//! The buffer core: document model, motions, viewport, and input handling.
//!
//! Deliberately free of `ratatui` so it can be exercised by requirements-based
//! tests without a terminal; the rendering half lives in
//! [`crate::components::buffer`].
//!
//! Developed to DO-178C DAL-C. Items implementing a low-level requirement carry
//! a `MJB-LLR-nnn` comment; see `docs/requirements/llr.md`.

pub mod command;
pub mod document;
pub mod encoding;
pub mod grapheme;
pub mod history;
pub mod keymap;
pub mod line_ending;
pub mod movement;
pub mod save;
pub mod selection;
pub mod transaction;
pub mod view;

use std::path::PathBuf;

use crossterm::event::KeyEvent;
use ropey::{LineType, RopeSlice};

use self::{
    command::Command,
    document::{Document, DocumentError},
    keymap::{Keymap, KeymapResult, self_insert_char},
    movement::{WordTarget, word_move},
    selection::{Range, Selection},
    transaction::Transaction,
    view::View,
};
use crate::config::{Config, Mode};

/// The line-break convention. `LF_CR` is what ropey enables by default, and
/// recognises LF, CR and CRLF — matching the endings [`line_ending`] detects.
pub const LINE_TYPE: LineType = LineType::LF_CR;

/// What the caller should do after a key was handled.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Outcome {
    /// Handled internally; nothing for the application to do.
    Consumed,
    /// The user asked to quit.
    Quit,
    /// The user asked to suspend.
    Suspend,
}

/// The editor state: one document, one viewport, one mode.
pub struct Buffer {
    pub document: Document,
    pub view: View,
    pub mode: Mode,
    keymap: Keymap,
    config: Config,
    /// Command-line contents while in [`Mode::Command`].
    pub command_line: String,
    /// Transient message shown on the status line.
    pub status: Option<String>,
    /// Viewport height last rendered, needed by paging commands.
    pub last_height: usize,
    pub last_width: usize,
}

impl Buffer {
    pub fn new(config: Config, path: Option<PathBuf>) -> Result<Self, DocumentError> {
        // MJB-LLR-111, MJB-LLR-112
        let document = match path {
            Some(p) => Document::open(&p)?,
            None => Document::empty(None),
        };
        let keymap = Keymap::new(&config.keybindings);

        Ok(Self {
            document,
            view: View::new(),
            mode: Mode::Normal,
            keymap,
            config,
            command_line: String::new(),
            status: None,
            last_height: 0,
            last_width: 0,
        })
    }

    pub fn config(&self) -> &Config {
        &self.config
    }

    /// Pending keys, for the status line.
    pub fn pending_keys(&self) -> &[KeyEvent] {
        self.keymap.pending()
    }

    /// Route one key according to the current mode.
    pub fn handle_key(&mut self, key: KeyEvent) -> Outcome {
        self.status = None;

        // MJB-LLR-158: command mode is a line editor, not a keymap consumer.
        // Its three bindings still resolve so Esc/Enter/Backspace stay
        // configurable, but anything else types into the line.
        if self.mode == Mode::Command {
            return self.handle_command_mode_key(key);
        }

        match self.keymap.resolve(self.mode, key) {
            KeymapResult::Pending => Outcome::Consumed,
            KeymapResult::Matched(cmd, count) => self.execute(cmd, count.unwrap_or(1)),
            KeymapResult::Cancelled(keys) => {
                // MJB-LLR-156: insert mode types the character; every other
                // mode discards it.
                if self.mode == Mode::Insert
                    && let Some(c) = self_insert_char(&keys)
                {
                    self.insert_char(c);
                }
                Outcome::Consumed
            }
        }
    }

    fn handle_command_mode_key(&mut self, key: KeyEvent) -> Outcome {
        use crossterm::event::{KeyCode, KeyModifiers};

