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
use std::cell::Cell;
enum UnfoldItem<A> {
Nil,
Item(A),
}
impl<A> Default for UnfoldItem<A> {
fn default() -> Self {
UnfoldItem::Nil
}
}
pub struct Unfold<F, S> {
f: F,
value: Cell<UnfoldItem<S>>,
}
impl<F, S, A> Iterator for Unfold<F, S>
where
F: Fn(S) -> Option<(A, S)>,
{
type Item = A;
fn next(&mut self) -> Option<Self::Item> {
match self.value.take() {
UnfoldItem::Nil => unreachable!(),
UnfoldItem::Item(value) => {
match (self.f)(value) {
None => None,
Some((next, value)) => {
self.value.set(UnfoldItem::Item(value));
Some(next)
}
}
}
}
}
}
pub fn unfold<F, S, A>(value: S, f: F) -> Unfold<F, S>
where
F: Fn(S) -> Option<(A, S)>,
{
Unfold {
f: f,
value: Cell::new(UnfoldItem::Item(value)),
}
}