opthash 0.6.0

use std::borrow::Borrow;
use std::fmt;
use std::hash::{BuildHasher, Hash};
use std::iter::FusedIterator;
use std::marker::PhantomData;
use std::mem;
use std::ops::Range;
use std::ptr;

use crate::common::config::{DEFAULT_RESERVE_FRACTION, INITIAL_CAPACITY};
use crate::common::control::{CTRL_EMPTY, CTRL_TOMBSTONE, ControlByte, ControlOps};
use crate::common::entry::{EntryView, OccupiedError as CommonOccupiedError};
use crate::common::iter::{
    IntoKeys as CommonIntoKeys, IntoValues as CommonIntoValues, Keys as CommonKeys,
    Values as CommonValues,
};
use crate::common::layout::{Entry as SlotEntry, GROUP_SIZE, RawTable, try_zeroed_boxed_slice};
use crate::common::math::{
    capacity_for, ceil_to_usize, fastmod_magic, fastmod_u32, floor_to_usize, level_salt,
    max_insertions, round_to_usize, round_up_to_group, round_up_to_pow2_groups,
    sanitize_reserve_fraction, usize_to_f64,
};
use crate::common::simd::{ProbeOps, prefetch_read};
use crate::common::{DefaultHashBuilder, TryReserveError};

pub(crate) const MAX_FUNNEL_RESERVE_FRACTION: f64 = 1.0 / 8.0;

/// Construction-time tuning for `FunnelHashMap`.
#[derive(Debug, Clone, Copy)]
pub struct FunnelOptions {
    /// Target initial capacity. Funnel caps load factor at 1/8 by design;
    /// useful capacity is `capacity * (1 - reserve_fraction)`.
    capacity: usize,
    /// Fraction kept free as headroom. Clamped to
    /// `MAX_FUNNEL_RESERVE_FRACTION` (1/8).
    reserve_fraction: f64,
    /// Max groups probed in the special-array primary before falling back to
    /// the fallback array. `None` derives from `reserve_fraction`.
    primary_probe_limit: Option<usize>,
}

impl Default for FunnelOptions {
    fn default() -> Self {
        Self {
            capacity: 0,
            reserve_fraction: DEFAULT_RESERVE_FRACTION,
            primary_probe_limit: None,
        }
    }
}

impl FunnelOptions {
    #[must_use]
    pub fn with_capacity(capacity: usize) -> Self {
        Self {
            capacity,
            ..Self::default()
        }
    }

    #[must_use]
    pub fn capacity(mut self, capacity: usize) -> Self {
        self.capacity = capacity;
        self
    }

    #[must_use]
    pub fn reserve_fraction(mut self, reserve_fraction: f64) -> Self {
        self.reserve_fraction = reserve_fraction;
        self
    }

    #[must_use]
    pub fn primary_probe_limit(mut self, primary_probe_limit: usize) -> Self {
        self.primary_probe_limit = Some(primary_probe_limit);
        self
    }
}

/// One level in the funnel array. Each level is a fixed grid of buckets;
/// inserts hash a key to one bucket and probe within that bucket only.
/// If the bucket is full the insert spills to the next level (or the
/// special array). Bucket-local probing keeps lookup cost bounded.
struct BucketLevel<K, V> {
    /// Structure of Arrays control bytes + entries.
    table: RawTable<SlotEntry<K, V>>,
    /// Live entry count.
    len: usize,
    /// Deleted-slot count.
    tombstones: usize,
    /// Slots per bucket.
    bucket_size: usize,
    /// Number of buckets in this level.
    bucket_count: usize,
    /// Per-level salt mixed into the key hash
    /// so each level distributes differently.
    salt: u64,
    /// Fastmod magic for `bucket_count`.
    bucket_count_magic: u64,
}

impl<K, V> BucketLevel<K, V> {
    fn with_bucket_count(bucket_count: usize, bucket_size: usize, salt: u64) -> Self {
        let total_capacity = bucket_count.saturating_mul(bucket_size);
        let bucket_count_magic = if bucket_count > 1 {
            fastmod_magic(bucket_count)
        } else {
            0
        };
        Self {
            table: RawTable::new(total_capacity),
            len: 0,
            tombstones: 0,
            bucket_size,
            bucket_count,
            salt,
            bucket_count_magic,
        }
    }

    /// Fallible counterpart to [`BucketLevel::with_bucket_count`].
    fn try_with_bucket_count(
        bucket_count: usize,
        bucket_size: usize,
        salt: u64,
    ) -> Result<Self, TryReserveError> {
        let total_capacity = bucket_count.saturating_mul(bucket_size);
        let bucket_count_magic = if bucket_count > 1 {
            fastmod_magic(bucket_count)
        } else {
            0
        };
        let table = RawTable::try_new(total_capacity).map_err(|()| TryReserveError::AllocError)?;
        Ok(Self {
            table,
            len: 0,
            tombstones: 0,
            bucket_size,
            bucket_count,
            salt,
            bucket_count_magic,
        })
    }

    #[inline]
    fn capacity(&self) -> usize {
        self.table.capacity()
    }

    /// Hash → bucket via fastmod,
    /// salted so each level distributes differently.
    #[inline]
    fn bucket_index(&self, key_hash: u64) -> usize {
        fastmod_u32(
            key_hash ^ self.salt,
            self.bucket_count_magic,
            self.bucket_count,
        )
    }

    /// Slot index range covering all entries in `bucket_idx`.
    #[inline]
    fn bucket_range(&self, bucket_idx: usize) -> Range<usize> {
        let start = bucket_idx * self.bucket_size;
        start..start + self.bucket_size
    }
}

impl<K, V> Drop for BucketLevel<K, V> {
    fn drop(&mut self) {
        for idx in 0..self.table.capacity() {
            if self.table.control_at(idx).is_occupied() {
                unsafe { self.table.drop_in_place(idx) };
            }
        }
    }
}

/// Fallback table for keys that didn't fit in any bucket level. SIMD-group
/// open addressing with per-key odd-step probing over pow2 `group_count`
/// (step coprime to `group_count` ⇒ permutation over all groups).
struct SpecialPrimary<K, V> {
    /// `SoA` control bytes + entries.
    table: RawTable<SlotEntry<K, V>>,
    /// Live entry count.
    len: usize,
    /// Total tombstones; drives the global 50%-capacity resize trigger.
    tombstones: usize,
    /// `group_count - 1`. Pow2 by construction.
    group_count_mask: usize,
    /// Per-group packed fingerprint metadata for fast scans.
    group_summaries: Box<[u128]>,
}

impl<K, V> SpecialPrimary<K, V> {
    fn with_capacity(capacity: usize) -> Self {
        let inflated = round_up_to_pow2_groups(capacity);
        let table = RawTable::new(inflated);
        let group_count = table.group_count();
        debug_assert!(
            group_count == 0 || group_count.is_power_of_two(),
            "SpecialPrimary group_count must be pow2",
        );
        Self {
            table,
            len: 0,
            tombstones: 0,
            group_count_mask: group_count.saturating_sub(1),
            group_summaries: vec![0; group_count].into_boxed_slice(),
        }
    }

    /// Fallible counterpart to [`SpecialPrimary::with_capacity`].
    fn try_with_capacity(capacity: usize) -> Result<Self, TryReserveError> {
        let inflated = round_up_to_pow2_groups(capacity);
        let table = RawTable::try_new(inflated).map_err(|()| TryReserveError::AllocError)?;
        let group_count = table.group_count();
        let group_summaries = try_zeroed_boxed_slice::<u128>(group_count)?;
        Ok(Self {
            table,
            len: 0,
            tombstones: 0,
            group_count_mask: group_count.saturating_sub(1),
            group_summaries,
        })
    }
}

impl<K, V> Drop for SpecialPrimary<K, V> {
    fn drop(&mut self) {
        for idx in 0..self.table.capacity() {
            if self.table.control_at(idx).is_occupied() {
                unsafe { self.table.drop_in_place(idx) };
            }
        }
    }
}

/// Last-resort table for keys that exhaust the special primary's probe
/// budget. Bucketed like `BucketLevel` but with larger buckets (`2 *
/// primary_probe_limit`) so a key that's been pushed this far almost
/// certainly fits.
struct SpecialFallback<K, V> {
    /// Structure of Arrays control bytes + entries.
    table: RawTable<SlotEntry<K, V>>,
    /// Live entry count.
    len: usize,
    /// Deleted-slot count.
    tombstones: usize,
    /// Slots per bucket. Larger than `BucketLevel` (`2 * primary_probe_limit`)
    /// since this is the last-resort table.
    bucket_size: usize,
    /// Number of buckets.
    bucket_count: usize,
}

impl<K, V> SpecialFallback<K, V> {
    fn with_capacity(capacity: usize, bucket_size: usize) -> Self {
        let bucket_count = if bucket_size == 0 {
            0
        } else {
            capacity.div_ceil(bucket_size)
        };
        Self {
            table: RawTable::new(capacity),
            len: 0,
            tombstones: 0,
            bucket_size,
            bucket_count,
        }
    }

    /// Fallible counterpart to [`SpecialFallback::with_capacity`].
    fn try_with_capacity(capacity: usize, bucket_size: usize) -> Result<Self, TryReserveError> {
        let bucket_count = if bucket_size == 0 {
            0
        } else {
            capacity.div_ceil(bucket_size)
        };
        let table = RawTable::try_new(capacity).map_err(|()| TryReserveError::AllocError)?;
        Ok(Self {
            table,
            len: 0,
            tombstones: 0,
            bucket_size,
            bucket_count,
        })
    }

    #[inline]
    fn capacity(&self) -> usize {
        self.table.capacity()
    }

    #[inline]
    fn bucket_count(&self) -> usize {
        self.bucket_count
    }

    #[inline]
    fn bucket_range(&self, bucket_idx: usize) -> Range<usize> {
        let start = bucket_idx * self.bucket_size;
        let end = (start + self.bucket_size).min(self.table.capacity());
        start..end
    }
}

impl<K, V> Drop for SpecialFallback<K, V> {
    fn drop(&mut self) {
        for idx in 0..self.table.capacity() {
            if self.table.control_at(idx).is_occupied() {
                unsafe { self.table.drop_in_place(idx) };
            }
        }
    }
}

/// Combines the special primary (probed first) and the special fallback
/// (when primary hits its probe limit). Together they catch keys that
/// overflowed every bucket level.
struct SpecialArray<K, V> {
    /// Probed first; bounded by `primary_probe_limit`.
    primary: SpecialPrimary<K, V>,
    /// Probed after primary hits its limit.
    fallback: SpecialFallback<K, V>,
}

impl<K, V> SpecialArray<K, V> {
    fn with_capacity(capacity: usize, primary_probe_limit: usize) -> Self {
        let fallback_bucket_size = (2usize.saturating_mul(primary_probe_limit)).max(2);
        let primary_capacity = capacity.div_ceil(2);
        let fallback_capacity = capacity.saturating_sub(primary_capacity);
        Self {
            primary: SpecialPrimary::with_capacity(primary_capacity),
            fallback: SpecialFallback::with_capacity(fallback_capacity, fallback_bucket_size),
        }
    }

    /// Fallible counterpart to [`SpecialArray::with_capacity`].
    fn try_with_capacity(
        capacity: usize,
        primary_probe_limit: usize,
    ) -> Result<Self, TryReserveError> {
        let fallback_bucket_size = (2usize.saturating_mul(primary_probe_limit)).max(2);
        let primary_capacity = capacity.div_ceil(2);
        let fallback_capacity = capacity.saturating_sub(primary_capacity);
        Ok(Self {
            primary: SpecialPrimary::try_with_capacity(primary_capacity)?,
            fallback: SpecialFallback::try_with_capacity(fallback_capacity, fallback_bucket_size)?,
        })
    }
}

/// Where in the funnel structure a key/slot lives. Returned by lookups,
/// consumed by inserts / removes to avoid recomputing the location.
#[derive(Clone, Copy, PartialEq, Eq)]
enum SlotLocation {
    Level { level_idx: usize, slot_idx: usize },
    SpecialPrimary { slot_idx: usize },
    SpecialFallback { slot_idx: usize },
}

/// Outcome of probing one bucket / group during lookup.
/// - `Found(slot_idx)`: key matched at slot.
/// - `Continue`: bucket has tombstones; keep probing for the key elsewhere.
/// - `StopSearch`: bucket has free space and no tombstones — key cannot
///   exist further along this hash chain, abort the search.
enum LookupStep {
    Found(usize),
    Continue,
    StopSearch,
}

/// Open-addressed hash map using funnel hashing.
///
/// Capacity is split between a stack of bucket-grouped `levels` (each level
/// half the size of the previous) and a `special` array catching overflow.
/// Inserts try level 0 first, then descend to deeper levels, then to
/// `special.primary`, then `special.fallback`. Lookups follow the same
/// order. The funnel structure trades a small probe budget per level for
/// hard worst-case guarantees on lookup cost.
pub struct FunnelHashMap<K, V> {
    /// Bucket-grouped levels, each half the size of the previous.
    levels: Vec<BucketLevel<K, V>>,
    /// Overflow-catching tables (primary + fallback).
    special: SpecialArray<K, V>,
    /// Total live entries across levels + special.
    len: usize,
    /// Total slot count.
    capacity: usize,
    /// Insert count that triggers `resize(2x)`.
    max_insertions: usize,
    /// Slot reserve fraction. See `FunnelOptions`.
    reserve_fraction: f64,
    /// Cap on groups probed in the special primary before fallback.
    primary_probe_limit: usize,
    /// Highest level index ever written; bounds the lookup probe loop.
    max_populated_level: usize,
    /// Hash builder. Cloned across resizes to preserve probe sequences.
    hash_builder: DefaultHashBuilder,
}

impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for FunnelHashMap<K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FunnelHashMap")
            .field("len", &self.len)
            .field("capacity", &self.capacity)
            .field("max_populated_level", &self.max_populated_level)
            .finish_non_exhaustive()
    }
}

impl<K, V> Default for FunnelHashMap<K, V>
where
    K: Eq + Hash,
{
    fn default() -> Self {
        Self::new()
    }
}

impl<K, V> FunnelHashMap<K, V>
where
    K: Eq + Hash,
{
    #[must_use]
    pub fn new() -> Self {
        Self::with_options(FunnelOptions::default())
    }

    #[must_use]
    pub fn with_capacity(capacity: usize) -> Self {
        Self::with_options(FunnelOptions::with_capacity(capacity))
    }

    #[must_use]
    pub fn with_hasher(hash_builder: DefaultHashBuilder) -> Self {
        Self::with_options_and_hasher(FunnelOptions::default(), hash_builder)
    }

    #[must_use]
    pub fn with_capacity_and_hasher(capacity: usize, hash_builder: DefaultHashBuilder) -> Self {
        Self::with_options_and_hasher(FunnelOptions::with_capacity(capacity), hash_builder)
    }

    #[must_use]
    pub fn with_options(options: FunnelOptions) -> Self {
        Self::with_options_and_hasher(options, DefaultHashBuilder::default())
    }

