//! The buffer widget — the only widget mojibake presents (MJB-HLR-019).
//!
//! Rendering is deliberately thin: it asks [`View::visible_lines`] for at most
//! `height` line slices and draws those. Nothing here walks the document, so
//! per-frame cost is O(viewport) however large the file (MJB-LLR-200).

use std::path::PathBuf;

use color_eyre::eyre::eyre;
use crossterm::event::KeyEvent;
use ratatui::{
    Frame,
    layout::{Constraint, Layout, Rect},
    style::{Style, Stylize},
    text::{Line, Span},
    widgets::Paragraph,
};
use tokio::sync::mpsc::UnboundedSender;

use super::Component;
use crate::{
    action::Action,
    buffer::{
        Buffer, LINE_TYPE, Outcome,
        grapheme::{display_column, grapheme_str, grapheme_width, next_grapheme_boundary},
    },
    config::{Config, Mode},
};

pub struct BufferComponent {
    buffer: Buffer,
    command_tx: Option<UnboundedSender<Action>>,
}

impl BufferComponent {
    pub fn new(config: Config, path: Option<PathBuf>) -> color_eyre::Result<Self> {
        let buffer = Buffer::new(config, path).map_err(|e| eyre!("{e}"))?;
        Ok(Self {
            buffer,
            command_tx: None,
        })
    }

    pub fn buffer(&self) -> &Buffer {
        &self.buffer
    }

    /// Width of the line-number gutter, sized to the largest line number.
    ///
    /// Counts digits arithmetically rather than formatting the number, since
    /// this runs every frame. `u16` cannot truncate here in practice — it would
    /// take more than 10^65000 lines — but the conversion is still checked
    /// rather than cast, and saturates to the terminal's own maximum width.
    fn gutter_width(&self) -> u16 {
        let lines = self.buffer.document.len_lines().max(1);
        let digits = lines.ilog10() as usize + 1;
        u16::try_from(digits + 1).unwrap_or(u16::MAX) // digits + one space of padding
    }

    /// MJB-LLR-202: mode, path, modified marker and cursor position.
    fn status_line(&self) -> String {
        let doc = &self.buffer.document;
        let name = doc
            .path()
            .map(|p| p.display().to_string())
            .unwrap_or_else(|| "[scratch]".to_owned());
        let modified = if doc.is_modified() { " [+]" } else { "" };

        let text = doc.slice();
        let cursor = doc.range().cursor(text);
        let line = text.byte_to_line_idx(cursor, LINE_TYPE);
        let line_start = text.line_to_byte_idx(line, LINE_TYPE);
        let col = display_column(text.line(line, LINE_TYPE), cursor - line_start);

        format!(
            " {}  {name}{modified}  {}:{} ",
            self.buffer.mode,
            line + 1,
            col + 1
        )
    }

    /// The bottom row: the command line while in command mode, otherwise any
    /// transient message, otherwise the pending key sequence.
    fn message_line(&self) -> String {
        if self.buffer.mode == Mode::Command {
            return format!(":{}", self.buffer.command_line);
        }
        if let Some(msg) = &self.buffer.status {
            return msg.clone();
        }
        if !self.buffer.pending_keys().is_empty() {
            let keys: String = self
                .buffer
                .pending_keys()
                .iter()
                .map(crate::config::key_event_to_string)
                .collect();
            return keys;
        }
        String::new()
    }
}

impl Component for BufferComponent {
    fn register_action_handler(&mut self, tx: UnboundedSender<Action>) -> color_eyre::Result<()> {
        self.command_tx = Some(tx);
        Ok(())
    }

    fn register_config_handler(&mut self, _config: Config) -> color_eyre::Result<()> {
        // The config is supplied at construction, because the document must be
        // opened with it; re-registering would discard buffer state.
        Ok(())
    }

