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use std::collections::HashMap;
use std::hash::Hash;
/// Group elements of a collection based on a key extracted by a provided function,
/// preserving the order of their first occurrence.
///
/// This function takes a slice of items and returns a `HashMap<U, Vec<T>>` where each key
/// corresponds to a group of items that share the same key.
///
/// **Note:** This implementation requires `U` to implement `Hash`, `Eq`, and `Clone`, and `T` to implement `Clone`.
///
/// # Arguments
///
/// * `collection` - A slice of items to be grouped.
/// * `iteratee` - A function that takes a reference to an item and returns a key of type `U`.
///
/// # Type Parameters
///
/// * `T` - The type of elements in the collection. Must implement `Clone`.
/// * `U` - The type of the key extracted from each element used to group the elements. Must implement `Hash`, `Eq`, and `Clone`.
/// * `F` - The type of the iteratee function. Must implement `Fn(&T) -> U`.
///
/// # Returns
///
/// * `HashMap<U, Vec<T>>` - A map where each key is associated with a vector of items that share the same key.
///
/// # Examples
///
/// ```rust
/// use lowdash::group_by;
///
/// let numbers = vec![1, 2, 2, 3, 4, 3, 5];
/// let grouped = group_by(&numbers, |x| *x % 2 == 0);
///
/// assert_eq!(grouped.get(&false), Some(&vec![1, 3, 3, 5]));
/// assert_eq!(grouped.get(&true), Some(&vec![2, 2, 4]));
/// ```
///
/// ```rust
/// use lowdash::group_by;
///
/// #[derive(Debug, PartialEq, Clone)]
/// struct Person {
/// name: String,
/// age: u32,
/// }
///
/// let people = vec![
/// Person { name: "Alice".to_string(), age: 25 },
/// Person { name: "Bob".to_string(), age: 30 },
/// Person { name: "Carol".to_string(), age: 25 },
/// ];
///
/// let grouped = group_by(&people, |p| p.age);
///
/// assert_eq!(grouped.get(&25), Some(&vec![
/// Person { name: "Alice".to_string(), age: 25 },
/// Person { name: "Carol".to_string(), age: 25 },
/// ]));
/// assert_eq!(grouped.get(&30), Some(&vec![
/// Person { name: "Bob".to_string(), age: 30 },
/// ]));
/// ```
pub fn group_by<T, U, F>(collection: &[T], iteratee: F) -> HashMap<U, Vec<T>>
where
T: Clone,
U: Eq + Hash + Clone,
F: Fn(&T) -> U,
{
let mut result: HashMap<U, Vec<T>> = HashMap::new();
for item in collection {
let key = iteratee(item);
result.entry(key).or_default().push(item.clone());
}
result
}
#[cfg(test)]
mod tests {
use super::*;
#[derive(Debug, PartialEq, Clone)]
struct Person {
name: String,
age: u32,
}
#[derive(Debug, PartialEq, Clone)]
struct Item {
id: u32,
value: String,
}
#[test]
fn test_group_by_integers() {
let numbers = vec![1, 2, 2, 3, 4, 3, 5];
let grouped = group_by(&numbers, |x| *x % 2 == 0);
assert_eq!(grouped.get(&false), Some(&vec![1, 3, 3, 5]));
assert_eq!(grouped.get(&true), Some(&vec![2, 2, 4]));
}
#[test]
fn test_group_by_strings() {
let strings = vec!["apple", "banana", "apple", "cherry", "banana"];
let grouped = group_by(&strings, |s| s.len());
assert_eq!(grouped.get(&5), Some(&vec!["apple", "apple"]));
assert_eq!(grouped.get(&6), Some(&vec!["banana", "cherry", "banana"]));
}
#[test]
fn test_group_by_with_structs() {
let people = vec![
Person {
name: "Alice".to_string(),
age: 25,
},
Person {
name: "Bob".to_string(),
age: 30,
},
Person {
name: "Carol".to_string(),
age: 25,
},
Person {
name: "Dave".to_string(),
age: 30,
},
];
