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.

Unsized coercions

The LazyHash type supports unsized payload types and coercions to such. For instance, a LazyHash<&'static str> can be coerced to a LazyHash<dyn YourTrait> when &'static str: YourTrait. When it is hashed, a LazyHash will always use the Hash impl of the underlying type. This underlying type changes through an unsized coercion. When coercing a LazyHash that has an already populated internal hash, you’ll thus get a cached hash that was hashed with another impl than a fresh hash would have used. To avoid this, when performing unsized coercions, avoid hashing the value before the coercion and overall try to minimize the timespan in which the original type is active. Typical usages of unsized coercions have a very minimal lifetime of the original type only upon construction.

Implementations

impl<T> LazyHash<T>

pub fn new(value: T) -> Self

Wraps an item without pre-computed hash.

pub fn into_inner(self) -> T

Returns the wrapped value.

Trait Implementations

impl<T: Clone + ?Sized> Clone for LazyHash<T>

fn clone(&self) -> LazyHash<T>

Returns a duplicate 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: ?Sized + 'static> DerefMut for LazyHash<T>

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

Mutably dereferences the value.

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

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.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type 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.