bounded_collections/

bounded_btree_set.rs

// This file is part of Substrate.

// Copyright (C) 2023 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// 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.

//! Traits, types and structs to support a bounded `BTreeSet`.

use crate::{Get, TryCollect};
use alloc::collections::BTreeSet;
use codec::{Compact, Decode, Encode, MaxEncodedLen};
use core::{borrow::Borrow, marker::PhantomData, ops::Deref};
#[cfg(feature = "serde")]
use serde::{
	de::{Error, SeqAccess, Visitor},
	Deserialize, Deserializer, Serialize,
};

/// A bounded set based on a B-Tree.
///
/// B-Trees represent a fundamental compromise between cache-efficiency and actually minimizing
/// the amount of work performed in a search. See [`BTreeSet`] for more details.
///
/// Unlike a standard `BTreeSet`, there is an enforced upper limit to the number of items in the
/// set. All internal operations ensure this bound is respected.
#[cfg_attr(feature = "serde", derive(Serialize), serde(transparent))]
#[derive(Encode, scale_info::TypeInfo)]
#[scale_info(skip_type_params(S))]
pub struct BoundedBTreeSet<T, S>(BTreeSet<T>, #[cfg_attr(feature = "serde", serde(skip_serializing))] PhantomData<S>);

#[cfg(feature = "serde")]
impl<'de, T, S: Get<u32>> Deserialize<'de> for BoundedBTreeSet<T, S>
where
	T: Ord + Deserialize<'de>,
	S: Clone,
{
	fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
	where
		D: Deserializer<'de>,
	{
		// Create a visitor to visit each element in the sequence
		struct BTreeSetVisitor<T, S>(PhantomData<(T, S)>);

		impl<'de, T, S> Visitor<'de> for BTreeSetVisitor<T, S>
		where
			T: Ord + Deserialize<'de>,
			S: Get<u32> + Clone,
		{
			type Value = BTreeSet<T>;

			fn expecting(&self, formatter: &mut core::fmt::Formatter) -> core::fmt::Result {
				formatter.write_str("a sequence")
			}

			fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
			where
				A: SeqAccess<'de>,
			{
				let size = seq.size_hint().unwrap_or(0);
				let max = match usize::try_from(S::get()) {
					Ok(n) => n,
					Err(_) => return Err(A::Error::custom("can't convert to usize")),
				};
				if size > max {
					Err(A::Error::custom("out of bounds"))
				} else {
					let mut values = BTreeSet::new();

					while let Some(value) = seq.next_element()? {
						if values.len() >= max {
							return Err(A::Error::custom("out of bounds"))
						}
						values.insert(value);
					}

					Ok(values)
				}
			}
		}

		let visitor: BTreeSetVisitor<T, S> = BTreeSetVisitor(PhantomData);
		deserializer
			.deserialize_seq(visitor)
			.map(|v| BoundedBTreeSet::<T, S>::try_from(v).map_err(|_| Error::custom("out of bounds")))?
	}
}

impl<T, S> Decode for BoundedBTreeSet<T, S>
where
	T: Decode + Ord,
	S: Get<u32>,
{
	fn decode<I: codec::Input>(input: &mut I) -> Result<Self, codec::Error> {
		// Same as the underlying implementation for `Decode` on `BTreeSet`, except we fail early if
		// the len is too big.
		let len: u32 = <Compact<u32>>::decode(input)?.into();
		if len > S::get() {
			return Err("BoundedBTreeSet exceeds its limit".into())
		}
		input.descend_ref()?;
		let inner = Result::from_iter((0..len).map(|_| Decode::decode(input)))?;
		input.ascend_ref();
		Ok(Self(inner, PhantomData))
	}

	fn skip<I: codec::Input>(input: &mut I) -> Result<(), codec::Error> {
		BTreeSet::<T>::skip(input)
	}
}

impl<T, S> BoundedBTreeSet<T, S>
where
	S: Get<u32>,
{
	/// Get the bound of the type in `usize`.
	pub fn bound() -> usize {
		S::get() as usize
	}
}

impl<T, S> BoundedBTreeSet<T, S>
where
	T: Ord,
	S: Get<u32>,
{
	/// Create `Self` from `t` without any checks.
	fn unchecked_from(t: BTreeSet<T>) -> Self {
		Self(t, Default::default())
	}