let grouped = group_by(&people, |p| p.age);
assert_eq!(
grouped.get(&25),
Some(&vec![
Person {
name: "Alice".to_string(),
age: 25
},
Person {
name: "Carol".to_string(),
age: 25
},
])
);
assert_eq!(
grouped.get(&30),
Some(&vec![
Person {
name: "Bob".to_string(),
age: 30
},
Person {
name: "Dave".to_string(),
age: 30
},
])
);
}
#[test]
fn test_group_by_with_empty_collection() {
let empty: Vec<i32> = vec![];
let grouped = group_by(&empty, |x| *x);
assert!(grouped.is_empty());
}
#[test]
fn test_group_by_no_duplicates() {
let collection = vec![1, 2, 3, 4, 5];
let grouped = group_by(&collection, |x| *x);
for x in &collection {
assert_eq!(grouped.get(x), Some(&vec![*x]));
}
}
#[test]
fn test_group_by_all_duplicates() {
let collection = vec![1, 1, 1, 1, 1];
let grouped = group_by(&collection, |x| *x);
assert_eq!(grouped.len(), 1);
assert_eq!(grouped.get(&1), Some(&vec![1, 1, 1, 1, 1]));
}
#[test]
fn test_group_by_with_floats() {
let float_collection = vec![1.1, 2.2, 2.2, 3.3, 4.4, 3.3, 5.5];
let grouped = group_by(&float_collection, |x| *x > 3.0);
assert_eq!(grouped.get(&false), Some(&vec![1.1, 2.2, 2.2]));
assert_eq!(grouped.get(&true), Some(&vec![3.3, 4.4, 3.3, 5.5]));
}
#[test]
fn test_group_by_with_characters() {
let chars = vec!['a', 'b', 'a', 'c', 'b', 'd'];
let grouped = group_by(&chars, |c| *c);
assert_eq!(grouped.get(&'a'), Some(&vec!['a', 'a']));
assert_eq!(grouped.get(&'b'), Some(&vec!['b', 'b']));
assert_eq!(grouped.get(&'c'), Some(&vec!['c']));
assert_eq!(grouped.get(&'d'), Some(&vec!['d']));
}
#[test]
fn test_group_by_preserves_order() {
let numbers = vec![3, 1, 2, 3, 2, 4, 1, 5];
let grouped = group_by(&numbers, |x| *x);
assert_eq!(grouped.get(&3), Some(&vec![3, 3]));
assert_eq!(grouped.get(&1), Some(&vec![1, 1]));
assert_eq!(grouped.get(&2), Some(&vec![2, 2]));
assert_eq!(grouped.get(&4), Some(&vec![4]));
assert_eq!(grouped.get(&5), Some(&vec![5]));
}
#[test]
fn test_group_by_with_mixed_types() {
let items = vec![
Item {
id: 1,
value: "one".to_string(),
},
Item {
id: 2,
value: "two".to_string(),
},
Item {
id: 1,
value: "one".to_string(),
},
Item {
id: 3,
value: "three".to_string(),
},
];
let grouped = group_by(&items, |item| item.id);
assert_eq!(
grouped.get(&1),
Some(&vec![
Item {
id: 1,
value: "one".to_string()
},
Item {
id: 1,
value: "one".to_string()
},
])
);
assert_eq!(
grouped.get(&2),
Some(&vec![Item {
id: 2,
value: "two".to_string()
},])
);
assert_eq!(
grouped.get(&3),
Some(&vec![Item {
id: 3,
value: "three".to_string()
},])
);
}
#[test]
fn test_group_by_with_optionals() {
let collection = vec![Some(1), None, Some(2), Some(1), None, Some(3), Some(2)];
let grouped = group_by(&collection, |x| x.clone());
assert_eq!(grouped.get(&Some(1)), Some(&vec![Some(1), Some(1)]));
assert_eq!(grouped.get(&None), Some(&vec![None, None]));
assert_eq!(grouped.get(&Some(2)), Some(&vec![Some(2), Some(2)]));
assert_eq!(grouped.get(&Some(3)), Some(&vec![Some(3)]));
}
#[test]
fn test_group_by_with_nan_floats() {
let float_collection = vec![std::f64::NAN, 2.2, std::f64::NAN, 4.4];
let grouped = group_by(&float_collection, |x| x.is_nan());
// Verify the NaN group
if let Some(nans) = grouped.get(&true) {
assert_eq!(nans.len(), 2);
assert!(nans[0].is_nan());
assert!(nans[1].is_nan());
} else {
panic!("Expected Some(&vec![NaN, NaN]) for key true");
}
// Verify the non-NaN group
assert_eq!(grouped.get(&false), Some(&vec![2.2, 4.4]));
}
}