_config_serde::load_rule_config::<MD063Config>(config);
        let md044_config =
            crate::rule_config_serde::load_rule_config::<crate::rules::md044_proper_names::MD044Config>(config);
        let mut rule = Self::from_config_struct(rule_config);
        rule.proper_names = md044_config.names;
        Box::new(rule)
    }
}

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

    fn create_rule() -> MD063HeadingCapitalization {
        let config = MD063Config {
            enabled: true,
            ..Default::default()
        };
        MD063HeadingCapitalization::from_config_struct(config)
    }

    fn create_rule_with_style(style: HeadingCapStyle) -> MD063HeadingCapitalization {
        let config = MD063Config {
            enabled: true,
            style,
            ..Default::default()
        };
        MD063HeadingCapitalization::from_config_struct(config)
    }

    // Title case tests
    #[test]
    fn test_title_case_basic() {
        let rule = create_rule();
        let content = "# hello world\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        assert!(result[0].message.contains("Hello World"));
    }

    #[test]
    fn test_title_case_lowercase_words() {
        let rule = create_rule();
        let content = "# the quick brown fox\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        // "The" should be capitalized (first word), "quick", "brown", "fox" should be capitalized
        assert!(result[0].message.contains("The Quick Brown Fox"));
    }

    #[test]
    fn test_title_case_already_correct() {
        let rule = create_rule();
        let content = "# The Quick Brown Fox\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(result.is_empty(), "Already correct heading should not be flagged");
    }

    #[test]
    fn test_title_case_hyphenated() {
        let rule = create_rule();
        let content = "# self-documenting code\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        assert!(result[0].message.contains("Self-Documenting Code"));
    }

    // Sentence case tests
    #[test]
    fn test_sentence_case_basic() {
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# The Quick Brown Fox\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        assert!(result[0].message.contains("The quick brown fox"));
    }

    #[test]
    fn test_sentence_case_already_correct() {
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# The quick brown fox\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(result.is_empty());
    }

    // All caps tests
    #[test]
    fn test_all_caps_basic() {
        let rule = create_rule_with_style(HeadingCapStyle::AllCaps);
        let content = "# hello world\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        assert!(result[0].message.contains("HELLO WORLD"));
    }

    // Preserve tests
    #[test]
    fn test_preserve_ignore_words() {
        let config = MD063Config {
            enabled: true,
            ignore_words: vec!["iPhone".to_string(), "macOS".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        let content = "# using iPhone on macOS\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        // iPhone and macOS should be preserved
        assert!(result[0].message.contains("iPhone"));
        assert!(result[0].message.contains("macOS"));
    }

    #[test]
    fn test_preserve_cased_words() {
        let rule = create_rule();
        let content = "# using GitHub actions\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        // GitHub should be preserved (has internal capital)
        assert!(result[0].message.contains("GitHub"));
    }

    // Inline code tests
    #[test]
    fn test_inline_code_preserved() {
        let rule = create_rule();
        let content = "# using `const` in javascript\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        // `const` should be preserved, rest capitalized
        assert!(result[0].message.contains("`const`"));
        assert!(result[0].message.contains("Javascript") || result[0].message.contains("JavaScript"));
    }

    // Level filter tests
    #[test]
    fn test_level_filter() {
        let config = MD063Config {
            enabled: true,
            min_level: 2,
            max_level: 4,
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        let content = "# h1 heading\n## h2 heading\n### h3 heading\n##### h5 heading\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();

        // Only h2 and h3 should be flagged (h1 < min_level, h5 > max_level)
        assert_eq!(result.len(), 2);
        assert_eq!(result[0].line, 2); // h2
        assert_eq!(result[1].line, 3); // h3
    }

    // Fix tests
    #[test]
    fn test_fix_atx_heading() {
        let rule = create_rule();
        let content = "# hello world\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(fixed, "# Hello World\n");
    }

    #[test]
    fn test_fix_multiple_headings() {
        let rule = create_rule();
        let content = "# first heading\n\n## second heading\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(fixed, "# First Heading\n\n## Second Heading\n");
    }

    // Setext heading tests
    #[test]
    fn test_setext_heading() {
        let rule = create_rule();
        let content = "hello world\n============\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        assert!(result[0].message.contains("Hello World"));
    }

    // Custom ID tests
    #[test]
    fn test_custom_id_preserved() {
        let rule = create_rule();
        let content = "# getting started {#intro}\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        // Custom ID should be preserved
        assert!(result[0].message.contains("{#intro}"));
    }

    // Acronym preservation tests
    #[test]
    fn test_skip_obsidian_tags_not_headings() {
        let rule = create_rule();

        // #tag (no space after #) is an Obsidian tag, not a heading
        let content = "# H1\n\n#tag\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Obsidian, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty() || result.iter().all(|w| w.line != 3),
            "Obsidian tag #tag should not be treated as a heading: {result:?}"
        );
    }

    #[test]
    fn test_skip_invalid_atx_headings_no_space() {
        let rule = create_rule();

        // #NoSpace is not a valid ATX heading (requires space after #)
        let content = "#notaheading\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "Invalid ATX heading without space should not be flagged: {result:?}"
        );
    }

    #[test]
    fn test_fix_skips_obsidian_tags() {
        let rule = create_rule();

        let content = "# hello world\n\n#tag\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Obsidian, None);
        let fixed = rule.fix(&ctx).unwrap();
        // Should fix the real heading but leave the tag alone
        assert!(fixed.contains("#tag"), "Fix should not modify Obsidian tag #tag");
        assert!(fixed.contains("# Hello World"), "Fix should still fix real headings");
    }

    #[test]
    fn test_preserve_all_caps_acronyms() {
        let rule = create_rule();
        let ctx = |c| LintContext::new(c, crate::config::MarkdownFlavor::Standard, None);

        // Basic acronyms should be preserved
        let fixed = rule.fix(&ctx("# using API in production\n")).unwrap();
        assert_eq!(fixed, "# Using API in Production\n");

        // Multiple acronyms
        let fixed = rule.fix(&ctx("# API and GPU integration\n")).unwrap();
        assert_eq!(fixed, "# API and GPU Integration\n");

        // Two-letter acronyms
        let fixed = rule.fix(&ctx("# IO performance guide\n")).unwrap();
        assert_eq!(fixed, "# IO Performance Guide\n");

        // Acronyms with numbers
        let fixed = rule.fix(&ctx("# HTTP2 and MD5 hashing\n")).unwrap();
        assert_eq!(fixed, "# HTTP2 and MD5 Hashing\n");
    }

    #[test]
    fn test_preserve_acronyms_in_hyphenated_words() {
        let rule = create_rule();
        let ctx = |c| LintContext::new(c, crate::config::MarkdownFlavor::Standard, None);

