tar for ComputerCraft

--[[
    CCTar - the ComputerCraft archiver
    Copyright (C) 2013  Yingtong Li (RunasSudo)

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
--]]

local function readString(tarHandle, length)
    local str = ""
    for i = 1, length do
        local byte = tarHandle.read()
        if byte > 0 then
            str = str .. string.char(byte)
        end
    end
    return str
end

local function skip (tarHandle, bytes)
    for i = 1, bytes do
        tarHandle.read()
    end
end

local function numChunks (length)
    if length % 512 == 0 then
        return math.floor(length / 512)
    end
    return math.floor(length / 512) + 1
end

local function readTar(tarHandle, folderName)
    while true do
        local firstByte = tarHandle.read()
        if firstByte <= 0 then
            return
        end

        local fileName = string.char(firstByte) .. readString(tarHandle, 99)
        print("Extracting " .. fileName)

        skip (tarHandle, 8) -- Mode
        skip (tarHandle, 8) -- Owner
        skip (tarHandle, 8) -- Group

        local fileSizeString = readString(tarHandle, 12):gsub(" ", "")
        local fileSize = tonumber(fileSizeString, 8)
        --print("Size: " .. fileSize .. " bytes")

        skip (tarHandle, 12) -- Last Modified
        skip (tarHandle, 8) -- Checksum
        skip (tarHandle, 1) -- Link Indicator
        skip (tarHandle, 100) -- Linked File

        skip (tarHandle, 512 - 257) -- Rest of header (NUL)

        local isFolder = (fileName:sub(fileName:len(), fileName:len()) == "/")
        if isFolder then
            fs.makeDir(fs.combine(folderName, fileName))
        else
            local fHandle = fs.open(fs.combine(folderName, fileName), "wb")
            for i = 1, fileSize do
                --write(string.char(tarHandle.read()))
                fHandle.write(tarHandle.read())
            end
            fHandle.close()
        end
        skip (tarHandle, (numChunks(fileSize) * 512) - fileSize);
    end
end

local tArgs = {...}

local tarHandle = fs.open(fs.combine(shell.dir(), tArgs[1]), "rb")
--local folderName = fs.combine(shell.dir(), tArgs[2])
local folderName = shell.dir()
fs.makeDir(folderName)
print("Extracting...")
readTar(tarHandle, folderName)
print("Finished!")