LZ4 Decompressor for Ada


This repository provides an Ada implementation of an LZ4 Decompressor (cf. https://lz4.github.io/lz4/ for general information on LZ4).

This implementation only supports decompression!

Performance is slower compared to reference and C implementations: On my system, the library decompresses slower than 1100 MiB/s. The Debian-supplied unlz4 program for instance, achieves over 3000 MiB/s. See section Performance.


This library is available under the Expat aka. MIT License. See LICENSE.txt or lz4ada.ads for details.


The following dependencies are required for building:



Run Tests

Running the test requires the following standard tools: bc, time


Note: When running tests like the supplied test suite directly from the console (rather than using the script), ulimit -s 60000 to increase the stack size!


It is advisable to generate a package by means of ant package. Alternatively, install the library directly using the following commands (or similar depending on your OS):

install -DsT lib/liblz4ada.so /usr/local/lib/x86_64-linux-gnu
install -m 644 -DT lib/lz4ada.ali /usr/local/lib/x86_64-linux-gnu/ada/adalib/lz4
install -m 644 -DT lib/lz4ada.ads /usr/local/share/ada/adainclude/lz4

The following instructions assume that files are below /usr/lib rather than /usr/local/lib. If you use the commands above, adapt accordingly.

Repository Structure

This repository contains multiple subdirectories for the various components of the library.

   ├── lib/
   │    │
   │    ├── lz4ada.adb                *** This is the implementation. ***
   │    │
   │    ├── lz4ada.ads                *** Implementation header file. ***
   │    │
   │    └── build.xml                 Build instructions
   ├── test_suite/                    Test Suite for LZ4 Library
   │    │
   │    ├── lz4test.adb               Test Suite Implementation
   │    │
   │    └── build.xml                 Build, run and coverage instructions
   ├── test_vectors_lz4/              Sample Data for Testing
   │    │
   │    ├── ....bin                   Original data
   │    │
   │    ├── ....lz4                   Compressee data
   │    │
   │    ├── ....err                   Invalid (manipulated) LZ4 data
   │    │
   │    └── ....eds                   Expected error messages for .err files
   ├── tool_unlz4ada/
   │    │
   │    └── unlz4ada.adb              Example of explicitly handling frames.
   ├── tool_lz4hdrinfo/
   │    │
   │    └── lz4hdrinfo.adb            Debugging tool to decode LZ4 frame header.
   ├── tool_unlz4ada_simple/
   │    │
   │    └── unlz4ada_simple.adb       Simple usage example for the library API.
   ├── tool_xxhash32ada/
   │    │
   │    └── xxhash32ada.adb           Auxiliary tool to demonstrate computing
   │                                  the XXHash32 Hash function.
   ├── test_benchmark.sh              Script to invoke a minimal benchmark.
   ├── test_run.sh                    Script to test against the test vectors.
   ├── README.md                      This file.
   ├── LICENSE.txt                    Expat license.
   └── build.xml                      Recursive antfile build instructions.

The important subdirectory regarding the library is lib. If you do not need tests or example programs, it is sufficient to compile and use only the files from that directory.

Sample Program

Unfortunately, given the interesting property of decompression that a small input can produce a larger output and given that the library is intended to achieve decent performance, setting up a minimal example is already nontrivial.

The following code unlz4ada_simple.adb demonstrates a fully-working LZ4 decompression using the library. Like the reference implementation, it decompresses any number of concatenated frames. It has a larger memory footprint than the more complex implementation provided in directory tool_unlz4ada.

with Ada.Text_IO;
with Ada.Text_IO.Text_Streams;
with Ada.Streams;
use  Ada.Streams;
with LZ4Ada;

procedure UnLZ4Ada_Simple is
    -- 1.
    Stdin:  constant access Root_Stream_Type'Class :=
    Stdout: constant access Root_Stream_Type'Class :=

    -- 2.
    Buf_In: Stream_Element_Array(0 .. 4095); -- 4k buffer
    Buf_Sz: Stream_Element_Offset;
    Ctx:    LZ4Ada.Decompressor := LZ4Ada.Init(Buf_Sz);

