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importall Base
# figure 5.2 from principles of parallel programming, ported to julia.
# sum a vector using a tree on top of local reductions.
function sum(v::DArray)
P = procs(v)
nodeval = { RemoteRef(p) for p=P }
answer = RemoteRef()
np = numel(P)
for i=0:np-1
@spawnat P[i+1] begin
stride=1
tally = sum(localpart(v))
while stride < np
if i%(2*stride) == 0
tally = tally + take(nodeval[i+stride])
stride = 2*stride
else
put(nodeval[i], tally)
break
end
end
if i==0
put(answer, tally)
end
end
end
fetch(answer)
end
function reduce(f, v::DArray)
mapreduce(fetch, f,
{ @spawnat p reduce(f,localpart(v)) for p = procs(v) })
end
# possibly-useful abstraction:
type RefGroup
refs::Array{RemoteRef,1}
RefGroup(P) = new([ RemoteRef(p) for p=P ])
end
getindex(r::RefGroup, i) = fetch(r.refs[i])
setindex!(r::RefGroup, v, i) = put(r.refs[i], v)
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