use minimad::{Composite, CompositeStyle, Compound, Line};
use ratatui::layout::{Constraint, Layout, Rect};
use ratatui::style::{Color, Modifier, Style, Stylize};
use ratatui::text::{Line as RLine, Span};
use ratatui::widgets::{Block, Borders, List, ListItem, Padding, Paragraph, Wrap};
use ratatui::Frame;
use tui_big_text::{BigText, PixelSize};

use crate::app::App;
use crate::views::highlight;

// To add a new help topic: drop a .md file in docs/ and add one line here.
const DOCS: &[(&str, &str)] = &[
    ("Welcome", include_str!("../../docs/welcome.md")),
    ("Audio Engine", include_str!("../../docs/audio_engine.md")),
    ("Keybindings", include_str!("../../docs/keybindings.md")),
    ("Sequencer", include_str!("../../docs/sequencer.md")),
    ("About", include_str!("../../docs/about.md")),
];

pub fn topic_count() -> usize {
    DOCS.len()
}

pub fn render(frame: &mut Frame, app: &App, area: Rect) {
    let [topics_area, content_area] =
        Layout::horizontal([Constraint::Length(18), Constraint::Fill(1)]).areas(area);

    render_topics(frame, app, topics_area);
    render_content(frame, app, content_area);
}

fn render_topics(frame: &mut Frame, app: &App, area: Rect) {
    let items: Vec<ListItem> = DOCS
        .iter()
        .enumerate()
        .map(|(i, (name, _))| {
            let selected = i == app.ui.help_topic;
            let style = if selected {
                Style::new().fg(Color::Cyan).add_modifier(Modifier::BOLD)
            } else {
                Style::new().fg(Color::White)
            };
            let prefix = if selected { "> " } else { "  " };
            ListItem::new(format!("{prefix}{name}")).style(style)
        })
        .collect();

    let list = List::new(items).block(Block::default().borders(Borders::ALL).title("Topics"));
    frame.render_widget(list, area);
}

const WELCOME_TOPIC: usize = 0;
const BIG_TITLE_HEIGHT: u16 = 6;

fn render_content(frame: &mut Frame, app: &App, area: Rect) {
    let (_, md) = DOCS[app.ui.help_topic];

    let is_welcome = app.ui.help_topic == WELCOME_TOPIC;
    let md_area = if is_welcome {
        let [title_area, rest] =
            Layout::vertical([Constraint::Length(BIG_TITLE_HEIGHT), Constraint::Fill(1)])
                .areas(area);
        let big_title = BigText::builder()
            .pixel_size(PixelSize::Quadrant)
            .style(Style::new().cyan().bold())
            .lines(vec!["CAGIRE".into()])
            .centered()
            .build();
        let subtitle = Paragraph::new(RLine::from(Span::styled(
            "A Forth Sequencer",
            Style::new().fg(Color::White),
        )))
        .alignment(ratatui::layout::Alignment::Center);
        let [big_area, subtitle_area] =
            Layout::vertical([Constraint::Length(4), Constraint::Length(2)]).areas(title_area);
        frame.render_widget(big_title, big_area);
        frame.render_widget(subtitle, subtitle_area);
        rest
    } else {
        area
    };
    let query = &app.ui.help_search_query;
    let has_query = !query.is_empty();
    let query_lower = query.to_lowercase();

    let lines = parse_markdown(md);

    let has_search_bar = app.ui.help_search_active || has_query;
    let search_bar_height: u16 = u16::from(has_search_bar);
    let visible_height = md_area.height.saturating_sub(6 + search_bar_height) as usize;
    let max_scroll = lines.len().saturating_sub(visible_height);
    let scroll = app.ui.help_scroll().min(max_scroll);

    let visible: Vec<RLine> = lines
        .into_iter()
        .skip(scroll)
        .take(visible_height)
        .map(|line| {
            if has_query {
                highlight_line(line, &query_lower)
            } else {
                line
            }
        })
        .collect();

