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Simplify Permutations #790

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merged 10 commits into from
Nov 1, 2023
Merged

Simplify Permutations #790

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bf0564f
Rename `PermutationState` variants
Philippe-Cholet Oct 30, 2023
ff1b1e3
`permutations`: simplify references
Philippe-Cholet Oct 30, 2023
839fa74
`permutations`: `advance` function
Philippe-Cholet Oct 30, 2023
c6de3e2
`PermutationState::size_hint_for`
Philippe-Cholet Oct 30, 2023
8ce6044
Merge `CompleteState` variants into `PermutationState`
Philippe-Cholet Nov 1, 2023
518811a
Unindent block in `Permutations::next`
Philippe-Cholet Nov 1, 2023
901e99b
Inline second inspection of state
Philippe-Cholet Nov 1, 2023
05a815f
Remove `PermutationState::LoadedStart`
Philippe-Cholet Nov 1, 2023
8f0d0ae
`Permutations::next`: simplify two vectors
Philippe-Cholet Nov 1, 2023
8ab3995
`impl FusedIterator for Permutations`
Philippe-Cholet Nov 1, 2023
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257 changes: 97 additions & 160 deletions src/permutations.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
use alloc::vec::Vec;
use std::fmt;
use std::iter::once;
use std::iter::FusedIterator;

use super::lazy_buffer::LazyBuffer;
use crate::size_hint::{self, SizeHint};
Expand All @@ -26,22 +27,17 @@ where

#[derive(Clone, Debug)]
enum PermutationState {
StartUnknownLen { k: usize },
OngoingUnknownLen { k: usize, min_n: usize },
Complete(CompleteState),
Empty,
}

#[derive(Clone, Debug)]
enum CompleteState {
Start {
n: usize,
k: usize,
},
Ongoing {
/// No permutation generated yet.
Start { k: usize },
/// Values from the iterator are not fully loaded yet so `n` is still unknown.
Buffered { k: usize, min_n: usize },
/// All values from the iterator are known so `n` is known.
Loaded {
indices: Vec<usize>,
cycles: Vec<usize>,
},
/// No permutation left to generate.
End,
}

impl<I> fmt::Debug for Permutations<I>
Expand All @@ -55,20 +51,13 @@ where
pub fn permutations<I: Iterator>(iter: I, k: usize) -> Permutations<I> {
let mut vals = LazyBuffer::new(iter);

if k == 0 {
// Special case, yields single empty vec; `n` is irrelevant
let state = PermutationState::Complete(CompleteState::Start { n: 0, k: 0 });

return Permutations { vals, state };
}

vals.prefill(k);
let enough_vals = vals.len() == k;

let state = if enough_vals {
PermutationState::StartUnknownLen { k }
PermutationState::Start { k }
} else {
PermutationState::Empty
PermutationState::End
};

Permutations { vals, state }
Expand All @@ -82,169 +71,117 @@ where
type Item = Vec<I::Item>;

fn next(&mut self) -> Option<Self::Item> {
{
let &mut Permutations {
ref mut vals,
ref mut state,
} = self;
match *state {
PermutationState::StartUnknownLen { k } => {
*state = PermutationState::OngoingUnknownLen { k, min_n: k };
}
PermutationState::OngoingUnknownLen { k, min_n } => {
if vals.get_next() {
*state = PermutationState::OngoingUnknownLen {
k,
min_n: min_n + 1,
};
} else {
let n = min_n;
let prev_iteration_count = n - k + 1;
let mut complete_state = CompleteState::Start { n, k };

// Advance the complete-state iterator to the correct point
for _ in 0..(prev_iteration_count + 1) {
complete_state.advance();
let Self { vals, state } = self;
match state {
PermutationState::Start { k: 0 } => {
*state = PermutationState::End;
Some(Vec::new())
}
&mut PermutationState::Start { k } => {
*state = PermutationState::Buffered { k, min_n: k };
Some(vals[0..k].to_vec())
}
PermutationState::Buffered { ref k, min_n } => {
if vals.get_next() {
let item = (0..*k - 1)
.chain(once(*min_n))
.map(|i| vals[i].clone())
.collect();
*min_n += 1;
Some(item)
} else {
let n = *min_n;
let prev_iteration_count = n - *k + 1;
let mut indices: Vec<_> = (0..n).collect();
let mut cycles: Vec<_> = (n - k..n).rev().collect();
// Advance the state to the correct point.
for _ in 0..prev_iteration_count {
if advance(&mut indices, &mut cycles) {
*state = PermutationState::End;
return None;
}

