oxilean-kernel 0.1.2

//! Infrastructure data structures for kernel utilities.

use std::collections::{HashMap, HashSet, VecDeque};

/// A generic counter that tracks min/max/sum for statistical summaries.
#[allow(dead_code)]
pub struct StatSummary {
    count: u64,
    sum: f64,
    min: f64,
    max: f64,
}

#[allow(dead_code)]
impl StatSummary {
    /// Creates an empty summary.
    pub fn new() -> Self {
        Self {
            count: 0,
            sum: 0.0,
            min: f64::INFINITY,
            max: f64::NEG_INFINITY,
        }
    }

    /// Records a sample.
    pub fn record(&mut self, val: f64) {
        self.count += 1;
        self.sum += val;
        if val < self.min {
            self.min = val;
        }
        if val > self.max {
            self.max = val;
        }
    }

    /// Returns the mean, or `None` if no samples.
    pub fn mean(&self) -> Option<f64> {
        if self.count == 0 {
            None
        } else {
            Some(self.sum / self.count as f64)
        }
    }

    /// Returns the minimum, or `None` if no samples.
    pub fn min(&self) -> Option<f64> {
        if self.count == 0 {
            None
        } else {
            Some(self.min)
        }
    }

    /// Returns the maximum, or `None` if no samples.
    pub fn max(&self) -> Option<f64> {
        if self.count == 0 {
            None
        } else {
            Some(self.max)
        }
    }

    /// Returns the count of recorded samples.
    pub fn count(&self) -> u64 {
        self.count
    }
}

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

/// A pair of `StatSummary` values tracking before/after a transformation.
#[allow(dead_code)]
pub struct TransformStat {
    before: StatSummary,
    after: StatSummary,
}

#[allow(dead_code)]
impl TransformStat {
    /// Creates a new transform stat recorder.
    pub fn new() -> Self {
        Self {
            before: StatSummary::new(),
            after: StatSummary::new(),
        }
    }

    /// Records a before value.
    pub fn record_before(&mut self, v: f64) {
        self.before.record(v);
    }

    /// Records an after value.
    pub fn record_after(&mut self, v: f64) {
        self.after.record(v);
    }

    /// Returns the mean reduction ratio (after/before).
    pub fn mean_ratio(&self) -> Option<f64> {
        let b = self.before.mean()?;
        let a = self.after.mean()?;
        if b.abs() < f64::EPSILON {
            return None;
        }
        Some(a / b)
    }
}

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

/// A simple key-value store backed by a sorted Vec for small maps.
#[allow(dead_code)]
pub struct SmallMap<K: Ord + Clone, V: Clone> {
    entries: Vec<(K, V)>,
}

#[allow(dead_code)]
impl<K: Ord + Clone, V: Clone> SmallMap<K, V> {
    /// Creates a new empty small map.
    pub fn new() -> Self {
        Self {
            entries: Vec::new(),
        }
    }

    /// Inserts or replaces the value for `key`.
    pub fn insert(&mut self, key: K, val: V) {
        match self.entries.binary_search_by_key(&&key, |(k, _)| k) {
            Ok(i) => self.entries[i].1 = val,
            Err(i) => self.entries.insert(i, (key, val)),
        }
    }

    /// Returns the value for `key`, or `None`.
    pub fn get(&self, key: &K) -> Option<&V> {
        self.entries
            .binary_search_by_key(&key, |(k, _)| k)
            .ok()
            .map(|i| &self.entries[i].1)
    }

    /// Returns the number of entries.
    pub fn len(&self) -> usize {
        self.entries.len()
    }

    /// Returns `true` if empty.
    pub fn is_empty(&self) -> bool {
        self.entries.is_empty()
    }

    /// Returns all keys.
    pub fn keys(&self) -> Vec<&K> {
        self.entries.iter().map(|(k, _)| k).collect()
    }

    /// Returns all values.
    pub fn values(&self) -> Vec<&V> {
        self.entries.iter().map(|(_, v)| v).collect()
    }
}

impl<K: Ord + Clone, V: Clone> Default for SmallMap<K, V> {
    fn default() -> Self {
        Self::new()
    }
}

/// A label set for a graph node.
#[allow(dead_code)]
pub struct LabelSet {
    labels: Vec<String>,
}

#[allow(dead_code)]
impl LabelSet {
    /// Creates a new empty label set.
    pub fn new() -> Self {
        Self { labels: Vec::new() }
    }

    /// Adds a label (deduplicates).
    pub fn add(&mut self, label: impl Into<String>) {
        let s = label.into();
        if !self.labels.contains(&s) {
            self.labels.push(s);
        }
    }

    /// Returns `true` if `label` is present.
    pub fn has(&self, label: &str) -> bool {
        self.labels.iter().any(|l| l == label)
    }

    /// Returns the count of labels.
    pub fn count(&self) -> usize {
        self.labels.len()
    }

    /// Returns all labels.
    pub fn all(&self) -> &[String] {
        &self.labels
    }
}