    /// Full constructor. `resize` also calls this with the existing
    /// `hash_builder` so all keys keep the same hash sequence across grows.
    #[must_use]
    pub fn with_options_and_hasher(
        options: FunnelOptions,
        hash_builder: DefaultHashBuilder,
    ) -> Self {
        let reserve_fraction =
            sanitize_reserve_fraction(options.reserve_fraction).min(MAX_FUNNEL_RESERVE_FRACTION);
        let capacity = options.capacity;
        let max_insertions = max_insertions(capacity, reserve_fraction);

        let level_count = compute_level_count(reserve_fraction);
        let bucket_width = round_up_to_group(compute_bucket_width(reserve_fraction));
        let primary_probe_limit = options
            .primary_probe_limit
            .unwrap_or_else(|| ProbeOps::log_log_probe_limit(capacity))
            .max(1);

        let mut special_capacity =
            choose_special_capacity(capacity, reserve_fraction, bucket_width);
        let mut main_capacity = capacity.saturating_sub(special_capacity);
        let main_remainder = main_capacity % bucket_width.max(1);
        if main_remainder != 0 {
            main_capacity = main_capacity.saturating_sub(main_remainder);
            special_capacity = capacity.saturating_sub(main_capacity);
        }

        let total_main_buckets = main_capacity.checked_div(bucket_width).unwrap_or(0);
        let level_bucket_counts = partition_funnel_buckets(total_main_buckets, level_count);
        let levels = level_bucket_counts
            .into_iter()
            .enumerate()
            .map(|(level_idx, bucket_count)| {
                BucketLevel::with_bucket_count(bucket_count, bucket_width, level_salt(level_idx))
            })
            .collect::<Vec<_>>();

        let special = SpecialArray::with_capacity(special_capacity, primary_probe_limit);

        Self {
            levels,
            special,
            len: 0,
            capacity,
            max_insertions,
            reserve_fraction,
            primary_probe_limit,
            max_populated_level: 0,
            hash_builder,
        }
    }

    #[must_use]
    pub fn len(&self) -> usize {
        self.len
    }

    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.len == 0
    }

    #[must_use]
    pub fn capacity(&self) -> usize {
        self.capacity
    }

    /// Grow capacity so at least `additional` more inserts fit without
    /// triggering an internal resize. No-op if already large enough.
    ///
    /// # Panics
    ///
    /// Panics on capacity overflow. Use [`Self::try_reserve`] for fallible
    /// growth.
    pub fn reserve(&mut self, additional: usize) {
        let needed = self.len.saturating_add(additional);
        if needed <= self.max_insertions {
            return;
        }
        let new_capacity = self.grow_capacity_for(needed).expect("capacity overflow");
        self.resize(new_capacity);
    }

    /// Fallible counterpart to [`Self::reserve`].
    ///
    /// # Errors
    ///
    /// [`TryReserveError::CapacityOverflow`] if `self.len + additional`
    /// overflows; [`TryReserveError::AllocError`] on allocator failure.
    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
        let needed = self
            .len
            .checked_add(additional)
            .ok_or(TryReserveError::CapacityOverflow)?;
        if needed <= self.max_insertions {
            return Ok(());
        }
        let new_capacity = self
            .grow_capacity_for(needed)
            .ok_or(TryReserveError::CapacityOverflow)?;
        self.try_resize(new_capacity)
    }

    /// Shrinks the capacity as much as possible while preserving all live
    /// entries. Mirrors [`std::collections::HashMap::shrink_to_fit`].
    pub fn shrink_to_fit(&mut self) {
        self.shrink_to(0);
    }

    /// Shrinks the capacity with a lower bound. The table won't shrink below
    /// the larger of `min_capacity` and `self.len`. Mirrors
    /// [`std::collections::HashMap::shrink_to`].
    ///
    /// # Panics
    ///
    /// Panics if no representable capacity satisfies
    /// `max_insertions(cap) >= min_capacity`.
    pub fn shrink_to(&mut self, min_capacity: usize) {
        if self.len == 0 && min_capacity == 0 {
            if self.capacity > 0 {
                self.resize(0);
            }
            return;
        }
        let lower = self.len.max(min_capacity).max(INITIAL_CAPACITY);
        let new_capacity = capacity_for(INITIAL_CAPACITY, lower, self.reserve_fraction)
            .expect("capacity overflow");
        if new_capacity >= self.capacity {
            return;
        }
        self.resize(new_capacity);
    }

    /// Round up to the smallest capacity whose `max_insertions` accommodates
    /// `needed` live entries. Returns `None` if no representable capacity
    /// suffices.
    fn grow_capacity_for(&self, needed: usize) -> Option<usize> {
        capacity_for(
            self.capacity.max(INITIAL_CAPACITY),
            needed,
            self.reserve_fraction,
        )
    }

    /// # Panics
    ///
    /// Panics if a resize succeeds but no free slot can be found for the new key.
    pub fn insert(&mut self, key: K, value: V) -> Option<V> {
        let key_hash = self.hash_key(&key);
        let key_fingerprint = ControlOps::control_fingerprint(key_hash);

        let level_candidate =
            match self.find_in_levels_with_candidate(&key, key_hash, key_fingerprint) {
                Ok(location) => return Some(self.replace_existing_value(location, value)),
                Err(level_candidate) => level_candidate,
            };

        if let Some(location) = level_candidate
            && self.special.primary.len == 0
            && self.special.fallback.len == 0
        {
            return self.insert_at_location_after_resize_check(
                Some(location),
                key_hash,
                key,
                value,
                key_fingerprint,
            );
        }

        let (primary_step, primary_candidate) =
            self.find_in_special_primary_with_candidate(key_hash, key_fingerprint, &key);
        match primary_step {
            LookupStep::Found(slot_idx) => {
                return Some(
                    self.replace_existing_value(SlotLocation::SpecialPrimary { slot_idx }, value),
                );
            }
            LookupStep::StopSearch => {
                if let Some(location) = level_candidate {
                    return self.insert_at_location_after_resize_check(
                        Some(location),
                        key_hash,
                        key,
                        value,
                        key_fingerprint,
                    );
                }
                return self.insert_at_location_after_resize_check(
                    primary_candidate.map(|slot_idx| SlotLocation::SpecialPrimary { slot_idx }),
                    key_hash,
                    key,
                    value,
                    key_fingerprint,
                );
            }
            LookupStep::Continue => {}
        }

        let (fallback_match, fallback_candidate) =
            self.find_in_special_fallback_with_candidate(key_hash, key_fingerprint, &key);
        if let Some(slot_idx) = fallback_match {
            return Some(
                self.replace_existing_value(SlotLocation::SpecialFallback { slot_idx }, value),
            );
        }

        let insertion_slot = level_candidate
            .or_else(|| primary_candidate.map(|slot_idx| SlotLocation::SpecialPrimary { slot_idx }))
            .or_else(|| {
                fallback_candidate.map(|slot_idx| SlotLocation::SpecialFallback { slot_idx })
            });
        self.insert_at_location_after_resize_check(
            insertion_slot,
            key_hash,
            key,
            value,
            key_fingerprint,
        )
    }

    pub fn get<Q>(&self, key: &Q) -> Option<&V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        let key_hash = self.hash_key(key);
        let key_fingerprint = ControlOps::control_fingerprint(key_hash);

        match self.find_slot_location_with_hash(key, key_hash, key_fingerprint)? {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => Some(unsafe { &self.levels[level_idx].table.get_ref(slot_idx).value }),
            SlotLocation::SpecialPrimary { slot_idx } => {
                Some(unsafe { &self.special.primary.table.get_ref(slot_idx).value })
            }
            SlotLocation::SpecialFallback { slot_idx } => {
                Some(unsafe { &self.special.fallback.table.get_ref(slot_idx).value })
            }
        }
    }

    /// Like [`Self::get`] but returns the stored key alongside its value.
    pub fn get_key_value<Q>(&self, key: &Q) -> Option<(&K, &V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        let key_hash = self.hash_key(key);
        let key_fingerprint = ControlOps::control_fingerprint(key_hash);

        let entry = match self.find_slot_location_with_hash(key, key_hash, key_fingerprint)? {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => unsafe { self.levels[level_idx].table.get_ref(slot_idx) },
            SlotLocation::SpecialPrimary { slot_idx } => unsafe {
                self.special.primary.table.get_ref(slot_idx)
            },
            SlotLocation::SpecialFallback { slot_idx } => unsafe {
                self.special.fallback.table.get_ref(slot_idx)
            },
        };
        Some((&entry.key, &entry.value))
    }

    pub fn get_mut<Q>(&mut self, key: &Q) -> Option<&mut V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        let key_hash = self.hash_key(key);
        let key_fingerprint = ControlOps::control_fingerprint(key_hash);

        match self.find_slot_location_with_hash(key, key_hash, key_fingerprint)? {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => Some(unsafe { &mut self.levels[level_idx].table.get_mut(slot_idx).value }),
            SlotLocation::SpecialPrimary { slot_idx } => {
                Some(unsafe { &mut self.special.primary.table.get_mut(slot_idx).value })
            }
            SlotLocation::SpecialFallback { slot_idx } => {
                Some(unsafe { &mut self.special.fallback.table.get_mut(slot_idx).value })
            }
        }
    }

    /// Returns `N` disjoint mutable references, mirroring
    /// [`std::collections::HashMap::get_disjoint_mut`]: `None` if any key
    /// misses, panic on aliasing.
    ///
    /// # Panics
    ///
    /// If two input keys resolve to the same physical slot.
    pub fn get_disjoint_mut<Q, const N: usize>(&mut self, keys: [&Q; N]) -> Option<[&mut V; N]>
    where
        K: Borrow<Q> + Eq,
        Q: Hash + Eq + ?Sized,
    {
        let mut locations: [SlotLocation; N] = [SlotLocation::SpecialPrimary { slot_idx: 0 }; N];
        for (i, key) in keys.iter().enumerate() {
            let key_hash = self.hash_key(*key);
            let key_fingerprint = ControlOps::control_fingerprint(key_hash);
            locations[i] = self.find_slot_location_with_hash(*key, key_hash, key_fingerprint)?;
        }

        // O(N^2) alias check; cheaper than a HashSet for the small N
        // (typically <= 16) std::get_disjoint_mut targets.
        for i in 0..N {
            for j in (i + 1)..N {
                assert!(
                    locations[i] != locations[j],
                    "get_disjoint_mut: duplicate keys resolve to the same entry",
                );
            }
        }

        // SAFETY: locations are unique (checked above) and point to occupied
        // slots. Raw pointers let us hand out disjoint borrows into `levels`
        // / `special` without reborrowing the slices each iteration.
        let levels_ptr: *mut BucketLevel<K, V> = self.levels.as_mut_ptr();
        let primary_ptr: *mut SpecialPrimary<K, V> = &raw mut self.special.primary;
        let fallback_ptr: *mut SpecialFallback<K, V> = &raw mut self.special.fallback;
        let mut out: core::mem::MaybeUninit<[&mut V; N]> = core::mem::MaybeUninit::uninit();
        let out_ptr = out.as_mut_ptr().cast::<&mut V>();
        for (i, loc) in locations.into_iter().enumerate() {
            let value_ref: &mut V = match loc {
                SlotLocation::Level {
                    level_idx,
                    slot_idx,
                } => unsafe {
                    let level = &mut *levels_ptr.add(level_idx);
                    &mut level.table.get_mut(slot_idx).value
                },
                SlotLocation::SpecialPrimary { slot_idx } => unsafe {
                    &mut (*primary_ptr).table.get_mut(slot_idx).value
                },
                SlotLocation::SpecialFallback { slot_idx } => unsafe {
                    &mut (*fallback_ptr).table.get_mut(slot_idx).value
                },
            };
            unsafe { out_ptr.add(i).write(value_ref) };
        }
        Some(unsafe { out.assume_init() })
    }

    pub fn contains_key<Q>(&self, key: &Q) -> bool
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        let key_hash = self.hash_key(key);
        let key_fingerprint = ControlOps::control_fingerprint(key_hash);
        self.find_slot_location_with_hash(key, key_hash, key_fingerprint)
            .is_some()
    }

    pub fn remove<Q>(&mut self, key: &Q) -> Option<V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        self.remove_inner(key).map(|(_, v)| v)
    }

    /// Like [`Self::remove`] but returns the stored key alongside its value.
    pub fn remove_entry<Q>(&mut self, key: &Q) -> Option<(K, V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        self.remove_inner(key)
    }

    fn remove_inner<Q>(&mut self, key: &Q) -> Option<(K, V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        let key_hash = self.hash_key(key);
        let key_fingerprint = ControlOps::control_fingerprint(key_hash);
        let location = self.find_slot_location_with_hash(key, key_hash, key_fingerprint)?;

        let (removed_entry, needs_resize) = match location {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => {
                let level = &mut self.levels[level_idx];
                let removed = unsafe { level.table.take(slot_idx) };
                level.table.mark_tombstone(slot_idx);
                level.len -= 1;
                level.tombstones += 1;
                let needs_resize = level.tombstones > level.capacity() / 2;
                (removed, needs_resize)
            }
            SlotLocation::SpecialPrimary { slot_idx } => {
                let primary = &mut self.special.primary;
                let removed = unsafe { primary.table.take(slot_idx) };
                primary.table.mark_tombstone(slot_idx);
                primary.len -= 1;
                primary.tombstones += 1;
                let needs_resize = primary.tombstones > primary.table.capacity() / 2;
                (removed, needs_resize)
            }
            SlotLocation::SpecialFallback { slot_idx } => {
                let fallback = &mut self.special.fallback;
                let removed = unsafe { fallback.table.take(slot_idx) };
                fallback.table.mark_tombstone(slot_idx);
                fallback.len -= 1;
                fallback.tombstones += 1;
                let needs_resize = fallback.tombstones > fallback.capacity() / 2;
                (removed, needs_resize)
            }
        };

        self.len -= 1;
        self.shrink_max_populated_level();
        if needs_resize {
            self.resize(self.capacity);
        }
        Some((removed_entry.key, removed_entry.value))
    }

    /// Borrowing iterator over `&K`. Order matches [`Self::iter`].
    #[must_use]
    pub fn keys(&self) -> Keys<'_, K, V> {
        Keys::new(self.iter())
    }

    /// Borrowing iterator over `&V`. Order matches [`Self::iter`].
    #[must_use]
    pub fn values(&self) -> Values<'_, K, V> {
        Values::new(self.iter())
    }

    /// Reference to the map's [`BuildHasher`].
    #[must_use]
    pub fn hasher(&self) -> &DefaultHashBuilder {
        &self.hash_builder
    }

    #[must_use]
    pub fn iter(&self) -> FunnelIter<'_, K, V> {
        FunnelIter {
            levels: &self.levels,
            primary: &self.special.primary,
            fallback: &self.special.fallback,
            phase: FunnelIterPhase::Levels,
            level_idx: 0,
            slot_idx: 0,
        }
    }

    /// Mutable iterator yielding `(&K, &mut V)`. Mirrors `HashMap::iter_mut`.
    pub fn iter_mut(&mut self) -> FunnelIterMut<'_, K, V> {
        let levels_len = self.levels.len();
        let levels = self.levels.as_mut_ptr();
        let primary = ptr::from_mut(&mut self.special.primary);
        let fallback = ptr::from_mut(&mut self.special.fallback);
        FunnelIterMut {
            levels,
            levels_len,
            primary,
            fallback,
            phase: FunnelIterPhase::Levels,
            level_idx: 0,
            slot_idx: 0,
            _marker: PhantomData,
        }
    }

    /// Mutable iterator yielding `&mut V`. Mirrors `HashMap::values_mut`.
    pub fn values_mut(&mut self) -> FunnelValuesMut<'_, K, V> {
        FunnelValuesMut {
            inner: self.iter_mut(),
        }
    }

    /// Consuming iterator yielding owned keys. Mirrors `HashMap::into_keys`.
    #[must_use]
    pub fn into_keys(self) -> FunnelIntoKeys<K, V> {
        FunnelIntoKeys::new(self.into_iter())
    }

    /// Consuming iterator yielding owned values. Mirrors `HashMap::into_values`.
    #[must_use]
    pub fn into_values(self) -> FunnelIntoValues<K, V> {
        FunnelIntoValues::new(self.into_iter())
    }

