LazyGroupBy: lazy, parallelizable and composable group-by operations

LazyGroupBy.jl exports a single API grouped. It can be used to run group-by operation using the dot-call syntax:

reducer.(..., grouped(key, collection), ...)

where reducer runs on each group (thus, grouped(key, collection) can be considered a as a key-value pairs with Dictionaries.jl-like broadcasting rule). Roughly speaking, grouped(key, collection) is equivalent to Dict(k_1 => [v_11, v_12, ...], k_2 => [v_21, v_22, ...], ...) where k_i is an output of value of key(v_ij) for v_ij in collection and each call of reducer is evaluated with a group "vector" [v_i1, v_i2, ...].

For example:

julia> using LazyGroupBy

julia> collect.(grouped(isodd, 1:7))
Transducers.GroupByViewDict{Bool,Array{Int64,1},…} with 2 entries:
  false => [2, 4, 6]
  true  => [1, 3, 5, 7]

julia> length.(grouped(isodd, 1:7))
Transducers.GroupByViewDict{Bool,Int64,…} with 2 entries:
  false => 3
  true  => 4

julia> keys.(grouped(isodd, [0, 7, 3, 1, 5, 9, 4, 3, 0, 5]))
Transducers.GroupByViewDict{Bool,Array{Int64,1},…} with 2 entries:
  false => [1, 7, 9]
  true  => [2, 3, 4, 5, 6, 8, 10]

julia> foldl.(tuple, grouped(isodd, [0, 7, 3, 1, 5, 9, 4, 3, 0, 5]))
Transducers.GroupByViewDict{Bool,Any,…} with 2 entries:
  false => ((0, 4), 0)
  true  => ((((((7, 3), 1), 5), 9), 3), 5)

julia> foldl.(tuple, grouped(isodd, [0, 7, 3, 1, 5, 9, 4, 3, 0, 5]); init = -1)
Transducers.GroupByViewDict{Bool,Tuple{Any,Int64},…} with 2 entries:
  false => (((-1, 0), 4), 0)
  true  => (((((((-1, 7), 3), 1), 5), 9), 3), 5)

julia> extrema_rf((min1, max1), (min2, max2)) = (min(min1, min2), max(max1, max2));

julia> mapfoldl.(x -> (x, x), extrema_rf, grouped(isodd, [0, 7, 3, 1, 5, 9, 4, 3, 0, 5]))
Transducers.GroupByViewDict{Bool,Tuple{Int64,Int64},…} with 2 entries:
  false => (0, 4)
  true  => (1, 9)

Following generic and standard reducers are supported:

• collect.(op, grouped(...)) → DICT{Key,Vector{...}}
• view.(grouped(_, array)) → DICT{Key,SubArray}
• map.(f, grouped(...))
• length.(op, grouped(...)) → DICT{Key,Int}
• count.([f,] op, grouped(...)) → DICT{Key,Int}
• sum.([f,] op, grouped(...)) → DICT{Key,Number}
• prod.([f,] op, grouped(...)) → DICT{Key,Number}
• any.(f, op, grouped(...)) → DICT{Key,Bool}
• all.(f, op, grouped(...)) → DICT{Key,Bool}
• minimum.([f,] op, grouped(...))
• maximum.([f,] op, grouped(...))
• extrema.([f,] op, grouped(...))
• keys.(op, grouped(_, collection)) → DICT{Key,Vector{keytype(collection)}}
• pairs.(op, grouped(_, collection)) → DICT{Key,DICT{keytype(collection),valtype(collection)}}
• findfirst.(f, grouped(_, array)) → DICT{Key,keytype(collection)}
• findlast.(f, grouped(_, array)) → DICT{Key,keytype(collection)}
• findall.(f, grouped(_, array)) → DICT{Key,Vector{keytype(collection)}}
• foldl.(op, grouped(...); [init])
• mapfoldl.(f, op, grouped(...); [init])

where DICT{K,V} above is a short-hand for AbstractDict{<:K,<:V} and Key is the type of the values returned from key function passed to grouped.

For more complex tasks, Transducers.jl and OnlineStats.jl can also be used:

• foldl.(op, xf, grouped(...); [init])
• foldxl.(op, [xf,] grouped(...); [init])
• foldxt.(op, [xf,] grouped(...); [init]) (multi-threaded)
• foldxd.(op, [xf,] grouped(...); [init]) (distributed)
• collect.(xf, grouped(...))
• tcollect.(xf, grouped(...)) (multi-threaded version of collect)
• dcollect.(xf, grouped(...)) (distributed version of collect)

where xf::Transducer is initiated for each group individually and op is either a two-argument function or an OnlineStat object (e.g., OnlineStats.Mean).

The dot-call syntax is used for defining the "domain-specific language" (DSL) and it is different from the standard semantics of broadcasting on arrays. In particular, reducer.(..., grouped(key, collection), ...) may not actually call reducer. Rather, it is pattern-matched and dispatched to an alternative definition based on Transducers.jl.

LazyGroupBy.jl is implemented as a direct transformation to foldl/foldxt/foldxd and GroupBy from Transducer.jl. Consider

foldl.(rf, xf, grouped(key, collection); init = init)

This is simply translated to

foldl(right, GroupBy(key, xf, rf, init), collection)

Other reducers like sum and collect are implemented in terms of above transformation.