        // Acronyms at start of hyphenated word
        let fixed = rule.fix(&ctx("# API-driven architecture\n")).unwrap();
        assert_eq!(fixed, "# API-Driven Architecture\n");

        // Multiple acronyms with hyphens
        let fixed = rule.fix(&ctx("# GPU-accelerated CPU-intensive tasks\n")).unwrap();
        assert_eq!(fixed, "# GPU-Accelerated CPU-Intensive Tasks\n");
    }

    #[test]
    fn test_single_letters_not_treated_as_acronyms() {
        let rule = create_rule();
        let ctx = |c| LintContext::new(c, crate::config::MarkdownFlavor::Standard, None);

        // Single uppercase letters should follow title case rules, not be preserved
        let fixed = rule.fix(&ctx("# i am a heading\n")).unwrap();
        assert_eq!(fixed, "# I Am a Heading\n");
    }

    #[test]
    fn test_lowercase_terms_need_ignore_words() {
        let ctx = |c| LintContext::new(c, crate::config::MarkdownFlavor::Standard, None);

        // Without ignore_words: npm gets capitalized
        let rule = create_rule();
        let fixed = rule.fix(&ctx("# using npm packages\n")).unwrap();
        assert_eq!(fixed, "# Using Npm Packages\n");

        // With ignore_words: npm preserved
        let config = MD063Config {
            enabled: true,
            ignore_words: vec!["npm".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);
        let fixed = rule.fix(&ctx("# using npm packages\n")).unwrap();
        assert_eq!(fixed, "# Using npm Packages\n");
    }

    #[test]
    fn test_acronyms_with_mixed_case_preserved() {
        let rule = create_rule();
        let ctx = |c| LintContext::new(c, crate::config::MarkdownFlavor::Standard, None);

        // Both acronyms (API, GPU) and mixed-case (GitHub) should be preserved
        let fixed = rule.fix(&ctx("# using API with GitHub\n")).unwrap();
        assert_eq!(fixed, "# Using API with GitHub\n");
    }

    #[test]
    fn test_real_world_acronyms() {
        let rule = create_rule();
        let ctx = |c| LintContext::new(c, crate::config::MarkdownFlavor::Standard, None);

        // Common technical acronyms from tested repositories
        let content = "# FFI bindings for CPU optimization\n";
        let fixed = rule.fix(&ctx(content)).unwrap();
        assert_eq!(fixed, "# FFI Bindings for CPU Optimization\n");

        let content = "# DOM manipulation and SSR rendering\n";
        let fixed = rule.fix(&ctx(content)).unwrap();
        assert_eq!(fixed, "# DOM Manipulation and SSR Rendering\n");

        let content = "# CVE security and RNN models\n";
        let fixed = rule.fix(&ctx(content)).unwrap();
        assert_eq!(fixed, "# CVE Security and RNN Models\n");
    }

    #[test]
    fn test_is_all_caps_acronym() {
        let rule = create_rule();

        // Should return true for all-caps with 2+ letters
        assert!(rule.is_all_caps_acronym("API"));
        assert!(rule.is_all_caps_acronym("IO"));
        assert!(rule.is_all_caps_acronym("GPU"));
        assert!(rule.is_all_caps_acronym("HTTP2")); // Numbers don't break it

        // Should return false for single letters
        assert!(!rule.is_all_caps_acronym("A"));
        assert!(!rule.is_all_caps_acronym("I"));

        // Should return false for words with lowercase
        assert!(!rule.is_all_caps_acronym("Api"));
        assert!(!rule.is_all_caps_acronym("npm"));
        assert!(!rule.is_all_caps_acronym("iPhone"));
    }

    #[test]
    fn test_sentence_case_ignore_words_first_word() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::SentenceCase,
            ignore_words: vec!["nvim".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // "nvim" as first word should be preserved exactly
        let content = "# nvim config\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "nvim in ignore-words should not be flagged. Got: {result:?}"
        );

        // Verify fix also preserves it
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(fixed, "# nvim config\n");
    }

    #[test]
    fn test_sentence_case_ignore_words_not_first() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::SentenceCase,
            ignore_words: vec!["nvim".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // "nvim" in middle should also be preserved
        let content = "# Using nvim editor\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "nvim in ignore-words should be preserved. Got: {result:?}"
        );
    }

    #[test]
    fn test_preserve_cased_words_ios() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::SentenceCase,
            preserve_cased_words: true,
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // "iOS" should be preserved (has mixed case: lowercase 'i' + uppercase 'OS')
        let content = "## This is iOS\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "iOS should be preserved with preserve-cased-words. Got: {result:?}"
        );

        // Verify fix also preserves it
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(fixed, "## This is iOS\n");
    }

    #[test]
    fn test_preserve_cased_words_ios_title_case() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            preserve_cased_words: true,
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // "iOS" should be preserved in title case too
        let content = "# developing for iOS\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(fixed, "# Developing for iOS\n");
    }

    #[test]
    fn test_has_internal_capitals_ios() {
        let rule = create_rule();

        // iOS should be detected as having internal capitals
        assert!(
            rule.has_internal_capitals("iOS"),
            "iOS has mixed case (lowercase i, uppercase OS)"
        );

        // Other mixed-case words
        assert!(rule.has_internal_capitals("iPhone"));
        assert!(rule.has_internal_capitals("macOS"));
        assert!(rule.has_internal_capitals("GitHub"));
        assert!(rule.has_internal_capitals("JavaScript"));
        assert!(rule.has_internal_capitals("eBay"));

        // All-caps should NOT be detected (handled by is_all_caps_acronym)
        assert!(!rule.has_internal_capitals("API"));
        assert!(!rule.has_internal_capitals("GPU"));

        // All-lowercase should NOT be detected
        assert!(!rule.has_internal_capitals("npm"));
        assert!(!rule.has_internal_capitals("config"));

        // Regular capitalized words should NOT be detected
        assert!(!rule.has_internal_capitals("The"));
        assert!(!rule.has_internal_capitals("Hello"));
    }

    #[test]
    fn test_lowercase_words_before_trailing_code() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec![
                "a".to_string(),
                "an".to_string(),
                "and".to_string(),
                "at".to_string(),
                "but".to_string(),
                "by".to_string(),
                "for".to_string(),
                "from".to_string(),
                "into".to_string(),
                "nor".to_string(),
                "on".to_string(),
                "onto".to_string(),
                "or".to_string(),
                "the".to_string(),
                "to".to_string(),
                "upon".to_string(),
                "via".to_string(),
                "vs".to_string(),
                "with".to_string(),
                "without".to_string(),
            ],
            preserve_cased_words: true,
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Test: "subtitle with a `app`" (all lowercase input)
        // Expected fix: "Subtitle With a `app`" - capitalize "Subtitle" and "With",
        // but keep "a" lowercase (it's in lowercase-words and not the last word)
        // Incorrect: "Subtitle with A `app`" (would incorrectly capitalize "a")
        let content = "## subtitle with a `app`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();

