reovim-kernel 0.14.4

//! Undo tree with branching support.
//!
//! Provides vim-style branching undo history where undoing and then
//! making new changes creates a new branch rather than losing history.

use {
    crate::mm::{Edit, Position},
    std::time::{Duration, Instant},
};

/// Origin of an edit for multi-client tracking (#471).
///
/// Tracks which client made an edit, enabling per-client undo where
/// each client only undoes their own changes.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
pub enum EditOrigin {
    /// Edit made by a specific client.
    ///
    /// The `usize` value corresponds to `ClientId::as_usize()`.
    Client(usize),
    /// System-generated edit (auto-format, LSP, macro playback, etc.).
    ///
    /// This is the default origin for backward compatibility.
    #[default]
    System,
}

/// A node in the undo tree.
///
/// Each node represents a set of edits made at a point in time,
/// along with cursor positions for proper restoration.
#[derive(Debug, Clone)]
pub struct UndoNode {
    /// Edits that were made (in order applied).
    edits: Vec<Edit>,
    /// Cursor position before these edits were applied.
    cursor_before: Position,
    /// Cursor position after these edits were applied.
    cursor_after: Position,
    /// When this node was created.
    timestamp: Instant,
    /// Parent node index (None for root).
    parent: Option<usize>,
    /// Child node indices (branches).
    children: Vec<usize>,
    /// Sequential change number.
    seq_num: u64,
    /// Origin of this edit (which client made it).
    origin: EditOrigin,
}

impl UndoNode {
    /// Get the edits in this node.
    #[must_use]
    pub fn edits(&self) -> &[Edit] {
        &self.edits
    }

    /// Get cursor position before edits.
    #[must_use]
    pub const fn cursor_before(&self) -> Position {
        self.cursor_before
    }

    /// Get cursor position after edits.
    #[must_use]
    pub const fn cursor_after(&self) -> Position {
        self.cursor_after
    }

    /// Get timestamp when this node was created.
    #[must_use]
    pub const fn timestamp(&self) -> Instant {
        self.timestamp
    }

    /// Get sequential change number.
    #[must_use]
    pub const fn seq_num(&self) -> u64 {
        self.seq_num
    }

    /// Check if this is the root node.
    #[must_use]
    pub const fn is_root(&self) -> bool {
        self.parent.is_none()
    }

    /// Get number of branches (children) from this node.
    #[must_use]
    pub const fn branch_count(&self) -> usize {
        self.children.len()
    }

    /// Get parent node index.
    #[must_use]
    pub const fn parent(&self) -> Option<usize> {
        self.parent
    }

    /// Get children node indices.
    #[must_use]
    pub fn children(&self) -> &[usize] {
        &self.children
    }

    /// Get the origin of this edit.
    ///
    /// Returns which client made this edit, or `EditOrigin::System` for
    /// system-generated edits.
    #[must_use]
    pub const fn origin(&self) -> EditOrigin {
        self.origin
    }
}

/// Result from an undo or redo operation.
///
/// Contains the edits to apply and the cursor position to restore.
#[derive(Debug, Clone)]
pub struct UndoResult {
    /// Edits to apply (already inverted for undo).
    pub edits: Vec<Edit>,
    /// Cursor position to restore.
    pub cursor: Position,
}

/// Undo tree with branching support.
///
/// Unlike a simple undo stack, an undo tree preserves all history.
/// When you undo and then make new changes, a new branch is created
/// rather than discarding the undone changes.
///
/// # Vim Comparison
///
/// This is similar to vim's `:undotree` feature. The `g-` and `g+`
/// commands traverse changes in time order (`seq_num`), while `u` and
/// `Ctrl-R` traverse the tree structure.
///
/// # Memory Management
///
/// The tree has a configurable maximum number of nodes. When exceeded,
/// the oldest nodes are pruned (but the path from root to current is
/// always preserved).
#[derive(Debug, Clone)]
pub struct UndoTree {
    /// All nodes in the tree.
    nodes: Vec<UndoNode>,
    /// Index of current position in the tree.
    current: usize,
    /// Sequential change counter.
    seq_counter: u64,
    /// Maximum number of nodes to retain.
    max_nodes: usize,
    /// Index of the preferred/active branch at each node.
    /// Used to remember which branch to follow on redo.
    active_branches: Vec<usize>,
}

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

impl UndoTree {
    /// Default maximum number of nodes.
    pub const DEFAULT_MAX_NODES: usize = 10000;

    /// Create a new undo tree.
    ///
    /// The tree starts with a root node representing the initial state.
    #[must_use]
    pub fn new() -> Self {
        Self::with_max_nodes(Self::DEFAULT_MAX_NODES)
    }

