marigold-impl 0.2.0

Internal logic for the marigold language.
Documentation 
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use futures::stream::StreamExt;
use marigold_impl::Permutable;

#[tokio::test]
async fn permutations_k_zero() {
    let result = futures::stream::iter(vec![1, 2, 3])
        .permutations(0)
        .await
        .collect::<Vec<_>>()
        .await;
    // P(3,0) = 1 empty permutation
    assert_eq!(result, vec![Vec::<i32>::new()]);
}

#[tokio::test]
async fn permutations_k_exceeds_n() {
    let result = futures::stream::iter(vec![1, 2])
        .permutations(5)
        .await
        .collect::<Vec<_>>()
        .await;
    assert!(result.is_empty());
}

#[tokio::test]
async fn permutations_empty_stream() {
    let result = futures::stream::iter(Vec::<i32>::new())
        .permutations(2)
        .await
        .collect::<Vec<_>>()
        .await;
    assert!(result.is_empty());
}

#[tokio::test]
async fn permutations_single_element_k1() {
    let result = futures::stream::iter(vec![7])
        .permutations(1)
        .await
        .collect::<Vec<_>>()
        .await;
    assert_eq!(result, vec![vec![7]]);
}

#[tokio::test]
async fn permutations_with_replacement_k_zero() {
    // itertools::multi_cartesian_product called on zero iterators yields an
    // empty iterator (observed behavior on itertools >=0.13). This diverges
    // from the mathematical Cartesian-product identity, but we assert the
    // observed library contract so the test is stable.
    //
    // Note: this differs from `permutations(0)` which yields vec![vec![]]
    // via itertools::permutations(0); the two code paths use different
    // itertools primitives and have different k=0 semantics.
    let result = futures::stream::iter(vec![1, 2])
        .permutations_with_replacement(0)
        .await
        .collect::<Vec<_>>()
        .await;
    assert!(result.is_empty());
}

#[tokio::test]
async fn permutations_with_replacement_single_element() {
    let result = futures::stream::iter(vec![5])
        .permutations_with_replacement(3)
        .await
        .collect::<Vec<_>>()
        .await;
    assert_eq!(result, vec![vec![5, 5, 5]]);
}

#[tokio::test]
async fn permutations_count_matches_formula() {
    // P(4,2) = 4!/(4-2)! = 12
    let result = futures::stream::iter(vec![1, 2, 3, 4])
        .permutations(2)
        .await
        .collect::<Vec<_>>()
        .await;
    assert_eq!(result.len(), 12);
}

#[tokio::test]
async fn permutations_with_replacement_count_matches_formula() {
    // n^k = 3^2 = 9
    let result = futures::stream::iter(vec![1, 2, 3])
        .permutations_with_replacement(2)
        .await
        .collect::<Vec<_>>()
        .await;
    assert_eq!(result.len(), 9);
}