NBT API

-- NBT API
-- by MysticT

local function readShort(file)
    local r = file.read() * 256 + file.read()
    if r >= 32768 then
        r = r - 65536
    end
    return r
end

local function readInt(file)
    local r = 0
    for i = 1, 4 do
        r = r * 256 + file.read()
    end
    if r >= 2147483648 then
        r = r - 4294967296
    end
    return r
end

local function readLong(file)
    local r = 0
    for i = 1, 8 do
        r = r * 256 + file.read()
    end
    if r >= 9223372036854775808 then
        r = r - 18446744073709551616
    end
    return r
end

local function readFloat(file)
    for i = 1, 4 do
        file.read()
    end
    return 0
end

local function readDouble(file)
    for i = 1, 4 do
        file.read()
    end
    return 0
end

local function readByteArray(file)
    local size = readInt(file)
    local array = {}
    for i = 1, size do
        table.insert(array, file.read())
    end
    return array
end

local function readString(file)
    local len = readShort(file)
    local s = ""
    for i = 1, len do
        local c = file.read()
        if not c then
            break
        end
        s = s..string.char(c)
    end
    return s
end

local function readIntArray(file)
    local size = readInt(file)
    local array = {}
    for i = 1, size do
        table.insert(array, readInt(file))
    end
    return array
end

local function readTags(file)
    local function readPayload(id)
        if id == 1 then
            -- TAG_Byte
            return file.read()
        elseif id == 2 then
            -- TAG_Short
            return readShort(file)
        elseif id == 3 then
            -- TAG_Int
            return readInt(file)
        elseif id == 4 then
            -- TAG_Long
            return readLong(file)
        elseif id == 5 then
            -- TAG_Float
            return readFloat(file)
        elseif id == 6 then
            -- TAG_Double
            return readDouble(file)
        elseif id == 7 then
            -- TAG_Byte_Array
            return readByteArray(file)
        elseif id == 8 then
            -- TAG_String
            return readString(file)
        elseif id == 9 then
            -- TAG_List
            local list = {}
            local id = file.read()
            local size = readInt(file)
            for i = 1, size do
                table.insert(list, readPayload(id))
            end
            return list
        elseif id == 10 then
            -- TAG_Compound
            return readTags(file)
        elseif id == 11 then
            -- TAG_Int_Array
            return readIntArray(file)
        end
    end
    local tags = {}
    while true do
        local tag = {}
        tag.id = file.read()
        if not tag.id then
            break
        end
        if tag.id == 0 then
            -- TAG_End
            tag.name = ""
            break
        elseif tag.id > 11 then
            return nil, "Unknown tag id: "..tostring(tag.id)
        end
        tag.name = readString(file)
        tag.payload = readPayload(tag.id)
        table.insert(tags, tag)
    end
    return tags
end

-- API functions

function readNBTFile(sPath)
    local file = fs.open(sPath, "rb")
    if file then
        local tags, err = readTags(file)
        file.close()
        return tags, err
    end
    return nil, "Error opening file "..sPath
end