Struct Span 

pub struct Span {
    pub at: u64,
    pub take: u64,
}

The rows a source is being asked for: take of them, starting at at.

This is what a graph reading from a source is handed as its input, and the input is the one value a cache hashes by content. Two numbers, so naming it is free; the rows are named by the source’s version instead.

And it is what makes a stream cacheable. A span is a position, and a position is repeatable: rows 400..500 are the same rows tomorrow, whatever has arrived since. What moves is not the source’s state, it is which spans exist. A source answering whatever is newest is the other thing, and the engine already refuses to cache under it.

Fields

at: u64

The first row, counting from zero.

take: u64

How many, at most: the last span of a dataset is short and that is not an error.

Implementations

impl Span

pub fn new(at: u64, take: u64) -> Self

take rows starting at at.

pub fn of(value: &Value) -> Result<Self, SpanError>

The span this value is, if it is one.

A Map of two numbers and not a pair of positions in a list: what a record shows is what was asked for, and {"at": 4096, "take": 64} says it where [4096, 64] needs the reader to remember the order.

pub fn value(&self) -> Value

As a value, which is how it is handed to a graph.

Trait Implementations

impl Clone for Span

fn clone(&self) -> Span

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for Span

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

Formats the value using the given formatter.

impl Hash for Span

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 PartialEq for Span

fn eq(&self, other: &Span) -> 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 Copy for Span

impl Eq for Span

impl StructuralPartialEq for Span