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FreeArc/Standard API for compression libraries

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Discussion: at encode.ru forum
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the main reason of freearc success is its use of leading compression algorithms. but not every great algorithm is open-source that forces advanced users to rely on "external compressors" feature, that isn't super-handy

external programs has advantage of being absolutely independent of me. everyone can develop compressor that will be usable standalone and at the same time easily integrated with FA while adding new algorithms to FA needs co-operation with me. now i think that by providing the same level of independence for compressors developed as dll we can make things better

so that i propose: standard API for compression dlls. once you have dll developed according to this API, you can just drop it to the FreeArc folder (or any other program supporting this standard) and immediately use it for compression and decompression. moreover, it will be possible to download-on-demand dlls required to decompress your archive just like now it's done in meda players


my proposal is based on experience of approving various algorithms for FA. it's highly flexible to allow further extensions w/o losing backward compatibility, at the same time i tried to simplify basic operations

1) library should be provided in dll with name cls-*.dll: it makes smpler to find all compatible libs in the large directory

2) the only function that should be exported is

int ClsMain(CALLBACK* cb, void* instance)

where

typedef int CALLBACK(char *what, void* instance, void *ptr, int n)

3) whole interaction with caller implemented via callbacks. string `what` describes operation what we ask to perform, instane allows to pass instantiation-specific parameters (important for multithreadung environments), while ptr and n are used to pass operation parameters. Operations requiring more params can use ptr as pointer to structure

4) the minimum set of operations, that should be supported, consists of:

cb("action", instance, buf, len) - puts "compress" or "decompress" in buf. required to determine what operation ClsMain should perform

cb("read", instance, buf, len) - allows to read input data into buf. returns >0 - amount of data read =0 - EOF <0 - errorcode

cb("write", instance, buf, len) - the same for writing data

compression methods supporting multiple output streams (such as bcj2) may add stream number to read or write: cb("write0", instance, buf, len) cb("write1", instance, buf, len) ...

the following action may be used to determine compression parameters: cb("parameters", instance, buf, len) - puts string representing compression parameters into buf


that's all for beginning. one interesting idea may be implemenatation of code that turns such ClsMain into standalone compressor. i.e. some standard shell with all those file/error/crc/cmdline mangling so that developer can focus on writing just compression code itself. this code may interact either with dlls or statically link with ClsMain-style library


Simplest codec:

#include "cls.h"

int ClsMain (int op, CLS_CALLBACK cb, void* instance)
{
    switch(op)
    {
    case CLS_COMPRESS:
    case CLS_DECOMPRESS:
        {   
            const int BUFSIZE = 4096;
            char buf[BUFSIZE];
            for (int len; (len=cb(instance, CLS_READ, buf, BUFSIZE)) != 0; )
            {
                if (len<0)  return len;  // Return errcode on error
                int ret = cb(instance, CLS_WRITE, buf, len);
                if (ret!=len)  return ret<0? ret : CLS_ERROR_WRITE;
            }
            return CLS_OK;
        }

    default:
        return CLS_ERROR_NOT_IMPLEMENTED;
    }
}


Minimal host:

#include <stdlib.h>
#include <io.h>

#include "cls.h"

int cb(void* instance, int op, void *ptr, int n)
{
    switch(op)
    {
    case CLS_READ:
        return read(0,ptr,n);
    case CLS_WRITE:
        return write(1,ptr,n);
    case CLS_MALLOC:
        *(void**)ptr = malloc(n);
        return *(void**)ptr? CLS_OK : CLS_ERROR_NOT_ENOUGH_MEMORY;
    case CLS_FREE:
        free(ptr);
        return CLS_OK;
    default:
        return CLS_ERROR_NOT_IMPLEMENTED;
    }
}

int main ()
{
    extern int ClsMain (int op, CLS_CALLBACK cb, void* instance);

    ClsMain(CLS_INIT, cb, NULL);
    int ret = ClsMain(CLS_COMPRESS, cb, NULL);
    ClsMain(CLS_DONE, cb, NULL);

    return ret;
}