    /// Returns an [`Entry`] for in-place manipulation of `key`'s slot.
    /// Mirrors [`std::collections::HashMap::entry`].
    pub fn entry(&mut self, key: K) -> Entry<'_, K, V> {
        let key_hash = self.hash_key(&key);
        let key_fingerprint = ControlOps::control_fingerprint(key_hash);
        if let Some(location) = self.find_slot_location_with_hash(&key, key_hash, key_fingerprint) {
            Entry::Occupied(OccupiedEntry {
                map: self,
                location,
            })
        } else {
            Entry::Vacant(VacantEntry {
                map: self,
                key,
                key_hash,
            })
        }
    }

    /// Inserts `key`/`value` if absent. Mirrors the unstable
    /// [`std::collections::HashMap::try_insert`].
    ///
    /// # Errors
    ///
    /// Returns [`OccupiedError`] if `key` was already present.
    pub fn try_insert(&mut self, key: K, value: V) -> Result<&mut V, OccupiedError<'_, K, V>> {
        match self.entry(key) {
            Entry::Occupied(entry) => Err(OccupiedError { entry, value }),
            Entry::Vacant(entry) => Ok(entry.insert(value)),
        }
    }

    /// Post-lookup insert for a key known to be absent. Returns the chosen
    /// slot so the caller can borrow into it without re-probing.
    fn insert_for_vacant_entry(&mut self, key: K, value: V, key_hash: u64) -> SlotLocation {
        let key_fingerprint = ControlOps::control_fingerprint(key_hash);

        let mut location = if self.len < self.max_insertions {
            self.choose_slot_for_new_key(key_hash)
        } else {
            None
        };

        if location.is_none() {
            let new_capacity = if self.capacity == 0 {
                INITIAL_CAPACITY
            } else {
                self.capacity.saturating_mul(2)
            };
            self.resize(new_capacity);
            location = Some(
                self.choose_slot_for_new_key(key_hash)
                    .expect("no free slot found after resize"),
            );
        }

        let final_location = location.expect("location set above");
        self.place_new_entry(final_location, key, value, key_fingerprint);
        final_location
    }

    /// Drops every entry for which `f(&K, &mut V)` returns `false`.
    /// Mirrors [`std::collections::HashMap::retain`].
    pub fn retain<F>(&mut self, mut f: F)
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        self.extract_if(|k, v| !f(k, v)).for_each(drop);
    }

    /// Returns a draining iterator that empties the map. Mirrors
    /// [`std::collections::HashMap::drain`].
    pub fn drain(&mut self) -> Drain<'_, K, V> {
        Drain {
            map: self,
            phase: DrainPhase::Levels,
            level_idx: 0,
            slot_idx: 0,
        }
    }

    /// Yields and removes `(K, V)` pairs where `f` returned `true`; kept
    /// entries remain in the map. Mirrors
    /// [`std::collections::HashMap::extract_if`].
    pub fn extract_if<F>(&mut self, f: F) -> ExtractIf<'_, K, V, F>
    where
        F: FnMut(&K, &mut V) -> bool,
    {
        ExtractIf {
            map: self,
            pred: f,
            phase: DrainPhase::Levels,
            level_idx: 0,
            slot_idx: 0,
        }
    }

    /// Triggers a no-grow rehash if any region crossed its cleanup threshold
    /// during a bulk op. Called once from each bulk-op iterator's `Drop`.
    fn resize_if_needed(&mut self) {
        let level_dirty = self
            .levels
            .iter()
            .any(|level| level.tombstones > level.capacity() / 2);
        let primary_dirty =
            self.special.primary.tombstones > self.special.primary.table.capacity() / 2;
        let fallback_dirty =
            self.special.fallback.tombstones > self.special.fallback.capacity() / 2;
        if level_dirty || primary_dirty || fallback_dirty {
            self.resize(self.capacity);
        }
    }

    pub fn clear(&mut self) {
        for level in &mut self.levels {
            for idx in 0..level.table.capacity() {
                if level.table.control_at(idx).is_occupied() {
                    unsafe { level.table.drop_in_place(idx) };
                }
            }
            level.table.clear_all_controls();
            level.len = 0;
            level.tombstones = 0;
        }

        for idx in 0..self.special.primary.table.capacity() {
            if self.special.primary.table.control_at(idx).is_occupied() {
                unsafe { self.special.primary.table.drop_in_place(idx) };
            }
        }
        self.special.primary.table.clear_all_controls();
        self.special.primary.len = 0;
        self.special.primary.tombstones = 0;
        self.special.primary.group_summaries.fill(0);

        for idx in 0..self.special.fallback.table.capacity() {
            if self.special.fallback.table.control_at(idx).is_occupied() {
                unsafe { self.special.fallback.table.drop_in_place(idx) };
            }
        }
        self.special.fallback.table.clear_all_controls();
        self.special.fallback.len = 0;
        self.special.fallback.tombstones = 0;

        self.len = 0;
        self.max_populated_level = 0;
    }

    /// Fallible counterpart to [`Self::resize`]. Allocates the new map
    /// before touching `self`, so `Err` leaves the map intact.
    fn try_resize(&mut self, new_capacity: usize) -> Result<(), TryReserveError> {
        let hash_builder = self.hash_builder.clone();
        let mut new_map = Self::try_with_options_and_hasher(
            FunnelOptions {
                capacity: new_capacity,
                reserve_fraction: self.reserve_fraction,
                primary_probe_limit: Some(self.primary_probe_limit),
            },
            hash_builder,
        )?;

        for level in &mut self.levels {
            for idx in 0..level.table.capacity() {
                if level.table.control_at(idx).is_occupied() {
                    let entry = unsafe { level.table.take(idx) };
                    new_map.insert_new_entry_unchecked(entry.key, entry.value);
                }
            }
            level.table.clear_all_controls();
            level.len = 0;
            level.tombstones = 0;
        }

        for idx in 0..self.special.primary.table.capacity() {
            if self.special.primary.table.control_at(idx).is_occupied() {
                let entry = unsafe { self.special.primary.table.take(idx) };
                new_map.insert_new_entry_unchecked(entry.key, entry.value);
            }
        }
        self.special.primary.table.clear_all_controls();
        self.special.primary.len = 0;
        self.special.primary.tombstones = 0;
        self.special.primary.group_summaries.fill(0);

        for idx in 0..self.special.fallback.table.capacity() {
            if self.special.fallback.table.control_at(idx).is_occupied() {
                let entry = unsafe { self.special.fallback.table.take(idx) };
                new_map.insert_new_entry_unchecked(entry.key, entry.value);
            }
        }
        self.special.fallback.table.clear_all_controls();
        self.special.fallback.len = 0;
        self.special.fallback.tombstones = 0;

        self.len = 0;
        self.max_populated_level = 0;
        *self = new_map;
        Ok(())
    }

    /// Fallible counterpart to [`Self::with_options_and_hasher`]. Returns
    /// `Err(TryReserveError::AllocError)` if any backing allocation fails.
    fn try_with_options_and_hasher(
        options: FunnelOptions,
        hash_builder: DefaultHashBuilder,
    ) -> Result<Self, TryReserveError> {
        let reserve_fraction =
            sanitize_reserve_fraction(options.reserve_fraction).min(MAX_FUNNEL_RESERVE_FRACTION);
        let capacity = options.capacity;
        let max_insertions = max_insertions(capacity, reserve_fraction);

        let level_count = compute_level_count(reserve_fraction);
        let bucket_width = round_up_to_group(compute_bucket_width(reserve_fraction));
        let primary_probe_limit = options
            .primary_probe_limit
            .unwrap_or_else(|| ProbeOps::log_log_probe_limit(capacity))
            .max(1);

        let mut special_capacity =
            choose_special_capacity(capacity, reserve_fraction, bucket_width);
        let mut main_capacity = capacity.saturating_sub(special_capacity);
        let main_remainder = main_capacity % bucket_width.max(1);
        if main_remainder != 0 {
            main_capacity = main_capacity.saturating_sub(main_remainder);
            special_capacity = capacity.saturating_sub(main_capacity);
        }

        let total_main_buckets = main_capacity.checked_div(bucket_width).unwrap_or(0);
        let level_bucket_counts = partition_funnel_buckets(total_main_buckets, level_count);
        let mut levels: Vec<BucketLevel<K, V>> = Vec::new();
        levels
            .try_reserve_exact(level_bucket_counts.len())
            .map_err(|_| TryReserveError::AllocError)?;
        for (level_idx, bucket_count) in level_bucket_counts.into_iter().enumerate() {
            levels.push(BucketLevel::try_with_bucket_count(
                bucket_count,
                bucket_width,
                level_salt(level_idx),
            )?);
        }

        let special = SpecialArray::try_with_capacity(special_capacity, primary_probe_limit)?;

        Ok(Self {
            levels,
            special,
            len: 0,
            capacity,
            max_insertions,
            reserve_fraction,
            primary_probe_limit,
            max_populated_level: 0,
            hash_builder,
        })
    }

    /// Drain all live entries (across levels + special), build a fresh map
    /// at `new_capacity`, reinsert. Also serves as a no-grow rehash when
    /// called with the current capacity.
    fn resize(&mut self, new_capacity: usize) {
        let mut entries = Vec::with_capacity(self.len);

        for level in &mut self.levels {
            for idx in 0..level.table.capacity() {
                if level.table.control_at(idx).is_occupied() {
                    let entry = unsafe { level.table.take(idx) };
                    entries.push((entry.key, entry.value));
                }
            }
            level.table.clear_all_controls();
            level.len = 0;
            level.tombstones = 0;
        }

        for idx in 0..self.special.primary.table.capacity() {
            if self.special.primary.table.control_at(idx).is_occupied() {
                let entry = unsafe { self.special.primary.table.take(idx) };
                entries.push((entry.key, entry.value));
            }
        }
        self.special.primary.table.clear_all_controls();
        self.special.primary.len = 0;
        self.special.primary.tombstones = 0;
        self.special.primary.group_summaries.fill(0);

        for idx in 0..self.special.fallback.table.capacity() {
            if self.special.fallback.table.control_at(idx).is_occupied() {
                let entry = unsafe { self.special.fallback.table.take(idx) };
                entries.push((entry.key, entry.value));
            }
        }
        self.special.fallback.table.clear_all_controls();
        self.special.fallback.len = 0;
        self.special.fallback.tombstones = 0;

        self.len = 0;
        self.max_populated_level = 0;

        let hash_builder = mem::take(&mut self.hash_builder);
        let mut new_map = Self::with_options_and_hasher(
            FunnelOptions {
                capacity: new_capacity,
                reserve_fraction: self.reserve_fraction,
                primary_probe_limit: Some(self.primary_probe_limit),
            },
            hash_builder,
        );

        for (key, value) in entries {
            new_map.insert_new_entry_unchecked(key, value);
        }

        *self = new_map;
    }

    #[inline]
    fn hash_key<Q>(&self, key: &Q) -> u64
    where
        Q: Hash + ?Sized,
    {
        self.hash_builder.hash_one(key)
    }

    /// Start group for the special primary probe sequence. `group_count`
    /// is pow2 by `SpecialPrimary::with_capacity` construction.
    #[inline]
    fn special_primary_group_start(&self, key_hash: u64) -> usize {
        let mask = self.special.primary.group_count_mask;
        if mask == 0 {
            return 0;
        }
        ProbeOps::hash_to_usize(key_hash.rotate_left(11)) & mask
    }

    /// Per-key odd step over the pow2 `group_count`. The `| 1` forces odd ⇒
    /// coprime to pow2 ⇒ `(group_idx + step) & mask` visits every group
    /// within `group_count` iterations.
    #[inline]
    fn special_primary_step(&self, key_hash: u64) -> usize {
        let mask = self.special.primary.group_count_mask;
        (ProbeOps::hash_to_usize(key_hash.rotate_left(43)) | 1) & mask
    }

    #[inline]
    fn special_fallback_bucket_a(key_hash: u64, bucket_count: usize) -> usize {
        ProbeOps::hash_to_usize(key_hash.rotate_left(19)) % bucket_count
    }

    #[inline]
    fn special_fallback_bucket_b(key_hash: u64, bucket_count: usize) -> usize {
        ProbeOps::hash_to_usize(key_hash.rotate_left(37)) % bucket_count
    }

    #[inline]
    fn choose_slot_for_new_key(&self, key_hash: u64) -> Option<SlotLocation> {
        for (level_idx, level) in self.levels.iter().enumerate() {
            if let Some(slot_idx) = Self::first_free_in_level_bucket(key_hash, level) {
                return Some(SlotLocation::Level {
                    level_idx,
                    slot_idx,
                });
            }
        }

        if let Some(slot_idx) = self.first_free_in_special_primary(key_hash) {
            return Some(SlotLocation::SpecialPrimary { slot_idx });
        }

        self.first_free_in_special_fallback(key_hash)
            .map(|slot_idx| SlotLocation::SpecialFallback { slot_idx })
    }

    /// Search bucket levels for `key`. Returns `Ok(SlotLocation)` on hit, or
    /// `Err(Some(insert_location))` with the first non-full bucket's
    /// candidate slot if known (used by insert to skip a re-search).
    /// `Err(None)` when no insert candidate was seen and the search exhausted.
    fn find_in_levels_with_candidate<Q>(
        &self,
        key: &Q,
        key_hash: u64,
        key_fingerprint: u8,
    ) -> Result<SlotLocation, Option<SlotLocation>>
    where
        K: Borrow<Q>,
        Q: Eq + ?Sized,
    {
        let search_limit = (self.max_populated_level + 1).min(self.levels.len());
        let mut candidate = None;

        for (level_idx, level) in self.levels[..search_limit].iter().enumerate() {
            let (lookup_step, slot_candidate) =
                Self::find_in_level_bucket_with_candidate(key_hash, key_fingerprint, key, level);
            if candidate.is_none() {
                candidate = slot_candidate.map(|slot_idx| SlotLocation::Level {
                    level_idx,
                    slot_idx,
                });
            }
            match lookup_step {
                LookupStep::Found(slot_idx) => {
                    return Ok(SlotLocation::Level {
                        level_idx,
                        slot_idx,
                    });
                }
                LookupStep::Continue => {}
                LookupStep::StopSearch => return Err(candidate),
            }
        }

        Err(candidate)
    }

    #[inline]
    fn replace_existing_value(&mut self, location: SlotLocation, value: V) -> V {
        match location {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => {
                let entry = unsafe { self.levels[level_idx].table.get_mut(slot_idx) };
                mem::replace(&mut entry.value, value)
            }
            SlotLocation::SpecialPrimary { slot_idx } => {
                let entry = unsafe { self.special.primary.table.get_mut(slot_idx) };
                mem::replace(&mut entry.value, value)
            }
            SlotLocation::SpecialFallback { slot_idx } => {
                let entry = unsafe { self.special.fallback.table.get_mut(slot_idx) };
                mem::replace(&mut entry.value, value)
            }
        }
    }

    #[inline]
    fn insert_new_entry_unchecked(&mut self, key: K, value: V) {
        let key_hash = self.hash_key(&key);
        let key_fingerprint = ControlOps::control_fingerprint(key_hash);
        let location = self
            .choose_slot_for_new_key(key_hash)
            .expect("resized funnel map should have free slot");
        self.place_new_entry(location, key, value, key_fingerprint);
    }

    #[inline]
    /// Insert `key`/`value` into a candidate slot, growing first via
    /// `resize` if `len >= max_insertions`. After resize, the candidate
    /// becomes stale, so this re-locates the slot from scratch.
    fn insert_at_location_after_resize_check(
        &mut self,
        location: Option<SlotLocation>,
        key_hash: u64,
        key: K,
        value: V,
        key_fingerprint: u8,
    ) -> Option<V> {
        let final_location = if self.len >= self.max_insertions || location.is_none() {
            let new_capacity = if self.capacity == 0 {
                INITIAL_CAPACITY
            } else {
                self.capacity.saturating_mul(2)
            };
            self.resize(new_capacity);
            self.choose_slot_for_new_key(key_hash)
                .expect("no free slot found after resize")
        } else {
            match location {
                Some(location) => location,
                None => unreachable!("checked for resize above"),
            }
        };

        self.place_new_entry(final_location, key, value, key_fingerprint);
        None
    }

