itertools/
powerset.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
use alloc::vec::Vec;
use std::fmt;
use std::iter::FusedIterator;

use super::combinations::{combinations, Combinations};
use crate::adaptors::checked_binomial;
use crate::size_hint::{self, SizeHint};

/// An iterator to iterate through the powerset of the elements from an iterator.
///
/// See [`.powerset()`](crate::Itertools::powerset) for more
/// information.
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub struct Powerset<I: Iterator> {
    combs: Combinations<I>,
}

impl<I> Clone for Powerset<I>
where
    I: Clone + Iterator,
    I::Item: Clone,
{
    clone_fields!(combs);
}

impl<I> fmt::Debug for Powerset<I>
where
    I: Iterator + fmt::Debug,
    I::Item: fmt::Debug,
{
    debug_fmt_fields!(Powerset, combs);
}

/// Create a new `Powerset` from a clonable iterator.
pub fn powerset<I>(src: I) -> Powerset<I>
where
    I: Iterator,
    I::Item: Clone,
{
    Powerset {
        combs: combinations(src, 0),
    }
}

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,
    I::Item: Clone,
{
    type Item = Vec<I::Item>;

    fn next(&mut self) -> Option<Self::Item> {
        if let Some(elt) = self.combs.next() {
            Some(elt)
        } 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.
        let (n_min, n_max) = self.combs.src().size_hint();
        let low = remaining_for(n_min, k).unwrap_or(usize::MAX);
        let upp = n_max.and_then(|n| remaining_for(n, k));
        size_hint::add(self.combs.size_hint(), (low, upp))
    }

    fn count(self) -> usize {
        let k = self.combs.k();
        let (n, combs_count) = self.combs.n_and_count();
        combs_count + remaining_for(n, k).unwrap()
    }

    fn fold<B, F>(self, mut init: B, mut f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    {
        let mut it = self.combs;
        if it.k() == 0 {
            init = it.by_ref().fold(init, &mut f);
            it.reset(1);
        }
        init = it.by_ref().fold(init, &mut f);
        // n is now known for sure because k >= 1 and all k-combinations have been generated.
        for k in it.k() + 1..=it.n() {
            it.reset(k);
            init = it.by_ref().fold(init, &mut f);
        }
        init
    }
}

impl<I> FusedIterator for Powerset<I>
where
    I: Iterator,
    I::Item: Clone,
{
}

fn remaining_for(n: usize, k: usize) -> Option<usize> {
    (k + 1..=n).try_fold(0usize, |sum, i| sum.checked_add(checked_binomial(n, i)?))
}