        match self.keymap.resolve(self.mode, key) {
            KeymapResult::Matched(Command::NormalMode, _) => {
                self.command_line.clear();
                self.set_mode(Mode::Normal);
                Outcome::Consumed
            }
            KeymapResult::Matched(Command::CommandSubmit, _) => {
                let line = std::mem::take(&mut self.command_line);
                self.set_mode(Mode::Normal);
                self.run_command_line(&line)
            }
            KeymapResult::Matched(Command::CommandBackspace, _) => {
                if self.command_line.pop().is_none() {
                    // Backspacing an empty line leaves command mode, as Helix does.
                    self.set_mode(Mode::Normal);
                }
                Outcome::Consumed
            }
            _ => {
                if let KeyCode::Char(c) = key.code
                    && !key.modifiers.contains(KeyModifiers::CONTROL)
                    && !key.modifiers.contains(KeyModifiers::ALT)
                {
                    self.command_line.push(c);
                }
                Outcome::Consumed
            }
        }
    }

    /// MJB-LLR-159, MJB-LLR-160: parse and run a command line.
    pub fn run_command_line(&mut self, line: &str) -> Outcome {
        let line = line.trim();
        let (name, _arg) = match line.split_once(char::is_whitespace) {
            Some((n, a)) => (n, Some(a.trim())),
            None => (line, None),
        };

        let insert_final_newline = self.config.editor.insert_final_newline;

        match name {
            "" => Outcome::Consumed,

            // MJB-LLR-159
            "w" | "write" => {
                self.save(false, insert_final_newline);
                Outcome::Consumed
            }
            "w!" | "write!" => {
                self.save(true, insert_final_newline);
                Outcome::Consumed
            }

            // MJB-LLR-160
            "q" | "quit" => {
                if self.document.is_modified() {
                    self.status =
                        Some("unsaved changes (use :q! to discard, :wq to save)".to_owned());
                    Outcome::Consumed
                } else {
                    Outcome::Quit
                }
            }
            "q!" | "quit!" => Outcome::Quit,

            "wq" | "x" | "write-quit" => {
                if self.save(false, insert_final_newline) {
                    Outcome::Quit
                } else {
                    Outcome::Consumed
                }
            }

            other => {
                // MJB-LLR-159: report, do not terminate.
                self.status = Some(format!("unknown command: {other}"));
                Outcome::Consumed
            }
        }
    }

    /// Returns whether the write succeeded.
    fn save(&mut self, force: bool, insert_final_newline: bool) -> bool {
        match self.document.save(force, insert_final_newline) {
            Ok(()) => {
                let name = self
                    .document
                    .path()
                    .map(|p| p.display().to_string())
                    .unwrap_or_else(|| "[no name]".to_owned());
                self.status = Some(format!("wrote {name}"));
                true
            }
            Err(e) => {
                self.status = Some(e.to_string());
                false
            }
        }
    }

    fn set_mode(&mut self, mode: Mode) {
        if self.mode != mode {
            self.mode = mode;
            // A half-typed sequence must not survive a mode change.
            self.keymap.reset();
        }
    }

    fn insert_char(&mut self, c: char) {
        let mut s = [0u8; 4];
        self.insert_text(c.encode_utf8(&mut s));
    }

    fn insert_text(&mut self, text: &str) {
        let t = Transaction::insert(self.document.text(), self.document.selection(), text);
        let at = self.document.range().cursor(self.document.slice());
        if self.apply(&t) {
            // Cursor advances past what was inserted.
            self.document.set_range(Range::point(at + text.len()));
        }
    }

    fn apply(&mut self, t: &Transaction) -> bool {
        match self.document.apply(t) {
            Ok(()) => true,
            Err(e) => {
                self.status = Some(e.to_string());
                false
            }
        }
    }

    /// Execute one command. `count` is at least 1.
    pub fn execute(&mut self, cmd: Command, count: usize) -> Outcome {
        use Command::*;

        let count = count.max(1);
        let extend = self.mode == Mode::Select;

        match cmd {
            Quit => return Outcome::Quit,
            Suspend => return Outcome::Suspend,

