Struct `SmallIndex`

struct SmallIndex(_)

A type that represents a "small" index.

The main idea of this type is to provide something that can index memory, but uses less memory than usize on 64-bit systems. Specifically, its representation is always a u32 and has repr(transparent) enabled. (So it is safe to transmute between a u32 and a SmallIndex.)

A small index is typically useful in cases where there is no practical way that the index will overflow a 32-bit integer. A good example of this is an NFA state. If you could somehow build an NFA with 2^30 states, its memory usage would be exorbitant and its runtime execution would be so slow as to be completely worthless. Therefore, this crate generally deems it acceptable to return an error if it would otherwise build an NFA that requires a slice longer than what a 32-bit integer can index. In exchange, we can use 32-bit indices instead of 64-bit indices in various places.

This type ensures this by providing a constructor that will return an error if its argument cannot fit into the type. This makes it much easier to handle these sorts of boundary cases that are otherwise extremely subtle.

On all targets, this type guarantees that its value will fit in a u32, i32, usize and an isize. This means that on 16-bit targets, for example, this type's maximum value will never overflow an isize, which means it will never overflow a i16 even though its internal representation is still a u32.

The purpose for making the type fit into even signed integer types like isize is to guarantee that the difference between any two small indices is itself also a small index. This is useful in certain contexts, e.g., for delta encoding.

Other types

The following types wrap SmallIndex to provide a more focused use case:

PatternID is for representing the identifiers of patterns.
StateID is for representing the identifiers of states in finite automata. It is used for both NFAs and DFAs.

Representation

This type is always represented internally by a u32 and is marked as repr(transparent). Thus, this type always has the same representation as a u32. It is thus safe to transmute between a u32 and a SmallIndex.

Indexing

For convenience, callers may use a SmallIndex to index slices.

Safety

While a SmallIndex is meant to guarantee that its value fits into usize without using as much space as a usize on all targets, callers must not rely on this property for safety. Callers may choose to rely on this property for correctness however. For example, creating a SmallIndex with an invalid value can be done in entirely safe code. This may in turn result in panics or silent logical errors.

Implementations

`impl SmallIndex`

fn new(index: usize) -> Result<SmallIndex, SmallIndexError>

Create a new small index.

If the given index exceeds SmallIndex::MAX, then this returns an error.

const fn new_unchecked(index: usize) -> SmallIndex

Create a new small index without checking whether the given value exceeds SmallIndex::MAX.

Using this routine with an invalid index value will result in unspecified behavior, but not undefined behavior. In particular, an invalid index value is likely to cause panics or possibly even silent logical errors.

Callers must never rely on a SmallIndex to be within a certain range for memory safety.

fn must(index: usize) -> SmallIndex

Like SmallIndex::new, but panics if the given index is not valid.

const fn as_usize(self: &Self) -> usize

Return this small index as a usize. This is guaranteed to never overflow usize.

const fn as_u64(self: &Self) -> u64

Return this small index as a u64. This is guaranteed to never overflow.

const fn as_u32(self: &Self) -> u32

Return the internal u32 of this small index. This is guaranteed to never overflow u32.

const fn as_i32(self: &Self) -> i32

Return the internal u32 of this small index represented as an i32. This is guaranteed to never overflow an i32.

fn one_more(self: &Self) -> usize

Returns one more than this small index as a usize.

Since a small index has constraints on its maximum value, adding 1 to it will always fit in a usize, u32 and a i32.

fn from_ne_bytes(bytes: [u8; 4]) -> Result<SmallIndex, SmallIndexError>

Decode this small index from the bytes given using the native endian byte order for the current target.

If the decoded integer is not representable as a small index for the current target, then this returns an error.

fn from_ne_bytes_unchecked(bytes: [u8; 4]) -> SmallIndex

Decode this small index from the bytes given using the native endian byte order for the current target.

This is analogous to SmallIndex::new_unchecked in that is does not check whether the decoded integer is representable as a small index.

fn to_ne_bytes(self: &Self) -> [u8; 4]

Return the underlying small index integer as raw bytes in native endian format.

`impl Clone for SmallIndex`

fn clone(self: &Self) -> SmallIndex

`impl Copy for SmallIndex`

`impl Debug for SmallIndex`

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

`impl Default for SmallIndex`

fn default() -> SmallIndex

`impl Eq for SmallIndex`

`impl Freeze for SmallIndex`

`impl From for SmallIndex`

fn from(index: u8) -> SmallIndex

`impl Hash for SmallIndex`

fn hash<__H: $crate::hash::Hasher>(self: &Self, state: &mut __H)

`impl Ord for SmallIndex`

fn cmp(self: &Self, other: &SmallIndex) -> Ordering

`impl PartialEq for SmallIndex`

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

`impl PartialOrd for SmallIndex`

fn partial_cmp(self: &Self, other: &SmallIndex) -> Option<Ordering>

`impl RefUnwindSafe for SmallIndex`

`impl Send for SmallIndex`

`impl StructuralPartialEq for SmallIndex`

`impl Sync for SmallIndex`

`impl TryFrom for SmallIndex`

fn try_from(index: u16) -> Result<SmallIndex, SmallIndexError>

`impl TryFrom for SmallIndex`

fn try_from(index: u32) -> Result<SmallIndex, SmallIndexError>

`impl TryFrom for SmallIndex`

fn try_from(index: u64) -> Result<SmallIndex, SmallIndexError>

`impl TryFrom for SmallIndex`

fn try_from(index: usize) -> Result<SmallIndex, SmallIndexError>

`impl Unpin for SmallIndex`

`impl UnsafeUnpin for SmallIndex`

`impl UnwindSafe for SmallIndex`

`impl<T> Any for SmallIndex`

fn type_id(self: &Self) -> TypeId

`impl<T> Borrow for SmallIndex`

fn borrow(self: &Self) -> &T

`impl<T> BorrowMut for SmallIndex`

fn borrow_mut(self: &mut Self) -> &mut T

`impl<T> CloneToUninit for SmallIndex`

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

`impl<T> From for SmallIndex`

fn from(t: T) -> T: Returns the argument unchanged.

`impl<T> ToOwned for SmallIndex`

fn to_owned(self: &Self) -> T
fn clone_into(self: &Self, target: &mut T)

`impl<T, U> Into for SmallIndex`

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

`impl<T, U> TryFrom for SmallIndex`

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

`impl<T, U> TryInto for SmallIndex`

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>