scapegoat

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

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 (no dependencies outside of the standard library) 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).into_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"]
);

Note

This project is an exercise in safe datastructure design. It's not as mature, fast, or memory efficient as the standard library's BTreeMap/BTreeSet.