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references: add explanatory commentary to solution #3058

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references: add explanatory commentary to solution #3058

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12 changes: 12 additions & 0 deletions src/references/solution.md
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{{#include exercise.rs:solution}}
```

This solution highlights the difference between shared and mutable references:

- **Shared References (`&`):** `magnitude` needs to read the vector components
but not modify them, so it takes a shared reference (`&[f64; 3]`).
- **Mutable References (`&mut`):** `normalize` modifies the vector in place, so
it requires an exclusive mutable reference (`&mut [f64; 3]`).
- **Dereferencing:** In the `normalize` loop, `item` is a `&mut f64`. To modify
the actual value, we must dereference it using `*item`.
- **Iteration:** When we iterate over a reference to an array (like `vector`),
the iterator yields references to the elements. In `magnitude`, `coord` is
`&f64`. In `normalize`, `item` is `&mut f64`.

<details>

- Note that in `normalize` we were able to do `*item /= mag` to modify each
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