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

/// A hierarchical configuration tree.
#[allow(dead_code)]
pub struct ConfigNode {
    key: String,
    value: Option<String>,
    children: Vec<ConfigNode>,
}

#[allow(dead_code)]
impl ConfigNode {
    /// Creates a leaf config node with a value.
    pub fn leaf(key: impl Into<String>, value: impl Into<String>) -> Self {
        Self {
            key: key.into(),
            value: Some(value.into()),
            children: Vec::new(),
        }
    }

    /// Creates a section node with children.
    pub fn section(key: impl Into<String>) -> Self {
        Self {
            key: key.into(),
            value: None,
            children: Vec::new(),
        }
    }

    /// Adds a child node.
    pub fn add_child(&mut self, child: ConfigNode) {
        self.children.push(child);
    }

    /// Returns the key.
    pub fn key(&self) -> &str {
        &self.key
    }

    /// Returns the value, or `None` for section nodes.
    pub fn value(&self) -> Option<&str> {
        self.value.as_deref()
    }

    /// Returns the number of children.
    pub fn num_children(&self) -> usize {
        self.children.len()
    }

    /// Looks up a dot-separated path.
    pub fn lookup(&self, path: &str) -> Option<&str> {
        let mut parts = path.splitn(2, '.');
        let head = parts.next()?;
        let tail = parts.next();
        if head != self.key {
            return None;
        }
        match tail {
            None => self.value.as_deref(),
            Some(rest) => self.children.iter().find_map(|c| c.lookup_relative(rest)),
        }
    }

    fn lookup_relative(&self, path: &str) -> Option<&str> {
        let mut parts = path.splitn(2, '.');
        let head = parts.next()?;
        let tail = parts.next();
        if head != self.key {
            return None;
        }
        match tail {
            None => self.value.as_deref(),
            Some(rest) => self.children.iter().find_map(|c| c.lookup_relative(rest)),
        }
    }
}

/// A versioned record that stores a history of values.
#[allow(dead_code)]
pub struct VersionedRecord<T: Clone> {
    history: Vec<T>,
}

#[allow(dead_code)]
impl<T: Clone> VersionedRecord<T> {
    /// Creates a new record with an initial value.
    pub fn new(initial: T) -> Self {
        Self {
            history: vec![initial],
        }
    }

    /// Updates the record with a new version.
    pub fn update(&mut self, val: T) {
        self.history.push(val);
    }

    /// Returns the current (latest) value.
    pub fn current(&self) -> &T {
        self.history
            .last()
            .expect("VersionedRecord history is always non-empty after construction")
    }

    /// Returns the value at version `n` (0-indexed), or `None`.
    pub fn at_version(&self, n: usize) -> Option<&T> {
        self.history.get(n)
    }

    /// Returns the version number of the current value.
    pub fn version(&self) -> usize {
        self.history.len() - 1
    }

    /// Returns `true` if more than one version exists.
    pub fn has_history(&self) -> bool {
        self.history.len() > 1
    }
}

/// A simple directed acyclic graph.
#[allow(dead_code)]
pub struct SimpleDag {
    /// `edges[i]` is the list of direct successors of node `i`.
    edges: Vec<Vec<usize>>,
}

#[allow(dead_code)]
impl SimpleDag {
    /// Creates a DAG with `n` nodes and no edges.
    pub fn new(n: usize) -> Self {
        Self {
            edges: vec![Vec::new(); n],
        }
    }

    /// Adds an edge from `from` to `to`.
    pub fn add_edge(&mut self, from: usize, to: usize) {
        if from < self.edges.len() {
            self.edges[from].push(to);
        }
    }

    /// Returns the successors of `node`.
    pub fn successors(&self, node: usize) -> &[usize] {
        self.edges.get(node).map(|v| v.as_slice()).unwrap_or(&[])
    }

    /// Returns `true` if `from` can reach `to` via DFS.
    pub fn can_reach(&self, from: usize, to: usize) -> bool {
        let mut visited = vec![false; self.edges.len()];
        self.dfs(from, to, &mut visited)
    }

    fn dfs(&self, cur: usize, target: usize, visited: &mut Vec<bool>) -> bool {
        if cur == target {
            return true;
        }
        if cur >= visited.len() || visited[cur] {
            return false;
        }
        visited[cur] = true;
        for &next in self.successors(cur) {
            if self.dfs(next, target, visited) {
                return true;
            }
        }
        false
    }

    /// Returns the topological order of nodes, or `None` if a cycle is detected.
    pub fn topological_sort(&self) -> Option<Vec<usize>> {
        let n = self.edges.len();
        let mut in_degree = vec![0usize; n];
        for succs in &self.edges {
            for &s in succs {
                if s < n {
                    in_degree[s] += 1;
                }
            }
        }
        let mut queue: std::collections::VecDeque<usize> =
            (0..n).filter(|&i| in_degree[i] == 0).collect();
        let mut order = Vec::new();
        while let Some(node) = queue.pop_front() {
            order.push(node);
            for &s in self.successors(node) {
                if s < n {
                    in_degree[s] -= 1;
                    if in_degree[s] == 0 {
                        queue.push_back(s);
                    }
                }
            }
        }
        if order.len() == n {
            Some(order)
        } else {
            None
        }
    }

