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#![doc = include_str!("README.md")]
#![no_std]
use core::marker::PhantomData;
type IdFn<X> = fn(X) -> X;
type DupFn<X> = fn(X) -> (X, X);
type Tpl2<X> = (X, X);
/// Iterator scan methods that don't suck.
pub trait IterScan: Iterator + Sized {
/// This iterator adapter holds an internal state and emit this state on each iteration.
///
/// This internal state can be [cloned](Clone).
///
/// `scan_clone()` takes 2 arguments:
/// * An initial value which seeds the internal state.
/// * A closure that:
/// - Takes 2 arguments: Copy of the internal state from the previous iteration and the current item.
/// - Returns the new state for the next iteration.
///
/// **Example:** Basic usage
///
/// ```rust
/// use iter_scan::IterScan;
/// let input = ['a', 'b', 'c', 'd', 'e', 'f'];
/// let output: Vec<_> = input
/// .into_iter()
/// .scan_clone(String::new(), |acc, x| format!("{acc}{x}"))
/// .collect();
/// assert_eq!(output, ["a", "ab", "abc", "abcd", "abcde", "abcdef"]);
/// ```
fn scan_clone<Compute, State>(
self,
initial: State,
compute: Compute,
) -> Scan<Self, DupFn<State>, Compute, State>
where
State: Clone,
Compute: FnMut(&State, Self::Item) -> State,
{
Scan {
compute,
iter: self,
state: initial,
duplicate: |x| (x.clone(), x),
_phantom: Default::default(),
}
}
/// This iterator adapter holds an internal state and emit this state on each iteration.
///
/// This internal state can be [copied](Copy).
///
/// `scan_copy()` takes 2 arguments:
/// * An initial value which seeds the internal state.
/// * A closure that:
/// - Takes 2 arguments: Copy of the internal state from the previous iteration and the current item.
/// - Returns the new state for the next iteration.
///
/// **Example:** Basic usage
///
/// ```rust
/// use iter_scan::IterScan;
/// let input = [2, 3, 4, 5];
/// let output: Vec<u64> = input
/// .into_iter()
/// .scan_copy(1, |acc, x| acc * x)
/// .collect();
/// assert_eq!(output, [2, 6, 24, 120]);
/// ```
fn scan_copy<Compute, State>(
self,
initial: State,
compute: Compute,
) -> Scan<Self, DupFn<State>, Compute, State>
where
State: Copy,
Compute: FnMut(&State, Self::Item) -> State,
{
Scan {
compute,
iter: self,
state: initial,
duplicate: |x| (x, x),
_phantom: Default::default(),
}
}
/// This iterator adapter holds an internal state and emit this state on each iteration.
///
/// This adapter should be used when the internal state can neither be [cloned](Clone) nor [copied](Copy).
///
/// `scan_with_tuple()` takes 2 arguments:
/// * An initial value which seeds the internal state.
/// * A closure that:
/// - Takes 2 arguments: Copy of the internal state from the previous iteration and the current item.
/// - Returns a tuple of the new state and its identical copy.
fn scan_with_tuple<Compute, State>(
self,
initial: State,
compute: Compute,
) -> Scan<Self, IdFn<Tpl2<State>>, Compute, State>
where
Compute: FnMut(&State, Self::Item) -> Tpl2<State>,
{
Scan {
compute,
iter: self,
state: initial,
duplicate: |x| x,
_phantom: Default::default(),
}
}
}
impl<X: Iterator + Sized> IterScan for X {}
#[derive(Clone)]
#[must_use = "iterators are lazy and do nothing unless consumed"]
pub struct Scan<Iter, Duplicate, Compute, State> {
iter: Iter,
state: State,
duplicate: Duplicate,
compute: Compute,
_phantom: PhantomData<State>,
}
impl<Iter, Duplicate, Compute, State, Intermediate> Iterator
for Scan<Iter, Duplicate, Compute, State>
where
Iter: Iterator,
Duplicate: FnMut(Intermediate) -> Tpl2<State>,
Compute: FnMut(&State, Iter::Item) -> Intermediate,
{
type Item = State;
#[inline]
fn next(&mut self) -> Option<Self::Item> {
let Scan {
iter,
state,
duplicate,
compute,
..
} = self;
let x = iter.next()?;
let (y1, y2) = duplicate(compute(state, x));
*state = y1;
Some(y2)
}
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let (_, upper) = self.iter.size_hint();
(0, upper) // can't know a lower bound, due to the scan function
}
}