        // Should flag it
        assert!(!result.is_empty(), "Should flag incorrect capitalization");
        let fixed = rule.fix(&ctx).unwrap();
        // "a" should remain lowercase (not "A") because inline code at end doesn't change lowercase-words behavior
        assert!(
            fixed.contains("with a `app`"),
            "Expected 'with a `app`' but got: {fixed:?}"
        );
        assert!(
            !fixed.contains("with A `app`"),
            "Should not capitalize 'a' to 'A'. Got: {fixed:?}"
        );
        // "Subtitle" should be capitalized, "with" and "a" should remain lowercase (they're in lowercase-words)
        assert!(
            fixed.contains("Subtitle with a `app`"),
            "Expected 'Subtitle with a `app`' but got: {fixed:?}"
        );
    }

    #[test]
    fn test_lowercase_words_preserved_before_trailing_code_variant() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string(), "with".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Another variant: "Title with the `code`"
        let content = "## Title with the `code`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "the" should remain lowercase
        assert!(
            fixed.contains("with the `code`"),
            "Expected 'with the `code`' but got: {fixed:?}"
        );
        assert!(
            !fixed.contains("with The `code`"),
            "Should not capitalize 'the' to 'The'. Got: {fixed:?}"
        );
    }

    #[test]
    fn test_last_word_capitalized_when_no_trailing_code() {
        // Verify that when there's NO trailing code, the last word IS capitalized
        // (even if it's in lowercase-words) - this is the normal title case behavior
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // "title with a word" - "word" is last, should be capitalized
        // "a" is in lowercase-words and not last, so should be lowercase
        let content = "## title with a word\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "a" should be lowercase, "word" should be capitalized (it's last)
        assert!(
            fixed.contains("With a Word"),
            "Expected 'With a Word' but got: {fixed:?}"
        );
    }

    #[test]
    fn test_multiple_lowercase_words_before_code() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec![
                "a".to_string(),
                "the".to_string(),
                "with".to_string(),
                "for".to_string(),
            ],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Multiple lowercase words before code - all should remain lowercase
        let content = "## Guide for the `user`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        assert!(
            fixed.contains("for the `user`"),
            "Expected 'for the `user`' but got: {fixed:?}"
        );
        assert!(
            !fixed.contains("For The `user`"),
            "Should not capitalize lowercase words before code. Got: {fixed:?}"
        );
    }

    #[test]
    fn test_code_in_middle_normal_rules_apply() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string(), "for".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Code in the middle - normal title case rules apply (last word capitalized)
        let content = "## Using `const` for the code\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "for" and "the" should be lowercase (middle), "code" should be capitalized (last)
        assert!(
            fixed.contains("for the Code"),
            "Expected 'for the Code' but got: {fixed:?}"
        );
    }

    #[test]
    fn test_link_at_end_same_as_code() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string(), "for".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Link at the end - same behavior as code (lowercase words before should remain lowercase)
        let content = "## Guide for the [link](./page.md)\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "for" and "the" should remain lowercase (not last word because link follows)
        assert!(
            fixed.contains("for the [Link]"),
            "Expected 'for the [Link]' but got: {fixed:?}"
        );
        assert!(
            !fixed.contains("for The [Link]"),
            "Should not capitalize 'the' before link. Got: {fixed:?}"
        );
    }

    #[test]
    fn test_multiple_code_segments() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string(), "with".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Multiple code segments - last segment is code, so lowercase words before should remain lowercase
        let content = "## Using `const` with a `variable`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "a" should remain lowercase (not last word because code follows)
        assert!(
            fixed.contains("with a `variable`"),
            "Expected 'with a `variable`' but got: {fixed:?}"
        );
        assert!(
            !fixed.contains("with A `variable`"),
            "Should not capitalize 'a' before trailing code. Got: {fixed:?}"
        );
    }

    #[test]
    fn test_code_and_link_combination() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string(), "for".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Code then link - last segment is link, so lowercase words before code should remain lowercase
        let content = "## Guide for the `code` [link](./page.md)\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "for" and "the" should remain lowercase (not last word because link follows)
        assert!(
            fixed.contains("for the `code`"),
            "Expected 'for the `code`' but got: {fixed:?}"
        );
    }

    #[test]
    fn test_text_after_code_capitalizes_last() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string(), "for".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Code in middle, text after - last word should be capitalized
        let content = "## Using `const` for the code\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "for" and "the" should be lowercase, "code" is last word, should be capitalized
        assert!(
            fixed.contains("for the Code"),
            "Expected 'for the Code' but got: {fixed:?}"
        );
    }

    #[test]
    fn test_preserve_cased_words_with_trailing_code() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string(), "for".to_string()],
            preserve_cased_words: true,
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Preserve-cased words should still work with trailing code
        let content = "## Guide for iOS `app`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "iOS" should be preserved, "for" should be lowercase
        assert!(
            fixed.contains("for iOS `app`"),
            "Expected 'for iOS `app`' but got: {fixed:?}"
        );
        assert!(
            !fixed.contains("For iOS `app`"),
            "Should not capitalize 'for' before trailing code. Got: {fixed:?}"
        );
    }

    #[test]
    fn test_ignore_words_with_trailing_code() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string(), "with".to_string()],
            ignore_words: vec!["npm".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Ignore-words should still work with trailing code
        let content = "## Using npm with a `script`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "npm" should be preserved, "with" and "a" should be lowercase
        assert!(
            fixed.contains("npm with a `script`"),
            "Expected 'npm with a `script`' but got: {fixed:?}"
        );
        assert!(
            !fixed.contains("with A `script`"),
            "Should not capitalize 'a' before trailing code. Got: {fixed:?}"
        );
    }

    #[test]
    fn test_empty_text_segment_edge_case() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "with".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Edge case: code at start, then text with lowercase word, then code at end
        let content = "## `start` with a `end`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "with" is first word in text segment, so capitalized (correct)
        // "a" should remain lowercase (not last word because code follows) - this is the key test
        assert!(fixed.contains("a `end`"), "Expected 'a `end`' but got: {fixed:?}");
        assert!(
            !fixed.contains("A `end`"),
            "Should not capitalize 'a' before trailing code. Got: {fixed:?}"
        );
    }