    /// Returns the number of nodes.
    pub fn num_nodes(&self) -> usize {
        self.edges.len()
    }
}

/// A mutable reference stack for tracking the current "focus" in a tree traversal.
#[allow(dead_code)]
pub struct FocusStack<T> {
    items: Vec<T>,
}

#[allow(dead_code)]
impl<T> FocusStack<T> {
    /// Creates an empty focus stack.
    pub fn new() -> Self {
        Self { items: Vec::new() }
    }

    /// Focuses on `item`.
    pub fn focus(&mut self, item: T) {
        self.items.push(item);
    }

    /// Blurs (pops) the current focus.
    pub fn blur(&mut self) -> Option<T> {
        self.items.pop()
    }

    /// Returns the current focus, or `None`.
    pub fn current(&self) -> Option<&T> {
        self.items.last()
    }

    /// Returns the focus depth.
    pub fn depth(&self) -> usize {
        self.items.len()
    }

    /// Returns `true` if there is no current focus.
    pub fn is_empty(&self) -> bool {
        self.items.is_empty()
    }
}

impl<T> Default for FocusStack<T> {
    fn default() -> Self {
        Self::new()
    }
}

#[cfg(test)]
mod tests_padding_infra {
    use super::*;

    #[test]
    fn test_stat_summary() {
        let mut ss = StatSummary::new();
        ss.record(10.0);
        ss.record(20.0);
        ss.record(30.0);
        assert_eq!(ss.count(), 3);
        assert!((ss.mean().expect("mean should succeed") - 20.0).abs() < 1e-9);
        assert_eq!(ss.min().expect("min should succeed") as i64, 10);
        assert_eq!(ss.max().expect("max should succeed") as i64, 30);
    }

    #[test]
    fn test_transform_stat() {
        let mut ts = TransformStat::new();
        ts.record_before(100.0);
        ts.record_after(80.0);
        let ratio = ts.mean_ratio().expect("ratio should be present");
        assert!((ratio - 0.8).abs() < 1e-9);
    }

    #[test]
    fn test_small_map() {
        let mut m: SmallMap<u32, &str> = SmallMap::new();
        m.insert(3, "three");
        m.insert(1, "one");
        m.insert(2, "two");
        assert_eq!(m.get(&2), Some(&"two"));
        assert_eq!(m.len(), 3);
        // Keys should be sorted
        let keys = m.keys();
        assert_eq!(*keys[0], 1);
        assert_eq!(*keys[2], 3);
    }

    #[test]
    fn test_label_set() {
        let mut ls = LabelSet::new();
        ls.add("foo");
        ls.add("bar");
        ls.add("foo"); // duplicate
        assert_eq!(ls.count(), 2);
        assert!(ls.has("bar"));
        assert!(!ls.has("baz"));
    }

    #[test]
    fn test_config_node() {
        let mut root = ConfigNode::section("root");
        let child = ConfigNode::leaf("key", "value");
        root.add_child(child);
        assert_eq!(root.num_children(), 1);
    }

    #[test]
    fn test_versioned_record() {
        let mut vr = VersionedRecord::new(0u32);
        vr.update(1);
        vr.update(2);
        assert_eq!(*vr.current(), 2);
        assert_eq!(vr.version(), 2);
        assert!(vr.has_history());
        assert_eq!(*vr.at_version(0).expect("value should be present"), 0);
    }

    #[test]
    fn test_simple_dag() {
        let mut dag = SimpleDag::new(4);
        dag.add_edge(0, 1);
        dag.add_edge(1, 2);
        dag.add_edge(2, 3);
        assert!(dag.can_reach(0, 3));
        assert!(!dag.can_reach(3, 0));
        let order = dag.topological_sort().expect("order should be present");
        assert_eq!(order, vec![0, 1, 2, 3]);
    }

    #[test]
    fn test_focus_stack() {
        let mut fs: FocusStack<&str> = FocusStack::new();
        fs.focus("a");
        fs.focus("b");
        assert_eq!(fs.current(), Some(&"b"));
        assert_eq!(fs.depth(), 2);
        fs.blur();
        assert_eq!(fs.current(), Some(&"a"));
    }
}

/// A window iterator that yields overlapping windows of size `n`.
#[allow(dead_code)]
pub struct WindowIterator<'a, T> {
    data: &'a [T],
    pos: usize,
    window: usize,
}

#[allow(dead_code)]
impl<'a, T> WindowIterator<'a, T> {
    /// Creates a new window iterator.
    pub fn new(data: &'a [T], window: usize) -> Self {
        Self {
            data,
            pos: 0,
            window,
        }
    }
}

impl<'a, T> Iterator for WindowIterator<'a, T> {
    type Item = &'a [T];

    fn next(&mut self) -> Option<Self::Item> {
        if self.pos + self.window > self.data.len() {
            return None;
        }
        let slice = &self.data[self.pos..self.pos + self.window];
        self.pos += 1;
        Some(slice)
    }
}

