local function extract_range(list)
    if #list == 0 then return "" end
    local sb = ""
    local first = list[1]
    local prev = first

    local function append(index)
        if first == prev then
            sb ..= tostring(prev)
        elseif first == prev - 1 then
            sb ..= tostring(first) .. "," .. tostring(prev)
        else
            sb ..= tostring(first) .. "-" .. tostring(prev)
        end
        if index < #list then sb ..= "," end
    end

    for i = 2, #list do
        if list[i] == prev + 1 then
            prev += 1
        else
            append(i)
            first = list[i]
            prev = first
        end
    end
    append(#list)
    return sb
end

local list1 = {-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20}print(extract_range(list1))
local list2 = {
    0,  1,  2,  4,  6,  7,  8, 11, 12, 14,
    15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
    25, 27, 28, 29, 30, 31, 32, 33, 35, 36,
    37, 38, 39
}
print(extract_range(list2))