structured_asn1/memory/

map.rs

//! Accounted storage and pre-admitted hashing for the existing hashbrown engine.

use core::fmt;
use core::hash::{BuildHasher, Hash};

use hashbrown::hash_map::RawEntryMut;
use hashbrown::{DefaultHashBuilder, HashMap, TryReserveError};

use super::allocator::AccountAllocator;
use super::{Memory, MemoryError, Work};

/// A trusted key type's upper bound for hashing and equality work.
///
/// Implementations must cover all bytes examined by Hash and Eq. This is a
/// contract for caller-provided code, not a value an untrusted document supplies.
/// The bound must remain unchanged and successful while a key is stored, just
/// as Hash and Eq must remain stable. Accounting retains this bound for rehashes.
pub trait HashWork: Hash + Eq {
    /// Work units for one hash/equality traversal, including constant overhead.
    fn hash_work(&self) -> Result<u64, MemoryError>;
}

impl HashWork for usize {
    fn hash_work(&self) -> Result<u64, MemoryError> {
        Ok(1)
    }
}

impl HashWork for (usize, usize, usize) {
    fn hash_work(&self) -> Result<u64, MemoryError> {
        Ok(3)
    }
}

impl HashWork for &[u8] {
    fn hash_work(&self) -> Result<u64, MemoryError> {
        Ok(self.len() as u64 + 1)
    }
}

impl HashWork for crate::Oid<'_> {
    fn hash_work(&self) -> Result<u64, MemoryError> {
        Ok(self.as_bytes().len() as u64 + 1)
    }
}

impl HashWork for (crate::Oid<'_>, crate::Oid<'_>) {
    fn hash_work(&self) -> Result<u64, MemoryError> {
        self.0
            .hash_work()?
            .checked_add(self.1.hash_work()?)
            .ok_or(MemoryError::WorkExceeded)
    }
}

/// A hash map whose backing allocation and operations share one resource domain.
///
/// Hashbrown supplies bucket/probing/SIMD machinery. Reservations include its
/// control bytes and alignment, not merely the initialized entries. Hashing and
/// actual collision comparisons are admitted before examining candidate keys.
pub struct OwnedMap<K: HashWork, V, W: Work = Memory> {
    values: HashMap<K, V, DefaultHashBuilder, AccountAllocator<W>>,
    execution: W,
    key_work: u64,
    maximum_capacity: usize,
}

impl<K: HashWork + fmt::Debug, V: fmt::Debug, W: Work> fmt::Debug for OwnedMap<K, V, W> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.values.fmt(f)
    }
}

impl<K: HashWork, V, W: Work> OwnedMap<K, V, W> {
    pub(super) fn new(execution: W) -> Self {
        Self {
            values: HashMap::with_hasher_in(
                DefaultHashBuilder::default(),
                AccountAllocator::new(execution.clone()),
            ),
            execution,
            key_work: 0,
            maximum_capacity: 0,
        }
    }

    /// The shared account retained until the table is deallocated.
    #[must_use]
    pub fn memory(&self) -> &Memory {
        self.execution.memory()
    }

    /// Number of initialized entries.
    #[must_use]
    pub fn len(&self) -> usize {
        self.values.len()
    }

    /// Whether there are no entries.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.values.is_empty()
    }

    /// Borrowed entries after admitting a full control-bucket traversal in
    /// the current execution domain, independently of retained storage.
    /// Sparse holes count too. Partial consumption retains the conservative
    /// prepayment; the opaque iterator exposes no cloning/restart operation.
    ///
    /// # Examples
    /// ```
    /// # extern crate alloc;
    /// # use alloc::sync::Arc;
    /// # use structured_asn1::memory::{Memory, MemoryAccount, MemoryError};
    /// # let memory = Memory::new(Arc::new(MemoryAccount::new(1024, 1024)));
    /// let mut map = memory.map();
    /// map.try_insert(1usize, 42)?;
    /// let mut entries = map.try_iter(&memory)?;
    /// assert_eq!(entries.next(), Some((&1, &42)));
    /// # Ok::<(), MemoryError>(())
    /// ```
    pub fn try_iter<'a, E: Work>(
        &'a self,
        execution: &E,
    ) -> Result<impl Iterator<Item = (&'a K, &'a V)> + 'a + use<'a, K, V, W, E>, MemoryError> {
        execution.charge_work(self.buckets()?)?;
        Ok(self.values.iter())
    }

    fn buckets(&self) -> Result<u64, MemoryError> {
        // Pinned hashbrown 0.17.1 raw::bucket_mask_to_capacity: tables with
        // four/eight buckets keep one empty slot; larger tables use 7/8 of
        // the buckets. Maximum capacity records each table immediately after
        // reserve, before tombstones can reduce the public capacity value.
        match self.maximum_capacity {
            0 => Ok(0),
            capacity @ 1..=7 => Ok(capacity as u64 + 1),
            capacity => (capacity as u64 / 7)
                .checked_mul(8)
                .ok_or(MemoryError::WorkExceeded),
        }
    }

