nu-utils 0.115.0

use serde::{Deserialize, Serialize};
use std::{fmt, ops, sync::Arc};

/// A container that transparently shares a value when possible, but clones on mutate.
///
/// Unlike `Arc`, this is only intended to help save memory usage and reduce the amount of effort
/// required to clone unmodified values with easy to use copy-on-write.
///
/// This should more or less reflect the API of [`std::borrow::Cow`] as much as is sensible.
#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Default)]
#[repr(transparent)]
pub struct SharedCow<T: Clone>(Arc<T>);

impl<T: Clone> SharedCow<T> {
    /// Create a new `Shared` value.
    pub fn new(value: T) -> SharedCow<T> {
        SharedCow(Arc::new(value))
    }

    /// Take ownership of the shared value if it has no other references.
    ///
    /// If the value is still shared, returns this [`SharedCow`] unchanged without cloning its
    /// contents.
    pub fn try_into_owned(self: SharedCow<T>) -> Result<T, SharedCow<T>> {
        Arc::try_unwrap(self.0).map_err(SharedCow)
    }

    /// Take an exclusive clone of the shared value, or move and take ownership if it wasn't shared.
    pub fn into_owned(self: SharedCow<T>) -> T {
        match self.try_into_owned() {
            Ok(value) => value,
            Err(shared) => (*shared.0).clone(),
        }
    }

    /// Get a mutable reference to the value inside the [`SharedCow`]. This will result in a clone
    /// being created only if the value was shared with multiple references.
    pub fn to_mut(&mut self) -> &mut T {
        Arc::make_mut(&mut self.0)
    }

    /// Convert the `Shared` value into an `Arc`
    pub fn into_arc(value: SharedCow<T>) -> Arc<T> {
        value.0
    }

    /// Return the number of references to the shared value.
    pub fn ref_count(value: &SharedCow<T>) -> usize {
        Arc::strong_count(&value.0)
    }
}

impl<T> From<T> for SharedCow<T>
where
    T: Clone,
{
    fn from(value: T) -> Self {
        SharedCow::new(value)
    }
}

impl<T> From<Arc<T>> for SharedCow<T>
where
    T: Clone,
{
    fn from(value: Arc<T>) -> Self {
        SharedCow(value)
    }
}

impl<T> fmt::Debug for SharedCow<T>
where
    T: fmt::Debug + Clone,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // Appears transparent
        (*self.0).fmt(f)
    }
}

impl<T> fmt::Display for SharedCow<T>
where
    T: fmt::Display + Clone,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (*self.0).fmt(f)
    }
}

impl<T: Clone> Serialize for SharedCow<T>
where
    T: Serialize,
{
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: serde::Serializer,
    {
        self.0.serialize(serializer)
    }
}

impl<'de, T: Clone> Deserialize<'de> for SharedCow<T>
where
    T: Deserialize<'de>,
{
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        T::deserialize(deserializer).map(Arc::new).map(SharedCow)
    }
}

impl<T: Clone> ops::Deref for SharedCow<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<T: Clone> AsRef<[T]> for SharedCow<Vec<T>> {
    fn as_ref(&self) -> &[T] {
        self.as_slice()
    }
}

impl<T: Clone> IntoIterator for SharedCow<Vec<T>> {
    type Item = T;
    type IntoIter = std::vec::IntoIter<T>;

    fn into_iter(self) -> Self::IntoIter {
        self.into_owned().into_iter()
    }
}

impl<'a, T: Clone> IntoIterator for &'a SharedCow<Vec<T>> {
    type Item = &'a T;
    type IntoIter = std::slice::Iter<'a, T>;

    fn into_iter(self) -> Self::IntoIter {
        self.iter()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn try_into_owned_returns_unique_value() {
        let value = vec![1, 2, 3];
        let original_ptr = value.as_ptr();
        let shared = SharedCow::new(value);

        let Ok(owned) = shared.try_into_owned() else {
            panic!("value should be uniquely owned");
        };

        assert_eq!(owned.as_ptr(), original_ptr);
    }

    #[test]
    fn try_into_owned_returns_shared_value_without_cloning() {
        let shared = SharedCow::new(vec![1, 2, 3]);
        let original_ptr = shared.as_ptr();
        let clone = shared.clone();

        let Err(still_shared) = clone.try_into_owned() else {
            panic!("value should still be shared");
        };

        assert_eq!(still_shared.as_ptr(), original_ptr);
        assert_eq!(SharedCow::ref_count(&still_shared), 2);
    }
}