    let content_area = if has_search_bar {
        let [content, search] =
            Layout::vertical([Constraint::Fill(1), Constraint::Length(1)]).areas(md_area);
        render_search_bar(frame, app, search);
        content
    } else {
        md_area
    };

    let para = Paragraph::new(visible)
        .block(
            Block::default()
                .borders(Borders::ALL)
                .padding(Padding::new(2, 2, 2, 2)),
        )
        .wrap(Wrap { trim: false });
    frame.render_widget(para, content_area);
}

fn render_search_bar(frame: &mut Frame, app: &App, area: Rect) {
    let style = if app.ui.help_search_active {
        Style::new().fg(Color::Yellow)
    } else {
        Style::new().fg(Color::DarkGray)
    };
    let cursor = if app.ui.help_search_active { "█" } else { "" };
    let text = format!(" /{}{cursor}", app.ui.help_search_query);
    frame.render_widget(Paragraph::new(text).style(style), area);
}

fn highlight_line<'a>(line: RLine<'a>, query: &str) -> RLine<'a> {
    let mut result: Vec<Span<'a>> = Vec::new();
    for span in line.spans {
        let lower = span.content.to_lowercase();
        if !lower.contains(query) {
            result.push(span);
            continue;
        }
        let content = span.content.to_string();
        let base_style = span.style;
        let hl_style = base_style.bg(Color::Yellow).fg(Color::Black);
        let mut start = 0;
        let lower_bytes = lower.as_bytes();
        let query_bytes = query.as_bytes();
        while let Some(pos) = find_bytes(&lower_bytes[start..], query_bytes) {
            let abs = start + pos;
            if abs > start {
                result.push(Span::styled(content[start..abs].to_string(), base_style));
            }
            result.push(Span::styled(
                content[abs..abs + query.len()].to_string(),
                hl_style,
            ));
            start = abs + query.len();
        }
        if start < content.len() {
            result.push(Span::styled(content[start..].to_string(), base_style));
        }
    }
    RLine::from(result)
}

fn find_bytes(haystack: &[u8], needle: &[u8]) -> Option<usize> {
    haystack.windows(needle.len()).position(|w| w == needle)
}

/// Find first line matching query across all topics. Returns (topic_index, line_index).
pub fn find_match(query: &str) -> Option<(usize, usize)> {
    let query = query.to_lowercase();
    for (topic_idx, (_, content)) in DOCS.iter().enumerate() {
        for (line_idx, line) in content.lines().enumerate() {
            if line.to_lowercase().contains(&query) {
                return Some((topic_idx, line_idx));
            }
        }
    }
    None
}

fn code_border_style() -> Style {
    Style::new().fg(Color::Rgb(60, 60, 70))
}

fn preprocess_underscores(md: &str) -> String {
    let mut out = String::with_capacity(md.len());
    for line in md.lines() {
        let mut result = String::with_capacity(line.len());
        let mut chars = line.char_indices().peekable();
        let bytes = line.as_bytes();
        while let Some((i, c)) = chars.next() {
            if c == '`' {
                result.push(c);
                for (_, ch) in chars.by_ref() {
                    result.push(ch);
                    if ch == '`' {
                        break;
                    }
                }
                continue;
            }
            if c == '_' {
                let before_is_space = i == 0 || bytes[i - 1] == b' ';
                if before_is_space {
                    if let Some(end) = line[i + 1..].find('_') {
                        let inner = &line[i + 1..i + 1 + end];
                        if !inner.is_empty() {
                            result.push('*');
                            result.push_str(inner);
                            result.push('*');
                            for _ in 0..end {
                                chars.next();
                            }
                            chars.next(); // skip closing _
                            continue;
                        }
                    }
                }
            }
            result.push(c);
        }
        out.push_str(&result);
        out.push('\n');
    }
    out
}

fn parse_markdown(md: &str) -> Vec<RLine<'static>> {
    let processed = preprocess_underscores(md);
    let text = minimad::Text::from(processed.as_str());
    let mut lines = Vec::new();