*state = PermutationState::Complete(complete_state);
}
let item = indices[0..*k].iter().map(|&i| vals[i].clone()).collect();
*state = PermutationState::Loaded { indices, cycles };
Some(item)
}
PermutationState::Complete(ref mut state) => {
state.advance();
}
PermutationState::Empty => {}
};
}
let &mut Permutations {
ref vals,
ref state,
} = self;
match *state {
PermutationState::StartUnknownLen { .. } => panic!("unexpected iterator state"),
PermutationState::OngoingUnknownLen { k, min_n } => {
let latest_idx = min_n - 1;
let indices = (0..(k - 1)).chain(once(latest_idx));

Some(indices.map(|i| vals[i].clone()).collect())
}
PermutationState::Complete(CompleteState::Ongoing {
ref indices,
ref cycles,
}) => {
PermutationState::Loaded { indices, cycles } => {
if advance(indices, cycles) {
*state = PermutationState::End;
return None;
}
let k = cycles.len();
Some(indices[0..k].iter().map(|&i| vals[i].clone()).collect())
}
PermutationState::Complete(CompleteState::Start { .. }) | PermutationState::Empty => {
None
}
PermutationState::End => None,
}
}

fn count(self) -> usize {
fn from_complete(complete_state: CompleteState) -> usize {
complete_state
.remaining()
.expect("Iterator count greater than usize::MAX")
}

let Permutations { vals, state } = self;
match state {
PermutationState::StartUnknownLen { k } => {
let n = vals.count();
let complete_state = CompleteState::Start { n, k };

from_complete(complete_state)
}
PermutationState::OngoingUnknownLen { k, min_n } => {
let prev_iteration_count = min_n - k + 1;
let n = vals.count();
let complete_state = CompleteState::Start { n, k };

from_complete(complete_state) - prev_iteration_count
}
PermutationState::Complete(state) => from_complete(state),
PermutationState::Empty => 0,
}
let Self { vals, state } = self;
let n = vals.count();
state.size_hint_for(n).1.unwrap()
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So, we go via SizeHint (i.e. (usize, Option<usize>)) to compute count. Fine given the simplification we gain by this.

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Indeed, size_hint_for mostly ends with (x.unwrap_or(usize::MAX), x) on which we either do .0 or .1.
At worst, it needlessly unwrapped an option one time.

}

fn size_hint(&self) -> SizeHint {
let at_start = |k| {
// At the beginning, there are `n!/(n-k)!` items to come (see `remaining`) but `n` might be unknown.
let (mut low, mut upp) = self.vals.size_hint();
low = CompleteState::Start { n: low, k }
.remaining()
.unwrap_or(usize::MAX);
upp = upp.and_then(|n| CompleteState::Start { n, k }.remaining());
(low, upp)
};
match self.state {
PermutationState::StartUnknownLen { k } => at_start(k),
PermutationState::OngoingUnknownLen { k, min_n } => {
// Same as `StartUnknownLen` minus the previously generated items.
size_hint::sub_scalar(at_start(k), min_n - k + 1)
}
PermutationState::Complete(ref state) => match state.remaining() {
Some(count) => (count, Some(count)),
None => (::std::usize::MAX, None),
},
PermutationState::Empty => (0, Some(0)),
}
let (mut low, mut upp) = self.vals.size_hint();
low = self.state.size_hint_for(low).0;
upp = upp.and_then(|n| self.state.size_hint_for(n).1);
Comment on lines +129 to +130
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This pattern seems familiar to me... Do you know if it occurs somewhere else? If so, should we introduce size_hint::map? (We can do this separately, it just occured to me.)