    Last:           Stream_Element_Offset := -1;
    Total_Consumed: Stream_Element_Offset := 0;
    Output_Buffer:  Stream_Element_Array(1 .. Buf_Sz);

    Consumed, Output_First, Output_Last: Stream_Element_Offset;
    -- 3.
        if Total_Consumed > Last then
            Read(Stdin.all, Buf_In, Last);
            exit when Last < 0;
            Total_Consumed := 0;
        end if;
        Ctx.Update(Buf_In(Total_Consumed .. Last), Consumed,
                Output_Buffer, Output_First, Output_Last);
        Write(Stdout.all, Output_Buffer(Output_First .. Output_Last));
        Total_Consumed := Total_Consumed + Consumed;
    end loop;
    -- 4.
    if LZ4Ada."="(Ctx.Is_End_Of_Frame, LZ4Ada.No) then
        raise Constraint_Error with "Input ended mid-frame.";
    end if;
end UnLZ4Ada_Simple;

Here is how the sample program works:

  1. Stdin and Stdout allow accessing the respective streams for binary input/output. Buf_In defines an input buffer with an arbitrary size. In this example, a 4 KiB buffer is allocated.
  2. The example makes use of the library API that allows initialization without supplying any data. This comes at the cost of allocating the buffer large enough to process the largest LZ4 blocks which means that two 8 MiB buffers are needed: One inside the library (as input buffer) and one external as output buffer.
  3. The data can now be processed in a loop:
    • If all buffered input data has been processed already (Total_Consumed > Last and initially) then new data is read from Stdin.
    • Ctx.Update is invoked with the current chunk of input data to process.
    • Write is called to output any data decompressed by the Update call.
  4. After processing, condition Ctx.Is_End_Of_Frame = No is checked because: If this is not the end of frame, processing has ended mid-frame and the output is most likely to be incomplete. An alternative exception that could be raised here insted of Constraint_Error is Data_Corruption as supplied by the library. The variant shown has the advantage of not needing to use any of the custom library types/exceptions except for the Decompressor itself.

Using the installed Library

Assuming the library is already installed on your system, you can compile and run the sample program from subdirectory tool_unlz4ada as follows:

gnatmake -o unlz4ada unlz4ada.adb \
    -aO/usr/lib/x86_64-linux-gnu/ada/adalib/lz4 \
    -aI/usr/share/ada/adainclude/lz4 \
    -largs -llz4ada
ulimit -s 60000
./unlz4ada < ../test_vectors_lz4/z1.lz4 | xxd

Output: 00000000: 00 .

Using the Library without Installation

If the library is not installed on your system, it can be integrated using multiple different approaches.

Easy Vendoring

The quickest way to get started is to just include the lz4ada.ads and lz4ada.adb files into the source tree.

cp ../lib/lz4ada.ad? .

Here is what the directory structure may look like then:

   ├── tool_unlz4ada/
   │    └── lz4ada.adb
   │    └── unlz4ada.adb
   │    └── lz4ada.ads

Compilation and invocation then become trivial:

gnatmake -o unlz4ada unlz4ada.adb
ulimit -s 60000
./unlz4ada < ../test_vectors_lz4/z1.lz4 | xxd

Output: 00000000: 00 .

Inclusion from different directory

It may not be suitable to just copy-over the files. In this case, it is also possible to import the compiled library from a different directory. Assume that the library is compiled but not installed, then the file structure may look as follows:

   ├── lib/
   │    ├── lz4ada.adb
   │    ├── lz4ada.ads
   │    ├── lz4ada.ali
   │    ├── build.xml
   │    └── liblz4ada.so
   ├── tool_unlz4ada/
   │    ├── unlz4ada.adb
   │    └── build.xml

Compilation and invocation then have to account for the library not being installed as follows:

gnatmake -o unlz4ada unlz4ada.adb -aO../lib -aI../lib -largs -llz4ada
ulimit -s 60000
LD_LIBRARY_PATH=$PWD/../lib ./unlz4ada < ../test_vectors_lz4/z1.lz4 | xxd

