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Fixing permutation of small lists, such that [], [x] -> [[]], [[x]] #13783

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Apr 27, 2014
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Fixing permutation of small lists, such that [], [x] -> [[]], [[x]] #13783

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55 changes: 48 additions & 7 deletions src/libstd/slice.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -307,6 +307,7 @@ pub struct ElementSwaps {
sdir: ~[SizeDirection],
/// If true, emit the last swap that returns the sequence to initial state
emit_reset: bool,
swaps_made : uint,
}

impl ElementSwaps {
Expand All @@ -319,7 +320,8 @@ impl ElementSwaps {
emit_reset: true,
sdir: range(0, length)
.map(|i| SizeDirection{ size: i, dir: Neg })
.collect::<~[_]>()
.collect::<~[_]>(),
swaps_made: 0
}
}
}
Expand Down Expand Up @@ -358,16 +360,30 @@ impl Iterator<(uint, uint)> for ElementSwaps {
x.dir = match x.dir { Pos => Neg, Neg => Pos };
}
}
self.swaps_made += 1;
Some((i, j))
},
None => if self.emit_reset && self.sdir.len() > 1 {
None => if self.emit_reset {
self.emit_reset = false;
Some((0, 1))
} else {
None
}
if self.sdir.len() > 1 {
// The last swap
self.swaps_made += 1;
Some((0, 1))
} else {
// Vector is of the form [] or [x], and the only permutation is itself
self.swaps_made += 1;
Some((0,0))
}
} else { None }
}
}

#[inline]
fn size_hint(&self) -> (uint, Option<uint>) {
// For a vector of size n, there are exactly n! permutations.
let n = range(2, self.sdir.len() + 1).product();
(n - self.swaps_made, Some(n - self.swaps_made))
}
}

/// An Iterator that uses `ElementSwaps` to iterate through
Expand All @@ -388,13 +404,19 @@ impl<T: Clone> Iterator<~[T]> for Permutations<T> {
fn next(&mut self) -> Option<~[T]> {
match self.swaps.next() {
None => None,
Some((0,0)) => Some(self.v.clone()),
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Did you find it necessary to have this case? I think that swap handles the case where the two indices are the same.

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@alexcrichton: I had two reasons:

  1. For an empty list, there is no item 0, and that would be bad.
  2. (Not as important) For a non-empty list, AFAICT there is no code in either slice::swap or ptr::swap stopping it from actually performing the swap, i.e. copying item 0 to a temporary value, moving item 0 to item 0, and then copying the temp value back in. That seemed unnecessary...

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Ah, of course!

Some((a, b)) => {
let elt = self.v.clone();
self.v.swap(a, b);
Some(elt)
}
}
}

#[inline]
fn size_hint(&self) -> (uint, Option<uint>) {
self.swaps.size_hint()
}
}

/// An iterator over the (overlapping) slices of length `size` within
Expand Down Expand Up @@ -2767,19 +2789,33 @@ mod tests {
{
let v: [int, ..0] = [];
let mut it = v.permutations();
let (min_size, max_opt) = it.size_hint();
assert_eq!(min_size, 1);
assert_eq!(max_opt.unwrap(), 1);
assert_eq!(it.next(), Some(v.as_slice().to_owned()));
assert_eq!(it.next(), None);
}
{
let v = ["Hello".to_owned()];
let mut it = v.permutations();
let (min_size, max_opt) = it.size_hint();
assert_eq!(min_size, 1);
assert_eq!(max_opt.unwrap(), 1);
assert_eq!(it.next(), Some(v.as_slice().to_owned()));
assert_eq!(it.next(), None);
}
{
let v = [1, 2, 3];
let mut it = v.permutations();
let (min_size, max_opt) = it.size_hint();
assert_eq!(min_size, 3*2);
assert_eq!(max_opt.unwrap(), 3*2);
assert_eq!(it.next(), Some(~[1,2,3]));
assert_eq!(it.next(), Some(~[1,3,2]));
assert_eq!(it.next(), Some(~[3,1,2]));
let (min_size, max_opt) = it.size_hint();
assert_eq!(min_size, 3);
assert_eq!(max_opt.unwrap(), 3);
assert_eq!(it.next(), Some(~[3,2,1]));
assert_eq!(it.next(), Some(~[2,3,1]));
assert_eq!(it.next(), Some(~[2,1,3]));
Expand All @@ -2789,10 +2825,15 @@ mod tests {
// check that we have N! permutations
let v = ['A', 'B', 'C', 'D', 'E', 'F'];
let mut amt = 0;
for _perm in v.permutations() {
let mut it = v.permutations();
let (min_size, max_opt) = it.size_hint();
for _perm in it {
amt += 1;
}
assert_eq!(amt, it.swaps.swaps_made);
assert_eq!(amt, min_size);
assert_eq!(amt, 2 * 3 * 4 * 5 * 6);
assert_eq!(amt, max_opt.unwrap());
}
}

Expand Down