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/// Finds all permutations of k elements from a collection.
///
/// # Arguments
/// * `items` - A slice of items to permute.
/// * `k` - The number of elements in each permutation.
///
/// # Returns
/// * `Vec<Vec<T>>` - A vector containing all permutations of k elements from the input.
///
/// # Examples
/// ```rust
/// use lowdash::permutation;
///
/// let items = vec![1, 2, 3];
/// let result = permutation(&items, 2);
/// // Expected permutations: [ [1,2], [1,3], [2,1], [2,3], [3,1], [3,2] ]
/// assert_eq!(result.len(), 6);
/// assert!(result.contains(&vec![2, 1]));
/// ```
pub fn permutation<T: Clone>(items: &[T], k: usize) -> Vec<Vec<T>> {
if k == 0 {
return vec![vec![]];
}
if k > items.len() {
return vec![];
}
let mut result = Vec::new();
for (i, item) in items.iter().enumerate() {
let mut remaining = items.to_vec();
remaining.remove(i);
for mut perm in permutation(&remaining, k - 1) {
let mut current = vec![item.clone()];
current.append(&mut perm);
result.push(current);
}
}
result
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_permutation_k_zero() {
let items: Vec<i32> = vec![];
assert_eq!(permutation(&items, 0), vec![vec![]]);
}
#[test]
fn test_permutation_k_greater_than_len() {
let items = vec![42];
assert_eq!(permutation(&items, 2), Vec::<Vec<i32>>::new());
}
#[test]
fn test_permutation_single_element() {
let items = vec![42];
assert_eq!(permutation(&items, 1), vec![vec![42]]);
}
#[test]
fn test_permutation_multiple() {
let items = vec![1, 2, 3];
let perms = permutation(&items, 2);
assert_eq!(perms.len(), 6);
assert!(perms.contains(&vec![2, 1]));
}
}