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Specialize sample for sparse weights #943

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Specialize sample for sparse weights #943

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ff780fd
Specialize `sample` for sparse weights
AntonOresten Nov 25, 2024
3b442b3
Update src/sampling.jl
AntonOresten Jan 7, 2026
db126d1
Update src/sampling.jl
AntonOresten Jan 7, 2026
b5f5af1
Add test for sampling without replacement
AntonOresten Jan 7, 2026
1f8f74d
fix test
AntonOresten Jan 7, 2026
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5 changes: 5 additions & 0 deletions src/sampling.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -606,6 +606,11 @@ sample(a::AbstractArray, wv::AbstractWeights) = sample(default_rng(), a, wv)
# Specialization for `UnitWeights`
sample(rng::AbstractRNG, wv::UnitWeights) = rand(rng, 1:length(wv))

function sample(rng::AbstractRNG, wv::AbstractWeights{<:Real,<:Real,<:SparseVector})
i = sample(rng, Weights(nonzeros(wv.values), sum(wv)))
return rowvals(wv.values)[i]
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@devmotion devmotion Jan 7, 2026

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The code is unsafe - in general AbstractWeights are not required to have a values field. It's just a few AbstractWeights subtypes in StatsBase that have an (undocumented and internal) values field.

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@nalimilan nalimilan Jan 8, 2026 •
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So actually better define this method only for types defines in Base. Probably using:

for W in (AnalyticWeights, FrequencyWeights, ProbabilityWeights, Weights)
    @eval function sample(rng::AbstractRNG, wv::W{<:Real,<:Real,<:SparseVector})
    ...

(I'm saying this because AFAICT there's no public API which allows accessing the backing array. And anyway I'm not aware of custom AbstractWeights types defined elsewhere so we don't really care to apply this optimization to them.)

end

"""
direct_sample!([rng], a::AbstractArray, wv::AbstractWeights, x::AbstractArray)

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7 changes: 6 additions & 1 deletion test/wsampling.jl
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@devmotion devmotion Jan 7, 2026

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The tests are insufficient - since the method is implemented for AbstractWeights, to be sure it works not only for Weights we should test all subtypes implemented in StatsBase and a custom subtype of AbstractWeights.

Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
using StatsBase
using Random, Test, OffsetArrays
using Random, Test, OffsetArrays, SparseArrays

Random.seed!(1234)

Expand Down Expand Up @@ -41,6 +41,7 @@ for wv in (
weights([0.2, 0.8, 0.4, 0.6]),
weights([2, 8, 4, 6]),
weights(Float32[0.2, 0.8, 0.4, 0.6]),
weights(sparsevec([0, 8, 0, 6])),
Weights(Float32[0.2, 0.8, 0.4, 0.6], 2),
Weights([2, 8, 4, 6], 20.0),
)
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Expand Down Expand Up @@ -130,6 +131,10 @@ test_rng_use(efraimidis_aexpj_wsample_norep!, 4:7, wv, zeros(Int, 2))
a = sample(4:7, wv, 3; replace=false, ordered=false)
check_wsample_norep(a, (4, 7), wv, -1; ordered=false)

wv_sparse = weights(sparsevec([0, 8, 4, 6]))
a_sparse = sample(4:7, wv_sparse, 3; replace=false, ordered=false)
check_wsample_norep(a_sparse, (4, 7), wv_sparse, -1; ordered=false)

for rev in (true, false), T in (Int, Int16, Float64, Float16, BigInt, ComplexF64, Rational{Int})
r = rev ? reverse(4:7) : (4:7)
r = T===Int ? r : T.(r)
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