	/// Create a new `BoundedBTreeSet`.
	///
	/// Does not allocate.
	pub fn new() -> Self {
		BoundedBTreeSet(BTreeSet::new(), PhantomData)
	}

	/// Consume self, and return the inner `BTreeSet`.
	///
	/// This is useful when a mutating API of the inner type is desired, and closure-based mutation
	/// such as provided by [`try_mutate`][Self::try_mutate] is inconvenient.
	pub fn into_inner(self) -> BTreeSet<T> {
		debug_assert!(self.0.len() <= Self::bound());
		self.0
	}

	/// Consumes self and mutates self via the given `mutate` function.
	///
	/// If the outcome of mutation is within bounds, `Some(Self)` is returned. Else, `None` is
	/// returned.
	///
	/// This is essentially a *consuming* shorthand [`Self::into_inner`] -> `...` ->
	/// [`Self::try_from`].
	pub fn try_mutate(mut self, mut mutate: impl FnMut(&mut BTreeSet<T>)) -> Option<Self> {
		mutate(&mut self.0);
		(self.0.len() <= Self::bound()).then(move || self)
	}

	/// Clears the set, removing all elements.
	pub fn clear(&mut self) {
		self.0.clear()
	}

	/// Exactly the same semantics as [`BTreeSet::insert`], but returns an `Err` (and is a noop) if
	/// the new length of the set exceeds `S`.
	///
	/// In the `Err` case, returns the inserted item so it can be further used without cloning.
	pub fn try_insert(&mut self, item: T) -> Result<bool, T> {
		if self.len() < Self::bound() || self.0.contains(&item) {
			Ok(self.0.insert(item))
		} else {
			Err(item)
		}
	}

	/// Remove an item from the set, returning whether it was previously in the set.
	///
	/// The item may be any borrowed form of the set's item type, but the ordering on the borrowed
	/// form _must_ match the ordering on the item type.
	pub fn remove<Q>(&mut self, item: &Q) -> bool
	where
		T: Borrow<Q>,
		Q: Ord + ?Sized,
	{
		self.0.remove(item)
	}

	/// Removes and returns the value in the set, if any, that is equal to the given one.
	///
	/// The value may be any borrowed form of the set's value type, but the ordering on the borrowed
	/// form _must_ match the ordering on the value type.
	pub fn take<Q>(&mut self, value: &Q) -> Option<T>
	where
		T: Borrow<Q> + Ord,
		Q: Ord + ?Sized,
	{
		self.0.take(value)
	}
}

impl<T, S> Default for BoundedBTreeSet<T, S>
where
	T: Ord,
	S: Get<u32>,
{
	fn default() -> Self {
		Self::new()
	}
}

impl<T, S> Clone for BoundedBTreeSet<T, S>
where
	BTreeSet<T>: Clone,
{
	fn clone(&self) -> Self {
		BoundedBTreeSet(self.0.clone(), PhantomData)
	}
}

impl<T, S> core::fmt::Debug for BoundedBTreeSet<T, S>
where
	BTreeSet<T>: core::fmt::Debug,
	S: Get<u32>,
{
	fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
		f.debug_tuple("BoundedBTreeSet").field(&self.0).field(&Self::bound()).finish()
	}
}

// Custom implementation of `Hash` since deriving it would require all generic bounds to also
// implement it.
#[cfg(feature = "std")]
impl<T: std::hash::Hash, S> std::hash::Hash for BoundedBTreeSet<T, S> {
	fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
		self.0.hash(state);
	}
}

impl<T, S1, S2> PartialEq<BoundedBTreeSet<T, S1>> for BoundedBTreeSet<T, S2>
where
	BTreeSet<T>: PartialEq,
	S1: Get<u32>,
	S2: Get<u32>,
{
	fn eq(&self, other: &BoundedBTreeSet<T, S1>) -> bool {
		S1::get() == S2::get() && self.0 == other.0
	}
}

impl<T, S> Eq for BoundedBTreeSet<T, S>
where
	BTreeSet<T>: Eq,
	S: Get<u32>,
{
}

impl<T, S> PartialEq<BTreeSet<T>> for BoundedBTreeSet<T, S>
where
	BTreeSet<T>: PartialEq,
	S: Get<u32>,
{
	fn eq(&self, other: &BTreeSet<T>) -> bool {
		self.0 == *other
	}
}

