local lfs = require("lfs")

-- Join should be used to construct paths in a portable way.
-- It takes any number of arguments and constructs a hierarchical path from them.
local function join(...)
    return table.concat({...}, "/"):gsub("//+", "/")
end

local p = join("dir1", "dir2", "filename")
print("p:", p)

-- You should always use join instead of concatenating "/" manually.
-- In addition to providing portability, join will also normalize paths
-- by removing superfluous separators and directory changes.
print(join("dir1//", "filename"))
print(join("dir1/../dir1", "filename"))

-- We can split a path to the directory and the file.
local function split(path)
    return path:match("(.*/)(.*)")
end

local dir, file = split(p)
print("Dir(p):", dir or "")
print("Base(p):", file or p)

-- We can check whether a path is absolute.
local function isabs(path)
    return path:sub(1, 1) == "/"
end

print(isabs("dir/file"))
print(isabs("/dir/file"))

local filename = "config.json"

-- Some file names have extensions following a dot.
-- We can split the extension out of such names.
local function ext(path)
    return path:match("%.%w+$") or ""
end

local extension = ext(filename)
print(extension)

-- To find the file's name with the extension removed,
-- use string.gsub to remove the extension.
print(filename:gsub("%"..extension.."$", ""))

-- Rel finds a relative path between a base and a target.
-- It returns nil if the target cannot be made relative to base.
local function rel(base, target)
    local b = base:gsub("/$", "")
    local t = target:gsub("/$", "")
    local bparts = {}
    local tparts = {}
    for part in b:gmatch("[^/]+") do table.insert(bparts, part) end
    for part in t:gmatch("[^/]+") do table.insert(tparts, part) end
    
    local i = 1
    while i <= #bparts and i <= #tparts and bparts[i] == tparts[i] do
        i = i + 1
    end
    
    local result = {}
    for j = i, #bparts do table.insert(result, "..") end
    for j = i, #tparts do table.insert(result, tparts[j]) end
    
    if #result == 0 then return "." end
    return table.concat(result, "/")
end

local relative = rel("a/b", "a/b/t/file")
print(relative)

relative = rel("a/b", "a/c/t/file")
print(relative)