    /// Create a new undo tree with custom maximum nodes.
    #[must_use]
    pub fn with_max_nodes(max_nodes: usize) -> Self {
        let root = UndoNode {
            edits: Vec::new(),
            cursor_before: Position::default(),
            cursor_after: Position::default(),
            timestamp: Instant::now(),
            parent: None,
            children: Vec::new(),
            seq_num: 0,
            origin: EditOrigin::System,
        };

        Self {
            nodes: vec![root],
            current: 0,
            seq_counter: 0,
            max_nodes: max_nodes.max(1), // At least 1 node
            active_branches: vec![0],
        }
    }

    /// Push a new change onto the tree with system origin.
    ///
    /// Creates a new node as a child of the current node.
    /// If the current node already has children (we're in the middle
    /// of the tree after an undo), this creates a new branch.
    ///
    /// This is equivalent to `push_with_origin(edits, cursor_before, cursor_after, EditOrigin::System)`.
    pub fn push(&mut self, edits: Vec<Edit>, cursor_before: Position, cursor_after: Position) {
        self.push_with_origin(edits, cursor_before, cursor_after, EditOrigin::System);
    }

    /// Push a new change onto the tree with specified origin.
    ///
    /// Creates a new node as a child of the current node, tagged with the
    /// specified origin. This enables per-client undo where each client
    /// can undo only their own changes.
    ///
    /// If the current node already has children (we're in the middle
    /// of the tree after an undo), this creates a new branch.
    pub fn push_with_origin(
        &mut self,
        edits: Vec<Edit>,
        cursor_before: Position,
        cursor_after: Position,
        origin: EditOrigin,
    ) {
        if edits.is_empty() {
            return;
        }

        self.seq_counter += 1;

        let new_node = UndoNode {
            edits,
            cursor_before,
            cursor_after,
            timestamp: Instant::now(),
            parent: Some(self.current),
            children: Vec::new(),
            seq_num: self.seq_counter,
            origin,
        };

        let new_idx = self.nodes.len();
        self.nodes.push(new_node);

        // Add as child of current node
        self.nodes[self.current].children.push(new_idx);

        // Update active branch for current node
        let branch_idx = self.nodes[self.current].children.len() - 1;
        // Extend active_branches if needed
        while self.active_branches.len() <= self.current {
            self.active_branches.push(0);
        }
        self.active_branches[self.current] = branch_idx;

        // Ensure active_branches has entry for new node
        while self.active_branches.len() <= new_idx {
            self.active_branches.push(0);
        }

        // Move to new node
        self.current = new_idx;

        // Prune if over limit
        self.prune_if_needed();
    }

    /// Undo the current change.
    ///
    /// Moves to the parent node and returns the inverse edits
    /// along with the cursor position to restore.
    ///
    /// Returns `None` if at the root (nothing to undo).
    pub fn undo(&mut self) -> Option<UndoResult> {
        let current_node = &self.nodes[self.current];

        // Can't undo past root
        let parent_idx = current_node.parent?;

        // Get inverse edits and cursor to restore
        let result = UndoResult {
            edits: current_node.edits.iter().rev().map(Edit::inverse).collect(),
            cursor: current_node.cursor_before,
        };

        // Move to parent
        self.current = parent_idx;

        Some(result)
    }

    /// Redo the last undone change.
    ///
    /// Moves to the first child node (or the active branch) and
    /// returns the edits along with cursor position to restore.
    ///
    /// Returns `None` if there's nothing to redo.
    pub fn redo(&mut self) -> Option<UndoResult> {
        let current_node = &self.nodes[self.current];

        if current_node.children.is_empty() {
            return None;
        }

        // Get active branch index
        let branch_idx = self
            .active_branches
            .get(self.current)
            .copied()
            .unwrap_or(0)
            .min(current_node.children.len() - 1);

        let child_idx = current_node.children[branch_idx];
        let child_node = &self.nodes[child_idx];

        let result = UndoResult {
            edits: child_node.edits.clone(),
            cursor: child_node.cursor_after,
        };

        // Move to child
        self.current = child_idx;

        Some(result)
    }

    /// Redo a specific branch.
    ///
    /// Like `redo()` but follows a specific branch index.
    #[cfg_attr(coverage_nightly, coverage(off))]
    pub fn redo_branch(&mut self, branch_idx: usize) -> Option<UndoResult> {
        let current_node = &self.nodes[self.current];

        if branch_idx >= current_node.children.len() {
            return None;
        }

        // Update active branch
        if self.current < self.active_branches.len() {
            self.active_branches[self.current] = branch_idx;
        }

        let child_idx = current_node.children[branch_idx];
        let child_node = &self.nodes[child_idx];

        let result = UndoResult {
            edits: child_node.edits.clone(),
            cursor: child_node.cursor_after,
        };

        self.current = child_idx;

