Accumulators and Counters

A accumulator, as defined below, is a data structure that maintains an accumulated number for each key. This is a counter when the accumulated values reflect the counts:

struct Accumulator{K, V<:Number}
    map::Dict{K, V}
end

There are different ways to construct an accumulator/counter:

a = Accumulator(K, V)    # construct an accumulator with key-type K and
                         # accumulated value type V

a = Accumulator(dict)    # construct an accumulator from a dictionary

a = counter(K)           # construct a counter, i.e. an accumulator with
                         # key type K and value type Int

a = counter(dict)        # construct a counter from a dictionary

a = counter(seq)         # construct a counter by counting keys in a sequence

a = counter(gen)         # construct a counter by counting keys in a generator

Usage of an accumulator/counter:

# let a and a2 be accumulators/counters

a[x]             # get the current value/count for x.
                 # if x was not added to a, it returns zero(V)

push!(a, x)      # increment the value/count for x by 1
push!(a, x, v)   # increment the value/count for x by v
push!(a, a2)     # add all counts from a2 to a1

pop!(a, x)       # remove a key x from a, and return its current value

merge(a, a2)     # return a new accumulator/counter that combines the
                 # values/counts in both a and a2