Add snapshot() for pre-packing directive subtrees#17
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Introduces a new `snapshot(ops)` constructor that pre-packs a directive array into its transfer encoding. The returned opaque `Op` can be spliced into any directive array, and during packing its bytes are copied directly into the command buffer without re-encoding. This enables higher-level frameworks to implement dirty tracking: unchanged component subtrees can reuse a cached snapshot, skipping the per-frame packing cost entirely.
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Motivation
Higher-level frameworks (like crankterm) need a way to skip re-packing unchanged component subtrees each frame. Currently, every
render()call re-encodes the entireOp[]array into the transfer encoding, even for components whose inputs haven't changed. This adds unnecessary overhead proportional to tree size rather than change size.Approach
Adds a new
snapshot(ops: Op[]): Opconstructor that pre-packs a directive array into its binary transfer encoding upfront. The returned value is an opaqueOpthat can appear anywhere in a directive array. Duringpack(), a snapshot's bytes are copied directly into the command buffer without re-encoding individual ops.snapshot()and its equivalence guarantee in Section 9.1ops.ts: NewSnapshotop type (not exported — opaque by design),packSize()for exact buffer allocation,snapshot()constructor, andpack()handler that memcpies pre-packed bytesAlternate Designs
Considered adding a
RenderInfomap toRenderResultwith per-element dimensions and packed byte slices. The snapshot-as-op approach is simpler and more composable — it lets frameworks own their dirty tracking without coupling to the renderer's return type.Possible Drawbacks or Risks
A snapshot captures the transfer encoding at creation time. If the transfer encoding changes in a future version, stale snapshots would produce incorrect results. This is acceptable since snapshots are intended to be short-lived (cached between frames, not persisted).