        Some(result)
    }

    /// Get the available branches (children) from current node.
    ///
    /// Returns indices that can be passed to `redo_branch()`.
    #[must_use]
    pub fn branches(&self) -> &[usize] {
        &self.nodes[self.current].children
    }

    /// Switch the active branch at current node.
    ///
    /// This affects which branch `redo()` will follow.
    /// Returns `false` if the branch index is invalid.
    pub fn switch_branch(&mut self, branch_idx: usize) -> bool {
        let current_node = &self.nodes[self.current];

        if branch_idx >= current_node.children.len() {
            return false;
        }

        while self.active_branches.len() <= self.current {
            self.active_branches.push(0);
        }
        self.active_branches[self.current] = branch_idx;

        true
    }

    /// Check if undo is possible.
    #[must_use]
    pub fn can_undo(&self) -> bool {
        self.nodes[self.current].parent.is_some()
    }

    /// Check if redo is possible.
    #[must_use]
    pub fn can_redo(&self) -> bool {
        !self.nodes[self.current].children.is_empty()
    }

    /// Get the current node.
    #[must_use]
    pub fn current_node(&self) -> &UndoNode {
        &self.nodes[self.current]
    }

    /// Get the current node index.
    #[must_use]
    pub const fn current_index(&self) -> usize {
        self.current
    }

    /// Get total number of nodes in the tree.
    #[must_use]
    pub const fn node_count(&self) -> usize {
        self.nodes.len()
    }

    /// Get node by index.
    ///
    /// Returns `None` if index is out of bounds.
    #[must_use]
    pub fn node(&self, index: usize) -> Option<&UndoNode> {
        self.nodes.get(index)
    }

    /// Get all node indices for iteration.
    ///
    /// Returns a range that can be used to iterate over all valid node indices.
    #[must_use]
    pub const fn node_indices(&self) -> std::ops::Range<usize> {
        0..self.nodes.len()
    }

    /// Get active branch index at a node.
    ///
    /// Returns the preferred child index that `redo()` would follow
    /// from the specified node. Returns `None` if the node doesn't exist
    /// or has no active branch set.
    #[must_use]
    pub fn active_branch_at(&self, node_index: usize) -> Option<usize> {
        self.active_branches.get(node_index).copied()
    }

    /// Get the maximum nodes limit.
    #[must_use]
    pub const fn max_nodes(&self) -> usize {
        self.max_nodes
    }

    /// Set the maximum nodes limit.
    pub fn set_max_nodes(&mut self, max: usize) {
        self.max_nodes = max.max(1);
        self.prune_if_needed();
    }

    /// Get the current sequential change number.
    #[must_use]
    pub const fn seq_counter(&self) -> u64 {
        self.seq_counter
    }

    /// Collect all edits applied between a node and the current head.
    ///
    /// Walks from the current position back to `from_idx` via parent links,
    /// collecting all edits from intermediate nodes in forward (application)
    /// order. The edits from the `from_idx` node itself are NOT included.
    ///
    /// Returns `Some(empty_vec)` if `from_idx` is the current position.
    /// Returns `None` if `from_idx` is not an ancestor of the current position
    /// or is out of bounds.
    #[must_use]
    pub fn edits_since(&self, from_idx: usize) -> Option<Vec<&Edit>> {
        if from_idx >= self.nodes.len() {
            return None;
        }

        if from_idx == self.current {
            return Some(Vec::new());
        }

        // Walk from current back to from_idx via parent links.
        let mut path_indices = Vec::new();
        let mut idx = self.current;

        loop {
            if idx == from_idx {
                break;
            }
            path_indices.push(idx);
            match self.nodes[idx].parent {
                Some(parent) => idx = parent,
                None => return None, // Reached root without finding from_idx
            }
        }

        // path_indices is in reverse order (current -> ... -> from_idx+1).
        // Reverse to get forward order (from_idx+1 -> ... -> current).
        path_indices.reverse();

        // Collect non-empty edits from each node in forward order.
        let mut edits = Vec::new();
        for node_idx in path_indices {
            for edit in &self.nodes[node_idx].edits {
                if !edit.is_empty() {
                    edits.push(edit);
                }
            }
        }

        Some(edits)
    }

    /// Prune old nodes if over the limit.
    ///
    /// This is a simple pruning strategy that removes the oldest
    /// nodes that are not on the path from root to current.
    #[cfg_attr(coverage_nightly, coverage(off))]
    fn prune_if_needed(&mut self) {
        if self.nodes.len() <= self.max_nodes {
            return;
        }

