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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151
pub struct Choose<'t, T: 't> { list: &'t [T], k: usize, indices: Vec<usize>, first: bool, } impl<'t, T> Choose<'t, T> { fn new(list: &'t [T], k: usize) -> Choose<'t, T> { Choose { list: list, k: k, indices: (0..k).collect(), first: true, } } fn n(&self) -> usize { self.list.len() } fn from_indices(&self) -> Vec<&'t T> { self.indices.iter().map(|&i| &self.list[i]).collect() } fn increment_indices(&mut self) -> Option<()> { let to_increment = (0..self.k) .rev() .filter(|&i| { let max_allowed = self.n() - self.k + i; self.indices[i] < max_allowed }) .next(); let to_increment = match to_increment { None => return None, Some(i) => i, }; self.indices[to_increment] += 1; for j in (to_increment + 1)..self.k { let previous_index = self.indices[j - 1]; self.indices[j] = previous_index + 1; } Some(()) } } impl<'t, T> Iterator for Choose<'t, T> { type Item = Vec<&'t T>; fn next(&mut self) -> Option<Self::Item> { if self.n() < self.k { return None; } if !self.first { match self.increment_indices() { None => return None, Some(()) => {} }; }; self.first = false; Some(self.from_indices()) } } pub trait Chooseable<'t, T> { fn choose(&'t self, k: usize) -> Choose<'t, T>; } impl<'t, T> Chooseable<'t, T> for [T] { fn choose(&'t self, k: usize) -> Choose<'t, T> { Choose::new(self, k) } } #[cfg(test)] mod tests { use super::*; fn assert_next(it: &mut Choose<usize>, items: Vec<usize>) { assert_eq!(it.next().unwrap(), items.iter().collect::<Vec<_>>()); } #[test] fn it_returns_none_when_choosing_one_from_an_empty_list() { let vector: Vec<()> = vec![]; let mut it = vector.choose(1); assert_eq!(it.next(), None); } #[test] fn it_returns_the_empty_vector_when_choosing_zero_from_an_empty_list() { let vector: Vec<usize> = vec![]; let mut it = vector.choose(0); assert_next(&mut it, vec![]); assert_eq!(it.next(), None); } #[test] fn it_returns_the_empty_vector_when_choosing_zero_from_a_list_with_elements() { let vector = vec![0, 1, 2]; let mut it = vector.choose(0); assert_eq!(it.next().unwrap().len(), 0); assert_eq!(it.next(), None); } #[test] fn it_returns_each_item_when_choosing_one_from_a_list_with_items() { let vector = vec![0, 1, 2]; let mut it = vector.choose(1); assert_next(&mut it, vec![0]); assert_next(&mut it, vec![1]); assert_next(&mut it, vec![2]); assert_eq!(it.next(), None); } #[test] fn it_returns_each_item_combination_when_choosing_two_from_a_list_with_items() { let vector = vec![0, 1, 2]; let mut it = vector.choose(2); assert_next(&mut it, vec![0, 1]); assert_next(&mut it, vec![0, 2]); assert_next(&mut it, vec![1, 2]); assert_eq!(it.next(), None); } #[test] fn it_returns_each_item_combination_when_choosing_two_from_a_list_with_five_items() { let vector = vec![0, 1, 2, 3, 4]; let mut it = vector.choose(2); assert_next(&mut it, vec![0, 1]); assert_next(&mut it, vec![0, 2]); assert_next(&mut it, vec![0, 3]); assert_next(&mut it, vec![0, 4]); assert_next(&mut it, vec![1, 2]); assert_next(&mut it, vec![1, 3]); assert_next(&mut it, vec![1, 4]); assert_next(&mut it, vec![2, 3]); assert_next(&mut it, vec![2, 4]); assert_next(&mut it, vec![3, 4]); assert_eq!(it.next(), None); } }