generate all combinations and output result using coroutines

combo-test.lua

local combo = require "combo"
function print_table(T)
    local str=""
    for i,v in ipairs(T) do
        str = str .. string.format("%d,",v)
    end
    print(str)
end
  
for p in combo.combo_gen({1,2,3,4,5,6,7}, 1) do
   print_table(p)
end
for p in combo.combo_gen({1,2,3,4,5,6,7}, 4) do
   print_table(p)
end
for p in combo.combo_gen({1,2,3,4,5,6,7}, 9) do
   print_table(p)
end
   

combo.lua

[[ solution to Exercise 24.2 from the book:
   Programming in Lua - Fourth Edition
   by Roberto Ierusalimschy
   ]]

local M={}
local function get_prefix(T,N)
    local result = {}
    for i = 1,N do
        table.insert(result,T[i])
    end
    return result
end

local function union(T,S)
    local R = {}
    for _,v in ipairs(S) do
        table.insert(R,v)
    end
    for _,v in ipairs(T) do
        table.insert(R,v)
    end
    return R
end
local function combo(set, N,K, part_result)
    if K == 0 then coroutine.yield(part_result) 
    elseif N==K then coroutine.yield (union(part_result,get_prefix(set,N))) 
    else
        set[1],set[N] = set[N], set[1]
        combo(set, N-1, K, part_result)

        combo(set, N-1, K-1,union(part_result,{set[N]}))

        set[1],set[N] = set[N], set[1]
    end
end

function M.combo_gen(set, K)
    if K <= #set then

        local co =  coroutine.create( function()
        combo(set, #set, K, {})
        end)
        return function () 
            local code,res = coroutine.resume(co) 
            return res
        end
    else
        return function() return nil end 
    end
end

return M