zrx-storage 0.0.4

// Copyright (c) 2025-2026 Zensical and contributors

// SPDX-License-Identifier: MIT
// All contributions are certified under the DCO

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// The above copyright notice and this permission notice shall be included in
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// ----------------------------------------------------------------------------

//! Union accessor.

use zrx_store::{Key, Value};

use crate::storage::Storage;

// ----------------------------------------------------------------------------
// Traits
// ----------------------------------------------------------------------------

/// Union accessor.
pub trait Union<'a, K, V> {
    /// Returns a reference to the value as a result of a set union.
    #[must_use]
    fn union(&self, key: &K) -> Option<&'a V>;
}

// ----------------------------------------------------------------------------
// Blanket implementations
// ----------------------------------------------------------------------------

impl<'a, K, V> Union<'a, K, V> for [&'a Storage<K, V>]
where
    K: Key,
    V: Value + PartialEq,
{
    /// Returns a reference to the value as a result of a set union.
    ///
    /// This method queries each storage for the given key and only returns a
    /// reference if any storage contains a value, and this value is the same
    /// across all storages. Otherwise, [`None`] is returned, indicating that
    /// the union is empty or values differ.
    ///
    /// # Examples
    ///
    /// ```
    /// use zrx_storage::accessor::Union;
    /// use zrx_storage::Storage;
    ///
    /// // Create storages from iterators
    /// let a = Storage::from_iter([("key", 42)]);
    /// let b = Storage::from_iter([("key", 42)]);
    ///
    /// // Obtain reference to value
    /// let value = [&a, &b].union(&"key");
    /// assert_eq!(value, Some(&42));
    /// ```
    fn union(&self, key: &K) -> Option<&'a V> {
        let iter = self.iter();
        iter.filter_map(|store| store.get(key))
            .try_fold(None, |check, value| match check {
                Some(check) => (check == value).then_some(Some(check)),
                None => Some(Some(value)),
            })
            .flatten()
    }
}