    #[inline]
    fn place_new_entry(&mut self, location: SlotLocation, key: K, value: V, key_fingerprint: u8) {
        match location {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => {
                let level = &mut self.levels[level_idx];
                level
                    .table
                    .write_with_control(slot_idx, SlotEntry { key, value }, key_fingerprint);
                level.len += 1;
                if level_idx > self.max_populated_level {
                    self.max_populated_level = level_idx;
                }
            }
            SlotLocation::SpecialPrimary { slot_idx } => {
                let group_idx = slot_idx / GROUP_SIZE;
                let primary = &mut self.special.primary;
                // Reusing a tombstone slot must decrement the counter;
                // otherwise resize triggers on stale-since-resize counts.
                let was_tombstone = primary.table.control_at(slot_idx) == CTRL_TOMBSTONE;
                primary.table.write_with_control(
                    slot_idx,
                    SlotEntry { key, value },
                    key_fingerprint,
                );
                primary.len += 1;
                primary.group_summaries[group_idx] |= ControlOps::fingerprint_bit(key_fingerprint);
                if was_tombstone {
                    primary.tombstones -= 1;
                }
            }
            SlotLocation::SpecialFallback { slot_idx } => {
                let fallback = &mut self.special.fallback;
                fallback.table.write_with_control(
                    slot_idx,
                    SlotEntry { key, value },
                    key_fingerprint,
                );
                fallback.len += 1;
            }
        }
        self.len += 1;
    }

    fn first_free_in_level_bucket(key_hash: u64, level: &BucketLevel<K, V>) -> Option<usize> {
        if level.len >= level.capacity() || level.bucket_count == 0 {
            return None;
        }

        let bucket_idx = level.bucket_index(key_hash);
        let bucket_range = level.bucket_range(bucket_idx);
        debug_assert_eq!(bucket_range.start % GROUP_SIZE, 0);
        let group_idx = bucket_range.start / GROUP_SIZE;
        level
            .table
            .group_free_mask(group_idx)
            .truncate_to(level.bucket_size)
            .lowest()
            .map(|offset| bucket_range.start + offset)
    }

    fn first_free_in_special_primary(&self, key_hash: u64) -> Option<usize> {
        let primary = &self.special.primary;
        if primary.len >= primary.table.capacity() {
            return None;
        }

        let group_count = primary.table.group_count();
        let group_limit = self.primary_probe_limit.min(group_count.max(1));
        let mask = primary.group_count_mask;
        let mut group_idx = self.special_primary_group_start(key_hash);
        let step = self.special_primary_step(key_hash);
        for _ in 0..group_limit {
            if let Some(slot_idx) = Self::first_free_in_special_primary_group(primary, group_idx) {
                return Some(slot_idx);
            }
            group_idx = (group_idx + step) & mask;
        }
        None
    }

    fn first_free_in_special_fallback(&self, key_hash: u64) -> Option<usize> {
        let fallback = &self.special.fallback;
        if fallback.len >= fallback.capacity() {
            return None;
        }

        let bucket_count = fallback.bucket_count();
        if bucket_count == 0 {
            return None;
        }

        let bucket_a = Self::special_fallback_bucket_a(key_hash, bucket_count);
        let bucket_b = Self::special_fallback_bucket_b(key_hash, bucket_count);

        for &bucket_idx in &[bucket_a, bucket_b] {
            let range = fallback.bucket_range(bucket_idx);
            for slot_idx in range {
                if fallback.table.control_at(slot_idx).is_free() {
                    return Some(slot_idx);
                }
            }
        }

        None
    }

    #[inline]
    fn first_free_in_special_primary_group(
        primary: &SpecialPrimary<K, V>,
        group_idx: usize,
    ) -> Option<usize> {
        primary.table.first_free_in_group(group_idx)
    }

    fn find_in_level_bucket<Q>(
        key_hash: u64,
        key_fingerprint: u8,
        key: &Q,
        level: &BucketLevel<K, V>,
    ) -> LookupStep
    where
        K: Borrow<Q>,
        Q: Eq + ?Sized,
    {
        if level.len == 0 || level.bucket_count == 0 {
            return LookupStep::Continue;
        }

        let bucket_idx = level.bucket_index(key_hash);
        let bucket_range = level.bucket_range(bucket_idx);
        debug_assert_eq!(bucket_range.start % GROUP_SIZE, 0);
        let group_idx = bucket_range.start / GROUP_SIZE;

        // SIMD fingerprint scan — same as _with_candidate.
        for relative_idx in level
            .table
            .group_match_mask(group_idx, key_fingerprint)
            .truncate_to(level.bucket_size)
        {
            let slot_idx = bucket_range.start + relative_idx;
            let entry = unsafe { level.table.get_ref(slot_idx) };
            if entry.key.borrow() == key {
                return LookupStep::Found(slot_idx);
            }
        }

        // StopSearch: bucket has an EMPTY byte → no key ever overflowed past
        // here. Tombstones in the bucket don't disable termination since the
        // empty byte still proves the probe chain terminated naturally.
        if level
            .table
            .group_match_mask(group_idx, CTRL_EMPTY)
            .truncate_to(level.bucket_size)
            .any()
        {
            return LookupStep::StopSearch;
        }

        LookupStep::Continue
    }

    fn find_in_level_bucket_with_candidate<Q>(
        key_hash: u64,
        key_fingerprint: u8,
        key: &Q,
        level: &BucketLevel<K, V>,
    ) -> (LookupStep, Option<usize>)
    where
        K: Borrow<Q>,
        Q: Eq + ?Sized,
    {
        if level.len == 0 {
            return (LookupStep::Continue, None);
        }

        if level.bucket_count == 0 {
            return (LookupStep::Continue, None);
        }

        let bucket_idx = level.bucket_index(key_hash);
        let bucket_range = level.bucket_range(bucket_idx);
        debug_assert_eq!(bucket_range.start % GROUP_SIZE, 0);
        let group_idx = bucket_range.start / GROUP_SIZE;

        // SIMD fingerprint scan over the bucket's control bytes.
        for relative_idx in level
            .table
            .group_match_mask(group_idx, key_fingerprint)
            .truncate_to(level.bucket_size)
        {
            let slot_idx = bucket_range.start + relative_idx;
            let entry = unsafe { level.table.get_ref(slot_idx) };
            if entry.key.borrow() == key {
                let free_candidate = level
                    .table
                    .group_free_mask(group_idx)
                    .truncate_to(level.bucket_size)
                    .lowest()
                    .map(|o| bucket_range.start + o);
                return (LookupStep::Found(slot_idx), free_candidate);
            }
        }

        // No match — compute free candidate and early-exit status.
        let free_candidate = level
            .table
            .group_free_mask(group_idx)
            .truncate_to(level.bucket_size)
            .lowest()
            .map(|o| bucket_range.start + o);

        let step = if level
            .table
            .group_match_mask(group_idx, CTRL_EMPTY)
            .truncate_to(level.bucket_size)
            .any()
        {
            LookupStep::StopSearch
        } else {
            LookupStep::Continue
        };

        (step, free_candidate)
    }

    /// Probe the special primary for `key` (lookup-only — no insert
    /// candidate tracking). Bounded by `primary_probe_limit` groups; if
    /// reached without a match and no tombstones seen, returns `StopSearch`
    /// so the caller skips fallback.
    fn find_in_special_primary<Q>(&self, key_hash: u64, key_fingerprint: u8, key: &Q) -> LookupStep
    where
        K: Borrow<Q>,
        Q: Eq + ?Sized,
    {
        let primary = &self.special.primary;
        if primary.len == 0 {
            return LookupStep::Continue;
        }

        let fingerprint_mask = ControlOps::fingerprint_bit(key_fingerprint);
        let group_count = primary.table.group_count();
        let group_limit = self.primary_probe_limit.min(group_count.max(1));
        let mask = primary.group_count_mask;
        let mut group_idx = self.special_primary_group_start(key_hash);
        let step = self.special_primary_step(key_hash);
        for _ in 0..group_limit {
            if primary.group_summaries[group_idx] & fingerprint_mask != 0 {
                for relative_idx in primary.table.group_match_mask(group_idx, key_fingerprint) {
                    let slot_idx = group_idx * GROUP_SIZE + relative_idx;
                    let entry = unsafe { primary.table.get_ref(slot_idx) };
                    if entry.key.borrow() == key {
                        return LookupStep::Found(slot_idx);
                    }
                }
            }
            if !primary
                .table
                .group_match_mask(group_idx, CTRL_TOMBSTONE)
                .any()
                && primary.table.group_match_mask(group_idx, CTRL_EMPTY).any()
            {
                return LookupStep::StopSearch;
            }
            let next = (group_idx + step) & mask;
            unsafe { prefetch_read(primary.table.group_data_ptr(next)) };
            group_idx = next;
        }
        LookupStep::Continue
    }

    /// Like `find_in_special_primary`, but also remembers the first
    /// FREE-or-TOMBSTONE slot seen so insert can land there without a re-scan.
    fn find_in_special_primary_with_candidate<Q>(
        &self,
        key_hash: u64,
        key_fingerprint: u8,
        key: &Q,
    ) -> (LookupStep, Option<usize>)
    where
        K: Borrow<Q>,
        Q: Eq + ?Sized,
    {
        let primary = &self.special.primary;
        if primary.table.capacity() == 0 {
            return (LookupStep::Continue, None);
        }
        if primary.len == 0 {
            return (
                LookupStep::Continue,
                self.first_free_in_special_primary(key_hash),
            );
        }

        let fingerprint_mask = ControlOps::fingerprint_bit(key_fingerprint);
        let group_count = primary.table.group_count();
        let group_limit = self.primary_probe_limit.min(group_count.max(1));
        let mask = primary.group_count_mask;
        let mut candidate = None;
        let mut group_idx = self.special_primary_group_start(key_hash);
        let step = self.special_primary_step(key_hash);
        for _ in 0..group_limit {
            // Cache the free-in-group result so candidate-population and
            // StopSearch share one SIMD scan per group.
            let free_in_group = if candidate.is_none() {
                let slot = primary.table.first_free_in_group(group_idx);
                if let Some(slot) = slot {
                    candidate = Some(slot);
                }
                slot.is_some()
            } else {
                primary.table.group_match_mask(group_idx, CTRL_EMPTY).any()
            };
            if primary.group_summaries[group_idx] & fingerprint_mask != 0 {
                for relative_idx in primary.table.group_match_mask(group_idx, key_fingerprint) {
                    let slot_idx = group_idx * GROUP_SIZE + relative_idx;
                    let entry = unsafe { primary.table.get_ref(slot_idx) };
                    if entry.key.borrow() == key {
                        return (LookupStep::Found(slot_idx), candidate);
                    }
                }
            }
            if free_in_group
                && !primary
                    .table
                    .group_match_mask(group_idx, CTRL_TOMBSTONE)
                    .any()
            {
                return (LookupStep::StopSearch, candidate);
            }
            let next = (group_idx + step) & mask;
            unsafe { prefetch_read(primary.table.group_data_ptr(next)) };
            group_idx = next;
        }
        (LookupStep::Continue, candidate)
    }

    /// Probe the special fallback for `key`. Bucket-local search like
    /// `BucketLevel`, but with larger buckets sized for primary spillover.
    fn find_in_special_fallback<Q>(
        &self,
        key_hash: u64,
        key_fingerprint: u8,
        key: &Q,
    ) -> Option<usize>
    where
        K: Borrow<Q>,
        Q: Eq + ?Sized,
    {
        let fallback = &self.special.fallback;
        if fallback.len == 0 {
            return None;
        }

        let bucket_count = fallback.bucket_count();
        if bucket_count == 0 {
            return None;
        }

        let bucket_a = Self::special_fallback_bucket_a(key_hash, bucket_count);
        let bucket_b = Self::special_fallback_bucket_b(key_hash, bucket_count);

        for bucket_idx in [bucket_a, bucket_b] {
            let range = fallback.bucket_range(bucket_idx);
            let controls = unsafe {
                std::slice::from_raw_parts(
                    fallback.table.group_data_ptr(0).add(range.start),
                    range.len(),
                )
            };

            let mut match_offset = 0;
            while let Some(relative_idx) = ControlOps::find_next_fingerprint_in_controls(
                controls,
                key_fingerprint,
                match_offset,
            ) {
                let slot_idx = range.start + relative_idx;
                let entry = unsafe { fallback.table.get_ref(slot_idx) };
                if entry.key.borrow() == key {
                    return Some(slot_idx);
                }
                match_offset = relative_idx + 1;
            }
        }

        None
    }

    /// Like `find_in_special_fallback`, but also tracks the first
    /// FREE-or-TOMBSTONE slot for insert.
    fn find_in_special_fallback_with_candidate<Q>(
        &self,
        key_hash: u64,
        key_fingerprint: u8,
        key: &Q,
    ) -> (Option<usize>, Option<usize>)
    where
        K: Borrow<Q>,
        Q: Eq + ?Sized,
    {
        let fallback = &self.special.fallback;
        if fallback.capacity() == 0 {
            return (None, None);
        }
        if fallback.len == 0 {
            return (None, self.first_free_in_special_fallback(key_hash));
        }

        let bucket_count = fallback.bucket_count();
        if bucket_count == 0 {
            return (None, None);
        }

        let bucket_a = Self::special_fallback_bucket_a(key_hash, bucket_count);
        let bucket_b = Self::special_fallback_bucket_b(key_hash, bucket_count);
        let mut candidate = None;

        // Find a free slot in either bucket.
        for &bucket_idx in &[bucket_a, bucket_b] {
            if candidate.is_some() {
                break;
            }
            let range = fallback.bucket_range(bucket_idx);
            for slot_idx in range {
                if fallback.table.control_at(slot_idx).is_free() {
                    candidate = Some(slot_idx);
                    break;
                }
            }
        }

        (
            self.find_in_special_fallback(key_hash, key_fingerprint, key),
            candidate,
        )
    }

    fn find_slot_location_with_hash<Q>(
        &self,
        key: &Q,
        key_hash: u64,
        key_fingerprint: u8,
    ) -> Option<SlotLocation>
    where
        K: Borrow<Q>,
        Q: Eq + ?Sized,
    {
        let search_limit = (self.max_populated_level + 1).min(self.levels.len());
        for (level_idx, level) in self.levels[..search_limit].iter().enumerate() {
            match Self::find_in_level_bucket(key_hash, key_fingerprint, key, level) {
                LookupStep::Found(slot_idx) => {
                    return Some(SlotLocation::Level {
                        level_idx,
                        slot_idx,
                    });
                }
                LookupStep::Continue => {}
                LookupStep::StopSearch => return None,
            }
        }

        if self.special.primary.len == 0 && self.special.fallback.len == 0 {
            return None;
        }

        match self.find_in_special_primary(key_hash, key_fingerprint, key) {
            LookupStep::Found(slot_idx) => return Some(SlotLocation::SpecialPrimary { slot_idx }),
            LookupStep::Continue => {}
            LookupStep::StopSearch => return None,
        }

        self.find_in_special_fallback(key_hash, key_fingerprint, key)
            .map(|slot_idx| SlotLocation::SpecialFallback { slot_idx })
    }

    fn shrink_max_populated_level(&mut self) {
        while self.max_populated_level > 0 && self.levels[self.max_populated_level].len == 0 {
            self.max_populated_level -= 1;
        }
    }
}

