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70 lines
2.3 KiB
Rust
70 lines
2.3 KiB
Rust
// This exercise explores the `Cow` (Clone-On-Write) smart pointer. It can
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// enclose and provide immutable access to borrowed data and clone the data
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// lazily when mutation or ownership is required. The type is designed to work
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// with general borrowed data via the `Borrow` trait.
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use std::borrow::Cow;
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fn abs_all(input: &mut Cow<[i32]>) {
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for ind in 0..input.len() {
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let value = input[ind];
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if value < 0 {
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// Clones into a vector if not already owned.
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input.to_mut()[ind] = -value;
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}
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}
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}
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fn main() {
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// You can optionally experiment here.
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn reference_mutation() {
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// Clone occurs because `input` needs to be mutated.
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let vec = vec![-1, 0, 1];
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let mut input = Cow::from(&vec);
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abs_all(&mut input);
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assert!(matches!(input, Cow::Owned(_)));
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}
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#[test]
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fn reference_no_mutation() {
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// No clone occurs because `input` doesn't need to be mutated.
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let vec = vec![0, 1, 2];
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let mut input = Cow::from(&vec);
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abs_all(&mut input);
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// TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
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assert!(matches!(input, todo!()));
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}
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#[test]
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fn owned_no_mutation() {
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// We can also pass `vec` without `&` so `Cow` owns it directly. In this
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// case, no mutation occurs (all numbers are already absolute) and thus
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// also no clone. But the result is still owned because it was never
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// borrowed or mutated.
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let vec = vec![0, 1, 2];
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let mut input = Cow::from(vec);
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abs_all(&mut input);
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// TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
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assert!(matches!(input, todo!()));
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}
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#[test]
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fn owned_mutation() {
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// Of course this is also the case if a mutation does occur (not all
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// numbers are absolute). In this case, the call to `to_mut()` in the
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// `abs_all` function returns a reference to the same data as before.
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let vec = vec![-1, 0, 1];
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let mut input = Cow::from(vec);
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abs_all(&mut input);
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// TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`.
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assert!(matches!(input, todo!()));
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}
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}
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