    fn control_work(&self) -> Result<u64, MemoryError> {
        let buckets = self.buckets()?;
        // The pinned engine's narrowest control group is four bytes. Wider
        // scalar/SIMD groups perform the same fixed group operations. One unit
        // admits one group visit; triangular probing visits each group once.
        Ok(buckets / 4)
    }

    fn rehash_work(&self) -> Result<u64, MemoryError> {
        // reserve(1) can double the current buckets or rehash in place. Every
        // retained key is hashed once; each slot search may visit all groups.
        // In-place rehash additionally scans every bucket and swaps entries.
        let buckets = self.buckets()?;
        let groups = buckets.checked_mul(2).ok_or(MemoryError::WorkExceeded)? / 4;
        let moves = (core::mem::size_of::<(K, V)>() as u64)
            .checked_mul(2)
            .ok_or(MemoryError::WorkExceeded)?;
        groups
            .checked_add(moves)
            .and_then(|per_entry| per_entry.checked_mul(self.values.len() as u64))
            .and_then(|work| work.checked_add(self.key_work))
            .and_then(|work| {
                buckets
                    .checked_mul(2)
                    .and_then(|scan| work.checked_add(scan))
            })
            .ok_or(MemoryError::WorkExceeded)
    }

    fn hash_key<E: Work>(&self, key: &K, execution: &E) -> Result<u64, MemoryError> {
        let work = key
            .hash_work()?
            .checked_add(self.control_work()?)
            .ok_or(MemoryError::WorkExceeded)?;
        execution.charge_work(work)?;
        Ok(self.values.hasher().hash_one(key))
    }

    /// Looks up a key after admitting its complete possible probe work.
    /// The current execution domain pays independently of stored capacity.
    ///
    /// # Examples
    /// ```
    /// # extern crate alloc;
    /// # use alloc::sync::Arc;
    /// # use structured_asn1::memory::{Memory, MemoryAccount, MemoryError};
    /// # let memory = Memory::new(Arc::new(MemoryAccount::new(1024, 1024)));
    /// # let mut map = memory.map();
    /// # map.try_insert(1usize, 42)?;
    /// assert_eq!(map.get(&1, &memory)?, Some(&42));
    /// assert_eq!(map.get(&2, &memory)?, None);
    /// # Ok::<(), MemoryError>(())
    /// ```
    pub fn get(&self, key: &K, execution: &impl Work) -> Result<Option<&V>, MemoryError> {
        let hash = self.hash_key(key, execution)?;
        let mut failure = None;
        let entry = self.values.raw_entry().from_hash(hash, |candidate| {
            equivalent(execution, key, candidate, &mut failure)
        });
        if let Some(reason) = failure {
            return Err(reason);
        }
        Ok(entry.map(|(_, value)| value))
    }

    /// Looks up an entry for mutation without exposing table growth.
    /// Hashing, probing and equality use the supplied execution domain.
    ///
    /// # Examples
    /// ```
    /// # extern crate alloc;
    /// # use alloc::sync::Arc;
    /// # use structured_asn1::memory::{Memory, MemoryAccount, MemoryError};
    /// # let memory = Memory::new(Arc::new(MemoryAccount::new(1024, 1024)));
    /// # let mut map = memory.map();
    /// # map.try_insert(1usize, 42)?;
    /// if let Some(value) = map.get_mut(&1, &memory)? { *value = 7; }
    /// assert_eq!(map.get(&1, &memory)?, Some(&7));
    /// # Ok::<(), MemoryError>(())
    /// ```
    pub fn get_mut(
        &mut self,
        key: &K,
        execution: &impl Work,
    ) -> Result<Option<&mut V>, MemoryError> {
        let hash = self.hash_key(key, execution)?;
        let mut failure = None;
        let entry = self.values.raw_entry_mut().from_hash(hash, |candidate| {
            equivalent(execution, key, candidate, &mut failure)
        });
        if let Some(reason) = failure {
            return Err(reason);
        }
        Ok(match entry {
            RawEntryMut::Occupied(entry) => Some(entry.into_mut()),
            RawEntryMut::Vacant(_) => None,
        })
    }

