Module alias 

12-bit alias generation, using the pseudo-random sequence suggested by the CAN Frame Transfer Standard (seeded from the Node ID).

Why CAN needs aliases

A CAN 2.0B extended identifier has 29 bits, of which OpenLCB can only spare 12 for the source address. A 48-bit NodeId does not fit, so on CAN each node picks a short alias (1..=0xFFF) and uses it as the source address in every frame header. Addressed messages likewise carry the destination’s alias rather than its Node ID. Aliases are local to one CAN segment; gateways and TCP links translate them.

Since aliases are chosen without coordination, two nodes may pick the same one. The CAN Frame Transfer Standard resolves this with a reservation protocol (CID frames, a 200 ms wait, RID, AMD; see crate::node) and by having each node generate candidates from a pseudo-random sequence seeded with its own Node ID. Different nodes then almost always try different aliases, and after a collision a node simply moves on to the next value in its sequence.

The sequence here is the 48-bit LFSR-like generator recommended by the standard (also used by OpenMRN and JMRI), so a given Node ID tends to receive the same alias after each power cycle.

use rolcb::alias::AliasGenerator;
use rolcb::NodeId;

let mut gen = AliasGenerator::new(NodeId::new(0x0501_0101_2200));
let first = gen.next_alias();
assert!((1..=0xFFF).contains(&first));
assert_ne!(gen.next_alias(), first);

// Deterministic: the same Node ID gives the same sequence.
assert_eq!(AliasGenerator::new(NodeId::new(0x0501_0101_2200)).next_alias(), first);

Structs

AliasGenerator

Produces a deterministic sequence of candidate aliases for a node.