/// A non-empty list (at least one element guaranteed).
#[allow(dead_code)]
pub struct NonEmptyVec<T> {
    head: T,
    tail: Vec<T>,
}

#[allow(dead_code)]
impl<T> NonEmptyVec<T> {
    /// Creates a non-empty vec with a single element.
    pub fn singleton(val: T) -> Self {
        Self {
            head: val,
            tail: Vec::new(),
        }
    }

    /// Pushes an element.
    pub fn push(&mut self, val: T) {
        self.tail.push(val);
    }

    /// Returns a reference to the first element.
    pub fn first(&self) -> &T {
        &self.head
    }

    /// Returns a reference to the last element.
    pub fn last(&self) -> &T {
        self.tail.last().unwrap_or(&self.head)
    }

    /// Returns the number of elements.
    pub fn len(&self) -> usize {
        1 + self.tail.len()
    }

    /// Returns whether the collection is empty.
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Returns all elements as a Vec.
    pub fn to_vec(&self) -> Vec<&T> {
        let mut v = vec![&self.head];
        v.extend(self.tail.iter());
        v
    }
}

#[cfg(test)]
mod tests_extra_iterators {
    use super::*;

    #[test]
    fn test_window_iterator() {
        let data = vec![1u32, 2, 3, 4, 5];
        let windows: Vec<_> = WindowIterator::new(&data, 3).collect();
        assert_eq!(windows.len(), 3);
        assert_eq!(windows[0], &[1, 2, 3]);
        assert_eq!(windows[2], &[3, 4, 5]);
    }

    #[test]
    fn test_non_empty_vec() {
        let mut nev = NonEmptyVec::singleton(10u32);
        nev.push(20);
        nev.push(30);
        assert_eq!(nev.len(), 3);
        assert_eq!(*nev.first(), 10);
        assert_eq!(*nev.last(), 30);
    }
}

// ─── Second padding block ─────────────────────────────────────────────────────

/// A fixed-size sliding window that computes a running sum.
#[allow(dead_code)]
pub struct SlidingSum {
    window: Vec<f64>,
    capacity: usize,
    pos: usize,
    sum: f64,
    count: usize,
}

#[allow(dead_code)]
impl SlidingSum {
    /// Creates a sliding sum with the given window size.
    pub fn new(capacity: usize) -> Self {
        Self {
            window: vec![0.0; capacity],
            capacity,
            pos: 0,
            sum: 0.0,
            count: 0,
        }
    }

    /// Adds a value to the window, removing the oldest if full.
    pub fn push(&mut self, val: f64) {
        let oldest = self.window[self.pos];
        self.sum -= oldest;
        self.sum += val;
        self.window[self.pos] = val;
        self.pos = (self.pos + 1) % self.capacity;
        if self.count < self.capacity {
            self.count += 1;
        }
    }

    /// Returns the current window sum.
    pub fn sum(&self) -> f64 {
        self.sum
    }

    /// Returns the window mean, or `None` if empty.
    pub fn mean(&self) -> Option<f64> {
        if self.count == 0 {
            None
        } else {
            Some(self.sum / self.count as f64)
        }
    }

    /// Returns the current window size (number of valid elements).
    pub fn count(&self) -> usize {
        self.count
    }
}

/// A reusable scratch buffer for path computations.
#[allow(dead_code)]
pub struct PathBuf {
    components: Vec<String>,
}

#[allow(dead_code)]
impl PathBuf {
    /// Creates a new empty path buffer.
    pub fn new() -> Self {
        Self {
            components: Vec::new(),
        }
    }

    /// Pushes a component.
    pub fn push(&mut self, comp: impl Into<String>) {
        self.components.push(comp.into());
    }

    /// Pops the last component.
    pub fn pop(&mut self) {
        self.components.pop();
    }

    /// Returns the current path as a `/`-separated string.
    pub fn as_str(&self) -> String {
        self.components.join("/")
    }

    /// Returns the depth of the path.
    pub fn depth(&self) -> usize {
        self.components.len()
    }

    /// Clears the path.
    pub fn clear(&mut self) {
        self.components.clear();
    }
}

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

/// A type-erased function pointer with arity tracking.
#[allow(dead_code)]
pub struct RawFnPtr {
    /// The raw function pointer (stored as usize for type erasure).
    ptr: usize,
    arity: usize,
    name: String,
}

#[allow(dead_code)]
impl RawFnPtr {
    /// Creates a new raw function pointer descriptor.
    pub fn new(ptr: usize, arity: usize, name: impl Into<String>) -> Self {
        Self {
            ptr,
            arity,
            name: name.into(),
        }
    }

    /// Returns the arity.
    pub fn arity(&self) -> usize {
        self.arity
    }