    #[test]
    fn test_sentence_case_with_trailing_code() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::SentenceCase,
            lowercase_words: vec!["a".to_string(), "the".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Sentence case should also respect lowercase words before code
        let content = "## guide for the `user`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // First word capitalized, rest lowercase including "the" before code
        assert!(
            fixed.contains("Guide for the `user`"),
            "Expected 'Guide for the `user`' but got: {fixed:?}"
        );
    }

    #[test]
    fn test_hyphenated_word_before_code() {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            lowercase_words: vec!["a".to_string(), "the".to_string(), "with".to_string()],
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Hyphenated word before code - last part should respect lowercase-words
        let content = "## Self-contained with a `feature`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        // "with" and "a" should remain lowercase (not last word because code follows)
        assert!(
            fixed.contains("with a `feature`"),
            "Expected 'with a `feature`' but got: {fixed:?}"
        );
    }

    // Issue #228: Sentence case with inline code at heading start
    // When a heading starts with inline code, the first word after the code
    // should NOT be capitalized because the heading already has a "first element"

    #[test]
    fn test_sentence_case_code_at_start_basic() {
        // The exact case from issue #228
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# `rumdl` is a linter\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // Should be correct as-is: code is first, "is" stays lowercase
        assert!(
            result.is_empty(),
            "Heading with code at start should not flag 'is' for capitalization. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_sentence_case_code_at_start_incorrect_capitalization() {
        // Verify we detect incorrect capitalization after code at start
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# `rumdl` Is a Linter\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // Should flag: "Is" and "Linter" should be lowercase
        assert_eq!(result.len(), 1, "Should detect incorrect capitalization");
        assert!(
            result[0].message.contains("`rumdl` is a linter"),
            "Should suggest lowercase after code. Got: {:?}",
            result[0].message
        );
    }

    #[test]
    fn test_sentence_case_code_at_start_fix() {
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# `rumdl` Is A Linter\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let fixed = rule.fix(&ctx).unwrap();
        assert!(
            fixed.contains("# `rumdl` is a linter"),
            "Should fix to lowercase after code. Got: {fixed:?}"
        );
    }

    #[test]
    fn test_sentence_case_text_at_start_still_capitalizes() {
        // Ensure normal headings still capitalize first word
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# the quick brown fox\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        assert!(
            result[0].message.contains("The quick brown fox"),
            "Text-first heading should capitalize first word. Got: {:?}",
            result[0].message
        );
    }

    #[test]
    fn test_sentence_case_link_at_start() {
        // Links at start: link text is lowercased, following text also lowercase
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        // Use lowercase link text to avoid link text case flagging
        let content = "# [api](api.md) reference guide\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // "reference" should be lowercase (link is first)
        assert!(
            result.is_empty(),
            "Heading with link at start should not capitalize 'reference'. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_sentence_case_link_preserves_acronyms() {
        // Acronyms in link text should be preserved (API, HTTP, etc.)
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# [API](api.md) Reference Guide\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        // "API" should be preserved (acronym), "Reference Guide" should be lowercased
        assert!(
            result[0].message.contains("[API](api.md) reference guide"),
            "Should preserve acronym 'API' but lowercase following text. Got: {:?}",
            result[0].message
        );
    }

    #[test]
    fn test_sentence_case_link_preserves_brand_names() {
        // Brand names with internal capitals should be preserved
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::SentenceCase,
            preserve_cased_words: true,
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);
        let content = "# [iPhone](iphone.md) Features Guide\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        // "iPhone" should be preserved, "Features Guide" should be lowercased
        assert!(
            result[0].message.contains("[iPhone](iphone.md) features guide"),
            "Should preserve 'iPhone' but lowercase following text. Got: {:?}",
            result[0].message
        );
    }

    #[test]
    fn test_sentence_case_link_lowercases_regular_words() {
        // Regular words in link text should be lowercased
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# [Documentation](docs.md) Reference\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        // "Documentation" should be lowercased (regular word)
        assert!(
            result[0].message.contains("[documentation](docs.md) reference"),
            "Should lowercase regular link text. Got: {:?}",
            result[0].message
        );
    }

    #[test]
    fn test_sentence_case_link_at_start_correct_already() {
        // Link with correct casing should not be flagged
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# [API](api.md) reference guide\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "Correctly cased heading with link should not be flagged. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_sentence_case_link_github_preserved() {
        // GitHub should be preserved (internal capitals)
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::SentenceCase,
            preserve_cased_words: true,
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);
        let content = "# [GitHub](gh.md) Repository Setup\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        assert!(
            result[0].message.contains("[GitHub](gh.md) repository setup"),
            "Should preserve 'GitHub'. Got: {:?}",
            result[0].message
        );
    }

    #[test]
    fn test_sentence_case_multiple_code_spans() {
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# `foo` and `bar` are methods\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // All text after first code should be lowercase
        assert!(
            result.is_empty(),
            "Should not capitalize words between/after code spans. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_sentence_case_code_only_heading() {
        // Heading with only code, no text
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# `rumdl`\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "Code-only heading should be fine. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_sentence_case_code_at_end() {
        // Heading ending with code, text before should still capitalize first word
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# install the `rumdl` tool\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // "install" should be capitalized (first word), rest lowercase
        assert_eq!(result.len(), 1);
        assert!(
            result[0].message.contains("Install the `rumdl` tool"),
            "First word should still be capitalized when text comes first. Got: {:?}",
            result[0].message
        );
    }

    #[test]
    fn test_sentence_case_code_in_middle() {
        // Code in middle, text at start should capitalize first word
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# using the `rumdl` linter for markdown\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // "using" should be capitalized, rest lowercase
        assert_eq!(result.len(), 1);
        assert!(
            result[0].message.contains("Using the `rumdl` linter for markdown"),
            "First word should be capitalized. Got: {:?}",
            result[0].message
        );
    }

    #[test]
    fn test_sentence_case_preserved_word_after_code() {
        // Preserved words (like iPhone) should stay preserved even after code
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::SentenceCase,
            preserve_cased_words: true,
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);
        let content = "# `swift` iPhone development\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // "iPhone" should be preserved, "development" lowercase
        assert!(
            result.is_empty(),
            "Preserved words after code should stay. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_title_case_code_at_start_still_capitalizes() {
        // Title case should still capitalize words even after code at start
        let rule = create_rule_with_style(HeadingCapStyle::TitleCase);
        let content = "# `api` quick start guide\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // Title case: all major words capitalized
        assert_eq!(result.len(), 1);
        assert!(
            result[0].message.contains("Quick Start Guide") || result[0].message.contains("quick Start Guide"),
            "Title case should capitalize major words after code. Got: {:?}",
            result[0].message
        );
    }

