Scan: carry state along dim 1, emitting per-step outputs.

The loop op behind scan_over() bodies (Kalman smoothers, EWMA, IIR filters, cumulative custom ops). At step t, body(carry, x) gets the current carry and the step-t slice of xs (taken along dim 1 with that unit axis dropped) and returns list(carry = , out = ); the outs are stacked into (length, ...) buffers. reverse = TRUE runs t = length..1, still reading and writing at position t (what a backward smoothing pass needs, e.g. Rauch-Tung-Striebel). Traced values route to anvl::nv_scan; plain R arrays take the pure-R reference implementation with identical semantics.

Usage

g_scan(init, body, xs = NULL, length = NULL, reverse = FALSE)

Arguments

init

Initial carry: array or (nested) named list of arrays.

body

Step function function(carry, x) -> list(carry, out); out may be an array, a (nested) list, or NULL.

xs

Per-step inputs sliced along dim 1 (array, nested list, or NULL).

length

Static trip count; required when xs is NULL.

reverse

Run steps in reverse order?

Value

list(carry = final carry, out = stacked outputs).

Details

The scanned axis must be dim 1 of every xs leaf (garry's canonical layout puts the scanned non-spatial axis first); bodies scanning a non-leading margin must permute first.