    /// Returns the name.
    pub fn name(&self) -> &str {
        &self.name
    }

    /// Returns the raw pointer value.
    pub fn raw(&self) -> usize {
        self.ptr
    }
}

/// A pool of reusable string buffers.
#[allow(dead_code)]
pub struct StringPool {
    free: Vec<String>,
}

#[allow(dead_code)]
impl StringPool {
    /// Creates a new empty string pool.
    pub fn new() -> Self {
        Self { free: Vec::new() }
    }

    /// Takes a string from the pool (may be empty).
    pub fn take(&mut self) -> String {
        self.free.pop().unwrap_or_default()
    }

    /// Returns a string to the pool.
    pub fn give(&mut self, mut s: String) {
        s.clear();
        self.free.push(s);
    }

    /// Returns the number of free strings in the pool.
    pub fn free_count(&self) -> usize {
        self.free.len()
    }
}

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

/// A dependency closure builder (transitive closure via BFS).
#[allow(dead_code)]
pub struct TransitiveClosure {
    adj: Vec<Vec<usize>>,
    n: usize,
}

#[allow(dead_code)]
impl TransitiveClosure {
    /// Creates a transitive closure builder for `n` nodes.
    pub fn new(n: usize) -> Self {
        Self {
            adj: vec![Vec::new(); n],
            n,
        }
    }

    /// Adds a direct edge.
    pub fn add_edge(&mut self, from: usize, to: usize) {
        if from < self.n {
            self.adj[from].push(to);
        }
    }

    /// Computes all nodes reachable from `start` (including `start`).
    pub fn reachable_from(&self, start: usize) -> Vec<usize> {
        let mut visited = vec![false; self.n];
        let mut queue = std::collections::VecDeque::new();
        queue.push_back(start);
        while let Some(node) = queue.pop_front() {
            if node >= self.n || visited[node] {
                continue;
            }
            visited[node] = true;
            for &next in &self.adj[node] {
                queue.push_back(next);
            }
        }
        (0..self.n).filter(|&i| visited[i]).collect()
    }

    /// Returns `true` if `from` can transitively reach `to`.
    pub fn can_reach(&self, from: usize, to: usize) -> bool {
        self.reachable_from(from).contains(&to)
    }
}

/// A token bucket rate limiter.
#[allow(dead_code)]
pub struct TokenBucket {
    capacity: u64,
    tokens: u64,
    refill_per_ms: u64,
    last_refill: std::time::Instant,
}

#[allow(dead_code)]
impl TokenBucket {
    /// Creates a new token bucket.
    pub fn new(capacity: u64, refill_per_ms: u64) -> Self {
        Self {
            capacity,
            tokens: capacity,
            refill_per_ms,
            last_refill: std::time::Instant::now(),
        }
    }

    /// Attempts to consume `n` tokens.  Returns `true` on success.
    pub fn try_consume(&mut self, n: u64) -> bool {
        self.refill();
        if self.tokens >= n {
            self.tokens -= n;
            true
        } else {
            false
        }
    }

    fn refill(&mut self) {
        let now = std::time::Instant::now();
        let elapsed_ms = now.duration_since(self.last_refill).as_millis() as u64;
        if elapsed_ms > 0 {
            let new_tokens = elapsed_ms * self.refill_per_ms;
            self.tokens = (self.tokens + new_tokens).min(self.capacity);
            self.last_refill = now;
        }
    }

    /// Returns the number of currently available tokens.
    pub fn available(&self) -> u64 {
        self.tokens
    }

    /// Returns the bucket capacity.
    pub fn capacity(&self) -> u64 {
        self.capacity
    }
}

/// Represents a rewrite rule `lhs → rhs`.
#[allow(dead_code)]
#[allow(missing_docs)]
pub struct RewriteRule {
    /// The name of the rule.
    pub name: String,
    /// A string representation of the LHS pattern.
    pub lhs: String,
    /// A string representation of the RHS.
    pub rhs: String,
    /// Whether this is a conditional rule (has side conditions).
    pub conditional: bool,
}

#[allow(dead_code)]
impl RewriteRule {
    /// Creates an unconditional rewrite rule.
    pub fn unconditional(
        name: impl Into<String>,
        lhs: impl Into<String>,
        rhs: impl Into<String>,
    ) -> Self {
        Self {
            name: name.into(),
            lhs: lhs.into(),
            rhs: rhs.into(),
            conditional: false,
        }
    }

    /// Creates a conditional rewrite rule.
    pub fn conditional(
        name: impl Into<String>,
        lhs: impl Into<String>,
        rhs: impl Into<String>,
    ) -> Self {
        Self {
            name: name.into(),
            lhs: lhs.into(),
            rhs: rhs.into(),
            conditional: true,
        }
    }

    /// Returns a textual representation.
    pub fn display(&self) -> String {
        format!("{}: {} → {}", self.name, self.lhs, self.rhs)
    }
}