    // ======== HTML TAG TESTS ========

    #[test]
    fn test_sentence_case_html_tag_at_start() {
        // HTML tag at start: text after should NOT capitalize first word
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# <kbd>Ctrl</kbd> is a Modifier Key\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // "is", "a", "Modifier", "Key" should all be lowercase (except preserved words)
        assert_eq!(result.len(), 1);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(
            fixed, "# <kbd>Ctrl</kbd> is a modifier key\n",
            "Text after HTML at start should be lowercase"
        );
    }

    #[test]
    fn test_sentence_case_html_tag_preserves_content() {
        // Content inside HTML tags should be preserved as-is
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# The <abbr>API</abbr> documentation guide\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // "The" is first, "API" inside tag preserved, rest lowercase
        assert!(
            result.is_empty(),
            "HTML tag content should be preserved. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_sentence_case_html_tag_at_start_with_acronym() {
        // HTML tag at start with acronym content
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# <abbr>API</abbr> Documentation Guide\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(
            fixed, "# <abbr>API</abbr> documentation guide\n",
            "Text after HTML at start should be lowercase, HTML content preserved"
        );
    }

    #[test]
    fn test_sentence_case_html_tag_in_middle() {
        // HTML tag in middle: first word still capitalized
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# using the <code>config</code> File\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(
            fixed, "# Using the <code>config</code> file\n",
            "First word capitalized, HTML preserved, rest lowercase"
        );
    }

    #[test]
    fn test_html_tag_strong_emphasis() {
        // <strong> tag handling
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# The <strong>Bold</strong> Way\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(
            fixed, "# The <strong>Bold</strong> way\n",
            "<strong> tag content should be preserved"
        );
    }

    #[test]
    fn test_html_tag_with_attributes() {
        // HTML tags with attributes should still be detected
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# <span class=\"highlight\">Important</span> Notice Here\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(
            fixed, "# <span class=\"highlight\">Important</span> notice here\n",
            "HTML tag with attributes should be preserved"
        );
    }

    #[test]
    fn test_multiple_html_tags() {
        // Multiple HTML tags in heading
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# <kbd>Ctrl</kbd>+<kbd>C</kbd> to Copy Text\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(
            fixed, "# <kbd>Ctrl</kbd>+<kbd>C</kbd> to copy text\n",
            "Multiple HTML tags should all be preserved"
        );
    }

    #[test]
    fn test_html_and_code_mixed() {
        // Mix of HTML tags and inline code
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# <kbd>Ctrl</kbd>+`v` Paste command\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(
            fixed, "# <kbd>Ctrl</kbd>+`v` paste command\n",
            "HTML and code should both be preserved"
        );
    }

    #[test]
    fn test_self_closing_html_tag() {
        // Self-closing tags like <br/>
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# Line one<br/>Line Two Here\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed = rule.fix(&ctx).unwrap();
        assert_eq!(
            fixed, "# Line one<br/>line two here\n",
            "Self-closing HTML tags should be preserved"
        );
    }

    #[test]
    fn test_title_case_with_html_tags() {
        // Title case with HTML tags
        let rule = create_rule_with_style(HeadingCapStyle::TitleCase);
        let content = "# the <kbd>ctrl</kbd> key is a modifier\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed = rule.fix(&ctx).unwrap();
        // "the" as first word should be "The", content inside <kbd> preserved
        assert!(
            fixed.contains("<kbd>ctrl</kbd>"),
            "HTML tag content should be preserved in title case. Got: {fixed}"
        );
        assert!(
            fixed.starts_with("# The ") || fixed.starts_with("# the "),
            "Title case should work with HTML. Got: {fixed}"
        );
    }

    // ======== CARET NOTATION TESTS ========

    #[test]
    fn test_sentence_case_preserves_caret_notation() {
        // Caret notation for control characters should be preserved
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "## Ctrl+A, Ctrl+R output ^A, ^R on zsh\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        // Should not flag - ^A and ^R are preserved
        assert!(
            result.is_empty(),
            "Caret notation should be preserved. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_sentence_case_caret_notation_various() {
        // Various caret notation patterns
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);

        // ^C for interrupt
        let content = "## Press ^C to cancel\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "^C should be preserved. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );

        // ^Z for suspend
        let content = "## Use ^Z for background\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "^Z should be preserved. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );

        // ^[ for escape
        let content = "## Press ^[ for escape\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "^[ should be preserved. Got: {:?}",
            result.iter().map(|w| &w.message).collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_caret_notation_detection() {
        let rule = create_rule();

        // Valid caret notation
        assert!(rule.is_caret_notation("^A"));
        assert!(rule.is_caret_notation("^Z"));
        assert!(rule.is_caret_notation("^C"));
        assert!(rule.is_caret_notation("^@")); // NUL
        assert!(rule.is_caret_notation("^[")); // ESC
        assert!(rule.is_caret_notation("^]")); // GS
        assert!(rule.is_caret_notation("^^")); // RS
        assert!(rule.is_caret_notation("^_")); // US

        // Not caret notation
        assert!(!rule.is_caret_notation("^a")); // lowercase
        assert!(!rule.is_caret_notation("A")); // no caret
        assert!(!rule.is_caret_notation("^")); // caret alone
        assert!(!rule.is_caret_notation("^1")); // digit
    }

    // MD044 proper names integration tests
    //
    // When MD063 (sentence case) and MD044 (proper names) are both active, MD063 must
    // preserve the exact capitalization of MD044 proper names rather than lowercasing them.
    // Without this, the two rules oscillate: MD044 re-capitalizes what MD063 lowercases.

    fn create_sentence_case_rule_with_proper_names(names: Vec<String>) -> MD063HeadingCapitalization {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::SentenceCase,
            ..Default::default()
        };
        let mut rule = MD063HeadingCapitalization::from_config_struct(config);
        rule.proper_names = names;
        rule
    }

    #[test]
    fn test_sentence_case_preserves_single_word_proper_name() {
        let rule = create_sentence_case_rule_with_proper_names(vec!["JavaScript".to_string()]);
        // "javascript" in non-first position should become "JavaScript", not "javascript"
        let content = "# installing javascript\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag the heading");
        let fix_text = result[0].fix.as_ref().unwrap().replacement.as_str();
        assert!(
            fix_text.contains("JavaScript"),
            "Fix should preserve proper name 'JavaScript', got: {fix_text:?}"
        );
        assert!(
            !fix_text.contains("javascript"),
            "Fix should not have lowercase 'javascript', got: {fix_text:?}"
        );
    }