            // --- Modes ---
            NormalMode => self.set_mode(Mode::Normal),
            InsertMode => {
                // MJB-LLR-009: `i` inserts before the selection.
                let from = self.document.range().from();
                self.document.set_range(Range::point(from));
                self.set_mode(Mode::Insert);
            }
            SelectMode => {
                self.set_mode(if self.mode == Mode::Select {
                    Mode::Normal
                } else {
                    Mode::Select
                });
            }
            CommandMode => {
                self.command_line.clear();
                self.set_mode(Mode::Command);
            }

            // --- Motion (MJB-HLR-006) ---
            MoveCharLeft => self.motion(extend, |t, r| movement::move_char_left(t, r, count)),
            MoveCharRight => self.motion(extend, |t, r| movement::move_char_right(t, r, count)),
            MoveLineUp => self.motion(extend, |t, r| movement::move_vertically(t, r, count, false)),
            MoveLineDown => self.motion(extend, |t, r| movement::move_vertically(t, r, count, true)),
            ExtendCharLeft => self.motion(true, |t, r| movement::move_char_left(t, r, count)),
            ExtendCharRight => self.motion(true, |t, r| movement::move_char_right(t, r, count)),
            ExtendLineUp => self.motion(true, |t, r| movement::move_vertically(t, r, count, false)),
            ExtendLineDown => self.motion(true, |t, r| movement::move_vertically(t, r, count, true)),

            // --- Word motion; these leave selections (MJB-HLR-007) ---
            MoveNextWordStart => self.word(count, WordTarget::NextStart, false),
            MovePrevWordStart => self.word(count, WordTarget::PrevStart, false),
            MoveNextWordEnd => self.word(count, WordTarget::NextEnd, false),
            MoveNextLongWordStart => self.word(count, WordTarget::NextStart, true),
            MovePrevLongWordStart => self.word(count, WordTarget::PrevStart, true),
            MoveNextLongWordEnd => self.word(count, WordTarget::NextEnd, true),
            ExtendNextWordStart => self.word(count, WordTarget::NextStart, false),
            ExtendPrevWordStart => self.word(count, WordTarget::PrevStart, false),
            ExtendNextWordEnd => self.word(count, WordTarget::NextEnd, false),

            // --- Goto (MJB-HLR-008) ---
            GotoFileStart => {
                let r = movement::goto_file_start(self.document.slice());
                self.put(r, extend);
            }
            GotoLastLine => {
                let r = movement::goto_last_line(self.document.slice());
                self.put(r, extend);
            }
            GotoLineStart => {
                let r = movement::goto_line_start(self.document.slice(), self.document.range());
                self.put(r, extend);
            }
            GotoLineEnd => {
                let r = movement::goto_line_end(self.document.slice(), self.document.range());
                self.put(r, extend);
            }
            GotoFirstNonWhitespace => {
                let r =
                    movement::goto_first_non_whitespace(self.document.slice(), self.document.range());
                self.put(r, extend);
            }

            // --- Selection manipulation ---
            ExtendLineBelow => self.extend_line_below(count),
            CollapseSelection => {
                let cursor = self.document.range().cursor(self.document.slice());
                self.document.set_range(Range::point(cursor));
            }
            FlipSelections => {
                let r = self.document.range().flipped();
                self.document.set_range(r);
            }
            SelectAll => {
                let len = self.document.text().len();
                self.document.set_range(Range::new(0, len));
            }

            // --- Entering insert mode (MJB-HLR-009) ---
            AppendMode => {
                // `a` inserts after the selection. In Helix a bare cursor is a
                // one-grapheme range, so appending lands *past* the grapheme
                // under it; our empty range has to step forward explicitly to
                // reproduce that.
                let text = self.document.slice();
                let r = self.document.range();
                let at = if r.is_empty() {
                    grapheme::next_grapheme_boundary(text, r.cursor(text))
                } else {
                    r.to()
                };
                self.document.set_range(Range::point(at));
                self.set_mode(Mode::Insert);
            }
            InsertAtLineStart => {
                let r =
                    movement::goto_first_non_whitespace(self.document.slice(), self.document.range());
                self.document.set_range(Range::point(r.head));
                self.set_mode(Mode::Insert);
            }
            InsertAtLineEnd => {
                let r = movement::goto_line_end(self.document.slice(), self.document.range());
                self.document.set_range(Range::point(r.head));
                self.set_mode(Mode::Insert);
            }
            OpenBelow => self.open_line(false),
            OpenAbove => self.open_line(true),