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This pattern occurs most of the time I think. I thought of a similar method in a messy PR but I had headaches about the conditions on f for the resulting size hint to be correct, so as I wrote several size hints I went with applying the pattern manually.

(low, upp)
}
}

impl CompleteState {
fn advance(&mut self) {
*self = match *self {
CompleteState::Start { n, k } => {
let indices = (0..n).collect();
let cycles = ((n - k)..n).rev().collect();

CompleteState::Ongoing { cycles, indices }
}
CompleteState::Ongoing {
ref mut indices,
ref mut cycles,
} => {
let n = indices.len();
let k = cycles.len();

for i in (0..k).rev() {
if cycles[i] == 0 {
cycles[i] = n - i - 1;

let to_push = indices.remove(i);
indices.push(to_push);
} else {
let swap_index = n - cycles[i];
indices.swap(i, swap_index);

cycles[i] -= 1;
return;
}
}
impl<I> FusedIterator for Permutations<I>
where
I: Iterator,
I::Item: Clone,
{
}

CompleteState::Start { n, k }
}
fn advance(indices: &mut [usize], cycles: &mut [usize]) -> bool {
let n = indices.len();
let k = cycles.len();
// NOTE: if `cycles` are only zeros, then we reached the last permutation.
for i in (0..k).rev() {
if cycles[i] == 0 {
cycles[i] = n - i - 1;
indices[i..].rotate_left(1);
} else {
let swap_index = n - cycles[i];
indices.swap(i, swap_index);
cycles[i] -= 1;
return false;
}
}
true
}

/// Returns the count of remaining permutations, or None if it would overflow.
fn remaining(&self) -> Option<usize> {
impl PermutationState {
fn size_hint_for(&self, n: usize) -> SizeHint {
// At the beginning, there are `n!/(n-k)!` items to come.
let at_start = |n, k| {
debug_assert!(n >= k);
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Could you shortly explain why this debug_assert holds?

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I wrote that in my previous branch some weeks ago so I was not much familiar with it, and take a glance at it is definitely not enough as it requires a bit of thinking.

This debug_assert! only occurs with Start and Buffered variants.
At definition, we prefill the lazy buffer with k values. It has enough values (or we would have the End variant) so vals.len() >= k (vals.len()==k at definition, more later).
size_hint_for is then called with:

  • in the case of count: n = vals.count() >= vals.len() (see lazy buffer for >=) ;
  • in the case of size_hint: n = vals.size_hint().0 >= vals.len() (see lazy buffer).
    Similar for n = vals.size_hint().1.

So in each case: n >= vals.len() >= k. Basically, it holds because we prefilled with k values.

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However, I'm considering to soon work on making all our iterators lazy (such as #602) and I'll surely turn that assertion into if n < k { return (0, Some(0)); } (and move the "prefill the lazy buffer" part).

let total = (n - k + 1..=n).try_fold(1usize, |acc, i| acc.checked_mul(i));
(total.unwrap_or(usize::MAX), total)
};
match *self {
CompleteState::Start { n, k } => {
if n < k {
return Some(0);
}
(n - k + 1..=n).try_fold(1usize, |acc, i| acc.checked_mul(i))
Self::Start { k } => at_start(n, k),
Self::Buffered { k, min_n } => {
// Same as `Start` minus the previously generated items.
size_hint::sub_scalar(at_start(n, k), min_n - k + 1)
}
CompleteState::Ongoing {
Self::Loaded {
ref indices,
ref cycles,
} => cycles.iter().enumerate().try_fold(0usize, |acc, (i, &c)| {
acc.checked_mul(indices.len() - i)
.and_then(|count| count.checked_add(c))
}),
} => {
let count = cycles.iter().enumerate().try_fold(0usize, |acc, (i, &c)| {
acc.checked_mul(indices.len() - i)
.and_then(|count| count.checked_add(c))
});
(count.unwrap_or(usize::MAX), count)
}
Self::End => (0, Some(0)),
}
}
}