[julia-users] cached iterator type (CachedIterator)

[Sheehan Olver]

I just put together a type that caches the values of a (possibly infinite) 
iterator, that I wanted to through up here in case its useful for others. 
 It assumes that the state of the input iterator is type stable.  Any 
suggestions for improving efficiency are welcome. 



## Cache iterator
type CachedIterator{T,IT,ST}
    iterator::IT
    storage::Vector{T}
    state::ST
    length::Int

    CachedIterator(it::IT) = new(it,Vector{it}(),start(it),0)
end

CachedIterator(it) = 
CachedIterator{eltype(it),typeof(it),typeof(start(it))}(it)

function Base.resize!(it::CachedIterator,n::Integer)
    m = it.length
    if n > m
        if n > length(it.storage)
            resize!(it.storage,2n)
        end

        for k = m+1:n
            if done(it.iterator,it.state)
                it.length = k-1
                return it
            end
            val,it.state = next(it.iterator,it.state)
            @inbounds it.storage[k] = val
        end

        it.length = n
    end
    it
end


Base.eltype{T}(it::CachedIterator{T}) = T
Base.start(it::CachedIterator) = 1
Base.next(it::CachedIterator,st::Int) = (it[st],st+1)
Base.done(it::CachedIterator,st::Int) = st == it.length + 1 &&
                                        done(it.iterator,it.state)


Base.getindex(it::CachedIterator,k::Int) = resize!(it,k).storage[k]