            // --- Modification (MJB-HLR-010) ---
            DeleteSelection => self.delete_selection(),
            ChangeSelection => {
                self.delete_selection();
                self.set_mode(Mode::Insert);
            }
            InsertNewline => self.insert_text("\n"),
            InsertTab => self.insert_text("\t"),
            DeleteCharBackward => self.delete_char_backward(),
            DeleteCharForward => self.delete_char_forward(),
            DeleteWordBackward => self.delete_word_backward(),
            KillToLineStart => self.kill_to_line_start(),

            // --- Undo / redo (MJB-HLR-011) ---
            Undo => match self.document.undo() {
                Ok(false) => self.status = Some("already at oldest change".to_owned()),
                Ok(true) => {}
                Err(e) => self.status = Some(e.to_string()),
            },
            Redo => match self.document.redo() {
                Ok(false) => self.status = Some("already at newest change".to_owned()),
                Ok(true) => {}
                Err(e) => self.status = Some(e.to_string()),
            },

            // --- Paging (MJB-HLR-013) ---
            PageCursorHalfUp => self.page(self.last_height / 2, false),
            PageCursorHalfDown => self.page(self.last_height / 2, true),
            PageUp => self.page(self.last_height, false),
            PageDown => self.page(self.last_height, true),

            // Handled by handle_command_mode_key; unreachable elsewhere but
            // must not panic if a user binds them outside command mode.
            CommandSubmit | CommandBackspace => {}
        }

        Outcome::Consumed
    }

    // --- helpers ---

    /// Run a motion and install its result, extending the selection or
    /// collapsing to a point per `extend`.
    ///
    /// Takes a closure rather than a function pointer so the direction and
    /// count stay visible at the call site, instead of hiding behind a family
    /// of near-identical adapter functions.
    fn motion(&mut self, extend: bool, f: impl FnOnce(RopeSlice, Range) -> Range) {
        let text = self.document.slice();
        let r = f(text, self.document.range());
        self.put(r, extend);
    }

    fn put(&mut self, target: Range, extend: bool) {
        let text = self.document.slice();
        let current = self.document.range();
        let r = current.put_cursor(text, target.cursor(text), extend);
        self.document.set_range(r);
    }

    /// MJB-LLR-065..067: word motions install the returned range directly,
    /// because the range *is* the result — collapsing it would destroy the
    /// selection-first behaviour that makes `wd` work.
    fn word(&mut self, count: usize, target: WordTarget, long: bool) {
        let text = self.document.slice();
        let r = word_move(text, self.document.range(), count, target, long);
        self.document.set_range(r);
    }

    /// Helix's `x`: select the current line; repeated, extend by one more.
    fn extend_line_below(&mut self, count: usize) {
        let text = self.document.slice();
        let r = self.document.range();
        let (start_line, end_line) = r.line_range(text);

        let already_whole_line = r.from() == text.line_to_byte_idx(start_line, LINE_TYPE)
            && r.to() == line_start_of_next(text, end_line);

        let (first, last) = if already_whole_line {
            (start_line, (end_line + count).min(last_line_index(text)))
        } else {
            (start_line, (end_line + count - 1).min(last_line_index(text)))
        };

        let from = text.line_to_byte_idx(first, LINE_TYPE);
        let to = line_start_of_next(text, last);
        self.document.set_range(Range::new(from, to));
    }

    fn open_line(&mut self, above: bool) {
        let text = self.document.slice();
        let line = self.document.range().cursor_line(text);

