Permutations iterator

permutations.jl

struct PermIter{T}
    v::Vector{T}
    infinite::Bool
end

function Base.iterate(iter::PermIter{T}, state = ones(Int, length(iter.v))) where T
    if state[end] == 2 return nothing end

    v = iter.v
    elem = Vector{T}(undef, length(iter.v))
    for (i, ind) in enumerate(state)
        elem[i] = v[ind]
        v = vcat(v[1:(ind - 1)], v[(ind + 1):end])
    end

    for i in length(iter.v):-1:1
        if state[i] == length(iter.v) - i + 1
            state[i] = 1
        else
            state[i] += 1
            break
        end
    end
    
    if !(iter.infinite) && (sum(state) == length(iter.v)) state[end] = 2 end
    
    return (elem, state)    
end

permutations(v::Vector{T}, inf = false) where T = PermIter(v, inf)

## Usage

for x in permutations(['a', 'b', 'c', 'd'])
    println(join(x))
end

for (i, x) in enumerate(permutations(['a', 'b', 'c', 'd'], true))
    println(join(x))
    if i == 40 break end
end