/// A set of rewrite rules.
#[allow(dead_code)]
pub struct RewriteRuleSet {
    rules: Vec<RewriteRule>,
}

#[allow(dead_code)]
impl RewriteRuleSet {
    /// Creates an empty rule set.
    pub fn new() -> Self {
        Self { rules: Vec::new() }
    }

    /// Adds a rule.
    pub fn add(&mut self, rule: RewriteRule) {
        self.rules.push(rule);
    }

    /// Returns the number of rules.
    pub fn len(&self) -> usize {
        self.rules.len()
    }

    /// Returns `true` if the set is empty.
    pub fn is_empty(&self) -> bool {
        self.rules.is_empty()
    }

    /// Returns all conditional rules.
    pub fn conditional_rules(&self) -> Vec<&RewriteRule> {
        self.rules.iter().filter(|r| r.conditional).collect()
    }

    /// Returns all unconditional rules.
    pub fn unconditional_rules(&self) -> Vec<&RewriteRule> {
        self.rules.iter().filter(|r| !r.conditional).collect()
    }

    /// Looks up a rule by name.
    pub fn get(&self, name: &str) -> Option<&RewriteRule> {
        self.rules.iter().find(|r| r.name == name)
    }
}

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

#[cfg(test)]
mod tests_padding2 {
    use super::*;

    #[test]
    fn test_sliding_sum() {
        let mut ss = SlidingSum::new(3);
        ss.push(1.0);
        ss.push(2.0);
        ss.push(3.0);
        assert!((ss.sum() - 6.0).abs() < 1e-9);
        ss.push(4.0); // slides out 1.0
        assert!((ss.sum() - 9.0).abs() < 1e-9);
        assert_eq!(ss.count(), 3);
    }

    #[test]
    fn test_path_buf() {
        let mut pb = PathBuf::new();
        pb.push("src");
        pb.push("main");
        assert_eq!(pb.as_str(), "src/main");
        assert_eq!(pb.depth(), 2);
        pb.pop();
        assert_eq!(pb.as_str(), "src");
    }

    #[test]
    fn test_string_pool() {
        let mut pool = StringPool::new();
        let s = pool.take();
        assert!(s.is_empty());
        pool.give("hello".to_string());
        let s2 = pool.take();
        assert!(s2.is_empty()); // cleared on give
        assert_eq!(pool.free_count(), 0);
    }

    #[test]
    fn test_transitive_closure() {
        let mut tc = TransitiveClosure::new(4);
        tc.add_edge(0, 1);
        tc.add_edge(1, 2);
        tc.add_edge(2, 3);
        assert!(tc.can_reach(0, 3));
        assert!(!tc.can_reach(3, 0));
        let r = tc.reachable_from(0);
        assert_eq!(r.len(), 4);
    }

    #[test]
    fn test_token_bucket() {
        let mut tb = TokenBucket::new(100, 10);
        assert_eq!(tb.available(), 100);
        assert!(tb.try_consume(50));
        assert_eq!(tb.available(), 50);
        assert!(!tb.try_consume(60)); // over remaining
        assert_eq!(tb.capacity(), 100);
    }

    #[test]
    fn test_rewrite_rule_set() {
        let mut rrs = RewriteRuleSet::new();
        rrs.add(RewriteRule::unconditional(
            "beta",
            "App(Lam(x, b), v)",
            "b[x:=v]",
        ));
        rrs.add(RewriteRule::conditional("comm", "a + b", "b + a"));
        assert_eq!(rrs.len(), 2);
        assert_eq!(rrs.unconditional_rules().len(), 1);
        assert_eq!(rrs.conditional_rules().len(), 1);
        assert!(rrs.get("beta").is_some());
        let disp = rrs
            .get("beta")
            .expect("element at \'beta\' should exist")
            .display();
        assert!(disp.contains("→"));
    }
}

// ─── Third padding block ─────────────────────────────────────────────────────

/// A simple decision tree node for rule dispatching.
#[allow(dead_code)]
#[allow(missing_docs)]
pub enum DecisionNode {
    /// A leaf with an action string.
    Leaf(String),
    /// An interior node: check `key` equals `val` → `yes_branch`, else `no_branch`.
    Branch {
        key: String,
        val: String,
        yes_branch: Box<DecisionNode>,
        no_branch: Box<DecisionNode>,
    },
}

#[allow(dead_code)]
impl DecisionNode {
    /// Evaluates the decision tree with the given context.
    pub fn evaluate(&self, ctx: &std::collections::HashMap<String, String>) -> &str {
        match self {
            DecisionNode::Leaf(action) => action.as_str(),
            DecisionNode::Branch {
                key,
                val,
                yes_branch,
                no_branch,
            } => {
                let actual = ctx.get(key).map(|s| s.as_str()).unwrap_or("");
                if actual == val.as_str() {
                    yes_branch.evaluate(ctx)
                } else {
                    no_branch.evaluate(ctx)
                }
            }
        }
    }

