local prevfile = require "prevfile"

local nextfile = {}

function nextfile.splitfilename(path)
	name, ext = path:match("^(.*)(%.%w*)")
	return name, ext
end

function nextfile.nextletter(letter)
	if letter == "z" then return "za" end

	local ascii = letter:byte()
	local nextascii = ascii + 1
	return string.char(nextascii)
end

function nextfile.nextfile(directory, extension)
	local previous = prevfile.prevfile(directory)
	if previous == nil then return nil end

	local name, ext = nextfile.splitfilename(previous)

	value = ""
	if tonumber(name) == nil then
		local lastletter = name:sub(#name)
		local nextletter = nextfile.nextletter(lastletter)
		value = value .. name:gsub(lastletter .. "$", nextletter)
	else
		value = value .. name + 1
	end
	
	if extension ~= nil then
		value = value .. "." .. extension
	else
		value = value .. ext
	end

	return value
end

return nextfile