/// A view into a single entry in a [`FunnelHashMap`], which may be either
/// vacant or occupied. Constructed via [`FunnelHashMap::entry`].
pub enum Entry<'a, K: 'a, V: 'a> {
    /// Slot is occupied; key already lives in the map.
    Occupied(OccupiedEntry<'a, K, V>),
    /// Slot is vacant; the supplied key does not exist in the map yet.
    Vacant(VacantEntry<'a, K, V>),
}

/// View of an occupied entry in a [`FunnelHashMap`].
pub struct OccupiedEntry<'a, K, V> {
    map: &'a mut FunnelHashMap<K, V>,
    location: SlotLocation,
}

impl<'a, K, V> OccupiedEntry<'a, K, V>
where
    K: Eq + Hash,
{
    /// Returns a reference to the entry's key.
    #[must_use]
    pub fn key(&self) -> &K {
        match self.location {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => unsafe { &self.map.levels[level_idx].table.get_ref(slot_idx).key },
            SlotLocation::SpecialPrimary { slot_idx } => unsafe {
                &self.map.special.primary.table.get_ref(slot_idx).key
            },
            SlotLocation::SpecialFallback { slot_idx } => unsafe {
                &self.map.special.fallback.table.get_ref(slot_idx).key
            },
        }
    }

    /// Returns a reference to the entry's value.
    #[must_use]
    pub fn get(&self) -> &V {
        match self.location {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => unsafe { &self.map.levels[level_idx].table.get_ref(slot_idx).value },
            SlotLocation::SpecialPrimary { slot_idx } => unsafe {
                &self.map.special.primary.table.get_ref(slot_idx).value
            },
            SlotLocation::SpecialFallback { slot_idx } => unsafe {
                &self.map.special.fallback.table.get_ref(slot_idx).value
            },
        }
    }

    /// Returns `&mut V`. Borrow is tied to `self`; for the map's lifetime
    /// use [`OccupiedEntry::into_mut`].
    pub fn get_mut(&mut self) -> &mut V {
        match self.location {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => unsafe { &mut self.map.levels[level_idx].table.get_mut(slot_idx).value },
            SlotLocation::SpecialPrimary { slot_idx } => unsafe {
                &mut self.map.special.primary.table.get_mut(slot_idx).value
            },
            SlotLocation::SpecialFallback { slot_idx } => unsafe {
                &mut self.map.special.fallback.table.get_mut(slot_idx).value
            },
        }
    }

    /// Consumes the entry and returns `&mut V` borrowed from the map.
    #[must_use]
    pub fn into_mut(self) -> &'a mut V {
        match self.location {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => unsafe { &mut self.map.levels[level_idx].table.get_mut(slot_idx).value },
            SlotLocation::SpecialPrimary { slot_idx } => unsafe {
                &mut self.map.special.primary.table.get_mut(slot_idx).value
            },
            SlotLocation::SpecialFallback { slot_idx } => unsafe {
                &mut self.map.special.fallback.table.get_mut(slot_idx).value
            },
        }
    }

    /// Replaces the entry's value and returns the old one.
    pub fn insert(&mut self, value: V) -> V {
        self.map.replace_existing_value(self.location, value)
    }

    /// Removes the entry and returns its value.
    #[must_use]
    pub fn remove(self) -> V {
        self.remove_entry().1
    }

    /// Removes the entry and returns the `(key, value)` pair.
    #[must_use]
    pub fn remove_entry(self) -> (K, V) {
        let (removed_entry, needs_resize) = match self.location {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => {
                let level = &mut self.map.levels[level_idx];
                let removed = unsafe { level.table.take(slot_idx) };
                level.table.mark_tombstone(slot_idx);
                level.len -= 1;
                level.tombstones += 1;
                let needs_resize = level.tombstones > level.capacity() / 2;
                (removed, needs_resize)
            }
            SlotLocation::SpecialPrimary { slot_idx } => {
                let primary = &mut self.map.special.primary;
                let removed = unsafe { primary.table.take(slot_idx) };
                primary.table.mark_tombstone(slot_idx);
                primary.len -= 1;
                primary.tombstones += 1;
                let needs_resize = primary.tombstones > primary.table.capacity() / 2;
                (removed, needs_resize)
            }
            SlotLocation::SpecialFallback { slot_idx } => {
                let fallback = &mut self.map.special.fallback;
                let removed = unsafe { fallback.table.take(slot_idx) };
                fallback.table.mark_tombstone(slot_idx);
                fallback.len -= 1;
                fallback.tombstones += 1;
                let needs_resize = fallback.tombstones > fallback.capacity() / 2;
                (removed, needs_resize)
            }
        };

        self.map.len -= 1;
        self.map.shrink_max_populated_level();
        if needs_resize {
            let capacity = self.map.capacity;
            self.map.resize(capacity);
        }
        (removed_entry.key, removed_entry.value)
    }
}

/// View of a vacant entry in a [`FunnelHashMap`].
pub struct VacantEntry<'a, K, V> {
    map: &'a mut FunnelHashMap<K, V>,
    key: K,
    key_hash: u64,
}

impl<'a, K, V> VacantEntry<'a, K, V>
where
    K: Eq + Hash,
{
    /// Returns a reference to the key that would be inserted.
    pub fn key(&self) -> &K {
        &self.key
    }

    /// Consumes the entry and returns the key without inserting.
    #[must_use]
    pub fn into_key(self) -> K {
        self.key
    }

    /// Inserts `value` for the entry's key, returning `&mut V`.
    pub fn insert(self, value: V) -> &'a mut V {
        let location = self
            .map
            .insert_for_vacant_entry(self.key, value, self.key_hash);
        match location {
            SlotLocation::Level {
                level_idx,
                slot_idx,
            } => unsafe { &mut self.map.levels[level_idx].table.get_mut(slot_idx).value },
            SlotLocation::SpecialPrimary { slot_idx } => unsafe {
                &mut self.map.special.primary.table.get_mut(slot_idx).value
            },
            SlotLocation::SpecialFallback { slot_idx } => unsafe {
                &mut self.map.special.fallback.table.get_mut(slot_idx).value
            },
        }
    }
}

impl<'a, K, V> Entry<'a, K, V>
where
    K: Eq + Hash,
{
    /// Returns a mutable reference to the entry's value, inserting `default`
    /// first if vacant.
    pub fn or_insert(self, default: V) -> &'a mut V {
        match self {
            Entry::Occupied(entry) => entry.into_mut(),
            Entry::Vacant(entry) => entry.insert(default),
        }
    }

    /// Like [`Entry::or_insert`] but the default is computed lazily.
    pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V {
        match self {
            Entry::Occupied(entry) => entry.into_mut(),
            Entry::Vacant(entry) => entry.insert(default()),
        }
    }

    /// Like [`Entry::or_insert_with`] but the default closure receives a
    /// reference to the key.
    pub fn or_insert_with_key<F: FnOnce(&K) -> V>(self, default: F) -> &'a mut V {
        match self {
            Entry::Occupied(entry) => entry.into_mut(),
            Entry::Vacant(entry) => {
                let value = default(entry.key());
                entry.insert(value)
            }
        }
    }

    /// Returns a reference to this entry's key.
    pub fn key(&self) -> &K {
        match self {
            Entry::Occupied(entry) => entry.key(),
            Entry::Vacant(entry) => entry.key(),
        }
    }

    /// Runs `f` against the value if the entry is occupied, then returns
    /// the entry for further chaining.
    #[must_use]
    pub fn and_modify<F: FnOnce(&mut V)>(self, f: F) -> Self {
        match self {
            Entry::Occupied(mut entry) => {
                f(entry.get_mut());
                Entry::Occupied(entry)
            }
            Entry::Vacant(entry) => Entry::Vacant(entry),
        }
    }
}

impl<'a, K, V> Entry<'a, K, V>
where
    K: Eq + Hash,
    V: Default,
{
    /// Returns a mutable reference to the value, inserting `V::default()`
    /// first if vacant.
    pub fn or_default(self) -> &'a mut V {
        match self {
            Entry::Occupied(entry) => entry.into_mut(),
            Entry::Vacant(entry) => entry.insert(V::default()),
        }
    }
}

/// Error returned by [`FunnelHashMap::try_insert`] on key collision.
pub type OccupiedError<'a, K, V> = CommonOccupiedError<OccupiedEntry<'a, K, V>, V>;

impl<K, V> EntryView for OccupiedEntry<'_, K, V>
where
    K: Eq + Hash,
{
    type Key = K;
    type Value = V;
    fn view_key(&self) -> &K {
        self.key()
    }
    fn view_value(&self) -> &V {
        self.get()
    }
}

/// Three-phase iterator state: walk all bucket levels, then the special
/// primary, then the special fallback.
#[derive(Debug, Clone, Copy)]
enum FunnelIterPhase {
    Levels,
    Primary,
    Fallback,
    Done,
}

/// Borrowing iterator over occupied entries.
/// Visits each region in funnel order: bucket levels → special primary → special fallback.
/// Skips FREE and TOMBSTONE control bytes.
#[derive(Clone)]
pub struct FunnelIter<'a, K, V> {
    levels: &'a [BucketLevel<K, V>],
    primary: &'a SpecialPrimary<K, V>,
    fallback: &'a SpecialFallback<K, V>,
    phase: FunnelIterPhase,
    level_idx: usize,
    slot_idx: usize,
}

impl<K, V> fmt::Debug for FunnelIter<'_, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FunnelIter")
            .field("phase", &self.phase)
            .field("level_idx", &self.level_idx)
            .field("slot_idx", &self.slot_idx)
            .finish_non_exhaustive()
    }
}

impl<'a, K, V> Iterator for FunnelIter<'a, K, V> {
    type Item = (&'a K, &'a V);

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            match self.phase {
                FunnelIterPhase::Levels => {
                    while self.level_idx < self.levels.len() {
                        let table = &self.levels[self.level_idx].table;
                        while self.slot_idx < table.capacity() {
                            let idx = self.slot_idx;
                            self.slot_idx += 1;
                            if table.control_at(idx).is_occupied() {
                                let entry = unsafe { table.get_ref(idx) };
                                return Some((&entry.key, &entry.value));
                            }
                        }
                        self.level_idx += 1;
                        self.slot_idx = 0;
                    }
                    self.phase = FunnelIterPhase::Primary;
                    self.slot_idx = 0;
                }
                FunnelIterPhase::Primary => {
                    let table = &self.primary.table;
                    while self.slot_idx < table.capacity() {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if table.control_at(idx).is_occupied() {
                            let entry = unsafe { table.get_ref(idx) };
                            return Some((&entry.key, &entry.value));
                        }
                    }
                    self.phase = FunnelIterPhase::Fallback;
                    self.slot_idx = 0;
                }
                FunnelIterPhase::Fallback => {
                    let table = &self.fallback.table;
                    while self.slot_idx < table.capacity() {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if table.control_at(idx).is_occupied() {
                            let entry = unsafe { table.get_ref(idx) };
                            return Some((&entry.key, &entry.value));
                        }
                    }
                    self.phase = FunnelIterPhase::Done;
                }
                FunnelIterPhase::Done => return None,
            }
        }
    }
}

impl<K, V> FusedIterator for FunnelIter<'_, K, V> {}

impl<'a, K, V> IntoIterator for &'a FunnelHashMap<K, V>
where
    K: Eq + Hash,
{
    type Item = (&'a K, &'a V);
    type IntoIter = FunnelIter<'a, K, V>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

/// Walk phase shared by `Drain` and `ExtractIf`: levels first, then the
/// special primary, then the special fallback.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum DrainPhase {
    Levels,
    Primary,
    Fallback,
    Done,
}

/// Draining iterator. Yields and removes every `(K, V)` entry; the map is
/// empty once the iterator is consumed or dropped. Returned by
/// [`FunnelHashMap::drain`].
pub struct Drain<'a, K, V> {
    map: &'a mut FunnelHashMap<K, V>,
    phase: DrainPhase,
    level_idx: usize,
    slot_idx: usize,
}

impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Drain<'_, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Drain").finish_non_exhaustive()
    }
}

impl<K, V> Iterator for Drain<'_, K, V> {
    type Item = (K, V);

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            match self.phase {
                DrainPhase::Levels => {
                    while self.level_idx < self.map.levels.len() {
                        let level = &mut self.map.levels[self.level_idx];
                        while self.slot_idx < level.table.capacity() {
                            let idx = self.slot_idx;
                            self.slot_idx += 1;
                            if level.table.control_at(idx).is_occupied() {
                                let entry = unsafe { level.table.take(idx) };
                                level.table.mark_tombstone(idx);
                                level.len -= 1;
                                level.tombstones += 1;
                                self.map.len -= 1;
                                return Some((entry.key, entry.value));
                            }
                        }
                        self.level_idx += 1;
                        self.slot_idx = 0;
                    }
                    self.phase = DrainPhase::Primary;
                    self.slot_idx = 0;
                }
                DrainPhase::Primary => {
                    let primary = &mut self.map.special.primary;
                    while self.slot_idx < primary.table.capacity() {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if primary.table.control_at(idx).is_occupied() {
                            let entry = unsafe { primary.table.take(idx) };
                            primary.table.mark_tombstone(idx);
                            primary.len -= 1;
                            primary.tombstones += 1;
                            self.map.len -= 1;
                            return Some((entry.key, entry.value));
                        }
                    }
                    self.phase = DrainPhase::Fallback;
                    self.slot_idx = 0;
                }
                DrainPhase::Fallback => {
                    let fallback = &mut self.map.special.fallback;
                    while self.slot_idx < fallback.table.capacity() {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if fallback.table.control_at(idx).is_occupied() {
                            let entry = unsafe { fallback.table.take(idx) };
                            fallback.table.mark_tombstone(idx);
                            fallback.len -= 1;
                            fallback.tombstones += 1;
                            self.map.len -= 1;
                            return Some((entry.key, entry.value));
                        }
                    }
                    self.phase = DrainPhase::Done;
                }
                DrainPhase::Done => return None,
            }
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.map.len, Some(self.map.len))
    }
}

impl<K, V> ExactSizeIterator for Drain<'_, K, V> {}
impl<K, V> FusedIterator for Drain<'_, K, V> {}

impl<K, V> Drop for Drain<'_, K, V> {
    fn drop(&mut self) {
        // Drain any unyielded entries so values run their `Drop`.
        for _ in &mut *self {}
        // All entries moved out via `next()`; wipe ctrl bytes + counters en bloc.
        for level in &mut self.map.levels {
            level.table.clear_all_controls();
            level.len = 0;
            level.tombstones = 0;
        }
        self.map.special.primary.table.clear_all_controls();
        self.map.special.primary.len = 0;
        self.map.special.primary.tombstones = 0;
        self.map.special.primary.group_summaries.fill(0);
        self.map.special.fallback.table.clear_all_controls();
        self.map.special.fallback.len = 0;
        self.map.special.fallback.tombstones = 0;
        self.map.len = 0;
        self.map.max_populated_level = 0;
    }
}

/// Filtering drain. Yields and removes entries for which the predicate
/// returns `true`; the rest stay in the map. Returned by
/// [`FunnelHashMap::extract_if`].
pub struct ExtractIf<'a, K, V, F>
where
    K: Eq + Hash,
    F: FnMut(&K, &mut V) -> bool,
{
    map: &'a mut FunnelHashMap<K, V>,
    pred: F,
    phase: DrainPhase,
    level_idx: usize,
    slot_idx: usize,
}

impl<K, V, F> fmt::Debug for ExtractIf<'_, K, V, F>
where
    K: Eq + Hash,
    F: FnMut(&K, &mut V) -> bool,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("ExtractIf").finish_non_exhaustive()
    }
}

impl<K, V, F> Iterator for ExtractIf<'_, K, V, F>
where
    K: Eq + Hash,
    F: FnMut(&K, &mut V) -> bool,
{
    type Item = (K, V);

