arch-diff

A git diff shows which lines changed. It does not show whether the structure changed. The most review-critical regressions in a layered backend are structural and invisible in a text diff:

• A handler that used to call a repo through a service now calls it directly.
• A domain package gains a dependency on infra.
• A refactor removes the last caller of a function, leaving it dead — fully tested, compiling, and looking alive in the diff.

arch-diff runs the archview engine on two git refs, runs a reachability pass on each, and diffs both the graph and the reachable-set. The result is a PR comment that shows architectural impact and dead code in seconds.

What it surfaces

In priority order:

1. Newly dead — reachable in base, dead in head. Your PR orphaned it. The headline.
2. New layer-crossing edge — e.g. OrderHandler.Create → InventoryRepo.Decrement, with the path it replaced.
3. Removed layer-crossing edge — e.g. a service hop that disappeared.
4. Revived — dead in base, reachable in head.
5. Changed — body hash differs.
6. New / removed nodes.

How it works

1. git worktree add for each ref into an isolated temp dir.
2. Load each worktree with the archview call-graph engine (raw, unpruned).
3. Resolve roots (routes, main, optionally exported API) from config.
4. Run reachability from roots, marking every node reachable or dead.
5. Set-diff the two graphs, and separately diff the two reachable-sets.
6. Render markdown plus a mermaid subgraph of the changed region.

Graphs are deterministic per commit SHA, so each is cached by SHA; the base branch is computed once and reused.

Node identity

Diff quality depends entirely on stable, location-independent node IDs. An ID is the package path + receiver + method — never file or line. Inserting a blank line above a function changes no ID, so only genuine structural change is visible.

github.com/x/eularix/internal/order/handler.(*OrderHandler).Create