    #[test]
    fn test_sentence_case_preserves_multi_word_proper_name() {
        let rule = create_sentence_case_rule_with_proper_names(vec!["Good Application".to_string()]);
        // "Good Application" is a proper name; sentence case must not lowercase "Application"
        let content = "# using good application features\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag the heading");
        let fix_text = result[0].fix.as_ref().unwrap().replacement.as_str();
        assert!(
            fix_text.contains("Good Application"),
            "Fix should preserve 'Good Application' as a phrase, got: {fix_text:?}"
        );
    }

    #[test]
    fn test_sentence_case_proper_name_at_start_of_heading() {
        let rule = create_sentence_case_rule_with_proper_names(vec!["Good Application".to_string()]);
        // The proper name "Good Application" starts the heading; both words must be canonical
        let content = "# good application overview\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag the heading");
        let fix_text = result[0].fix.as_ref().unwrap().replacement.as_str();
        assert!(
            fix_text.contains("Good Application"),
            "Fix should produce 'Good Application' at start of heading, got: {fix_text:?}"
        );
        assert!(
            fix_text.contains("overview"),
            "Non-proper-name word 'overview' should be lowercase, got: {fix_text:?}"
        );
    }

    #[test]
    fn test_sentence_case_with_proper_names_no_oscillation() {
        // This is the core convergence test: applying the fix once must produce
        // output that is already correct (no further changes needed).
        let rule = create_sentence_case_rule_with_proper_names(vec!["Good Application".to_string()]);

        // First application of fix
        let content = "# installing good application on your system\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed_heading = result[0].fix.as_ref().unwrap().replacement.as_str();

        // The fixed heading should contain the proper name preserved
        assert!(
            fixed_heading.contains("Good Application"),
            "After fix, proper name must be preserved: {fixed_heading:?}"
        );

        // Second application: must produce no further warnings (convergence)
        let fixed_line = format!("{fixed_heading}\n");
        let ctx2 = LintContext::new(&fixed_line, crate::config::MarkdownFlavor::Standard, None);
        let result2 = rule.check(&ctx2).unwrap();
        assert!(
            result2.is_empty(),
            "After one fix, heading must already satisfy both MD063 and MD044 - no oscillation. \
             Second pass warnings: {result2:?}"
        );
    }

    #[test]
    fn test_sentence_case_proper_names_already_correct() {
        let rule = create_sentence_case_rule_with_proper_names(vec!["Good Application".to_string()]);
        // Heading already has correct sentence case with proper name preserved
        let content = "# Installing Good Application\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "Correct sentence-case heading with proper name should not be flagged, got: {result:?}"
        );
    }

    #[test]
    fn test_sentence_case_multiple_proper_names_in_heading() {
        let rule = create_sentence_case_rule_with_proper_names(vec!["TypeScript".to_string(), "React".to_string()]);
        let content = "# using typescript with react\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fix_text = result[0].fix.as_ref().unwrap().replacement.as_str();
        assert!(
            fix_text.contains("TypeScript"),
            "Fix should preserve 'TypeScript', got: {fix_text:?}"
        );
        assert!(
            fix_text.contains("React"),
            "Fix should preserve 'React', got: {fix_text:?}"
        );
    }

    #[test]
    fn test_sentence_case_unicode_casefold_expansion_before_proper_name() {
        // Regression for Unicode case-fold expansion: `İ` lowercases to `i̇` (2 code points),
        // so matching offsets must be computed from the original text, not from a lowercased copy.
        let rule = create_sentence_case_rule_with_proper_names(vec!["Österreich".to_string()]);
        let content = "# İ österreich guide\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);

        // Should not panic and should preserve canonical proper-name casing.
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag heading for canonical proper-name casing");
        let fix_text = result[0].fix.as_ref().unwrap().replacement.as_str();
        assert!(
            fix_text.contains("Österreich"),
            "Fix should preserve canonical 'Österreich', got: {fix_text:?}"
        );
    }

    #[test]
    fn test_sentence_case_preserves_trailing_punctuation_on_proper_name() {
        let rule = create_sentence_case_rule_with_proper_names(vec!["JavaScript".to_string()]);
        let content = "# using javascript, today\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag heading");
        let fix_text = result[0].fix.as_ref().unwrap().replacement.as_str();
        assert!(
            fix_text.contains("JavaScript,"),
            "Fix should preserve trailing punctuation, got: {fix_text:?}"
        );
    }

    // Title case + MD044 conflict tests
    //
    // In title case, short words like "the", "a", "of" are kept lowercase by MD063.
    // If those words are part of an MD044 proper name (e.g. "The Rolling Stones"),
    // the same oscillation problem occurs.  The fix must extend to title case too.

    fn create_title_case_rule_with_proper_names(names: Vec<String>) -> MD063HeadingCapitalization {
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            ..Default::default()
        };
        let mut rule = MD063HeadingCapitalization::from_config_struct(config);
        rule.proper_names = names;
        rule
    }

    #[test]
    fn test_title_case_preserves_proper_name_with_lowercase_article() {
        // "The" is in the lowercase_words list for title case, so "the" in the middle
        // of a heading would normally stay lowercase.  But "The Rolling Stones" is a
        // proper name that must be capitalised exactly.
        let rule = create_title_case_rule_with_proper_names(vec!["The Rolling Stones".to_string()]);
        let content = "# listening to the rolling stones today\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag the heading");
        let fix_text = result[0].fix.as_ref().unwrap().replacement.as_str();
        assert!(
            fix_text.contains("The Rolling Stones"),
            "Fix should preserve proper name 'The Rolling Stones', got: {fix_text:?}"
        );
    }

    #[test]
    fn test_title_case_proper_name_no_oscillation() {
        // One fix pass must produce output that title case already accepts.
        let rule = create_title_case_rule_with_proper_names(vec!["The Rolling Stones".to_string()]);
        let content = "# listening to the rolling stones today\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fixed_heading = result[0].fix.as_ref().unwrap().replacement.as_str();

        let fixed_line = format!("{fixed_heading}\n");
        let ctx2 = LintContext::new(&fixed_line, crate::config::MarkdownFlavor::Standard, None);
        let result2 = rule.check(&ctx2).unwrap();
        assert!(
            result2.is_empty(),
            "After one title-case fix, heading must already satisfy both rules. \
             Second pass warnings: {result2:?}"
        );
    }

    #[test]
    fn test_title_case_unicode_casefold_expansion_before_proper_name() {
        let rule = create_title_case_rule_with_proper_names(vec!["Österreich".to_string()]);
        let content = "# İ österreich guide\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag the heading");
        let fix_text = result[0].fix.as_ref().unwrap().replacement.as_str();
        assert!(
            fix_text.contains("Österreich"),
            "Fix should preserve canonical proper-name casing, got: {fix_text:?}"
        );
    }

    // End-to-end integration test: from_config wires MD044 names into MD063
    //
    // This tests the actual code path used in production, where both rules are
    // configured in a rumdl.toml and the rule registry calls from_config.