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            match self.phase {
                DrainPhase::Levels => {
                    while self.level_idx < self.map.levels.len() {
                        let level = &mut self.map.levels[self.level_idx];
                        while self.slot_idx < level.table.capacity() {
                            let idx = self.slot_idx;
                            self.slot_idx += 1;
                            if !level.table.control_at(idx).is_occupied() {
                                continue;
                            }
                            let entry = unsafe { level.table.get_mut(idx) };
                            if (self.pred)(&entry.key, &mut entry.value) {
                                let removed = unsafe { level.table.take(idx) };
                                level.table.mark_tombstone(idx);
                                level.len -= 1;
                                level.tombstones += 1;
                                self.map.len -= 1;
                                return Some((removed.key, removed.value));
                            }
                        }
                        self.level_idx += 1;
                        self.slot_idx = 0;
                    }
                    self.phase = DrainPhase::Primary;
                    self.slot_idx = 0;
                }
                DrainPhase::Primary => {
                    let primary = &mut self.map.special.primary;
                    while self.slot_idx < primary.table.capacity() {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if !primary.table.control_at(idx).is_occupied() {
                            continue;
                        }
                        let entry = unsafe { primary.table.get_mut(idx) };
                        if (self.pred)(&entry.key, &mut entry.value) {
                            let removed = unsafe { primary.table.take(idx) };
                            primary.table.mark_tombstone(idx);
                            primary.len -= 1;
                            primary.tombstones += 1;
                            self.map.len -= 1;
                            return Some((removed.key, removed.value));
                        }
                    }
                    self.phase = DrainPhase::Fallback;
                    self.slot_idx = 0;
                }
                DrainPhase::Fallback => {
                    let fallback = &mut self.map.special.fallback;
                    while self.slot_idx < fallback.table.capacity() {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if !fallback.table.control_at(idx).is_occupied() {
                            continue;
                        }
                        let entry = unsafe { fallback.table.get_mut(idx) };
                        if (self.pred)(&entry.key, &mut entry.value) {
                            let removed = unsafe { fallback.table.take(idx) };
                            fallback.table.mark_tombstone(idx);
                            fallback.len -= 1;
                            fallback.tombstones += 1;
                            self.map.len -= 1;
                            return Some((removed.key, removed.value));
                        }
                    }
                    self.phase = DrainPhase::Done;
                }
                DrainPhase::Done => return None,
            }
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (0, Some(self.map.len))
    }
}

impl<K, V, F> Drop for ExtractIf<'_, K, V, F>
where
    K: Eq + Hash,
    F: FnMut(&K, &mut V) -> bool,
{
    fn drop(&mut self) {
        // Dropping `extract_if` early leaves unvisited entries in the map.
        // Only consolidate any tombstone backlog from already-removed entries.
        self.map.resize_if_needed();
    }
}

/// `&K` iterator returned by [`FunnelHashMap::keys`].
pub type Keys<'a, K, V> = CommonKeys<FunnelIter<'a, K, V>>;
/// `&V` iterator returned by [`FunnelHashMap::values`].
pub type Values<'a, K, V> = CommonValues<FunnelIter<'a, K, V>>;

/// `(&K, &mut V)` iterator. Visits bucket levels → special primary →
/// special fallback. Skips FREE / TOMBSTONE.
///
/// SAFETY: raw pointers to each region + `PhantomData<&'a mut FunnelHashMap>`
/// tie the iterator to the map's exclusive borrow. Each `next()` returns a
/// borrow of a strictly newer slot ⇒ disjoint.
pub struct FunnelIterMut<'a, K, V> {
    levels: *mut BucketLevel<K, V>,
    levels_len: usize,
    primary: *mut SpecialPrimary<K, V>,
    fallback: *mut SpecialFallback<K, V>,
    phase: FunnelIterPhase,
    level_idx: usize,
    slot_idx: usize,
    _marker: PhantomData<&'a mut FunnelHashMap<K, V>>,
}

// SAFETY: `FunnelIterMut` acts as an exclusive borrow of the underlying
// map regions for its lifetime, matching `&mut FunnelHashMap<K, V>`.
unsafe impl<K: Send, V: Send> Send for FunnelIterMut<'_, K, V> {}
unsafe impl<K: Sync, V: Sync> Sync for FunnelIterMut<'_, K, V> {}

impl<'a, K, V> Iterator for FunnelIterMut<'a, K, V> {
    type Item = (&'a K, &'a mut V);

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            match self.phase {
                FunnelIterPhase::Levels => {
                    while self.level_idx < self.levels_len {
                        // SAFETY: `level_idx < levels_len`, and `self.levels`
                        // points at a slice of `levels_len` initialized
                        // `BucketLevel`s owned by the borrowed map. We hold
                        // an exclusive borrow for `'a` (via PhantomData).
                        let level = unsafe { &mut *self.levels.add(self.level_idx) };
                        let cap = level.table.capacity();
                        while self.slot_idx < cap {
                            let idx = self.slot_idx;
                            self.slot_idx += 1;
                            if level.table.control_at(idx).is_occupied() {
                                // SAFETY: occupied control byte ⇒ valid
                                // `Entry<K, V>`. We never revisit this slot,
                                // so the returned borrows stay disjoint.
                                let entry = unsafe { level.table.get_mut(idx) };
                                let key: &'a K = unsafe { &*ptr::from_ref(&entry.key) };
                                let val: &'a mut V =
                                    unsafe { &mut *ptr::from_mut(&mut entry.value) };
                                return Some((key, val));
                            }
                        }
                        self.level_idx += 1;
                        self.slot_idx = 0;
                    }
                    self.phase = FunnelIterPhase::Primary;
                    self.slot_idx = 0;
                }
                FunnelIterPhase::Primary => {
                    // SAFETY: `self.primary` points at the borrowed map's
                    // `SpecialPrimary` for `'a`. We mutably alias only via
                    // this iterator, and only one outstanding `&mut` exists.
                    let primary = unsafe { &mut *self.primary };
                    let cap = primary.table.capacity();
                    while self.slot_idx < cap {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if primary.table.control_at(idx).is_occupied() {
                            // SAFETY: see above; occupied ⇒ valid entry,
                            // distinct slot each call.
                            let entry = unsafe { primary.table.get_mut(idx) };
                            let key: &'a K = unsafe { &*ptr::from_ref(&entry.key) };
                            let val: &'a mut V = unsafe { &mut *ptr::from_mut(&mut entry.value) };
                            return Some((key, val));
                        }
                    }
                    self.phase = FunnelIterPhase::Fallback;
                    self.slot_idx = 0;
                }
                FunnelIterPhase::Fallback => {
                    // SAFETY: same as the Primary arm, for `self.fallback`.
                    let fallback = unsafe { &mut *self.fallback };
                    let cap = fallback.table.capacity();
                    while self.slot_idx < cap {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if fallback.table.control_at(idx).is_occupied() {
                            // SAFETY: occupied ⇒ valid entry, distinct slot.
                            let entry = unsafe { fallback.table.get_mut(idx) };
                            let key: &'a K = unsafe { &*ptr::from_ref(&entry.key) };
                            let val: &'a mut V = unsafe { &mut *ptr::from_mut(&mut entry.value) };
                            return Some((key, val));
                        }
                    }
                    self.phase = FunnelIterPhase::Done;
                }
                FunnelIterPhase::Done => return None,
            }
        }
    }
}

impl<K, V> fmt::Debug for FunnelIterMut<'_, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FunnelIterMut")
            .field("level_idx", &self.level_idx)
            .field("slot_idx", &self.slot_idx)
            .finish_non_exhaustive()
    }
}

impl<'a, K, V> IntoIterator for &'a mut FunnelHashMap<K, V>
where
    K: Eq + Hash,
{
    type Item = (&'a K, &'a mut V);
    type IntoIter = FunnelIterMut<'a, K, V>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter_mut()
    }
}

/// `&mut V` iterator returned by [`FunnelHashMap::values_mut`].
pub struct FunnelValuesMut<'a, K, V> {
    inner: FunnelIterMut<'a, K, V>,
}

impl<'a, K, V> Iterator for FunnelValuesMut<'a, K, V> {
    type Item = &'a mut V;

    fn next(&mut self) -> Option<Self::Item> {
        self.inner.next().map(|(_, v)| v)
    }
}

impl<K, V> fmt::Debug for FunnelValuesMut<'_, K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FunnelValuesMut")
            .field("level_idx", &self.inner.level_idx)
            .field("slot_idx", &self.inner.slot_idx)
            .finish_non_exhaustive()
    }
}

/// Owned `(K, V)` iterator returned by `FunnelHashMap::into_iter`.
///
/// SAFETY: each yielded slot is immediately tombstoned, so the map's
/// `Drop` never revisits it. `Drop` drains the remainder per std semantics.
pub struct FunnelIntoIter<K, V> {
    map: FunnelHashMap<K, V>,
    phase: FunnelIterPhase,
    level_idx: usize,
    slot_idx: usize,
}

impl<K, V> Iterator for FunnelIntoIter<K, V> {
    type Item = (K, V);

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            match self.phase {
                FunnelIterPhase::Levels => {
                    while self.level_idx < self.map.levels.len() {
                        let level = &mut self.map.levels[self.level_idx];
                        let cap = level.table.capacity();
                        while self.slot_idx < cap {
                            let idx = self.slot_idx;
                            self.slot_idx += 1;
                            if level.table.control_at(idx).is_occupied() {
                                // SAFETY: occupied ⇒ valid entry. Move it
                                // out and tombstone the slot so the map's
                                // `Drop` skips it.
                                let entry = unsafe { level.table.take(idx) };
                                level.table.mark_tombstone(idx);
                                return Some((entry.key, entry.value));
                            }
                        }
                        self.level_idx += 1;
                        self.slot_idx = 0;
                    }
                    self.phase = FunnelIterPhase::Primary;
                    self.slot_idx = 0;
                }
                FunnelIterPhase::Primary => {
                    let table = &mut self.map.special.primary.table;
                    let cap = table.capacity();
                    while self.slot_idx < cap {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if table.control_at(idx).is_occupied() {
                            // SAFETY: same as Levels arm.
                            let entry = unsafe { table.take(idx) };
                            table.mark_tombstone(idx);
                            return Some((entry.key, entry.value));
                        }
                    }
                    self.phase = FunnelIterPhase::Fallback;
                    self.slot_idx = 0;
                }
                FunnelIterPhase::Fallback => {
                    let table = &mut self.map.special.fallback.table;
                    let cap = table.capacity();
                    while self.slot_idx < cap {
                        let idx = self.slot_idx;
                        self.slot_idx += 1;
                        if table.control_at(idx).is_occupied() {
                            // SAFETY: same as Levels arm.
                            let entry = unsafe { table.take(idx) };
                            table.mark_tombstone(idx);
                            return Some((entry.key, entry.value));
                        }
                    }
                    self.phase = FunnelIterPhase::Done;
                }
                FunnelIterPhase::Done => return None,
            }
        }
    }
}

impl<K, V> FusedIterator for FunnelIntoIter<K, V> {}

impl<K, V> Drop for FunnelIntoIter<K, V> {
    fn drop(&mut self) {
        // Drain the remainder so each owned `(K, V)` runs its `Drop`. After
        // this, the map's own `Drop` finds only tombstones and is a no-op
        // over the entries.
        for _ in self.by_ref() {}
    }
}

impl<K, V> fmt::Debug for FunnelIntoIter<K, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FunnelIntoIter")
            .field("level_idx", &self.level_idx)
            .field("slot_idx", &self.slot_idx)
            .finish_non_exhaustive()
    }
}

impl<K, V> IntoIterator for FunnelHashMap<K, V>
where
    K: Eq + Hash,
{
    type Item = (K, V);
    type IntoIter = FunnelIntoIter<K, V>;

    fn into_iter(self) -> Self::IntoIter {
        FunnelIntoIter {
            map: self,
            phase: FunnelIterPhase::Levels,
            level_idx: 0,
            slot_idx: 0,
        }
    }
}

/// Owned `K` iterator returned by [`FunnelHashMap::into_keys`].
pub type FunnelIntoKeys<K, V> = CommonIntoKeys<FunnelIntoIter<K, V>>;
/// Owned `V` iterator returned by [`FunnelHashMap::into_values`].
pub type FunnelIntoValues<K, V> = CommonIntoValues<FunnelIntoIter<K, V>>;

/// Number of bucket levels for a given reserve fraction. Tighter reserve →
/// more levels (more probing budget per insert).
fn compute_level_count(reserve_fraction: f64) -> usize {
    ceil_to_usize((4.0 * (1.0 / reserve_fraction).log2() + 10.0).max(1.0))
}

/// Per-bucket slot count. Wider buckets reduce overflow into deeper levels
/// at the cost of more in-bucket probing.
fn compute_bucket_width(reserve_fraction: f64) -> usize {
    ceil_to_usize((2.0 * (1.0 / reserve_fraction).log2()).max(1.0))
}

/// Carve out the special-array capacity from the total. Returns
/// `(level_capacity, special_capacity)` such that levels get the bulk and
/// special gets a fraction sized to absorb expected overflow.
fn choose_special_capacity(
    total_capacity: usize,
    reserve_fraction: f64,
    bucket_size: usize,
) -> usize {
    if total_capacity == 0 {
        return 0;
    }

    let total_capacity_f64 = usize_to_f64(total_capacity);
    let lower_bound = ceil_to_usize((reserve_fraction * total_capacity_f64) / 2.0);
    let upper_bound = floor_to_usize((3.0 * reserve_fraction * total_capacity_f64) / 4.0);
    let lower_bound = lower_bound.min(total_capacity);
    let upper_bound = upper_bound.min(total_capacity);

    if lower_bound <= upper_bound {
        for special_capacity in (lower_bound..=upper_bound).rev() {
            if (total_capacity - special_capacity).is_multiple_of(bucket_size.max(1)) {
                return special_capacity;
            }
        }
    }

    let target = round_to_usize(
        ((5.0 * reserve_fraction * total_capacity_f64) / 8.0).clamp(0.0, total_capacity_f64),
    );

    let mut best_special_capacity = total_capacity % bucket_size.max(1);
    let mut best_distance = usize::MAX;

    for main_capacity in (0..=total_capacity).step_by(bucket_size.max(1)) {
        let special_capacity = total_capacity - main_capacity;
        let distance = special_capacity.abs_diff(target);
        if distance < best_distance {
            best_distance = distance;
            best_special_capacity = special_capacity;
        }
    }

    best_special_capacity
}

fn partition_funnel_buckets(total_buckets: usize, level_count: usize) -> Vec<usize> {
    if level_count == 0 {
        return Vec::new();
    }

    if total_buckets == 0 {
        return vec![0; level_count];
    }

    let first_level_guess = {
        let ratio = 0.75f64;
        let denom = 1.0 - ratio.powi(i32::try_from(level_count).expect("level count fits in i32"));
        if denom <= 0.0 {
            total_buckets.max(1)
        } else {
            round_to_usize((((usize_to_f64(total_buckets)) * (1.0 - ratio)) / denom).max(0.0))
        }
    };

    for radius in 0..=total_buckets {
        let lower = first_level_guess.saturating_sub(radius);
        if let Some(bucket_counts) = build_funnel_bucket_sequence(total_buckets, level_count, lower)
        {
            return bucket_counts;
        }

        let upper = first_level_guess.saturating_add(radius).min(total_buckets);
        if upper != lower
            && let Some(bucket_counts) =
                build_funnel_bucket_sequence(total_buckets, level_count, upper)
        {
            return bucket_counts;
        }
    }

    let mut fallback_counts = vec![0; level_count];
    fallback_counts[0] = total_buckets;
    fallback_counts
}