impl<T, S> PartialOrd for BoundedBTreeSet<T, S>
where
	BTreeSet<T>: PartialOrd,
	S: Get<u32>,
{
	fn partial_cmp(&self, other: &Self) -> Option<core::cmp::Ordering> {
		self.0.partial_cmp(&other.0)
	}
}

impl<T, S> Ord for BoundedBTreeSet<T, S>
where
	BTreeSet<T>: Ord,
	S: Get<u32>,
{
	fn cmp(&self, other: &Self) -> core::cmp::Ordering {
		self.0.cmp(&other.0)
	}
}

impl<T, S> IntoIterator for BoundedBTreeSet<T, S> {
	type Item = T;
	type IntoIter = alloc::collections::btree_set::IntoIter<T>;

	fn into_iter(self) -> Self::IntoIter {
		self.0.into_iter()
	}
}

impl<'a, T, S> IntoIterator for &'a BoundedBTreeSet<T, S> {
	type Item = &'a T;
	type IntoIter = alloc::collections::btree_set::Iter<'a, T>;

	fn into_iter(self) -> Self::IntoIter {
		self.0.iter()
	}
}

impl<T, S> MaxEncodedLen for BoundedBTreeSet<T, S>
where
	T: MaxEncodedLen,
	S: Get<u32>,
{
	fn max_encoded_len() -> usize {
		Self::bound()
			.saturating_mul(T::max_encoded_len())
			.saturating_add(codec::Compact(S::get()).encoded_size())
	}
}

impl<T, S> Deref for BoundedBTreeSet<T, S>
where
	T: Ord,
{
	type Target = BTreeSet<T>;

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

impl<T, S> AsRef<BTreeSet<T>> for BoundedBTreeSet<T, S>
where
	T: Ord,
{
	fn as_ref(&self) -> &BTreeSet<T> {
		&self.0
	}
}

impl<T, S> From<BoundedBTreeSet<T, S>> for BTreeSet<T>
where
	T: Ord,
{
	fn from(set: BoundedBTreeSet<T, S>) -> Self {
		set.0
	}
}

impl<T, S> TryFrom<BTreeSet<T>> for BoundedBTreeSet<T, S>
where
	T: Ord,
	S: Get<u32>,
{
	type Error = ();

	fn try_from(value: BTreeSet<T>) -> Result<Self, Self::Error> {
		(value.len() <= Self::bound())
			.then(move || BoundedBTreeSet(value, PhantomData))
			.ok_or(())
	}
}

impl<T, S> codec::DecodeLength for BoundedBTreeSet<T, S> {
	fn len(self_encoded: &[u8]) -> Result<usize, codec::Error> {
		// `BoundedBTreeSet<T, S>` is stored just a `BTreeSet<T>`, which is stored as a
		// `Compact<u32>` with its length followed by an iteration of its items. We can just use
		// the underlying implementation.
		<BTreeSet<T> as codec::DecodeLength>::len(self_encoded)
	}
}

impl<T, S> codec::EncodeLike<BTreeSet<T>> for BoundedBTreeSet<T, S> where BTreeSet<T>: Encode {}

impl<I, T, Bound> TryCollect<BoundedBTreeSet<T, Bound>> for I
where
	T: Ord,
	I: ExactSizeIterator + Iterator<Item = T>,
	Bound: Get<u32>,
{
	type Error = &'static str;

	fn try_collect(self) -> Result<BoundedBTreeSet<T, Bound>, Self::Error> {
		if self.len() > Bound::get() as usize {
			Err("iterator length too big")
		} else {
			Ok(BoundedBTreeSet::<T, Bound>::unchecked_from(self.collect::<BTreeSet<T>>()))
		}
	}
}

#[cfg(test)]
mod test {
	use super::*;
	use crate::ConstU32;
	use alloc::{vec, vec::Vec};
	use codec::CompactLen;

	fn set_from_keys<T>(keys: &[T]) -> BTreeSet<T>
	where
		T: Ord + Copy,
	{
		keys.iter().copied().collect()
	}

	fn boundedset_from_keys<T, S>(keys: &[T]) -> BoundedBTreeSet<T, S>
	where
		T: Ord + Copy,
		S: Get<u32>,
	{
		set_from_keys(keys).try_into().unwrap()
	}