    #[test]
    fn test_from_config_loads_md044_names_into_md063() {
        use crate::config::{Config, RuleConfig};
        use crate::rule::Rule;
        use std::collections::BTreeMap;

        let mut config = Config::default();

        // Configure MD063 with sentence_case
        let mut md063_values = BTreeMap::new();
        md063_values.insert("style".to_string(), toml::Value::String("sentence_case".to_string()));
        md063_values.insert("enabled".to_string(), toml::Value::Boolean(true));
        config.rules.insert(
            "MD063".to_string(),
            RuleConfig {
                values: md063_values,
                severity: None,
            },
        );

        // Configure MD044 with a proper name
        let mut md044_values = BTreeMap::new();
        md044_values.insert(
            "names".to_string(),
            toml::Value::Array(vec![toml::Value::String("Good Application".to_string())]),
        );
        config.rules.insert(
            "MD044".to_string(),
            RuleConfig {
                values: md044_values,
                severity: None,
            },
        );

        // Build MD063 via the production code path
        let rule = MD063HeadingCapitalization::from_config(&config);

        // Verify MD044 names were loaded: the fix must preserve "Good Application"
        let content = "# using good application features\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag the heading");
        let fix_text = result[0].fix.as_ref().unwrap().replacement.as_str();
        assert!(
            fix_text.contains("Good Application"),
            "from_config should wire MD044 names into MD063; fix should preserve \
             'Good Application', got: {fix_text:?}"
        );
    }

    #[test]
    fn test_title_case_short_word_not_confused_with_substring() {
        // Verify that short preposition matching ("in") does not trigger on
        // substrings of longer words ("insert"). Title case must capitalize
        // "insert" while keeping "in" lowercase.
        let rule = create_rule_with_style(HeadingCapStyle::TitleCase);

        // "in" is a short preposition (should be lowercase in title case)
        // "insert" contains "in" as substring but is a regular word (should be capitalized)
        let content = "# in the insert\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag the heading");
        let fix = result[0].fix.as_ref().expect("Fix should be present");
        // "In" capitalized as first word, "the" lowercase as article, "Insert" capitalized
        assert!(
            fix.replacement.contains("In the Insert"),
            "Expected 'In the Insert', got: {:?}",
            fix.replacement
        );
    }

    #[test]
    fn test_title_case_or_not_confused_with_orchestra() {
        let rule = create_rule_with_style(HeadingCapStyle::TitleCase);

        // "or" is a conjunction (should be lowercase in title case)
        // "orchestra" contains "or" as substring but is a regular word
        let content = "# or the orchestra\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag the heading");
        let fix = result[0].fix.as_ref().expect("Fix should be present");
        // "Or" capitalized as first word, "the" lowercase, "Orchestra" capitalized
        assert!(
            fix.replacement.contains("Or the Orchestra"),
            "Expected 'Or the Orchestra', got: {:?}",
            fix.replacement
        );
    }

    #[test]
    fn test_all_caps_preserves_all_words() {
        let rule = create_rule_with_style(HeadingCapStyle::AllCaps);

        let content = "# in the insert\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1, "Should flag the heading");
        let fix = result[0].fix.as_ref().expect("Fix should be present");
        assert!(
            fix.replacement.contains("IN THE INSERT"),
            "All caps should uppercase all words, got: {:?}",
            fix.replacement
        );
    }

    // Numbered prefix tests — words following a period-terminated token must be capitalized
    #[test]
    fn test_title_case_numbered_prefix_lowercase_word() {
        // "to" follows "1." and must be treated as the start of a new phrase
        let rule = create_rule();
        let content = "## 1. To Be a Thing\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "Should not flag '## 1. To Be a Thing', got: {result:?}"
        );

        let content_lower = "## 1. to be a thing\n";
        let ctx2 = LintContext::new(content_lower, crate::config::MarkdownFlavor::Standard, None);
        let result2 = rule.check(&ctx2).unwrap();
        assert!(!result2.is_empty(), "Should flag '## 1. to be a thing'");
        let fix = result2[0].fix.as_ref().expect("Should have a fix");
        assert!(
            fix.replacement.contains("1. To Be a Thing"),
            "Fix should capitalize 'To', got: {:?}",
            fix.replacement
        );
    }

    #[test]
    fn test_title_case_numbered_prefix_article() {
        // "a" follows "2." and must be capitalized as the first word of the phrase
        let rule = create_rule();
        let content = "## 2. A Guide to the Galaxy\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "Should not flag '## 2. A Guide to the Galaxy', got: {result:?}"
        );

        let content_lower = "## 2. a guide to the galaxy\n";
        let ctx2 = LintContext::new(content_lower, crate::config::MarkdownFlavor::Standard, None);
        let result2 = rule.check(&ctx2).unwrap();
        assert!(!result2.is_empty(), "Should flag '## 2. a guide to the galaxy'");
        let fix = result2[0].fix.as_ref().expect("Should have a fix");
        assert!(
            fix.replacement.contains("2. A Guide to the Galaxy"),
            "Fix should capitalize 'A', got: {:?}",
            fix.replacement
        );
    }

    #[test]
    fn test_title_case_mid_sentence_period_word() {
        // "introduction" follows "1." embedded in a phrase — must be capitalized
        let rule = create_rule();
        let content = "## Step 1. Introduction to the Problem\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "Should not flag '## Step 1. Introduction to the Problem', got: {result:?}"
        );

        let content_lower = "## Step 1. introduction to the problem\n";
        let ctx2 = LintContext::new(content_lower, crate::config::MarkdownFlavor::Standard, None);
        let result2 = rule.check(&ctx2).unwrap();
        assert!(
            !result2.is_empty(),
            "Should flag '## Step 1. introduction to the problem'"
        );
        let fix = result2[0].fix.as_ref().expect("Should have a fix");
        assert!(
            fix.replacement.contains("Step 1. Introduction to the Problem"),
            "Fix should capitalize 'Introduction', got: {:?}",
            fix.replacement
        );
    }

    #[test]
    fn test_title_case_numbered_prefix_in_link_text() {
        // apply_title_case (link text path) must also respect after_period.
        // A heading whose only content is a link: ## [1. to be a thing](url)
        let config = MD063Config {
            enabled: true,
            style: HeadingCapStyle::TitleCase,
            ..Default::default()
        };
        let rule = MD063HeadingCapitalization::from_config_struct(config);