fn build_funnel_bucket_sequence(
    total_buckets: usize,
    level_count: usize,
    first_level_bucket_count: usize,
) -> Option<Vec<usize>> {
    if level_count == 0 || first_level_bucket_count > total_buckets {
        return None;
    }

    let mut bucket_counts = Vec::with_capacity(level_count);
    bucket_counts.push(first_level_bucket_count);
    let mut remaining = total_buckets.saturating_sub(first_level_bucket_count);
    let mut previous_bucket_count = first_level_bucket_count;

    for level_idx in 1..level_count {
        let levels_after = level_count - level_idx - 1;
        let (min_next_bucket_count, max_next_bucket_count) =
            next_bucket_count_bounds(previous_bucket_count);
        let ideal_next_bucket_count = ((3 * previous_bucket_count + 2) / 4)
            .clamp(min_next_bucket_count, max_next_bucket_count);

        let mut chosen_bucket_count = None;
        let mut best_distance = usize::MAX;
        let candidate_upper_bound = max_next_bucket_count.min(remaining);
        for candidate_bucket_count in min_next_bucket_count..=candidate_upper_bound {
            let remaining_after_candidate = remaining - candidate_bucket_count;
            let (tail_min_sum, tail_max_sum) =
                possible_tail_sum_range(candidate_bucket_count, levels_after);
            if remaining_after_candidate < tail_min_sum || remaining_after_candidate > tail_max_sum
            {
                continue;
            }

            let distance = candidate_bucket_count.abs_diff(ideal_next_bucket_count);
            if distance < best_distance {
                best_distance = distance;
                chosen_bucket_count = Some(candidate_bucket_count);
                if distance == 0 {
                    break;
                }
            }
        }
        let chosen_bucket_count = chosen_bucket_count?;

        bucket_counts.push(chosen_bucket_count);
        remaining -= chosen_bucket_count;
        previous_bucket_count = chosen_bucket_count;
    }

    if remaining == 0 {
        Some(bucket_counts)
    } else {
        None
    }
}

fn next_bucket_count_bounds(current_bucket_count: usize) -> (usize, usize) {
    let scaled = current_bucket_count.saturating_mul(3);
    let min_next_bucket_count = scaled.saturating_sub(4).div_ceil(4);
    let max_next_bucket_count = scaled.saturating_add(4) / 4;
    (
        min_next_bucket_count,
        max_next_bucket_count.max(min_next_bucket_count),
    )
}

fn possible_tail_sum_range(start_bucket_count: usize, levels_after: usize) -> (usize, usize) {
    let mut min_sum = 0;
    let mut max_sum = 0;
    let mut min_previous = start_bucket_count;
    let mut max_previous = start_bucket_count;

    for _ in 0..levels_after {
        let (next_min, _) = next_bucket_count_bounds(min_previous);
        let (_, next_max) = next_bucket_count_bounds(max_previous);
        min_sum += next_min;
        max_sum += next_max;
        min_previous = next_min;
        max_previous = next_max;
    }

    (min_sum, max_sum)
}

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

    #[test]
    fn funnel_layout_covers_capacity() {
        // SpecialPrimary rounds up to pow2 group_count for odd-step probing,
        // so total slots may exceed the requested capacity. The invariant is
        // "covers at least the request, bounded inflation".
        let capacity = 257;
        let map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(capacity);
        let level_capacity = map.levels.iter().map(BucketLevel::capacity).sum::<usize>();
        let special_capacity =
            map.special.primary.table.capacity() + map.special.fallback.table.capacity();
        let total = level_capacity + special_capacity;
        assert!(
            total >= capacity,
            "total={total} below requested={capacity}"
        );
        assert!(
            total <= capacity * 2,
            "total={total} exceeds 2x of requested={capacity}",
        );
    }

    #[test]
    fn insert_get_and_update_work() {
        let mut map = FunnelHashMap::with_capacity(512);

        for key in 0..20 {
            assert_eq!(map.insert(key, key * 10), None);
        }
        for key in 0..20 {
            assert_eq!(map.get(&key), Some(&(key * 10)));
        }

        let replaced = map.insert(7, 777).expect("update should succeed");
        assert_eq!(replaced, 70);
        assert_eq!(map.get(&7), Some(&777));
    }

    #[test]
    fn remove_and_clear_work_with_borrowed_keys() {
        let mut map: FunnelHashMap<String, i32> = FunnelHashMap::with_capacity(256);
        assert_eq!(map.insert("alpha".to_string(), 1), None);
        assert_eq!(map.insert("beta".to_string(), 2), None);

        assert_eq!(map.remove("alpha"), Some(1));
        assert_eq!(map.remove("alpha"), None);
        map.clear();
        assert_eq!(map.get("beta"), None);
        assert!(map.is_empty());
    }

    #[test]
    fn new_starts_empty() {
        let map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        assert_eq!(map.capacity(), 0);
        assert_eq!(map.len(), 0);
    }

    #[test]
    fn insert_resizes_from_zero_capacity() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        assert_eq!(map.get(&1), Some(&10));
        assert!(map.capacity() > 0);
    }

    #[test]
    fn insert_resizes_when_threshold_is_reached() {
        let capacity = 64;
        let mut map = FunnelHashMap::with_capacity(capacity);
        let max_insertions = max_insertions(capacity, DEFAULT_RESERVE_FRACTION);

        for key in 0..max_insertions + 10 {
            let _ = map.insert(key, key * 10);
        }
        for key in 0..max_insertions + 10 {
            assert_eq!(map.get(&key), Some(&(key * 10)));
        }

        assert!(map.capacity() > capacity);
    }

    #[test]
    fn options_constructor_preserves_capacity() {
        let map: FunnelHashMap<i32, i32> = FunnelHashMap::with_options(FunnelOptions {
            capacity: 320,
            reserve_fraction: DEFAULT_RESERVE_FRACTION,
            primary_probe_limit: Some(4),
        });
        assert_eq!(map.capacity(), 320);
    }

    #[test]
    fn delete_heavy_preserves_correctness() {
        let mut map = FunnelHashMap::with_capacity(400);
        for i in 0..200 {
            map.insert(i, i * 10);
        }
        for i in 0..160 {
            assert_eq!(map.remove(&i), Some(i * 10));
        }
        for i in 160..200 {
            assert_eq!(
                map.get(&i),
                Some(&(i * 10)),
                "key {i} missing after deletes"
            );
        }
        assert_eq!(map.len(), 40);
        // Re-insert into tombstone-heavy map.
        for i in 1000..1100 {
            assert_eq!(map.insert(i, i), None);
        }
        for i in 1000..1100 {
            assert_eq!(map.get(&i), Some(&i), "key {i} missing after re-insert");
        }
    }

    #[test]
    fn large_map_correctness() {
        let n = 10_000;
        let mut map = FunnelHashMap::with_capacity(n * 2);
        for i in 0..n {
            assert_eq!(map.insert(i, i), None);
        }
        for i in 0..n {
            assert_eq!(map.get(&i), Some(&i), "key {i} missing");
        }
        assert_eq!(map.len(), n);
    }

    #[test]
    fn interleaved_insert_delete_correctness() {
        let mut map = FunnelHashMap::with_capacity(256);
        // Insert 100, delete odd keys, verify even keys survive.
        for i in 0..100 {
            map.insert(i, i);
        }
        for i in (1..100).step_by(2) {
            assert!(map.remove(&i).is_some());
        }
        for i in (0..100).step_by(2) {
            assert_eq!(map.get(&i), Some(&i), "even key {i} missing");
        }
        for i in (1..100).step_by(2) {
            assert_eq!(map.get(&i), None, "odd key {i} should be gone");
        }
    }

    #[test]
    fn special_primary_single_group_edge_case() {
        // Smallest capacity exercises the SpecialPrimary path with
        // `group_count == 1` (`mask == 0`). The odd-step probe must still
        // make forward progress (loop terminates via `group_limit`).
        let mut map: FunnelHashMap<u64, u64> = FunnelHashMap::with_capacity(1);
        for i in 0..16 {
            map.insert(i, i * 3);
        }
        for i in 0..16 {
            assert_eq!(map.get(&i), Some(&(i * 3)));
        }
        for i in 0..8 {
            assert_eq!(map.remove(&i), Some(i * 3));
        }
        for i in 0..8 {
            assert_eq!(map.get(&i), None);
        }
        for i in 8..16 {
            assert_eq!(map.get(&i), Some(&(i * 3)));
        }
    }

    #[test]
    fn delete_insert_cycles_trigger_rebuild() {
        // Exercises the tombstone cleanup path: 6000 remove+insert cycles
        // on a 12K map forces level.tombstones > capacity/2.
        let n = 12_000;
        let mut map = FunnelHashMap::with_capacity(n * 2);
        for i in 0..n {
            map.insert(i, i);
        }

        for i in 0..6000 {
            assert!(map.remove(&i).is_some(), "remove {i} failed");
            map.insert(i + n, i + n);
        }

        assert_eq!(map.len(), n);
        // Verify all remaining keys are findable.
        for i in 6000..n {
            assert_eq!(map.get(&i), Some(&i), "original key {i} missing");
        }
        for i in 0..6000 {
            assert_eq!(
                map.get(&(i + n)),
                Some(&(i + n)),
                "new key {} missing",
                i + n
            );
        }
    }

    #[test]
    fn iter_yields_every_inserted_pair_once() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(128);
        for i in 0..80 {
            map.insert(i, i * 7);
        }
        let mut collected: Vec<(i32, i32)> = map.iter().map(|(&k, &v)| (k, v)).collect();
        collected.sort();
        let expected: Vec<(i32, i32)> = (0..80).map(|i| (i, i * 7)).collect();
        assert_eq!(collected, expected);
    }

    #[test]
    fn iter_skips_tombstones_after_remove() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(64);
        for i in 0..40 {
            map.insert(i, i);
        }
        for i in (0..40).step_by(3) {
            map.remove(&i);
        }
        let keys: Vec<i32> = map.iter().map(|(&k, _)| k).collect();
        assert_eq!(keys.len(), map.len());
    }

    #[test]
    fn iter_empty_map_is_empty() {
        let map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        assert_eq!(map.iter().count(), 0);
    }

    // Regression: removing one of two keys that both routed into a deep level
    // (level 0 unallocated) must not orphan the surviving sibling.
    #[test]
    fn remove_preserves_sibling_when_level_zero_empty() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::default();
        map.insert(1, 10);
        map.insert(2, 20);
        map.remove(&1);
        assert_eq!(map.get(&2), Some(&20));
        assert_eq!(map.len(), 1);
    }

    #[test]
    fn get_disjoint_mut_returns_all_refs_on_hits() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(64);
        for i in 0..16 {
            map.insert(i, i * 10);
        }

        let got = map.get_disjoint_mut([&1, &3, &7, &15]).expect("all hits");
        assert_eq!(*got[0], 10);
        assert_eq!(*got[1], 30);
        assert_eq!(*got[2], 70);
        assert_eq!(*got[3], 150);
    }

    #[test]
    fn get_disjoint_mut_returns_none_if_any_missing() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(32);
        for i in 0..8 {
            map.insert(i, i);
        }

        assert!(map.get_disjoint_mut([&0, &1, &99]).is_none());
    }

    #[test]
    #[should_panic(expected = "duplicate keys")]
    fn get_disjoint_mut_panics_on_duplicate_keys() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(32);
        map.insert(1, 100);
        map.insert(2, 200);
        let _ = map.get_disjoint_mut([&1, &1]);
    }

    #[test]
    fn get_disjoint_mut_zero_keys_is_some_empty() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(16);
        map.insert(1, 1);
        let got: [&mut i32; 0] = map
            .get_disjoint_mut::<i32, 0>([])
            .expect("zero-key returns Some");
        assert_eq!(got.len(), 0);
    }

    #[test]
    fn get_disjoint_mut_mutation_propagates() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(32);
        for i in 0..8 {
            map.insert(i, i);
        }
        {
            let got = map.get_disjoint_mut([&2, &5]).expect("hit");
            *got[0] = 222;
            *got[1] = 555;
        }
        assert_eq!(map.get(&2), Some(&222));
        assert_eq!(map.get(&5), Some(&555));
    }

    #[test]
    fn keys_yields_inserted_keys_only() {
        use std::collections::HashSet;
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(128);
        for i in 0..50 {
            map.insert(i, i * 7);
        }
        let got: HashSet<i32> = map.keys().copied().collect();
        let expected: HashSet<i32> = (0..50).collect();
        assert_eq!(got, expected);
    }

    #[test]
    fn values_yields_inserted_values_only() {
        use std::collections::HashSet;
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(128);
        for i in 0..50 {
            map.insert(i, i * 7);
        }
        let got: HashSet<i32> = map.values().copied().collect();
        let expected: HashSet<i32> = (0..50).map(|i| i * 7).collect();
        assert_eq!(got, expected);
    }

    #[test]
    fn hasher_returns_consistent_handle() {
        let map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        let a: *const _ = map.hasher();
        let b: *const _ = map.hasher();
        assert!(std::ptr::eq(a, b));
    }

    #[test]
    fn get_key_value_returns_both_on_hit_none_on_miss() {
        let mut map: FunnelHashMap<String, i32> = FunnelHashMap::with_capacity(16);
        map.insert("alpha".to_string(), 1);
        map.insert("beta".to_string(), 2);

        let (k, v) = map.get_key_value("alpha").expect("hit");
        assert_eq!(k, "alpha");
        assert_eq!(*v, 1);

        assert!(map.get_key_value("missing").is_none());
    }

    #[test]
    fn remove_entry_returns_both_and_actually_removes() {
        let mut map: FunnelHashMap<String, i32> = FunnelHashMap::with_capacity(16);
        map.insert("alpha".to_string(), 1);
        map.insert("beta".to_string(), 2);

        let (k, v) = map.remove_entry("alpha").expect("hit");
        assert_eq!(k, "alpha");
        assert_eq!(v, 1);
        assert_eq!(map.len(), 1);
        assert!(map.get("alpha").is_none());

        assert!(map.remove_entry("alpha").is_none());
    }

    #[test]
    fn iter_mut_yields_mutable_values_in_some_order() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(128);
        for i in 0..50 {
            map.insert(i, i);
        }
        for (_, v) in &mut map {
            *v *= 2;
        }
        for i in 0..50 {
            assert_eq!(map.get(&i), Some(&(i * 2)), "key {i} not doubled");
        }
    }

    #[test]
    fn try_reserve_grows_when_needed() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        assert_eq!(map.capacity(), 0);
        map.try_reserve(1024).expect("alloc should succeed");
        let cap = map.capacity();
        assert!(cap >= 1024, "reserve under-allocated: cap={cap}");
        for i in 0..1024 {
            map.insert(i, i * 2);
        }
        for i in 0..1024 {
            assert_eq!(map.get(&i), Some(&(i * 2)));
        }
        assert_eq!(map.len(), 1024);
    }

    #[test]
    fn try_reserve_zero_additional_is_noop() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(128);
        let cap_before = map.capacity();
        map.try_reserve(0).expect("noop");
        assert_eq!(map.capacity(), cap_before);
    }

    #[test]
    fn try_reserve_overflow_returns_error() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 1);
        assert_eq!(
            map.try_reserve(usize::MAX),
            Err(TryReserveError::CapacityOverflow)
        );
    }

    #[test]
    fn shrink_to_below_len_clamps_to_len() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(4096);
        for i in 0..200 {
            map.insert(i, i);
        }
        let cap_before = map.capacity();
        map.shrink_to(0);
        let cap_after = map.capacity();
        assert!(cap_after < cap_before);
        assert!(cap_after >= map.len());
        for i in 0..200 {
            assert_eq!(map.get(&i), Some(&i));
        }
    }

