sample.lua

local function sample(it, count, random)
    local answer = {}

    do
        -- first element
        local element = it()
        if element == nil then
            return answer
        end
        answer[1] = element
    end

    for i = 2, count do
        local element = it()
        if element == nil then
            return answer
        end
        local j = random(1, i - 1)
        answer[i] = answer[j]
        answer[j] = element
    end

    local i = count + 1
    while true do
        local element = it()
        if element == nil then
            return answer
        end
        local j = random(1, i - 1)
        if j <= count then
            answer[j] = element
        end
        i = i + 1
    end
end

return sample