forked from enlightenment/efl
185 lines
7.4 KiB
C
185 lines
7.4 KiB
C
/**
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@file
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@brief Eet Data Handling Library Public API Calls
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These routines are used for Eet Library interaction
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*/
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/**
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@mainpage Eet Library Documentation
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@image html eet.png
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@version 0.9.0
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@author Carsten Haitzler <raster@rasterman.com>
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@date 2000-2002
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@section intro What is Eet?
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It is a tiny library designed to write an arbitary set of chunks of data
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to a file and optionally compress each chunk (very much like a zip file)
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and allow fast random-access reading of the file later on. It does not
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do zip as a zip itself has more complexity than is needed, and it was much
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simpler to impliment this once here.
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Eet is extremely fast, small and simple. Eet files can be very small and
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highly compressed, making them very optimal for just sending across the
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internet without having to archive, compress or decompress and install them.
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They allow for lightning-fast random-acess reads once created, making them
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perfect for storing data that is written once (or rarely) and read many
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times, but the program does not want to have to read it all in at once.
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It also can encode and decode data structures in memory, as well as image
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data for saving to Eet files or sending across the network to other
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machines, or just writing to arbitary files on the system. All data is
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encoded in a platform independant way and can be written and read by any
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architecture.
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@section example A simple example on using Eet
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Here is a simple example on how to use Eet to save a series of strings to a
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file and load them again. The advantage of using Eet over just fprintf() and
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fscanf() is that not only can these entries be strings, they need no special
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parsing to handle delimiter characters or escaping, they can be binary data,
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image data, data structures containing integers, strings, other data
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structures, linked lists and much more, without the programmer having to
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worry about parsing, and best of all, Eet is very fast.
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@code
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#include <Eet.h>
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int
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main(int argc, char **argv)
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{
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Eet_File *ef;
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int i;
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char buf[32];
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char *ret;
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int size;
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char **entries =
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{
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"Entry 1",
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"Big text string here compared to others",
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"Eet is cool"
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};
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// blindly open an file for output and write strings with their NUL char
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ef = eet_open("test.eet", EET_FILE_MODE_WRITE);
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eet_write(ef, "Entry 1", entries[0], strlen(entries[0]) + 1, 0);
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eet_write(ef, "Entry 2", entries[1], strlen(entries[1]) + 1, 1);
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eet_write(ef, "Entry 3", entries[2], strlen(entries[2]) + 1, 0);
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eet_close(ef);
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// open the file again and blindly get the entries we wrote
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ef = eet_open("test.eet", EET_FILE_MODE_READ);
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ret = eet_read(ef, "Entry 1", &size);
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printf("%s\n", ret);
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ret = eet_read(ef, "Entry 2", &size);
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printf("%s\n", ret);
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ret = eet_read(ef, "Entry 3", &size);
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printf("%s\n", ret);
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eet_close(ef);
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}
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@endcode
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@section format What does an Eet file look like?
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The file format is very simple. There is a directory block at the start of
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the file listing entries and offsets into the file where they are stored,
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their sizes, compression flags etc. followed by all the entry data strung one
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element after the other.
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All Eet files start with t a 4 byte magic number. It is written using network
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byte-order (big endian, or from most significant byte first to least
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significant byte last) and is 0x1ee7ff00 (or byte by byte 0:1e 1:e7 2:ff
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3:00). The next 4 bytes are an integer (in big endian notation) indicating
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how many entries are stored in the Eet file. 0 indicates it is empty. This is
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a signed integer and thus values less than 0 are invalid, limiting the number
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of entries in an Eet file to 0x7fffffff entries at most. The next 4 bytes is
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the size of the directory table, in bytes, encoded in big-endian format. This
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is a signed integer and cannot be less than 0.
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The directory table for the file follows immediately, with a continuous list
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of all entries in the Eet file, their offset in the file etc. The order of
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these entries is not important, but convention would have them be from first
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to last entry in the file. Each directory entry consiste of 5 integers, one
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after the other, each stored as a signed, big endian integer. The first is
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the offset in the file that the data for this entry is stored at (based from
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the very start of the file, not relative to the end of the directory block).
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The second integer holds flags for the entry. currently only the least
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significant bit (bit 0) holds any useful information, and it is set to 1 if
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the entry is compressed using zlib compression calls, or 0 if it is not
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compressed. The next integer is the size of the entry in bytes stored in the
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file. The next integer is the size of the data when decompressed (if it was
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compressed) in bytes. This may be the same as the previous integer if the
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entry was not compressed. The final integer is the number of bytes used by
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the string identifier for the entry, without the NUL byte terminator, which
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is not stored. The next series of bytes is the string name of the entry, with
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the number of bytes being the same as specified in the last integer above.
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This list of entries continues until there are no more entries left to list.
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To read an entry from an Eet file, simply find the appropriate entry in the
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directory table, find it's offset and size, and read it into memory. If it is
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compressed, decompress it using zlib and then use that data.
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Here is a data map of an Eet file. All integers are encoded using big-endian
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notation (most significant byte first) and are signed. There is no alignment
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of data, so all data types follow immediately on, one after the other. All
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compressed data is compressed using the zlib compress2() function, and
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decompressed using the zlib uncompress() function. Please see zlib
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documentation for more information as to the encoding of compressed data.
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@verbatim
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HEADER:
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[INT] Magic number (0x1ee7ff00)
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[INT] Number of entries in the directory table
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[INT] The size of the directory table, in bytes
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DIRECTORY TABLE ENTRIES (as many as specified in the header):
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[INT] Offest from file start at which entry is stored (in bytes)
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[INT] Entry flags (1 = compressed, 0 = not compressed)
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[INT] Size of data chunk in file (in bytes)
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[INT] Size of the data chunk once decompressed (or the same as above, if not)
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[INT] The length of the string itendifier, in bytes, without NUL terminator
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[STR] Series of bytes for the string identifier, no NUL terminator
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... more directory entries
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DATA STORED, ONE AFTER ANOTHER:
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[DAT] DATA ENTRY 1...
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[DAT] DATA ENTRY 2...
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[DAT] DATA ENTRY 3...
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... more data chunks
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@endverbatim
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The contents of each entry in an Eet file has no defined format as such. It
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is an opaque chunk of data, that is up to the application to deocde, unless
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it is an image, ecoded by Eet, or a data structure encoded by Eet. The data
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itself for these entries can be encoded and decoded by Eet with extra helper
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functions in Eet. eet_data_image_read() and eet_data_image_write() are used
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to handle reading and writing image data from a known Eet file entry name.
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eet_data_read() and eet_data_write() are used to decode and encode program
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data structures from an Eet file, making the loading and saving of program
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information stored in data structures a simple 1 function call process.
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Please see src/lib/eet_data.c for information on the format of these
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specially encoded data entries in an Eet file (for now).
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@todo Add hash table, fixed and variable array encode/decode support.
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@todo Document data format for images and data structures.
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*/
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