    #[test]
    fn iter_mut_yields_each_entry_exactly_once() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(128);
        for i in 0..80 {
            map.insert(i, i * 3);
        }
        let mut collected: Vec<(i32, i32)> = map.iter_mut().map(|(&k, v)| (k, *v)).collect();
        collected.sort_unstable();
        let expected: Vec<(i32, i32)> = (0..80).map(|i| (i, i * 3)).collect();
        assert_eq!(collected, expected);
    }

    #[test]
    fn iter_mut_skips_tombstones() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(64);
        for i in 0..40 {
            map.insert(i, i);
        }
        for i in (0..40).step_by(2) {
            map.remove(&i);
        }
        let count = map.iter_mut().count();
        assert_eq!(count, map.len());
    }

    #[test]
    fn iter_mut_empty_map_is_empty() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        assert_eq!(map.iter_mut().count(), 0);
    }

    #[test]
    fn values_mut_mutates_in_place() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(64);
        for i in 0..30 {
            map.insert(i, i);
        }
        for v in map.values_mut() {
            *v += 100;
        }
        for i in 0..30 {
            assert_eq!(map.get(&i), Some(&(i + 100)));
        }
    }

    #[test]
    fn retain_keeps_matching_drops_rest() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(256);
        for i in 0..80 {
            map.insert(i, i * 10);
        }
        map.retain(|k, _| k % 2 == 0);
        assert_eq!(map.len(), 40);
        for i in 0..80 {
            if i % 2 == 0 {
                assert_eq!(map.get(&i), Some(&(i * 10)));
            } else {
                assert!(map.get(&i).is_none());
            }
        }
    }

    #[test]
    fn retain_with_empty_map_is_noop() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        let mut called = false;
        map.retain(|_, _| {
            called = true;
            true
        });
        assert!(!called);
        assert!(map.is_empty());
    }

    #[test]
    fn retain_can_mutate_values_in_place() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(256);
        for i in 0..40 {
            map.insert(i, i);
        }
        map.retain(|k, v| {
            *v += 100;
            k % 2 == 0
        });
        assert_eq!(map.len(), 20);
        for i in (0..40).step_by(2) {
            assert_eq!(map.get(&i), Some(&(i + 100)));
        }
    }

    #[test]
    fn into_iter_yields_all_entries() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(128);
        for i in 0..60 {
            map.insert(i, i * 11);
        }
        let mut collected: Vec<(i32, i32)> = map.into_iter().collect();
        collected.sort_unstable();
        let expected: Vec<(i32, i32)> = (0..60).map(|i| (i, i * 11)).collect();
        assert_eq!(collected, expected);
    }

    #[test]
    fn into_iter_skips_tombstones() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(128);
        for i in 0..60 {
            map.insert(i, i);
        }
        for i in (0..60).step_by(3) {
            map.remove(&i);
        }
        let expected_len = map.len();
        let collected: Vec<(i32, i32)> = map.into_iter().collect();
        assert_eq!(collected.len(), expected_len);
    }

    #[test]
    fn into_keys_yields_all_keys() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(64);
        for i in 0..30 {
            map.insert(i, i);
        }
        let mut keys: Vec<i32> = map.into_keys().collect();
        keys.sort_unstable();
        assert_eq!(keys, (0..30).collect::<Vec<_>>());
    }

    #[test]
    fn into_values_yields_all_values() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(64);
        for i in 0..30 {
            map.insert(i, i * 5);
        }
        let mut vals: Vec<i32> = map.into_values().collect();
        vals.sort_unstable();
        let expected: Vec<i32> = (0..30).map(|i| i * 5).collect();
        assert_eq!(vals, expected);
    }

    #[test]
    fn into_keys_drops_values() {
        use std::sync::Arc;
        use std::sync::atomic::{AtomicUsize, Ordering};

        struct DropCounter {
            counter: Arc<AtomicUsize>,
        }
        impl Drop for DropCounter {
            fn drop(&mut self) {
                self.counter.fetch_add(1, Ordering::SeqCst);
            }
        }

        let counter = Arc::new(AtomicUsize::new(0));
        let n: usize = 32;
        let mut map: FunnelHashMap<usize, DropCounter> = FunnelHashMap::with_capacity(64);
        for i in 0..n {
            map.insert(
                i,
                DropCounter {
                    counter: Arc::clone(&counter),
                },
            );
        }
        let keys: Vec<usize> = map.into_keys().collect();
        assert_eq!(keys.len(), n);
        assert_eq!(counter.load(Ordering::SeqCst), n);
    }

    #[test]
    fn into_values_drops_keys() {
        use std::sync::Arc;
        use std::sync::atomic::{AtomicUsize, Ordering};

        struct DropKey {
            id: usize,
            counter: Arc<AtomicUsize>,
        }
        impl std::hash::Hash for DropKey {
            fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
                self.id.hash(state);
            }
        }
        impl PartialEq for DropKey {
            fn eq(&self, other: &Self) -> bool {
                self.id == other.id
            }
        }
        impl Eq for DropKey {}
        impl Drop for DropKey {
            fn drop(&mut self) {
                self.counter.fetch_add(1, Ordering::SeqCst);
            }
        }

        let counter = Arc::new(AtomicUsize::new(0));
        let n: usize = 32;
        let mut map: FunnelHashMap<DropKey, usize> = FunnelHashMap::with_capacity(64);
        for i in 0..n {
            map.insert(
                DropKey {
                    id: i,
                    counter: Arc::clone(&counter),
                },
                i,
            );
        }
        let vals: Vec<usize> = map.into_values().collect();
        assert_eq!(vals.len(), n);
        assert_eq!(counter.load(Ordering::SeqCst), n);
    }

    #[test]
    fn iter_mut_partial_consume_then_drop() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(128);
        for i in 0..40 {
            map.insert(i, i);
        }
        {
            let mut it = map.iter_mut();
            for _ in 0..7 {
                if let Some((_, v)) = it.next() {
                    *v += 1000;
                }
            }
            // it drops here; map is still consistent.
        }
        assert_eq!(map.len(), 40);
        for i in 0..40 {
            assert!(map.get(&i).is_some(), "key {i} disappeared");
        }
    }

    #[test]
    fn into_iter_partial_drop_drops_remaining() {
        use std::sync::Arc;
        use std::sync::atomic::{AtomicUsize, Ordering};

        struct DropCounter {
            counter: Arc<AtomicUsize>,
        }
        impl Drop for DropCounter {
            fn drop(&mut self) {
                self.counter.fetch_add(1, Ordering::SeqCst);
            }
        }

        let counter = Arc::new(AtomicUsize::new(0));
        let n: usize = 50;
        let mut map: FunnelHashMap<usize, DropCounter> = FunnelHashMap::with_capacity(128);
        for i in 0..n {
            map.insert(
                i,
                DropCounter {
                    counter: Arc::clone(&counter),
                },
            );
        }
        let take = 12;
        let mut it = map.into_iter();
        let mut taken: Vec<(usize, DropCounter)> = Vec::with_capacity(take);
        for _ in 0..take {
            taken.push(it.next().expect("element"));
        }
        drop(it);
        assert_eq!(counter.load(Ordering::SeqCst), n - take);
        drop(taken);
        assert_eq!(counter.load(Ordering::SeqCst), n);
    }

    #[test]
    fn entry_or_insert_creates_when_missing() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        let value = map.entry(1).or_insert(10);
        assert_eq!(*value, 10);
        assert_eq!(map.get(&1), Some(&10));
        assert_eq!(map.len(), 1);
    }

    #[test]
    fn entry_or_insert_returns_existing() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        let value = map.entry(1).or_insert(99);
        assert_eq!(*value, 10);
        assert_eq!(map.get(&1), Some(&10));
        assert_eq!(map.len(), 1);
    }

    #[test]
    fn entry_or_insert_with_lazy_default_not_called_on_hit() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        let mut called = false;
        let value = map.entry(1).or_insert_with(|| {
            called = true;
            42
        });
        assert_eq!(*value, 10);
        assert!(!called, "default closure must not run on occupied entry");
    }

    #[test]
    fn entry_or_insert_with_key_uses_key_in_default() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        let value = map.entry(7).or_insert_with_key(|k| k * 100);
        assert_eq!(*value, 700);
        assert_eq!(map.get(&7), Some(&700));
    }

    #[test]
    fn entry_and_modify_runs_on_occupied() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        let value = map.entry(1).and_modify(|v| *v += 5).or_insert(0);
        assert_eq!(*value, 15);
        assert_eq!(map.get(&1), Some(&15));
    }

    #[test]
    fn entry_and_modify_skips_on_vacant() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        let mut touched = false;
        let value = map.entry(1).and_modify(|_| touched = true).or_insert(42);
        assert_eq!(*value, 42);
        assert!(!touched);
        assert_eq!(map.get(&1), Some(&42));
    }

    #[test]
    fn entry_occupied_get_mut_mutates() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        if let Entry::Occupied(mut occ) = map.entry(1) {
            *occ.get_mut() = 99;
            assert_eq!(*occ.get(), 99);
        } else {
            panic!("expected occupied");
        }
        assert_eq!(map.get(&1), Some(&99));
    }

    #[test]
    fn entry_occupied_into_mut_outlives_entry_borrow() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        let value: &mut i32 = match map.entry(1) {
            Entry::Occupied(occ) => occ.into_mut(),
            Entry::Vacant(_) => panic!("expected occupied"),
        };
        *value = 123;
        assert_eq!(map.get(&1), Some(&123));
    }

    #[test]
    fn entry_occupied_insert_returns_old_and_replaces() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        if let Entry::Occupied(mut occ) = map.entry(1) {
            let old = occ.insert(99);
            assert_eq!(old, 10);
        } else {
            panic!("expected occupied");
        }
        assert_eq!(map.get(&1), Some(&99));
    }

    #[test]
    fn entry_occupied_remove_returns_value() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        map.insert(2, 20);
        if let Entry::Occupied(occ) = map.entry(1) {
            assert_eq!(occ.remove(), 10);
        } else {
            panic!("expected occupied");
        }
        assert!(map.get(&1).is_none());
        assert_eq!(map.get(&2), Some(&20));
        assert_eq!(map.len(), 1);
    }

    #[test]
    fn entry_vacant_insert_returns_mut_ref() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        let value: &mut i32 = match map.entry(5) {
            Entry::Vacant(vac) => vac.insert(50),
            Entry::Occupied(_) => panic!("expected vacant"),
        };
        *value += 1;
        assert_eq!(map.get(&5), Some(&51));
    }

    #[test]
    fn try_insert_succeeds_when_missing() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        let value = map.try_insert(1, 10).expect("vacant should succeed");
        assert_eq!(*value, 10);
        assert_eq!(map.get(&1), Some(&10));
    }

    #[test]
    fn try_insert_fails_with_occupied_error_when_present() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        let err = map.try_insert(1, 99).expect_err("occupied must error");
        assert_eq!(err.entry.key(), &1);
        assert_eq!(err.entry.get(), &10);
        assert_eq!(map.get(&1), Some(&10));
    }

    #[test]
    fn try_insert_occupied_error_carries_rejected_value() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::new();
        map.insert(1, 10);
        let err = map.try_insert(1, 99).expect_err("occupied must error");
        assert_eq!(err.value, 99);
    }

    #[test]
    fn drain_yields_all_entries_then_empties_map() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(256);
        for i in 0..60 {
            map.insert(i, i * 7);
        }
        let mut collected: Vec<(i32, i32)> = map.drain().collect();
        collected.sort_unstable();
        let expected: Vec<(i32, i32)> = (0..60).map(|i| (i, i * 7)).collect();
        assert_eq!(collected, expected);
        assert!(map.is_empty());
        assert_eq!(map.iter().count(), 0);
        // Reuse after drain.
        map.insert(999, 999);
        assert_eq!(map.get(&999), Some(&999));
    }

    #[test]
    fn drain_partial_consume_then_drop_still_empties_map() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(256);
        for i in 0..60 {
            map.insert(i, i);
        }
        {
            let mut drain = map.drain();
            let _first = drain.next();
            let _second = drain.next();
            // Drop here without exhausting; remaining entries must still be
            // freed and the map emptied (std semantics).
        }
        assert!(map.is_empty());
        assert_eq!(map.iter().count(), 0);
    }

    #[test]
    fn extract_if_yields_only_matching_and_leaves_rest() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(256);
        for i in 0..80 {
            map.insert(i, i);
        }
        let mut extracted: Vec<(i32, i32)> = map.extract_if(|k, _| k % 3 == 0).collect();
        extracted.sort_unstable();
        let expected: Vec<(i32, i32)> = (0..80).filter(|i| i % 3 == 0).map(|i| (i, i)).collect();
        assert_eq!(extracted, expected);
        assert_eq!(map.len(), 80 - expected.len());
        for i in 0..80 {
            if i % 3 == 0 {
                assert!(map.get(&i).is_none());
            } else {
                assert_eq!(map.get(&i), Some(&i));
            }
        }
    }

    #[test]
    fn extract_if_partial_consume_then_drop_keeps_remaining_in_map() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(256);
        for i in 0..60 {
            map.insert(i, i);
        }
        let original_len = map.len();
        let extracted_count;
        {
            let mut it = map.extract_if(|_, _| true);
            assert!(it.next().is_some());
            assert!(it.next().is_some());
            extracted_count = 2;
        }
        assert_eq!(map.len(), original_len - extracted_count);
        let remaining: Vec<i32> = map.iter().map(|(&k, _)| k).collect();
        assert_eq!(remaining.len(), original_len - extracted_count);
    }

    #[test]
    fn retain_does_not_trigger_mid_iter_resize_with_clustered_tombstones() {
        // Pack the map to a high load and retain half. The bulk-op iterators
        // bypass `remove`, so the per-remove resize check can't fire mid-walk;
        // this test guards that invariant.
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(1024);
        let max = i32::try_from(max_insertions(map.capacity(), DEFAULT_RESERVE_FRACTION))
            .expect("test capacity fits i32");
        for i in 0..max {
            map.insert(i, i);
        }
        let initial_capacity = map.capacity();
        map.retain(|k, _| k % 2 == 0);

        let expected_count = (0..max).filter(|i| i % 2 == 0).count();
        assert_eq!(map.len(), expected_count);
        for i in 0..max {
            if i % 2 == 0 {
                assert_eq!(map.get(&i), Some(&i), "kept key {i} missing");
            } else {
                assert!(map.get(&i).is_none(), "dropped key {i} survived");
            }
        }
        assert!(map.capacity() <= initial_capacity * 2);
    }

    #[test]
    fn shrink_to_above_capacity_is_noop() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(256);
        for i in 0..20 {
            map.insert(i, i);
        }
        let cap = map.capacity();
        map.shrink_to(cap * 4);
        assert_eq!(map.capacity(), cap);
    }

    #[test]
    fn shrink_to_fit_reduces_capacity_when_sparse() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(4096);
        for i in 0..2000 {
            map.insert(i, i);
        }
        for i in 0..1800 {
            map.remove(&i);
        }
        let cap_before = map.capacity();
        map.shrink_to_fit();
        assert!(map.capacity() < cap_before);
        for i in 1800..2000 {
            assert_eq!(map.get(&i), Some(&i));
        }
    }

    #[test]
    fn shrink_then_insert_works() {
        let mut map: FunnelHashMap<i32, i32> = FunnelHashMap::with_capacity(2048);
        for i in 0..400 {
            map.insert(i, i * 3);
        }
        for i in 0..300 {
            map.remove(&i);
        }
        map.shrink_to_fit();
        for i in 0..100 {
            assert_eq!(map.insert(i, i * 5), None);
        }
        for i in 0..100 {
            assert_eq!(map.get(&i), Some(&(i * 5)));
        }
        for i in 300..400 {
            assert_eq!(map.get(&i), Some(&(i * 3)));
        }
    }
}