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diff --git a/docs/LAR.md b/docs/LAR.md new file mode 100644 index 000000000..457b076f9 --- /dev/null +++ b/docs/LAR.md @@ -0,0 +1,87 @@ +LAR is a simple archive format to pack multiple lua source files and arbitrary other resources into a single file. + + +# Format Specification + +A LAR archive file is divided into two parts: the payload and the index lookup table. +All segments of the archive are 4 Byte aligned to ease reading and processing of the format. +All integers are stored in network byte order, so an implementation has to use htonl() and htons() to properly read them. + +Schema: + + <payload: + <member: + <N*4 bytes: path of file #1> + <N*4 bytes: data of file #1> + > + + <member: + <N*4 bytes: path of file #2> + <N*4 bytes: data of file #2> + > + + ... + + <member: + <N*4 bytes: path of file #N> + <N*4 bytes: data of file #N> + > + > + + <index table: + <entry: + <uint32: offset for path of file #1> <uint32: length for path of file #1> + <uint32: offset for data of file #1> <uint32: length for data of file #1> + <uint16: type of file #1> <uint16: flags of file #1> + > + + <entry: + <uint32: offset for path of file #2> <uint32: length for path of file #2> + <uint32: offset for data of file #2> <uint32: length for data of file #2> + <uint16: type of file #2> <uint16: flags of file #2> + > + + ... + + <entry: + <uint32: offset for path of file #N> <uint32: length for path of file #N> + <uint32: offset for data of file #N> <uint32: length for data of file #N> + <uint16: type of file #N> <uint16: flags of file #N> + > + > + + <uint32: offset for begin of index table> + + + +# Processing + +In order to process an LAR archive, an implementation would have to do the following steps: + +## Read Index + +1. Locate and open the archive file +1. Seek to end of file - 4 bytes +1. Read 32bit index offset and swap from network to native byte order +1. Seek to index offset, calculate index length: filesize - index offset - 4 +1. Initialize a linked list for index table entries +1. Read each index entry until the index length is reached, read and byteswap 4 * 32bit int and 2 * 16bit int +1. Seek to begin of file + +## Read Member + +1. Read the archive index +1. Iterate through the linked index list, perform the following steps for each entry +1. Seek to the specified file path offset +1. Read as much bytes as specified in the file path length into a buffer +1. Compare the contents of the buffer against the path of the searched member +1. If buffer and searched path are equal, seek to the specified file data offset +1. Read data until the file data length is reached, return +1. Select the next index table entry and repeat from step 3, if there is no next entry then return + +# Reference implementation + +A reference implementation can be found here: +http://luci.subsignal.org/trac/browser/luci/trunk/contrib/lar + +The lar.pl script is a simple packer for LAR archives and cli.c provides a utility to list and dump packed LAR archives. |