permute_str.lua

local ffi = require 'ffi'

local function permute_inner(arr, l, r)
    if l == r then
        coroutine.yield(ffi.string(arr))
    else
        for i = l, r do
            arr[l], arr[i] = arr[i], arr[l]
            permute_inner(arr, l + 1, r)
            arr[l], arr[i] = arr[i], arr[l]
        end
    end
end

local function permute(str)
    local arr = ffi.new('char[?]', #str + 1, str)
    return coroutine.wrap(function() permute_inner(arr, 0, #str - 1) end)
end