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use std::fmt;
use super::lazy_buffer::LazyBuffer;
use alloc::vec::Vec;
/// An iterator to iterate through all the `k`-length combinations in an iterator.
///
/// See [`.combinations()`](../trait.Itertools.html#method.combinations) for more information.
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub struct Combinations<I: Iterator> {
indices: Vec<usize>,
pool: LazyBuffer<I>,
first: bool,
}
impl<I> Clone for Combinations<I>
where I: Clone + Iterator,
I::Item: Clone,
{
clone_fields!(indices, pool, first);
}
impl<I> fmt::Debug for Combinations<I>
where I: Iterator + fmt::Debug,
I::Item: fmt::Debug,
{
debug_fmt_fields!(Combinations, indices, pool, first);
}
/// Create a new `Combinations` from a clonable iterator.
pub fn combinations<I>(iter: I, k: usize) -> Combinations<I>
where I: Iterator
{
let mut pool = LazyBuffer::new(iter);
pool.prefill(k);
Combinations {
indices: (0..k).collect(),
pool,
first: true,
}
}
impl<I: Iterator> Combinations<I> {
/// Returns the length of a combination produced by this iterator.
#[inline]
pub fn k(&self) -> usize { self.indices.len() }
/// Returns the (current) length of the pool from which combination elements are
/// selected. This value can change between invocations of [`next`].
///
/// [`next`]: #method.next
#[inline]
pub fn n(&self) -> usize { self.pool.len() }
/// Returns a reference to the source iterator.
#[inline]
pub(crate) fn src(&self) -> &I { &self.pool.it }
/// Resets this `Combinations` back to an initial state for combinations of length
/// `k` over the same pool data source. If `k` is larger than the current length
/// of the data pool an attempt is made to prefill the pool so that it holds `k`
/// elements.
pub(crate) fn reset(&mut self, k: usize) {
self.first = true;
if k < self.indices.len() {
self.indices.truncate(k);
for i in 0..k {
self.indices[i] = i;
}
} else {
for i in 0..self.indices.len() {
self.indices[i] = i;
}
self.indices.extend(self.indices.len()..k);
self.pool.prefill(k);
}
}
}
impl<I> Iterator for Combinations<I>
where I: Iterator,
I::Item: Clone
{
type Item = Vec<I::Item>;
fn next(&mut self) -> Option<Self::Item> {
if self.first {
if self.k() > self.n() {
return None;
}
self.first = false;
} else if self.indices.is_empty() {
return None;
} else {
// Scan from the end, looking for an index to increment
let mut i: usize = self.indices.len() - 1;
// Check if we need to consume more from the iterator
if self.indices[i] == self.pool.len() - 1 {
self.pool.get_next(); // may change pool size
}
while self.indices[i] == i + self.pool.len() - self.indices.len() {
if i > 0 {
i -= 1;
} else {
// Reached the last combination
return None;
}
}
// Increment index, and reset the ones to its right
self.indices[i] += 1;
for j in i+1..self.indices.len() {
self.indices[j] = self.indices[j - 1] + 1;
}
}
// Create result vector based on the indices
Some(self.indices.iter().map(|i| self.pool[*i].clone()).collect())
}
}
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