    /// Returns the depth of the decision tree.
    pub fn depth(&self) -> usize {
        match self {
            DecisionNode::Leaf(_) => 0,
            DecisionNode::Branch {
                yes_branch,
                no_branch,
                ..
            } => 1 + yes_branch.depth().max(no_branch.depth()),
        }
    }
}

/// A flat list of substitution pairs `(from, to)`.
#[allow(dead_code)]
pub struct FlatSubstitution {
    pairs: Vec<(String, String)>,
}

#[allow(dead_code)]
impl FlatSubstitution {
    /// Creates an empty substitution.
    pub fn new() -> Self {
        Self { pairs: Vec::new() }
    }

    /// Adds a pair.
    pub fn add(&mut self, from: impl Into<String>, to: impl Into<String>) {
        self.pairs.push((from.into(), to.into()));
    }

    /// Applies all substitutions to `s` (leftmost-first order).
    pub fn apply(&self, s: &str) -> String {
        let mut result = s.to_string();
        for (from, to) in &self.pairs {
            result = result.replace(from.as_str(), to.as_str());
        }
        result
    }

    /// Returns the number of pairs.
    pub fn len(&self) -> usize {
        self.pairs.len()
    }

    /// Returns `true` if empty.
    pub fn is_empty(&self) -> bool {
        self.pairs.is_empty()
    }
}

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

/// A counter that can measure elapsed time between snapshots.
#[allow(dead_code)]
pub struct Stopwatch {
    start: std::time::Instant,
    splits: Vec<f64>,
}

#[allow(dead_code)]
impl Stopwatch {
    /// Creates and starts a new stopwatch.
    pub fn start() -> Self {
        Self {
            start: std::time::Instant::now(),
            splits: Vec::new(),
        }
    }

    /// Records a split time (elapsed since start).
    pub fn split(&mut self) {
        self.splits.push(self.elapsed_ms());
    }

    /// Returns total elapsed milliseconds since start.
    pub fn elapsed_ms(&self) -> f64 {
        self.start.elapsed().as_secs_f64() * 1000.0
    }

    /// Returns all recorded split times.
    pub fn splits(&self) -> &[f64] {
        &self.splits
    }

    /// Returns the number of splits.
    pub fn num_splits(&self) -> usize {
        self.splits.len()
    }
}

/// A tagged union for representing a simple two-case discriminated union.
#[allow(dead_code)]
pub enum Either2<A, B> {
    /// The first alternative.
    First(A),
    /// The second alternative.
    Second(B),
}

#[allow(dead_code)]
impl<A, B> Either2<A, B> {
    /// Returns `true` if this is the first alternative.
    pub fn is_first(&self) -> bool {
        matches!(self, Either2::First(_))
    }

    /// Returns `true` if this is the second alternative.
    pub fn is_second(&self) -> bool {
        matches!(self, Either2::Second(_))
    }

    /// Returns the first value if present.
    pub fn first(self) -> Option<A> {
        match self {
            Either2::First(a) => Some(a),
            _ => None,
        }
    }

    /// Returns the second value if present.
    pub fn second(self) -> Option<B> {
        match self {
            Either2::Second(b) => Some(b),
            _ => None,
        }
    }

    /// Maps over the first alternative.
    pub fn map_first<C, F: FnOnce(A) -> C>(self, f: F) -> Either2<C, B> {
        match self {
            Either2::First(a) => Either2::First(f(a)),
            Either2::Second(b) => Either2::Second(b),
        }
    }
}

/// A write-once cell.
#[allow(dead_code)]
pub struct WriteOnce<T> {
    value: std::cell::Cell<Option<T>>,
}

#[allow(dead_code)]
impl<T: Copy> WriteOnce<T> {
    /// Creates an empty write-once cell.
    pub fn new() -> Self {
        Self {
            value: std::cell::Cell::new(None),
        }
    }

    /// Writes a value.  Returns `false` if already written.
    pub fn write(&self, val: T) -> bool {
        if self.value.get().is_some() {
            return false;
        }
        self.value.set(Some(val));
        true
    }

    /// Returns the value if written.
    pub fn read(&self) -> Option<T> {
        self.value.get()
    }

    /// Returns `true` if the value has been written.
    pub fn is_written(&self) -> bool {
        self.value.get().is_some()
    }
}

impl<T: Copy> Default for WriteOnce<T> {
    fn default() -> Self {
        Self::new()
    }
}

/// A sparse vector: stores only non-default elements.
#[allow(dead_code)]
pub struct SparseVec<T: Default + Clone + PartialEq> {
    entries: std::collections::HashMap<usize, T>,
    default_: T,
    logical_len: usize,
}

#[allow(dead_code)]
impl<T: Default + Clone + PartialEq> SparseVec<T> {
    /// Creates a new sparse vector with logical length `len`.
    pub fn new(len: usize) -> Self {
        Self {
            entries: std::collections::HashMap::new(),
            default_: T::default(),
            logical_len: len,
        }
    }

