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//! Backends for the [`StringInterner`](`crate::StringInterner`).
//!
//! The backend is the method or strategy that handles the actual interning.
//! There are trade-offs for the different kinds of backends. A user should
//! find the backend that suits their use case best.

mod bucket;
mod buffer;
mod string;

#[cfg(feature = "backends")]
pub use self::{bucket::BucketBackend, buffer::BufferBackend, string::StringBackend};
use crate::Symbol;

/// The default backend recommended for general use.
#[cfg(feature = "backends")]
pub type DefaultBackend = StringBackend<crate::DefaultSymbol>;

/// Types implementing this trait may act as backends for the string interner.
///
/// The job of a backend is to actually store, manage and organize the interned
/// strings. Different backends have different trade-offs. Users should pick
/// their backend with hinsight of their personal use-case.
pub trait Backend: Default {
    /// The symbol used by the string interner backend.
    type Symbol: Symbol;

    /// The iterator over the symbols and their strings.
    type Iter<'a>: Iterator<Item = (Self::Symbol, &'a str)>
    where
        Self: 'a;

    /// Creates a new backend for the given capacity.
    ///
    /// The capacity denotes how many strings are expected to be interned.
    fn with_capacity(cap: usize) -> Self;

    /// Interns the given string and returns its interned ref and symbol.
    ///
    /// # Note
    ///
    /// The backend must make sure that the returned symbol maps back to the
    /// original string in its [`resolve`](`Backend::resolve`) method.
    fn intern(&mut self, string: &str) -> Self::Symbol;

    /// Interns the given static string and returns its interned ref and symbol.
    ///
    /// # Note
    ///
    /// The backend must make sure that the returned symbol maps back to the
    /// original string in its [`resolve`](`Backend::resolve`) method.
    #[inline]
    fn intern_static(&mut self, string: &'static str) -> Self::Symbol {
        // The default implementation simply forwards to the normal [`intern`]
        // implementation. Backends that can optimize for this use case should
        // implement this method.
        self.intern(string)
    }

    /// Shrink backend capacity to fit interned symbols exactly.
    fn shrink_to_fit(&mut self);

    /// Resolves the given symbol to its original string contents.
    fn resolve(&self, symbol: Self::Symbol) -> Option<&str>;

    /// Resolves the given symbol to its original string contents.
    ///
    /// # Safety
    ///
    /// Does not perform validity checks on the given symbol and relies
    /// on the caller to be provided with a symbol that has been generated
    /// by the [`intern`](`Backend::intern`) or
    /// [`intern_static`](`Backend::intern_static`) methods of the same
    /// interner backend.
    unsafe fn resolve_unchecked(&self, symbol: Self::Symbol) -> &str;

    /// Creates an iterator that yields all interned strings and their symbols.
    fn iter(&self) -> Self::Iter<'_>;
}