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/// Flatten a collection of slices into a single vector, preserving the order of elements.
///
/// This function takes a slice of slices and concatenates all inner slices into a single `Vec<T>`.
/// The order of elements is preserved based on their original ordering in the input collection.
///
/// **Time Complexity:**
/// O(n), where n is the total number of elements across all inner slices.
///
/// # Arguments
///
/// * `collection` - A slice of slices to be flattened.
///
/// # Type Parameters
///
/// * `T` - The type of elements in the slices. Must implement `Clone`.
/// * `Slice` - The type of the inner slices. Must implement `AsRef<[T]>`.
///
/// # Returns
///
/// * `Vec<T>` - A vector containing all elements from the input slices, flattened into a single collection.
///
/// # Examples
///
/// ```rust
/// use lowdash::flatten;
///
/// let nested = vec![vec![1, 2], vec![3, 4], vec![5]];
/// let flat = flatten(&nested);
/// assert_eq!(flat, vec![1, 2, 3, 4, 5]);
/// ```
///
/// ```rust
/// use lowdash::flatten;
///
/// #[derive(Debug, PartialEq, Clone)]
/// struct Person {
/// name: String,
/// age: u32,
/// }
///
/// let people_groups = vec![
/// vec![
/// Person { name: "Alice".to_string(), age: 25 },
/// Person { name: "Bob".to_string(), age: 30 },
/// ],
/// vec![
/// Person { name: "Carol".to_string(), age: 35 },
/// Person { name: "Dave".to_string(), age: 40 },
/// ],
/// ];
///
/// let flat_people = flatten(&people_groups);
/// assert_eq!(
/// flat_people,
/// vec![
/// Person { name: "Alice".to_string(), age: 25 },
/// Person { name: "Bob".to_string(), age: 30 },
/// Person { name: "Carol".to_string(), age: 35 },
/// Person { name: "Dave".to_string(), age: 40 },
/// ]
/// );
/// ```
pub fn flatten<T, Slice>(collection: &[Slice]) -> Vec<T>
where
Slice: AsRef<[T]>,
T: Clone,
{
let total_len = collection.iter().map(|slice| slice.as_ref().len()).sum();
let mut result = Vec::with_capacity(total_len);
for slice in collection {
result.extend(slice.as_ref().iter().cloned());
}
result
}
#[cfg(test)]
mod tests {
use super::*;
#[derive(Debug, PartialEq, Clone)]
struct Person {
name: String,
age: u32,
}
#[test]
fn test_flatten_integers() {
let nested = vec![vec![1, 2], vec![3, 4], vec![5]];
let flat = flatten(&nested);
assert_eq!(flat, vec![1, 2, 3, 4, 5]);
}
#[test]
fn test_flatten_strings() {
let nested = vec![
vec!["apple".to_string(), "banana".to_string()],
vec!["cherry".to_string()],
vec!["date".to_string(), "elderberry".to_string()],
];
let flat = flatten(&nested);
assert_eq!(
flat,
vec![
"apple".to_string(),
"banana".to_string(),
"cherry".to_string(),
"date".to_string(),
"elderberry".to_string()
]
);
}
#[test]
fn test_flatten_with_structs() {
let people_groups = vec![
vec![
Person {
name: "Alice".to_string(),
age: 25,
},
Person {
name: "Bob".to_string(),
age: 30,
},
],
vec![
Person {
name: "Carol".to_string(),
age: 35,
},
Person {
name: "Dave".to_string(),
age: 40,
},
],
];
let flat_people = flatten(&people_groups);
assert_eq!(
flat_people,
vec![
Person {
name: "Alice".to_string(),
age: 25
},
Person {
name: "Bob".to_string(),
age: 30
},
Person {
name: "Carol".to_string(),
age: 35
},
Person {
name: "Dave".to_string(),
age: 40
},
]
);
}
#[test]
fn test_flatten_with_empty_inner_slices() {
let nested: Vec<Vec<i32>> = vec![vec![1, 2], vec![], vec![3], vec![]];
let flat = flatten(&nested);
assert_eq!(flat, vec![1, 2, 3]);
}
#[test]
fn test_flatten_with_all_empty_slices() {
let nested: Vec<Vec<i32>> = vec![vec![], vec![], vec![]];
let flat = flatten(&nested);
assert_eq!(flat, Vec::<i32>::new());
}
#[test]
fn test_flatten_with_single_inner_slice() {
let nested = vec![vec![1, 2, 3, 4, 5]];
let flat = flatten(&nested);
assert_eq!(flat, vec![1, 2, 3, 4, 5]);
}
#[test]
fn test_flatten_with_multiple_empty_and_non_empty_slices() {
let nested = vec![
vec![],
vec![1],
vec![],
vec![2, 3],
vec![],
vec![4, 5, 6],
vec![],
];
let flat = flatten(&nested);
assert_eq!(flat, vec![1, 2, 3, 4, 5, 6]);
}
#[test]
fn test_flatten_with_nested_empty_vectors() {
let nested: Vec<Vec<i32>> = vec![vec![]];
let flat = flatten(&nested);
assert_eq!(flat, Vec::<i32>::new());
}
#[test]
fn test_flatten_with_optionals() {
let nested = vec![vec![Some(1), None], vec![Some(2), Some(3)], vec![None]];
let flat = flatten(&nested);
assert_eq!(flat, vec![Some(1), None, Some(2), Some(3), None]);
}
#[test]
fn test_flatten_with_floats() {
let nested = vec![vec![1.1, 2.2], vec![3.3], vec![4.4, 5.5]];
let flat = flatten(&nested);
assert_eq!(flat, vec![1.1, 2.2, 3.3, 4.4, 5.5]);
}
}