1 use std::ops::Index;
2 use alloc::vec::Vec;
3 
4 #[derive(Debug, Clone)]
5 pub struct LazyBuffer<I: Iterator> {
6     pub it: I,
7     done: bool,
8     buffer: Vec<I::Item>,
9 }
10 
11 impl<I> LazyBuffer<I>
12 where
13     I: Iterator,
14 {
new(it: I) -> LazyBuffer<I>15     pub fn new(it: I) -> LazyBuffer<I> {
16         LazyBuffer {
17             it,
18             done: false,
19             buffer: Vec::new(),
20         }
21     }
22 
len(&self) -> usize23     pub fn len(&self) -> usize {
24         self.buffer.len()
25     }
26 
get_next(&mut self) -> bool27     pub fn get_next(&mut self) -> bool {
28         if self.done {
29             return false;
30         }
31         if let Some(x) = self.it.next() {
32             self.buffer.push(x);
33             true
34         } else {
35             self.done = true;
36             false
37         }
38     }
39 
prefill(&mut self, len: usize)40     pub fn prefill(&mut self, len: usize) {
41         let buffer_len = self.buffer.len();
42 
43         if !self.done && len > buffer_len {
44             let delta = len - buffer_len;
45 
46             self.buffer.extend(self.it.by_ref().take(delta));
47             self.done = self.buffer.len() < len;
48         }
49     }
50 }
51 
52 impl<I, J> Index<J> for LazyBuffer<I>
53 where
54     I: Iterator,
55     I::Item: Sized,
56     Vec<I::Item>: Index<J>
57 {
58     type Output = <Vec<I::Item> as Index<J>>::Output;
59 
index(&self, index: J) -> &Self::Output60     fn index(&self, index: J) -> &Self::Output {
61         self.buffer.index(index)
62     }
63 }
64