emt — Epoch Merkle Tree

Tier L4 — an instantiation. EMT is the general-purpose mutable tree: the polydigest combinator over the single-algorithm canonical-mt engine — polydigest(canonical-mt) — instantiated at arity k = 2, with no prefix. It is the mutable peer of the append-only eml log.

Role

EMT fixes the proof-spine arity to 2 and pairs the combinator with a concrete unprefixed SHA-256 hasher, so an application gets a ready mutable tree in a few lines:

use emt::Emt;

let mut tree = Emt::new();
tree.set(0, b"genesis".to_vec()).unwrap();
tree.set(1, b"second".to_vec()).unwrap();
let root = tree.root().expect("a non-empty tree has a root");
assert_eq!(tree.get(0), Some(b"genesis".as_slice()));

Because it is mutable (set / get), EMT keeps no frontier and offers no consistency proofs — that is inherited from cmt. It does offer inclusion, non-membership, and leaf proofs, and per-node multi-hash add.

A consumer can compose a single principal tree — a mutable EMT outer tree with an embedded append-only eml commit log, joined by the spine's opaque subtree embedding. That composition lives at the consumer's layer; this crate only supplies the outer mutable tree.

Place in the layered model

merkle-spine → canonical-mt → polydigest → EMT  (this: polydigest(canonical-mt) @ k=2)

Compare the sibling instantiation:

• EML (eml) — the append-only peer, polydigest(canonical-ml) @ k = 2.

Further reading

• polydigest — the combinator EMT instantiates.
• canonical-mt — the single-algorithm mutable engine underneath.
• ../docs/architecture.md — the full design.