scapegoat

Ordered set and map data structures via an arena-based scapegoat tree (memory-efficient, self-balancing binary search tree).

This library is #![no_std] compatible by default, strictly #![forbid(unsafe_code)], and verified using differential fuzzing.

About

Three APIs:

• Ordered Set API (SGSet)
• Ordered Map API (SGMap)
• Binary Tree API (SGTree)

Strives for two properties:

• Maximal safety: strong memory safety guarantees.

  • Compile-time safety: no unsafe (no raw pointer dereference, etc.).
  • Debug-time safety: debug_assert! for logical invariants exercised in testing.
  • Runtime safety: no interior mutability (e.g. no need for Rc<RefCell<T>>'s runtime check).

• Minimal footprint: small binary with low resource use.

  • Memory-efficient: nodes have only child index metadata, node memory is re-used.
  • Recursion-free: all operations are iterative, so stack use and runtime are both minimized.
  • Zero-copy: rebuild/removal re-point in-place, nodes are never copied or cloned.

Other features:

• Generic: map keys and set elements can be any type that implements the Ord trait.
• Arbitrarily mutable: elements can be insert and removed, map values can be mutated.

Usage

SGMap non-exhaustive API example (would work identically for std::collections::BTreeMap):

use scapegoat::SGMap;

let mut example = SGMap::new();

example.insert(3, String::from("the"));
example.insert(2, String::from("don't blame"));
example.insert(1, String::from("Please"));
example.insert(4, String::from("borrow checker"));

assert_eq!(
    example.iter().map(|(_, v)| v).collect::<Vec<&String>>(),
    vec!["Please","don't blame","the","borrow checker"]
);

assert_eq!(example[&3], "the");

let please_tuple = example.pop_first().unwrap();
assert_eq!(please_tuple, (1, String::from("Please")));

example.insert(5, String::from("! :P"));

let dont_blame = example.get_mut(&2).unwrap();
dont_blame.remove(0);
dont_blame.insert(0, 'D');

assert_eq!(
    example.into_iter().map(|(_, v)| v).collect::<Vec<String>>(),
    vec!["Don't blame","the","borrow checker","! :P"]
);

Configuring a Stack Storage Limit

The maximum number of stack-stored elements (set) or key-value pairs (map/tree) is determined at compile-time, via the environment variable SG_MAX_STACK_ELEMS. Valid values are in the range [0, 32] and powers of 2 up to 2048. For example, to store up to 1024 items on the stack:

export SG_MAX_STACK_ELEMS=1024
cargo build --release

Please note:

• If the SG_MAX_STACK_ELEMS environment variable is not set, it will default to 2048.

• For any system with dynamic (heap) memory: the first SG_MAX_STACK_ELEMS elements are stack-allocated and the remainder will be automatically heap-allocated.

• For embedded systems without dynamic memory: SG_MAX_STACK_ELEMS is a hard maximum - attempting to insert beyond this limit will cause a panic.

  • Use feature high_assurance to ensure error handling and avoid panic (see below).

The high_assurance Feature

For embedded use cases prioritizing robustness, enabling the high_assurance feature makes two changes:

1. Front-end, API Tweak: insert and append APIs now return Result. Err indicates stack storage is already at maximum capacity, so caller must handle. No heap use, no panic potential on insert.

2. Back-end, Integer Packing: Because the fixed/max size of the stack arena is known, indexing integers (metadata stored at every node!) can be size-optimized. This memory micro-optimization honors the original design goals of the scapegoat data structure.

That second change is a subtle but interesting one. Example of packing saving 53% (but in reality only 61 KB) of RAM usage:

use scapegoat::SGMap;
use core::mem::size_of;

// If you're on a 64-bit system, you can compile-time check the below numbers yourself!
// Just do:
//
// $ cargo test --doc
// $ cargo test --doc --features="high_assurance"
//
// One command per set of `cfg` macros below.
// Internally, this compile time struct packing is done with the `smallnum` crate:
// https://crates.io/crates/smallnum

// This code assumes `SG_MAX_STACK_ELEMS == 2048` (default)
let temp: SGMap<u64, u64> = SGMap::new();
assert!(temp.capacity() == 2048);

// Without packing
#[cfg(target_pointer_width = "64")]
#[cfg(not(feature = "high_assurance"))]
{
    assert_eq!(size_of::<SGMap<u64, u64>>(), 114_776);
}

// With packing
#[cfg(target_pointer_width = "64")]
#[cfg(feature = "high_assurance")]
{
    assert_eq!(size_of::<SGMap<u64, u64>>(), 53_304);
}

Trusted Dependencies

This library has three dependencies, each of which have no dependencies of their own (e.g. exactly three total dependencies).

• smallvec - !#[no_std] compatible Vec alternative. Used in Mozilla's Servo browser engine.
• micromath - !#[no_std], #![forbid(unsafe_code)] floating point approximations.
• smallnum - !#[no_std], #![forbid(unsafe_code)] integer packing.

Considerations

This project is an exercise in safe data structure design. It's not as mature, fast, or memory efficient as the standard library's BTreeMap/BTreeSet. It does, however, offer:

• Best-effort Compatibility: APIs are a subset of BTreeMap's/BTreeSet's, making it a somewhat "drop-in" replacement for !#[no_std] systems. Please open an issue if an API you need isn't yet supported!

• Dynamic Verification: Coverage-guided differential fuzzing is used to verify that this implementation is logically equivalent and equally reliable.