        // `above` inserts the terminator at the line's start, so the blank line
        // appears *at* that offset. `below` inserts it after the line's content
        // — deliberately at the content end rather than at the next line's
        // start, because a final line with no trailing newline has no next line
        // to anchor to, and the blank line then lands one byte later.
        let (at, cursor) = if above {
            let start = text.line_to_byte_idx(line, LINE_TYPE);
            (start, start)
        } else {
            let eol = movement::line_end_byte(text, line);
            (eol, eol + 1)
        };

        let t = Transaction::change(self.document.text(), [(at, at, Some("\n".to_owned()))]);
        if self.apply(&t) {
            self.document.set_range(Range::point(cursor));
            self.set_mode(Mode::Insert);
        }
    }

    fn delete_selection(&mut self) {
        let r = self.document.range();
        if r.is_empty() {
            // MJB-LLR-050 robustness: `d` with nothing selected deletes the
            // grapheme under the cursor rather than doing nothing.
            let text = self.document.slice();
            let to = grapheme::next_grapheme_boundary(text, r.cursor(text));
            if to == r.from() {
                return;
            }
            let t = Transaction::change(self.document.text(), [(r.from(), to, None)]);
            let from = r.from();
            if self.apply(&t) {
                self.document.set_range(Range::point(from));
            }
            return;
        }

        let from = r.from();
        let t = Transaction::delete(self.document.text(), self.document.selection());
        if self.apply(&t) {
            self.document.set_range(Range::point(from));
        }
    }

    fn delete_char_backward(&mut self) {
        let text = self.document.slice();
        let cursor = self.document.range().cursor(text);
        let from = grapheme::prev_grapheme_boundary(text, cursor);
        if from == cursor {
            return; // at the start of the buffer
        }
        let t = Transaction::change(self.document.text(), [(from, cursor, None)]);
        if self.apply(&t) {
            self.document.set_range(Range::point(from));
        }
    }

    fn delete_char_forward(&mut self) {
        let text = self.document.slice();
        let cursor = self.document.range().cursor(text);
        let to = grapheme::next_grapheme_boundary(text, cursor);
        if to == cursor {
            return; // at the end of the buffer
        }
        let t = Transaction::change(self.document.text(), [(cursor, to, None)]);
        if self.apply(&t) {
            self.document.set_range(Range::point(cursor));
        }
    }

    fn delete_word_backward(&mut self) {
        let text = self.document.slice();
        let cursor = self.document.range().cursor(text);
        if cursor == 0 {
            return;
        }
        let target = word_move(text, Range::point(cursor), 1, WordTarget::PrevStart, false);
        let from = target.from();
        if from >= cursor {
            return;
        }
        let t = Transaction::change(self.document.text(), [(from, cursor, None)]);
        if self.apply(&t) {
            self.document.set_range(Range::point(from));
        }
    }

    fn kill_to_line_start(&mut self) {
        let text = self.document.slice();
        let cursor = self.document.range().cursor(text);
        let line = text.byte_to_line_idx(cursor, LINE_TYPE);
        let from = text.line_to_byte_idx(line, LINE_TYPE);
        if from >= cursor {
            return;
        }
        let t = Transaction::change(self.document.text(), [(from, cursor, None)]);
        if self.apply(&t) {
            self.document.set_range(Range::point(from));
        }
    }

    fn page(&mut self, lines: usize, down: bool) {
        if lines == 0 {
            return;
        }
        let text = self.document.slice();
        let r = self.view.page(text, self.document.range(), lines, down);
        self.document.set_range(r);
    }

    /// Re-anchor the viewport for a viewport of `height` rows.
    pub fn update_view(&mut self, width: usize, height: usize) {
        self.last_width = width;
        self.last_height = height;
        let text = self.document.slice();
        let range = self.document.range();
        self.view
            .ensure_cursor_in_view(text, range, height, self.config.editor.scrolloff);
        self.view.ensure_horizontal_in_view(text, range, width);
    }

    /// Replace the whole selection state — used by tests and by `%`.
    pub fn set_selection(&mut self, selection: Selection) {
        self.document.set_selection(selection);
    }
}

fn line_start_of_next(text: RopeSlice, line: usize) -> usize {
    let total = text.len_lines(LINE_TYPE);
    if line + 1 < total {
        text.line_to_byte_idx(line + 1, LINE_TYPE)
    } else {
        text.len()
    }
}

fn last_line_index(text: RopeSlice) -> usize {
    text.len_lines(LINE_TYPE).saturating_sub(1)
}