Struct LazyHash

pub struct LazyHash<T: ?Sized> { /* private fields */ }

A wrapper type with lazily-computed hash.

This is useful if you want to pass large values of T to memoized functions. Especially recursive structures like trees benefit from intermediate prehashed nodes.

Note that for a value v of type T, hash(v) is not necessarily equal to hash(LazyHash::new(v)). Writing the precomputed hash into a hasher’s state produces different output than writing the value’s parts directly. However, that seldom matters as you are typically either dealing with values of type T or with values of type LazyHash<T>, not a mix of both.

Equality

Because Typst uses high-quality 128 bit hashes in all places, the risk of a hash collision is reduced to an absolute minimum. Therefore, this type additionally provides PartialEq and Eq implementations that compare by hash instead of by value. For this to be correct, your hash implementation must feed all information relevant to the PartialEq impl to the hasher.

Usage

If the value is expected to be cloned, it is best used inside of an Arc or Rc to best re-use the hash once it has been computed.

Implementations

impl<T> LazyHash<T>

pub fn new(value: T) -> Self

Wraps an item without pre-computed hash.

pub fn reuse<U: ?Sized>(value: T, existing: &LazyHash<U>) -> Self

Wrap an item with a pre-computed hash.

Important: The hash must be correct for the value. This cannot be enforced at compile time, so use with caution.

pub fn into_inner(self) -> T

Returns the wrapped value.

Trait Implementations

impl<T: Hash + Clone + 'static> Clone for LazyHash<T>

fn clone(&self) -> Self

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Debug> Debug for LazyHash<T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T: Default> Default for LazyHash<T>

fn default() -> Self

Returns the “default value” for a type.

impl<T: ?Sized> Deref for LazyHash<T>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.

impl<T: Hash + ?Sized + 'static> DerefMut for LazyHash<T>

fn deref_mut(&mut self) -> &mut Self::Target

Mutably dereferences the value.

impl<T> From<T> for LazyHash<T>

fn from(value: T) -> Self

Converts to this type from the input type.

impl<T: Hash + ?Sized + 'static> Hash for LazyHash<T>

fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given Hasher.

impl<T: Hash + ?Sized + 'static> PartialEq for LazyHash<T>

fn eq(&self, other: &Self) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T: Hash + ?Sized + 'static> Eq for LazyHash<T>