1	import Base
2	import ../shape/core.bend as Shape
3	import ../tensor/core.bend as Core
4	import ../tensor/contracts/fold.bend as Fold
5	import ../tensor/contracts/sum.bend as Sum
6	import ../tensor/contracts/broadcast.bend as Broadcast
7	
8	# Ordered-fold family, reduce instance: target cell `index` of reduce_to sums,
9	# in increasing position, the contribution words whose target offset is
10	# `index`.
11	
12	type Cell is Data:
13	  Cell{plan: Shape.BroadcastPlan, index: U32}
14	
15	def target_offset(plan: Shape.BroadcastPlan, position: Nat) -> Nat:
16	  Broadcast.left_map(plan, position)
17	
18	def contribution_offset(plan: Shape.BroadcastPlan, position: Nat) -> Nat:
19	  Broadcast.right_map(plan, position)
20	
21	def source(cell: Cell, position: Nat) -> Nat:
22	  Cell{+plan, index} = cell
23	  contribution_offset(plan, position)
24	
25	def lands(cell: Cell, position: Nat) -> Bool:
26	  Cell{+plan, +index} = cell
27	  Nat.is_eq(target_offset(plan, position), U32.to_nat(index))
28	
29	def cell(+plan: Shape.BroadcastPlan, +index: U32, input: Core.Dynamic) -> Core.Dynamic & Result<&2, &1, Core.Error, F32>:
30	  Fold.run(~Unit, ~Sum.add, ~Cell, ~source, ~lands, Unit{}, Cell{plan, index}, Shape.elements(Shape.right_shape(plan)), input)