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Specialize Powerset::nth #924

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May 7, 2024
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39 changes: 22 additions & 17 deletions src/combinations.rs
Original file line number Diff line number Diff line change
Expand Up @@ -143,6 +143,27 @@ impl<I: Iterator> Combinations<I> {
// If we've made it this far, we haven't run out of combos
false
}

/// Returns the n-th item or the number of successful steps.
pub(crate) fn try_nth(&mut self, n: usize) -> Result<<Self as Iterator>::Item, usize>
where
I::Item: Clone,
{
let done = if self.first {
self.init()
} else {
self.increment_indices()
};
if done {
return Err(0);
}
for i in 0..n {
if self.increment_indices() {
return Err(i + 1);
}
}
Ok(self.pool.get_at(&self.indices))
}
}

impl<I> Iterator for Combinations<I>
Expand All @@ -166,23 +187,7 @@ where
}

fn nth(&mut self, n: usize) -> Option<Self::Item> {
let done = if self.first {
self.init()
} else {
self.increment_indices()
};

if done {
return None;
}

for _ in 0..n {
if self.increment_indices() {
return None;
}
}

Some(self.pool.get_at(&self.indices))
self.try_nth(n).ok()
}

fn size_hint(&self) -> (usize, Option<usize>) {
Expand Down
29 changes: 27 additions & 2 deletions src/powerset.rs
Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,18 @@ where
}
}

impl<I: Iterator> Powerset<I> {
/// Returns true if `k` has been incremented, false otherwise.
fn increment_k(&mut self) -> bool {
if self.combs.k() < self.combs.n() || self.combs.k() == 0 {
self.combs.reset(self.combs.k() + 1);
true
} else {
false
}
}
}

impl<I> Iterator for Powerset<I>
where
I: Iterator,
Expand All @@ -52,14 +64,27 @@ where
fn next(&mut self) -> Option<Self::Item> {
if let Some(elt) = self.combs.next() {
Some(elt)
} else if self.combs.k() < self.combs.n() || self.combs.k() == 0 {
self.combs.reset(self.combs.k() + 1);
} else if self.increment_k() {
self.combs.next()
} else {
None
}
}

fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
loop {
match self.combs.try_nth(n) {
Ok(item) => return Some(item),
Err(steps) => {
if !self.increment_k() {
return None;
}
n -= steps;
}
}
}
}

fn size_hint(&self) -> SizeHint {
let k = self.combs.k();
// Total bounds for source iterator.
Expand Down