    /// Inserts a value fallibly, or replaces the value for an existing key.
    /// This storage mutation runs in the collection's original resource
    /// domain, including hashing, rehashing, allocation and copying work.
    /// To mutate under a different domain, build a new collection there from
    /// explicitly admitted borrowed entries; retained capacity is not rebound.
    pub fn try_insert(&mut self, key: K, value: V) -> Result<Option<V>, MemoryError> {
        let hash = self.hash_key(&key, &self.execution)?;
        let mut failure = None;
        let entry = self.values.raw_entry_mut().from_hash(hash, |candidate| {
            equivalent(&self.execution, &key, candidate, &mut failure)
        });
        if let Some(reason) = failure {
            return Err(reason);
        }
        if let RawEntryMut::Occupied(entry) = entry {
            let moves = (core::mem::size_of::<V>() as u64)
                .checked_mul(2)
                .ok_or(MemoryError::WorkExceeded)?;
            self.execution.charge_work(moves)?;
            return Ok(Some(core::mem::replace(entry.into_mut(), value)));
        }
        let key_work = self
            .key_work
            .checked_add(key.hash_work()?)
            .ok_or(MemoryError::WorkExceeded)?;
        if self.values.len() == self.values.capacity() {
            // Include collision-dependent slot searches, not just key hashes.
            self.execution.charge_work(self.rehash_work()?)?;
            self.values.allocator().clear_failure();
            self.values.try_reserve(1).map_err(|error| match error {
                TryReserveError::CapacityOverflow => MemoryError::CapacityOverflow,
                TryReserveError::AllocError { .. } => self.values.allocator().failure(),
            })?;
            self.maximum_capacity = self.maximum_capacity.max(self.values.capacity());
        }
        // Reuse the already computed hash after reserve. No key equality is
        // needed: the first lookup established absence and resize preserves keys.
        // raw_entry searches for a match; insert_hashed_nocheck subsequently
        // searches for an insertion slot. Admit both control traversals.
        self.execution.charge_work(
            self.control_work()?
                .checked_mul(2)
                .and_then(|work| work.checked_add(core::mem::size_of::<(K, V)>() as u64))
                .ok_or(MemoryError::WorkExceeded)?,
        )?;
        let RawEntryMut::Vacant(entry) = self.values.raw_entry_mut().from_hash(hash, |_| false)
        else {
            unreachable!("a predicate that always refuses cannot find an entry");
        };
        entry.insert_hashed_nocheck(hash, key, value);
        self.key_work = key_work;
        Ok(None)
    }

    /// Removes an entry, retaining table capacity and its storage charge.
    /// The supplied execution domain pays for lookup and removal before mutation.
    ///
    /// # Examples
    /// ```
    /// # extern crate alloc;
    /// # use alloc::sync::Arc;
    /// # use structured_asn1::memory::{Memory, MemoryAccount, MemoryError};
    /// # let memory = Memory::new(Arc::new(MemoryAccount::new(1024, 1024)));
    /// # let mut map = memory.map();
    /// # map.try_insert(1usize, 42)?;
    /// let capacity_charge = memory.used();
    /// assert_eq!(map.remove(&1, &memory)?, Some(42));
    /// assert_eq!(memory.used(), capacity_charge);
    /// # Ok::<(), MemoryError>(())
    /// ```
    pub fn remove(&mut self, key: &K, execution: &impl Work) -> Result<Option<V>, MemoryError> {
        let hash = self.hash_key(key, execution)?;
        let mut failure = None;
        let entry = self.values.raw_entry_mut().from_hash(hash, |candidate| {
            equivalent(execution, key, candidate, &mut failure)
        });
        if let Some(reason) = failure {
            return Err(reason);
        }
        let entry = match entry {
            RawEntryMut::Occupied(entry) => {
                // Erase examines the adjacent control groups; returning the
                // pair moves its storage. Admit both before changing the table.
                let work = (core::mem::size_of::<(K, V)>() as u64)
                    .checked_mul(2)
                    .and_then(|work| work.checked_add(2))
                    .ok_or(MemoryError::WorkExceeded)?;
                let removed_work = entry.key().hash_work()?;
                execution.charge_work(work)?;
                let (_, value) = entry.remove_entry();
                self.key_work -= removed_work;
                Some(value)
            }
            RawEntryMut::Vacant(_) => None,
        };
        Ok(entry)
    }
}

/// A synthetic match stops hashbrown immediately on exhaustion. The caller
/// checks failure before exposing or mutating the matched entry, so no false
/// equality escapes and no expensive comparison runs after a refusal.
fn equivalent<K: HashWork, W: Work>(
    memory: &W,
    query: &K,
    candidate: &K,
    failure: &mut Option<MemoryError>,
) -> bool {
    let cost = query.hash_work().and_then(|query| {
        candidate.hash_work().and_then(|candidate| {
            query
                .checked_add(candidate)
                .ok_or(MemoryError::WorkExceeded)
        })
    });
    let admitted = cost.and_then(|cost| memory.charge_work(cost));
    if let Err(reason) = admitted {
        *failure = Some(reason);
        return true;
    }
    query == candidate
}