Function compute_diff 

pub fn compute_diff(
    sources: &[Rope],
    algorithm: DiffAlgorithm,
) -> Result<HunkIndex, DiffEngineError>

D.8.a: the engine’s only public dispatch entry. Routes by sources.len() to the appropriate algorithm, returning a typed error for arities outside the supported range.

• N=0: Err(DiffEngineError::Empty) — there’s nothing to diff. Callers should never reach this; defensive.
• N=1: empty HunkIndex — a single participant has no peers to compare against. The session is “dormant” in the subsystem’s terms (D.8.e); the first :diffthis lands here, before a second :diffthis extends it to N=2.
• N=2: dispatches to [two_way].
• N=3: dispatches to [three_way].
• N≥4: Err(DiffEngineError::Unsupported { n }). v1 cap. The reference table in §12 of docs/dev/architecture/n-way-diff-membership.md surveys how other editors handle this — Helix/Zed/VSCode also don’t support N≥4; Vim does via pairwise-vs-anchor. The cap moves when user-feedback signals N>3 is wanted.

compute_diff is the only public entry — all external callers go through it. The dispatch decision lives entirely inside this function so consumers never branch on arity.