Scan along a named dim, preserving it. Barrier over over.

Created by scan_over(). The IR shape between MapNode and ReduceNode: carry state sequentially along one non-spatial axis while emitting a same-length series (Kalman smoothers, EWMA, IIR filters, cumulative ops). The output grid is the parent grid unchanged (the scanned axis survives), optionally with a dtype override.

Usage

ScanNode(
  id = integer(0),
  parents = integer(0),
  grid = GridSpec(),
  role = character(0),
  over = character(0),
  direction = character(0),
  fn = list(),
  dtype = character(0)
)

Arguments

id

Integer node id (assigned by graph_add()).

parents

Integer ids of parent nodes.

grid

Output GridSpec of this node.

role

Optional semantic role tag (e.g. "mask", set by mask()). Pure metadata: never read by the planner or executors; surfaced by draw() and plan_view().

over

Single dim name to scan along ("t" or "band").

direction

One of "forward", "backward", "bidir".

fn

Length-1 list holding the scan body fn(xs, margin).

dtype

Length-0 (parent's dtype) or length-1 dtype override.

Value

A ScanNode.

Details

The body kernel is fn(xs, margin) -> y: xs is the LIST of parent chunk values (length >= 1; multi-parent scans read several cubes in lockstep), margin is the 1-based position of the scanned axis in the chunk's dimension order (outer dims first, then y, x), and y has the same shape as xs[[1]]. Inside, the body uses g_scan() to carry state along margin; everything else in the chunk (the spatial axes) is batched through the carry. Like a custom reducer, a scan cannot be decomposed across spatial chunks, so it is supported over t/band only (each spatial chunk holds the full axis).

direction is declarative metadata (drawn, hashed into the kernel signature): "bidir" bodies run a forward and a reverse g_scan() internally as ONE fused kernel, so forward state never crosses a node boundary.