        // Build set of nodes on path from root to current
        let mut protected = vec![false; self.nodes.len()];
        let mut idx = self.current;
        loop {
            protected[idx] = true;
            match self.nodes[idx].parent {
                Some(parent) => idx = parent,
                None => break,
            }
        }

        // Find nodes to remove (oldest first, not protected)
        let to_remove = self.nodes.len() - self.max_nodes;
        let mut removed = 0;
        let mut remove_indices = Vec::new();

        for (i, node) in self.nodes.iter().enumerate() {
            if removed >= to_remove {
                break;
            }
            if !protected[i] && node.children.is_empty() {
                remove_indices.push(i);
                removed += 1;
            }
        }

        // Actually remove nodes (in reverse order to maintain indices)
        for &idx in remove_indices.iter().rev() {
            self.remove_node(idx);
        }
    }

    /// Remove a node from the tree.
    ///
    /// Updates parent's children list and adjusts indices.
    #[cfg_attr(coverage_nightly, coverage(off))]
    fn remove_node(&mut self, idx: usize) {
        if idx == 0 || idx == self.current {
            return; // Never remove root or current
        }

        // Remove from parent's children
        if let Some(parent_idx) = self.nodes[idx].parent {
            self.nodes[parent_idx].children.retain(|&c| c != idx);
        }

        // Update indices in all nodes
        for node in &mut self.nodes {
            if let Some(ref mut parent) = node.parent
                && *parent > idx
            {
                *parent -= 1;
            }
            for child in &mut node.children {
                if *child > idx {
                    *child -= 1;
                }
            }
        }

        // Update current if needed
        if self.current > idx {
            self.current -= 1;
        }

        // Update active_branches
        for branch in &mut self.active_branches {
            if *branch > idx {
                *branch -= 1;
            }
        }

        // Remove the node
        self.nodes.remove(idx);

        // Trim active_branches
        while self.active_branches.len() > self.nodes.len() {
            self.active_branches.pop();
        }
    }

    /// Clear all history except root.
    pub fn clear(&mut self) {
        let root_cursor = self.nodes[0].cursor_after;
        *self = Self::with_max_nodes(self.max_nodes);
        self.nodes[0].cursor_after = root_cursor;
    }

    /// Reconstruct an `UndoTree` from serialized data.
    ///
    /// This is used by the persistence layer to restore undo history from disk.
    /// Timestamps are reconstructed relative to "now" using the provided durations.
    ///
    /// # Arguments
    ///
    /// * `nodes_data` - Vector of tuples containing node data:
    ///   - `Vec<Edit>`: The edits in this node
    ///   - `Position`: Cursor before edits
    ///   - `Position`: Cursor after edits
    ///   - `Duration`: Relative time from tree creation
    ///   - `Option<usize>`: Parent node index
    ///   - `Vec<usize>`: Child node indices
    ///   - `u64`: Sequential change number
    ///   - `EditOrigin`: Origin of this edit
    /// * `current` - Index of current position in tree
    /// * `seq_counter` - Current sequential change counter
    /// * `max_nodes` - Maximum nodes to retain
    /// * `active_branches` - Preferred branch at each node
    ///
    /// # Panics
    ///
    /// Panics if `nodes_data` is empty (tree must have at least a root node).
    #[must_use]
    #[allow(clippy::type_complexity)] // Complex tuple is intentional for persistence API
    pub fn from_serializable(
        nodes_data: Vec<(
            Vec<Edit>,     // edits
            Position,      // cursor_before
            Position,      // cursor_after
            Duration,      // relative_time from root
            Option<usize>, // parent
            Vec<usize>,    // children
            u64,           // seq_num
            EditOrigin,    // origin
        )>,
        current: usize,
        seq_counter: u64,
        max_nodes: usize,
        active_branches: Vec<usize>,
    ) -> Self {
        assert!(!nodes_data.is_empty(), "UndoTree must have at least a root node");

        let now = Instant::now();

        let nodes: Vec<UndoNode> = nodes_data
            .into_iter()
            .map(
                |(
                    edits,
                    cursor_before,
                    cursor_after,
                    relative_time,
                    parent,
                    children,
                    seq_num,
                    origin,
                )| {
                    UndoNode {
                        edits,
                        cursor_before,
                        cursor_after,
                        // Reconstruct timestamp: now + relative offset
                        // Note: For a tree loaded at time T, all nodes have timestamps
                        // relative to T. The ordering is preserved.
                        timestamp: now + relative_time,
                        parent,
                        children,
                        seq_num,
                        origin,
                    }
                },
            )
            .collect();

        Self {
            nodes,
            current,
            seq_counter,
            max_nodes: max_nodes.max(1), // At least 1 node
            active_branches,
        }
    }
}