fenwick in ruby (ig)

fenwick.rb

class Fenwick
    def initialize(n)
        @size=n
        @data=[0]*(n+1)
    end
    def add(idx,v)
        while idx<=@size
            @data[idx]+=v
            idx+=idx&-idx
        end
    end
    def sum(idx)
        res=0
        while idx>0
            res+=@data[idx]
            idx-=idx&-idx
        end
        res
    end
    def size()
        @size
    end
end