Add mappedarrayreduce

README.md

@@ -210,3 +210,16 @@ julia> arraysT = map(A->of_eltype(Float64, A), arrays)
 This construct is inferrable (type-stable), so it can be a useful
 means to "coerce" arrays to a common type. This can sometimes solve
 type-stability problems without requiring that one copy the data.
+
+### mappedarrayreduce
+
+This package provides a "lazy" `mapreduce` operation in the form of the function `mappedarrayreduce`, where the `map` is evaluated as a `MappedArray` and is not materialized. This therefore might be more performant than a standard `mapreduce`.
+
+Note that `mappedarrayreduce` follows the same signature as `mapreduce`, and does not accept an inverse function.
+
+An example of its usage:
+
+```julia
+julia> mappedarrayreduce(x -> x^2, +, 1:3) # == 1^2 + 2^2 + 3^2
+14
+```

src/MappedArrays.jl

@@ -2,7 +2,7 @@ module MappedArrays
 
 using Base: @propagate_inbounds
 
-export AbstractMappedArray, MappedArray, ReadonlyMappedArray, mappedarray, of_eltype
+export AbstractMappedArray, MappedArray, ReadonlyMappedArray, mappedarray, of_eltype, mappedarrayreduce
 
 abstract type AbstractMappedArray{T,N} <: AbstractArray{T,N} end
 abstract type AbstractMultiMappedArray{T,N} <: AbstractMappedArray{T,N} end
@@ -261,4 +261,25 @@ eltypes(A::AbstractArray) = Tuple{eltype(A)}
 ## Deprecations
 @deprecate mappedarray(f_finv::Tuple{Any,Any}, args::AbstractArray...) mappedarray(f_finv[1], f_finv[2], args...)
 
+
+# mapreduce
+
+"""
+    mappedarrayreduce(f, op, A...; kw...)
+
+Perform a "lazy" `mapreduce` without allocating an intermediate array. This might 
+be more performant than a standard `mapreduce`. Functionally this is equivalent to 
+`reduce(op, mappedarray(f, A...); kw...)`.
+
+# Examples
+```jldoctest
+julia> mappedarrayreduce(x -> x^2, +, 1:10) # == 1^2 + 2^2 + 3^2
+385
+```
+"""
+mappedarrayreduce(f, op, A...; kw...) = reduce(op, mappedarray(f, A...); kw...)
+mappedarrayreduce(f, finv::Function, op::Function, A...; kw...) = error(
+    "mappedarrayreduce does not support an inverse function, "*
+    "please use the signature mappedarrayreduce(f, op, A...; kw...)")
+
 end # module

test/runtests.jl

@@ -177,3 +177,25 @@ end
     str = String(take!(io))
     @test occursin("x1 + x2", str)
 end
+
+@testset "mapreduce" begin
+    for T in [Int, Float64]
+        x = rand(T, 10); y = similar(x);
+
+        f = x->x^2; op = +
+        @test mapreduce(f, op, x) == mappedarrayreduce(f, op, x)
+        @test mapreduce(f, op, x, init = zero(T)) == mappedarrayreduce(f, op, x, init = zero(T))
+        @test mapreduce(f, op, x, init = zero(T), dims = 1) == mappedarrayreduce(f, op, x, init = zero(T), dims = 1)
+        @test mapreduce(f, op, x, init = zero(T), dims = :) == mappedarrayreduce(f, op, x, init = zero(T), dims = :)
+
+        @test_throws Exception mappedarrayreduce(x->x^2, sqrt, op, x)
+
+        if VERSION >= v"1.2"
+            f = ==; op = +
+            @test mapreduce(f, op, x, y) == mappedarrayreduce(f, op, x, y)
+            @test mapreduce(f, op, x, y, init = 0) == mappedarrayreduce(f, op, x, y, init = 0)
+            @test mapreduce(f, op, x, y, init = 0, dims = 1) == mappedarrayreduce(f, op, x, y, init = 0, dims = 1)
+            @test mapreduce(f, op, x, y, init = 0, dims = :) == mappedarrayreduce(f, op, x, y, init = 0, dims = :)
+        end
+    end
+end