	#[test]
	fn encoding_same_as_unbounded_set() {
		let b = boundedset_from_keys::<u32, ConstU32<7>>(&[1, 2, 3, 4, 5, 6]);
		let m = set_from_keys(&[1, 2, 3, 4, 5, 6]);

		assert_eq!(b.encode(), m.encode());
	}

	#[test]
	fn try_insert_works() {
		let mut bounded = boundedset_from_keys::<u32, ConstU32<4>>(&[1, 2, 3]);
		bounded.try_insert(0).unwrap();
		assert_eq!(*bounded, set_from_keys(&[1, 0, 2, 3]));

		assert!(bounded.try_insert(9).is_err());
		assert_eq!(*bounded, set_from_keys(&[1, 0, 2, 3]));
	}

	#[test]
	fn deref_coercion_works() {
		let bounded = boundedset_from_keys::<u32, ConstU32<7>>(&[1, 2, 3]);
		// these methods come from deref-ed vec.
		assert_eq!(bounded.len(), 3);
		assert!(bounded.iter().next().is_some());
		assert!(!bounded.is_empty());
	}

	#[test]
	fn try_mutate_works() {
		let bounded = boundedset_from_keys::<u32, ConstU32<7>>(&[1, 2, 3, 4, 5, 6]);
		let bounded = bounded
			.try_mutate(|v| {
				v.insert(7);
			})
			.unwrap();
		assert_eq!(bounded.len(), 7);
		assert!(bounded
			.try_mutate(|v| {
				v.insert(8);
			})
			.is_none());
	}

	#[test]
	fn btree_map_eq_works() {
		let bounded = boundedset_from_keys::<u32, ConstU32<7>>(&[1, 2, 3, 4, 5, 6]);
		assert_eq!(bounded, set_from_keys(&[1, 2, 3, 4, 5, 6]));
	}

	#[test]
	fn too_big_fail_to_decode() {
		let v: Vec<u32> = vec![1, 2, 3, 4, 5];
		assert_eq!(
			BoundedBTreeSet::<u32, ConstU32<4>>::decode(&mut &v.encode()[..]),
			Err("BoundedBTreeSet exceeds its limit".into()),
		);
	}

	#[test]
	fn dont_consume_more_data_than_bounded_len() {
		let s = set_from_keys(&[1, 2, 3, 4, 5, 6]);
		let data = s.encode();
		let data_input = &mut &data[..];

		BoundedBTreeSet::<u32, ConstU32<4>>::decode(data_input).unwrap_err();
		assert_eq!(data_input.len(), data.len() - Compact::<u32>::compact_len(&(data.len() as u32)));
	}

	#[test]
	fn unequal_eq_impl_insert_works() {
		// given a struct with a strange notion of equality
		#[derive(Debug)]
		struct Unequal(u32, bool);

		impl PartialEq for Unequal {
			fn eq(&self, other: &Self) -> bool {
				self.0 == other.0
			}
		}
		impl Eq for Unequal {}

		impl Ord for Unequal {
			fn cmp(&self, other: &Self) -> core::cmp::Ordering {
				self.0.cmp(&other.0)
			}
		}

		impl PartialOrd for Unequal {
			fn partial_cmp(&self, other: &Self) -> Option<core::cmp::Ordering> {
				Some(self.cmp(other))
			}
		}

		let mut set = BoundedBTreeSet::<Unequal, ConstU32<4>>::new();

		// when the set is full

		for i in 0..4 {
			set.try_insert(Unequal(i, false)).unwrap();
		}

		// can't insert a new distinct member
		set.try_insert(Unequal(5, false)).unwrap_err();

		// but _can_ insert a distinct member which compares equal, though per the documentation,
		// neither the set length nor the actual member are changed
		set.try_insert(Unequal(0, true)).unwrap();
		assert_eq!(set.len(), 4);
		let zero_item = set.get(&Unequal(0, true)).unwrap();
		assert_eq!(zero_item.0, 0);
		assert_eq!(zero_item.1, false);
	}

	#[test]
	fn eq_works() {
		// of same type
		let b1 = boundedset_from_keys::<u32, ConstU32<7>>(&[1, 2]);
		let b2 = boundedset_from_keys::<u32, ConstU32<7>>(&[1, 2]);
		assert_eq!(b1, b2);