    fn handle_key_event(&mut self, key: KeyEvent) -> color_eyre::Result<Option<Action>> {
        Ok(match self.buffer.handle_key(key) {
            Outcome::Consumed => None,
            Outcome::Quit => Some(Action::Quit),
            Outcome::Suspend => Some(Action::Suspend),
        })
    }

    fn draw(&mut self, frame: &mut Frame, area: Rect) -> color_eyre::Result<()> {
        // MJB-LLR-202: reserve the last two rows for status and message.
        let [text_area, status_area, message_area] = Layout::vertical([
            Constraint::Min(0),
            Constraint::Length(1),
            Constraint::Length(1),
        ])
        .areas(area);

        let gutter = self.gutter_width();
        let [gutter_area, content_area] =
            Layout::horizontal([Constraint::Length(gutter), Constraint::Min(0)]).areas(text_area);

        let height = content_area.height as usize;
        let width = content_area.width as usize;
        self.buffer.update_view(width, height);

        let mode = self.buffer.mode;
        let cfg = self.buffer.config();
        let cursor_style = cfg.style(mode, "cursor");
        let selection_style = cfg.style(mode, "selection");
        let linenr_style = cfg.style(mode, "linenr");
        let status_style = cfg.style(mode, "statusline");

        let doc = &self.buffer.document;
        let text = doc.slice();
        let range = doc.range();
        let cursor_byte = range.cursor(text);
        let sel_from = range.from();
        let sel_to = range.to();

        let (top_line, count) = self.buffer.view.visible_line_range(text, height);
        let h_offset = self.buffer.view.offset.horizontal_offset;

        let mut rows: Vec<Line> = Vec::with_capacity(count);
        let mut gutter_rows: Vec<Line> = Vec::with_capacity(count);

        // MJB-LLR-200: only the visible lines are touched.
        for (i, line) in self.buffer.view.visible_lines(text, height).enumerate() {
            let line_idx = top_line + i;
            let line_start = text.line_to_byte_idx(line_idx, LINE_TYPE);

            gutter_rows.push(Line::from(Span::styled(
                format!("{:>w$} ", line_idx + 1, w = (gutter as usize).saturating_sub(1)),
                linenr_style,
            )));

            // MJB-LLR-201: style the cursor and the selection span.
            let mut spans: Vec<Span> = Vec::new();
            let mut column = 0usize;
            let mut byte = 0usize;
            let line_len = line.len();

            while byte < line_len {
                let next = next_grapheme_boundary(line, byte);
                if next <= byte {
                    break;
                }
                // Borrows from the rope unless the grapheme straddles a chunk.
                let g = grapheme_str(line, byte..next);
                if matches!(g.as_ref(), "\n" | "\r\n" | "\r") {
                    break;
                }

                let abs = line_start + byte;
                let w = grapheme_width(&g, column);

                let style = if abs == cursor_byte {
                    cursor_style
                } else if abs >= sel_from && abs < sel_to {
                    selection_style
                } else {
                    Style::default()
                };

                // Horizontal scrolling: skip graphemes left of the offset.
                if column + w > h_offset {
                    // A tab is stored as one byte but occupies `w` columns.
                    let rendered = if g == "\t" {
                        " ".repeat(w)
                    } else {
                        g.into_owned()
                    };
                    spans.push(Span::styled(rendered, style));
                }

                column += w;
                byte = next;
            }

            // A cursor sitting past the last character (end of line, or an
            // empty line) still needs a visible block.
            if cursor_byte == line_start + byte && cursor_byte >= line_start {
                spans.push(Span::styled(" ", cursor_style));
            }

            rows.push(Line::from(spans));
        }

        frame.render_widget(Paragraph::new(gutter_rows), gutter_area);
        frame.render_widget(Paragraph::new(rows), content_area);
        frame.render_widget(
            Paragraph::new(Line::from(Span::styled(self.status_line(), status_style))),
            status_area,
        );
        frame.render_widget(
            Paragraph::new(Line::from(self.message_line())).dim(),
            message_area,
        );

        Ok(())
    }
}