        // Correct heading — link text already title-cased after numbered prefix
        let content = "## [1. To Be a Thing](https://example.com)\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(
            result.is_empty(),
            "Should not flag '## [1. To Be a Thing](url)', got: {result:?}"
        );

        // Incorrect heading — "to" in link text must be capitalized after "1."
        let content_lower = "## [1. to be a thing](https://example.com)\n";
        let ctx2 = LintContext::new(content_lower, crate::config::MarkdownFlavor::Standard, None);
        let result2 = rule.check(&ctx2).unwrap();
        assert!(!result2.is_empty(), "Should flag '## [1. to be a thing](url)'");
        let fix = result2[0].fix.as_ref().expect("Should have a fix");
        assert!(
            fix.replacement.contains("1. To Be a Thing"),
            "Fix should capitalize 'To' in link text, got: {:?}",
            fix.replacement
        );
    }

    // Numeric-ordinal tests (issue #608): "1st", "2nd", "3rd", "4th", "21st"
    // and so on must keep their alphabetic suffix lower-cased in title case
    // and must be normalised back from mis-cased forms like "5Th".

    #[test]
    fn test_is_numeric_ordinal_recognises_canonical_forms() {
        for word in &[
            "1st", "2nd", "3rd", "4th", "5th", "11th", "21st", "22nd", "23rd", "100th", "1ST", "5Th", "21St", "21sT",
        ] {
            assert!(
                MD063HeadingCapitalization::is_numeric_ordinal(word),
                "expected `{word}` to be detected as a numeric ordinal"
            );
        }
    }

    #[test]
    fn test_is_numeric_ordinal_rejects_non_ordinals() {
        // Words without a digit prefix, an unrecognised alphabetic suffix,
        // or a non-ordinal alpha tail are all rejected. Compound forms with
        // hyphens are handled by `handle_hyphenated_word` so the helper's
        // behaviour on them is intentionally unconstrained.
        for word in &[
            "first", "1stop", "ist", "5", "th", "abc", "4G", "4K", "30s", "100k", "5x", "1.5", "iPhone6S",
        ] {
            assert!(
                !MD063HeadingCapitalization::is_numeric_ordinal(word),
                "expected `{word}` NOT to be detected as a numeric ordinal"
            );
        }
    }

    #[test]
    fn test_is_numeric_ordinal_strips_trailing_punctuation() {
        for word in &["5th.", "1st,", "21st!", "3rd:", "4th)", "5th's"] {
            assert!(
                MD063HeadingCapitalization::is_numeric_ordinal(word),
                "expected `{word}` to be detected as a numeric ordinal (with punctuation)"
            );
        }
    }

    #[test]
    fn test_title_case_ordinal_first_word_not_flagged() {
        let rule = create_rule();
        for content in &[
            "# 1st Place\n",
            "# 2nd Edition\n",
            "# 3rd Time\n",
            "# 5th Avenue\n",
            "# 21st Century Skills\n",
            "# 100th Customer\n",
        ] {
            let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
            let result = rule.check(&ctx).unwrap();
            assert!(result.is_empty(), "Should not flag {content:?}, got: {result:?}");
        }
    }

    #[test]
    fn test_title_case_ordinal_mid_heading_not_flagged() {
        let rule = create_rule();
        for content in &[
            "# May 3rd Notes\n",
            "# Top 100th Customer\n",
            "# Notes for the 5th of May\n",
        ] {
            let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
            let result = rule.check(&ctx).unwrap();
            assert!(result.is_empty(), "Should not flag {content:?}, got: {result:?}");
        }
    }

    #[test]
    fn test_title_case_ordinal_corrupted_form_is_fixed() {
        // The "sticky" case: a heading already mangled by the buggy
        // capitaliser must be flagged and corrected back, not left alone.
        let rule = create_rule();
        for (input, expected) in &[
            ("# 1St Place\n", "1st Place"),
            ("# 5Th Avenue\n", "5th Avenue"),
            ("# 21St Century Skills\n", "21st Century Skills"),
            ("# May 3Rd Notes\n", "May 3rd Notes"),
            ("# 22Nd Edition\n", "22nd Edition"),
        ] {
            let ctx = LintContext::new(input, crate::config::MarkdownFlavor::Standard, None);
            let result = rule.check(&ctx).unwrap();
            assert!(!result.is_empty(), "Should flag {input:?}");
            let fix = result[0].fix.as_ref().expect("should have a fix");
            assert!(
                fix.replacement.contains(expected),
                "Fix for {input:?} should contain {expected:?}, got: {:?}",
                fix.replacement
            );
        }
    }

    #[test]
    fn test_title_case_ordinal_lowercase_other_words_capitalised() {
        // Non-ordinal words around an ordinal still need title-casing.
        let rule = create_rule();
        let content = "# 5th avenue\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fix = result[0].fix.as_ref().expect("should have a fix");
        assert!(
            fix.replacement.contains("5th Avenue"),
            "Fix should produce '5th Avenue', got: {:?}",
            fix.replacement
        );
    }

    #[test]
    fn test_title_case_ordinal_with_trailing_punctuation() {
        let rule = create_rule();
        let content = "# Released on the 5th.\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert!(result.is_empty(), "Should not flag {content:?}, got: {result:?}");
    }

    #[test]
    fn test_title_case_ordinal_hyphenated() {
        let rule = create_rule();
        for content in &["# 21st-Century Skills\n", "# A 19th-Century Novel\n"] {
            let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
            let result = rule.check(&ctx).unwrap();
            assert!(result.is_empty(), "Should not flag {content:?}, got: {result:?}");
        }
    }

    #[test]
    fn test_sentence_case_ordinal_corrupted_form_is_fixed() {
        let rule = create_rule_with_style(HeadingCapStyle::SentenceCase);
        let content = "# 5Th avenue\n";
        let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
        let result = rule.check(&ctx).unwrap();
        assert_eq!(result.len(), 1);
        let fix = result[0].fix.as_ref().expect("should have a fix");
        assert!(
            fix.replacement.contains("5th avenue"),
            "Fix should produce '5th avenue', got: {:?}",
            fix.replacement
        );
    }

    #[test]
    fn test_title_case_digit_acronym_unchanged() {
        // Non-ordinal digit-prefixed tokens (4G, 4K) must still be preserved
        // as all-caps acronyms — the ordinal carve-out must not catch them.
        let rule = create_rule();
        for content in &["# 4G Networks\n", "# 4K Streaming\n"] {
            let ctx = LintContext::new(content, crate::config::MarkdownFlavor::Standard, None);
            let result = rule.check(&ctx).unwrap();
            assert!(result.is_empty(), "Should not flag {content:?}, got: {result:?}");
        }
    }
}