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// Copyright 2019-2022 Parity Technologies (UK) Ltd.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//! Interning data structure and associated symbol definitions.
//!
//! The interner is used by the registry in order to deduplicate strings and type
//! definitions. Strings are uniquely identified by their contents while types
//! are uniquely identified by their respective type identifiers.
//!
//! The interners provide a strict ordered sequence of cached (interned)
//! elements and is later used for space-efficient serialization within the
//! registry.
use crate::prelude::{
collections::btree_map::{
BTreeMap,
Entry,
},
marker::PhantomData,
vec::Vec,
};
#[cfg(feature = "serde")]
use serde::{
Deserialize,
Serialize,
};
/// A symbol that is not lifetime tracked.
///
/// This can be used by self-referential types but
/// can no longer be used to resolve instances.
#[derive(
Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, scale::Encode, scale::Decode,
)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
#[cfg_attr(feature = "serde", serde(transparent))]
pub struct UntrackedSymbol<T> {
/// The index to the symbol in the interner table.
#[codec(compact)]
id: u32,
#[cfg_attr(feature = "serde", serde(skip))]
marker: PhantomData<fn() -> T>,
}
impl<T> UntrackedSymbol<T> {
/// Returns the index to the symbol in the interner table.
pub fn id(&self) -> u32 {
self.id
}
}
impl<T> From<u32> for UntrackedSymbol<T> {
fn from(id: u32) -> Self {
Self {
id,
marker: Default::default(),
}
}
}
/// A symbol from an interner.
///
/// Can be used to resolve to the associated instance.
#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
#[cfg_attr(feature = "serde", derive(Serialize))]
#[cfg_attr(feature = "serde", serde(transparent))]
pub struct Symbol<'a, T> {
id: u32,
#[cfg_attr(feature = "serde", serde(skip))]
marker: PhantomData<fn() -> &'a T>,
}
impl<T> Symbol<'_, T> {
/// Removes the lifetime tracking for this symbol.
///
/// # Note
///
/// - This can be useful in situations where a data structure owns all
/// symbols and interners and can verify accesses by itself.
/// - For further safety reasons an untracked symbol can no longer be used
/// to resolve from an interner. It is still useful for serialization
/// purposes.
///
/// # Safety
///
/// Although removing lifetime constraints this operation can be
/// considered to be safe since untracked symbols can no longer be
/// used to resolve their associated instance from the interner.
pub fn into_untracked(self) -> UntrackedSymbol<T> {
UntrackedSymbol {
id: self.id,
marker: PhantomData,
}
}
}
/// Interning data structure generic over the element type.
///
/// For the sake of simplicity and correctness we are using a rather naive
/// implementation.
///
/// # Usage
///
/// This is used in order to quite efficiently cache strings and type
/// definitions uniquely identified by their associated type identifiers.
#[derive(Debug, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(Serialize))]
#[cfg_attr(feature = "serde", serde(transparent))]
pub struct Interner<T> {
/// A mapping from the interned elements to their respective space-efficient
/// identifiers.
///
/// The idenfitiers can be used to retrieve information about the original
/// element from the interner.
#[cfg_attr(feature = "serde", serde(skip))]
map: BTreeMap<T, usize>,
/// The ordered sequence of cached elements.
///
/// This is used to efficiently provide access to the cached elements and
/// to establish a strict ordering upon them since each is uniquely
/// identified later by its position in the vector.
vec: Vec<T>,
}
impl<T> Interner<T>
where
T: Ord,
{
/// Creates a new empty interner.
pub fn new() -> Self {
Self {
map: BTreeMap::new(),
vec: Vec::new(),
}
}
}
impl<T: Ord> Default for Interner<T> {
fn default() -> Self {
Self::new()
}
}
impl<T> Interner<T>
where
T: Ord + Clone,
{
/// Interns the given element or returns its associated symbol if it has
/// already been interned.
pub fn intern_or_get(&mut self, s: T) -> (bool, Symbol<T>) {
let next_id = self.vec.len();
let (inserted, sym_id) = match self.map.entry(s.clone()) {
Entry::Vacant(vacant) => {
vacant.insert(next_id);
self.vec.push(s);
(true, next_id)
}
Entry::Occupied(occupied) => (false, *occupied.get()),
};
(
inserted,
Symbol {
id: sym_id as u32,
marker: PhantomData,
},
)
}
/// Returns the symbol of the given element or `None` if it hasn't been
/// interned already.
pub fn get(&self, sym: &T) -> Option<Symbol<T>> {
self.map.get(sym).map(|&id| {
Symbol {
id: id as u32,
marker: PhantomData,
}
})
}
/// Resolves the original element given its associated symbol or
/// returns `None` if it has not been interned yet.
pub fn resolve(&self, sym: Symbol<T>) -> Option<&T> {
let idx = sym.id as usize;
if idx >= self.vec.len() {
return None
}
self.vec.get(idx)
}
/// Returns the ordered sequence of interned elements.
pub fn elements(&self) -> &[T] {
&self.vec
}
}
#[cfg(test)]
mod tests {
use super::*;
type StringInterner = Interner<&'static str>;
fn assert_id(
interner: &mut StringInterner,
new_symbol: &'static str,
expected_id: u32,
) {
let actual_id = interner.intern_or_get(new_symbol).1.id;
assert_eq!(actual_id, expected_id,);
}
fn assert_resolve<E>(interner: &mut StringInterner, symbol_id: u32, expected_str: E)
where
E: Into<Option<&'static str>>,
{
let actual_str = interner.resolve(Symbol {
id: symbol_id,
marker: PhantomData,
});
assert_eq!(actual_str.cloned(), expected_str.into(),);
}
#[test]
fn simple() {
let mut interner = StringInterner::new();
assert_id(&mut interner, "Hello", 0);
assert_id(&mut interner, ", World!", 1);
assert_id(&mut interner, "1 2 3", 2);
assert_id(&mut interner, "Hello", 0);
assert_resolve(&mut interner, 0, "Hello");
assert_resolve(&mut interner, 1, ", World!");
assert_resolve(&mut interner, 2, "1 2 3");
assert_resolve(&mut interner, 3, None);
}
}