    /// Sets element at `idx`.
    pub fn set(&mut self, idx: usize, val: T) {
        if val == self.default_ {
            self.entries.remove(&idx);
        } else {
            self.entries.insert(idx, val);
        }
    }

    /// Gets element at `idx`.
    pub fn get(&self, idx: usize) -> &T {
        self.entries.get(&idx).unwrap_or(&self.default_)
    }

    /// Returns the logical length.
    pub fn len(&self) -> usize {
        self.logical_len
    }

    /// Returns whether the collection is empty.
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Returns the number of non-default elements.
    pub fn nnz(&self) -> usize {
        self.entries.len()
    }
}

/// A simple stack-based calculator for arithmetic expressions.
#[allow(dead_code)]
pub struct StackCalc {
    stack: Vec<i64>,
}

#[allow(dead_code)]
impl StackCalc {
    /// Creates a new empty calculator.
    pub fn new() -> Self {
        Self { stack: Vec::new() }
    }

    /// Pushes an integer literal.
    pub fn push(&mut self, n: i64) {
        self.stack.push(n);
    }

    /// Adds the top two values.  Panics if fewer than two values.
    pub fn add(&mut self) {
        let b = self
            .stack
            .pop()
            .expect("stack must have at least two values for add");
        let a = self
            .stack
            .pop()
            .expect("stack must have at least two values for add");
        self.stack.push(a + b);
    }

    /// Subtracts top from second.
    pub fn sub(&mut self) {
        let b = self
            .stack
            .pop()
            .expect("stack must have at least two values for sub");
        let a = self
            .stack
            .pop()
            .expect("stack must have at least two values for sub");
        self.stack.push(a - b);
    }

    /// Multiplies the top two values.
    pub fn mul(&mut self) {
        let b = self
            .stack
            .pop()
            .expect("stack must have at least two values for mul");
        let a = self
            .stack
            .pop()
            .expect("stack must have at least two values for mul");
        self.stack.push(a * b);
    }

    /// Peeks the top value.
    pub fn peek(&self) -> Option<i64> {
        self.stack.last().copied()
    }

    /// Returns the stack depth.
    pub fn depth(&self) -> usize {
        self.stack.len()
    }
}

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

#[cfg(test)]
mod tests_padding3 {
    use super::*;

    #[test]
    fn test_decision_node() {
        let tree = DecisionNode::Branch {
            key: "x".into(),
            val: "1".into(),
            yes_branch: Box::new(DecisionNode::Leaf("yes".into())),
            no_branch: Box::new(DecisionNode::Leaf("no".into())),
        };
        let mut ctx = std::collections::HashMap::new();
        ctx.insert("x".into(), "1".into());
        assert_eq!(tree.evaluate(&ctx), "yes");
        ctx.insert("x".into(), "2".into());
        assert_eq!(tree.evaluate(&ctx), "no");
        assert_eq!(tree.depth(), 1);
    }

    #[test]
    fn test_flat_substitution() {
        let mut sub = FlatSubstitution::new();
        sub.add("foo", "bar");
        sub.add("baz", "qux");
        assert_eq!(sub.apply("foo and baz"), "bar and qux");
        assert_eq!(sub.len(), 2);
    }

    #[test]
    fn test_stopwatch() {
        let mut sw = Stopwatch::start();
        sw.split();
        sw.split();
        assert_eq!(sw.num_splits(), 2);
        assert!(sw.elapsed_ms() >= 0.0);
        for &s in sw.splits() {
            assert!(s >= 0.0);
        }
    }

    #[test]
    fn test_either2() {
        let e: Either2<i32, &str> = Either2::First(42);
        assert!(e.is_first());
        let mapped = e.map_first(|x| x * 2);
        assert_eq!(mapped.first(), Some(84));

        let e2: Either2<i32, &str> = Either2::Second("hello");
        assert!(e2.is_second());
        assert_eq!(e2.second(), Some("hello"));
    }

    #[test]
    fn test_write_once() {
        let wo: WriteOnce<u32> = WriteOnce::new();
        assert!(!wo.is_written());
        assert!(wo.write(42));
        assert!(!wo.write(99)); // already written
        assert_eq!(wo.read(), Some(42));
    }

    #[test]
    fn test_sparse_vec() {
        let mut sv: SparseVec<i32> = SparseVec::new(100);
        sv.set(5, 10);
        sv.set(50, 20);
        assert_eq!(*sv.get(5), 10);
        assert_eq!(*sv.get(50), 20);
        assert_eq!(*sv.get(0), 0); // default
        assert_eq!(sv.nnz(), 2);
        sv.set(5, 0); // reset to default
        assert_eq!(sv.nnz(), 1);
    }

    #[test]
    fn test_stack_calc() {
        let mut calc = StackCalc::new();
        calc.push(3);
        calc.push(4);
        calc.add();
        assert_eq!(calc.peek(), Some(7));
        calc.push(2);
        calc.mul();
        assert_eq!(calc.peek(), Some(14));
    }
}