reduce

Folds a run of elements to one element per group: the sum, least, greatest, mean, count, or the index of the least or greatest, of the whole run or of each segment of it.

Specialize	Options	Meaning
T	f32, i32, vec2 to vec4 of either	the element; a vector folds lane by lane
FOLD	add, min, max, mean, count, argmin, argmax	the fold

The dispatch size n is the number of elements. One workgroup folds one group at a time, so a segmented fold runs its segments side by side and an unsegmented one is a single group; any workgroup count from 1 up folds every group.

params.segments above 0 makes starts partition the run into that many segments: it holds where each segment begins (a segment ends where the next begins, the last at n), and dst holds one element per segment. With segments 0 the whole run is one group, starts is any one-word run, and dst holds one element. An empty group answers 0 for every fold. argmin and argmax write the lowest index among equal elements to picked (for a vector element, the element that beats the best before it in every lane); every other fold writes -1 there. The mean of an integer run is the integer quotient. The bounds of a run are its min and its max.

Exact: every group folds its elements in a fixed order and combines the 256 partial results in a fixed tree, so the same run gives the same bits on every dispatch, and an integer fold is the exact integer answer.

Over a live count (COUNT=device) n is the capacity and the first domain_count elements are the run: the last segment ends at the live count. Over live segments (SEGMENTS=device) params.segments is the most segments there can be, which sizes dst and picked, and the first segment_count of them are live; a group past them is not written, and a run of no live segments folds nothing.

Nodes that fold columns run it, among them core.reduce, core.select, core.arclength, core.normals.face, attribute.promote and sim.particles.