		// of different type, but same value and bound.
		crate::parameter_types! {
			B1: u32 = 7;
			B2: u32 = 7;
		}
		let b1 = boundedset_from_keys::<u32, B1>(&[1, 2]);
		let b2 = boundedset_from_keys::<u32, B2>(&[1, 2]);
		assert_eq!(b1, b2);
	}

	#[test]
	fn can_be_collected() {
		let b1 = boundedset_from_keys::<u32, ConstU32<5>>(&[1, 2, 3, 4]);
		let b2: BoundedBTreeSet<u32, ConstU32<5>> = b1.iter().map(|k| k + 1).try_collect().unwrap();
		assert_eq!(b2.into_iter().collect::<Vec<_>>(), vec![2, 3, 4, 5]);

		// can also be collected into a collection of length 4.
		let b2: BoundedBTreeSet<u32, ConstU32<4>> = b1.iter().map(|k| k + 1).try_collect().unwrap();
		assert_eq!(b2.into_iter().collect::<Vec<_>>(), vec![2, 3, 4, 5]);

		// can be mutated further into iterators that are `ExactSizedIterator`.
		let b2: BoundedBTreeSet<u32, ConstU32<5>> = b1.iter().map(|k| k + 1).rev().skip(2).try_collect().unwrap();
		// note that the binary tree will re-sort this, so rev() is not really seen
		assert_eq!(b2.into_iter().collect::<Vec<_>>(), vec![2, 3]);

		let b2: BoundedBTreeSet<u32, ConstU32<5>> = b1.iter().map(|k| k + 1).take(2).try_collect().unwrap();
		assert_eq!(b2.into_iter().collect::<Vec<_>>(), vec![2, 3]);

		// but these worn't work
		let b2: Result<BoundedBTreeSet<u32, ConstU32<3>>, _> = b1.iter().map(|k| k + 1).try_collect();
		assert!(b2.is_err());

		let b2: Result<BoundedBTreeSet<u32, ConstU32<1>>, _> = b1.iter().map(|k| k + 1).skip(2).try_collect();
		assert!(b2.is_err());
	}

	// Just a test that structs containing `BoundedBTreeSet` can derive `Hash`. (This was broken
	// when it was deriving `Hash`).
	#[test]
	#[cfg(feature = "std")]
	fn container_can_derive_hash() {
		#[derive(Hash, Default)]
		struct Foo {
			bar: u8,
			set: BoundedBTreeSet<String, ConstU32<16>>,
		}
		let _foo = Foo::default();
	}

	#[cfg(feature = "serde")]
	mod serde {
		use super::*;
		use crate::alloc::string::ToString as _;

		#[test]
		fn test_serializer() {
			let mut c = BoundedBTreeSet::<u32, ConstU32<6>>::new();
			c.try_insert(0).unwrap();
			c.try_insert(1).unwrap();
			c.try_insert(2).unwrap();

			assert_eq!(serde_json::json!(&c).to_string(), r#"[0,1,2]"#);
		}

		#[test]
		fn test_deserializer() {
			let c: Result<BoundedBTreeSet<u32, ConstU32<6>>, serde_json::error::Error> =
				serde_json::from_str(r#"[0,1,2]"#);
			assert!(c.is_ok());
			let c = c.unwrap();

			assert_eq!(c.len(), 3);
			assert!(c.contains(&0));
			assert!(c.contains(&1));
			assert!(c.contains(&2));
		}

		#[test]
		fn test_deserializer_bound() {
			let c: Result<BoundedBTreeSet<u32, ConstU32<3>>, serde_json::error::Error> =
				serde_json::from_str(r#"[0,1,2]"#);
			assert!(c.is_ok());
			let c = c.unwrap();

			assert_eq!(c.len(), 3);
			assert!(c.contains(&0));
			assert!(c.contains(&1));
			assert!(c.contains(&2));
		}

		#[test]
		fn test_deserializer_failed() {
			let c: Result<BoundedBTreeSet<u32, ConstU32<4>>, serde_json::error::Error> =
				serde_json::from_str(r#"[0,1,2,3,4]"#);

			match c {
				Err(msg) => assert_eq!(msg.to_string(), "out of bounds at line 1 column 11"),
				_ => unreachable!("deserializer must raise error"),
			}
		}
	}
}