Layout diff

reticle-diff answers “what changed between these two versions of a layout?” It computes a pure geometric diff between two Document snapshots, and the app paints the result over the canvas: shapes added in green, removed in red, changed in amber. Like the metrology and DRC crates, it runs on the CPU over the flattened top cell, reads only the public APIs of reticle-model and reticle-geometry, and never mutates a document.

The pure diff

The one entry point is diff(before, after) -> LayoutDiff, where

LayoutDiff { added: Vec<DiffShape>, removed: Vec<DiffShape>, changed: Vec<DiffShape> }
DiffShape { layer, rect, label }

The comparison runs over each document’s flattened top cell and treats the two as multisets keyed by (layer, exact geometry):

• A shape present in after with no match in before is added.
• A shape present in before with no match in after is removed.
• Matched shapes, including matched duplicates (the counts are compared, not just presence), are reported as neither.

Because the key is the exact geometry, not a bounding box, two shapes match only when they are geometrically identical. A shape that merely moved therefore reads as one removed plus one added. Flattening happens first, so the diff compares leaf geometry regardless of how the two documents structured their cells; DiffShape’s rect is the shape’s bounding box (what the overlay paints) and label names the flattened top cell.

changed is deferred in v1

Telling a moved or resized shape apart from an independent add-plus-remove is a fuzzy match, and emitting it wrongly is worse than not emitting it at all. v1 therefore reports every geometric difference as an add or a remove and always leaves changed empty. The field exists so the overlay and any future revision keep a stable shape; a same-place resize shows today as a red box plus a green box.

Correctness

The diff is pinned by property tests, the same discipline the DRC and metrology crates use:

• diff(d, d) is empty for any document.
• diff(empty, d) is all-added, with the added count equal to the flattened shape count of d.
• diff(d, empty) is all-removed.
• The single-insertion oracle: appending one rectangle to any base document yields exactly one added shape and zero removed, whatever the base and whatever the rectangle (a duplicate still raises the multiset count by exactly one).

The app overlay

The app consumes the diff through a diff_overlay module (egui-free logic, unit tested, mirroring the DRC panel split) and a “Layout diff” side panel. The comparison document comes from a two-snapshot flow rather than a second file open, since no clean comparison-document loader exists in this build:

1. Snapshot captures the current document as the baseline (the before).
2. Edit the layout.
3. Diff vs snapshot compares the baseline against the now-current document and paints the difference on the canvas.

A Show diff overlay checkbox hides or shows the painted rectangles without discarding the computed diff, and Clear drops the baseline and the diff. Loading a new document clears the baseline, since it snapshotted the previous one.

A comparison-document file loader and a true per-instance (unflattened) diff are open work; both can reuse the LayoutDiff surface unchanged. See ADR 0079.