    let mut code_line_nr: usize = 0;

    for line in text.lines {
        match line {
            Line::Normal(composite) if composite.style == CompositeStyle::Code => {
                code_line_nr += 1;
                let raw: String = composite.compounds.iter().map(|c: &minimad::Compound| c.src).collect();
                let mut spans = vec![
                    Span::styled(format!(" {code_line_nr:>2} "), code_border_style()),
                    Span::styled("│ ", code_border_style()),
                ];
                spans.extend(
                    highlight::highlight_line(&raw)
                        .into_iter()
                        .map(|(style, text)| Span::styled(text, style)),
                );
                lines.push(RLine::from(spans));
            }
            Line::Normal(composite) => {
                code_line_nr = 0;
                lines.push(composite_to_line(composite));
            }
            _ => {
                lines.push(RLine::from(""));
            }
        }
    }

    lines
}

fn composite_to_line(composite: Composite) -> RLine<'static> {
    let base_style = match composite.style {
        CompositeStyle::Header(1) => Style::new()
            .fg(Color::Cyan)
            .add_modifier(Modifier::BOLD | Modifier::UNDERLINED),
        CompositeStyle::Header(2) => Style::new().fg(Color::Yellow).add_modifier(Modifier::BOLD),
        CompositeStyle::Header(_) => Style::new().fg(Color::Magenta).add_modifier(Modifier::BOLD),
        CompositeStyle::ListItem(_) => Style::new().fg(Color::White),
        CompositeStyle::Quote => Style::new().fg(Color::Rgb(150, 150, 150)),
        CompositeStyle::Code => Style::new().fg(Color::Green),
        CompositeStyle::Paragraph => Style::new().fg(Color::White),
    };

    let prefix = match composite.style {
        CompositeStyle::ListItem(_) => "  • ",
        CompositeStyle::Quote => "  │ ",
        _ => "",
    };

    let mut spans: Vec<Span<'static>> = Vec::new();
    if !prefix.is_empty() {
        spans.push(Span::styled(prefix.to_string(), base_style));
    }

    for compound in composite.compounds {
        compound_to_spans(compound, base_style, &mut spans);
    }

    RLine::from(spans)
}

fn compound_to_spans(compound: Compound, base: Style, out: &mut Vec<Span<'static>>) {
    let mut style = base;

    if compound.bold {
        style = style.add_modifier(Modifier::BOLD);
    }
    if compound.italic {
        style = style.add_modifier(Modifier::ITALIC);
    }
    if compound.code {
        style = Style::new().fg(Color::Green);
    }
    if compound.strikeout {
        style = style.add_modifier(Modifier::CROSSED_OUT);
    }

    let src = compound.src.to_string();
    let link_style = Style::new()
        .fg(Color::Rgb(120, 200, 180))
        .add_modifier(Modifier::UNDERLINED);

    let mut rest = src.as_str();
    while let Some(start) = rest.find('[') {
        let after_bracket = &rest[start + 1..];
        if let Some(text_end) = after_bracket.find("](") {
            let url_start = start + 1 + text_end + 2;
            if let Some(url_end) = rest[url_start..].find(')') {
                if start > 0 {
                    out.push(Span::styled(rest[..start].to_string(), style));
                }
                let text = &rest[start + 1..start + 1 + text_end];
                let url = &rest[url_start..url_start + url_end];
                if text == url {
                    out.push(Span::styled(url.to_string(), link_style));
                } else {
                    out.push(Span::styled(text.to_string(), link_style));
                    out.push(Span::styled(
                        format!(" ({url})"),
                        Style::new().fg(Color::Rgb(100, 100, 100)),
                    ));
                }
                rest = &rest[url_start + url_end + 1..];
                continue;
            }
        }
        out.push(Span::styled(rest[..start + 1].to_string(), style));
        rest = &rest[start + 1..];
    }
    if !rest.is_empty() {
        out.push(Span::styled(rest.to_string(), style));
    }
}