local path = {}

function path.Common_Root( Left, Right, Separator )
    local Sep = Separator or "/"
    local smallest_string = nil
    local largest_string = nil
    if string.len(Left) > string.len(Right) then
        smallest_string = Right
        largest_string = Left
    else
        smallest_string = Left
        largest_string = Right
    end

    local last_sep_idx = 1
    local sep = string.byte(Sep)
    for i = 1, #smallest_string do
        if smallest_string:byte(i) == sep then
            last_sep_idx = i
        end

        if smallest_string:byte(i) ~= largest_string:byte(i) then
            break
        end
    end

    local match = string.sub(smallest_string, 1, last_sep_idx - 1)

    if match == '' then
        return nil
    else
        return match
    end
end

function path.Get_Extension( P )
    if P == nil then
        return nil
    end

    local rev_string = string.reverse(P)
    local start_idx = 0
    local end_idx = 0
    start_idx, end_idx = string.find(rev_string, ".", 1, true)

    if start_idx == nil then
        return nil
    else
        return string.sub(P, #P - (start_idx -2))
    end
end

function path.Get_Extension_From_List( P, Extension_List )
    if Extension_List == nil or P == nil then
        return nil
    end

    local ext_length = 0
    for _,item in ipairs(Extension_List) do
        local current_ext_length = #item
        if current_ext_length > ext_length then
            ext_length = current_ext_length
        end
    end

    local rev_string = string.reverse(P)
    local start_idx = 0
    local end_idx = 0
    local match = nil
    while ( true ) do
        start_idx, end_idx = string.find(rev_string, ".", start_idx+1, true)
        if start_idx == nil or (start_idx-1) > ext_length then
            break
        end

        for _,exts in ipairs(Extension_List) do
            if exts == string.sub(P, #P - (start_idx -2)) then
                match = exts
            end
        end
    end

    return match
end

function path.Get_Ext( P, Extension_List )
    if Extension_List == nil then
        return path.Get_Extension(P)
    else
        return path.Get_Extension_From_List(P, Extension_List)
    end
end

print(path.Common_Root("/home/folkert/Documents/Projects","/home/folkert/Downloads/sub/sub2/sub3/sub4/hello.txt"))
print(path.Get_Ext("/home/folkert/Documents/projects.wo.cp", {"o.cp"}))


return path