Decompression API

This section describes the decompression API provided by this library. There is also an API to make use of the XXHash32 checksum directly, see XXHash32 API further down. Some of the data types are shared among both of the APIs and only described here.


subtype U8  is Interfaces.Unsigned_8;
subtype U32 is Interfaces.Unsigned_32;
subtype U64 is Interfaces.Unsigned_64;
type Octets is array (Integer range <>) of U8;
type End_Of_Frame is (Yes, No, Maybe);
type Flexible_Memory_Reservation is (SZ_64_KiB, SZ_256_KiB, SZ_1_MiB,
                SZ_4_MiB, SZ_8_MiB, Use_First, Single_Frame);
subtype Memory_Reservation is Flexible_Memory_Reservation range
                            SZ_64_KiB .. SZ_8_MiB;
For_Modern: constant Memory_Reservation := SZ_4_MiB;
For_All:    constant Memory_Reservation := SZ_8_MiB;
type End_Of_Frame is (Yes, No, Maybe);
type Decompressor(In_Last: Integer) is tagged limited private;

U8, U32, Octets, Decompressor


A memory reservation is used to limit how much stack space the library allocates for processing data. On “large” machines and to be able to process all kinds of LZ4 frames the default setting of 8 MiB (For_All constant) may be a good choice.

If you are hitting memory limit issues but want to keep things simple, the For_Modern constant corresponding to 4 MiB buffers already halves the amount of memory needed only by sacrificing compatibility with legacy frames.

For cases where strict limits are needed, any of the SZ_ enum values can be used as memory reservation. If input data is too large to be processed using the allocated buffers, exception Too_Little_Memory is raised to clearly indicate this to the API user.


While a constant value memory limit like SZ_64_KiB may be a good choice for some (larger) embedded systems, there are also cases where the memory allocation should follow the size of the input data. For these cases, a Flexible_Memory_Reservation can be used in conjunction with API function Init_With_Header. The following flexible memory reservations are available:

As Memory_Reservation is a subtype of Flexible_Memory_Reservation, any constant Memory_Reservation can also be passed to Init_With_Header.


This type represents the state of processing. It is a tristate value Yes/No/Maybe with the following meaning:


The following UML-like diagram shows an overview about the exceptions provided by this library:

                   │ (cannot process the given data) │
                      │                            │
          ┌───────────┴─────┐                 ┌────┴────────────────────┐
          │ Data_Corruption │                 │ (library usage related) │
          └──△───────────△──┘                 └────△───────────────△────┘
             │           │                         │               │
┌────────────┴───┐ ┌─────┴─────────┐ ┌─────────────┴────────┐ ┌────┴──────────────┐
│ Checksum_Error │ │ Not_Supported │ │ Too_Few_Header_Bytes │ │ Too_Little_Memory │
└────────────────┘ └───────────────┘ └──────────────────────┘ └───────────────────┘

The labels in parentheses serve as additional information and do not correspond to exceptions in the library.

These exceptions occur because certain restrictions (e.g. Memory_Reservation) are passed to the API in a way that contradicts what the actual supplied data requires. If an application declares reservations in a way that they “should” be OK, then any of these exceptions can be treated like a Data_Corruption or Not_Supported.

API Rationale

This section describes some of the thoughts behind the API design. They may help API users understand the overall idea behind the API better and are less focused on the API usage.

Min_Buffer_Size Requirement

The minimum output buffer size is at least a single block size. If the output buffer were possible to be chosen even smaller, internal computation would be much more complicated since it would be necessary to pause and resume output mid-buffer. Allowing the routines to assume that there is enough space for at least one block, makes the handling less complicated without impacting performance.

About Buffer and Num_Consumed

A small input may lead to a large output. To avoid using unbounded memory amounts one must limit the output buffer size. The library implementation ensures this by passing a target buffer as an in out parameter rather than a return value. A supplied input’s decompressed size may exceed the output buffer capacity. In order to allow output for the remainder of the input to be generated, it may become necessary to supply part of the same input again. This is achieved by signalling the number of consumed bytes back to the caller as Num_Consumed.

In addition to the output data block, the buffer also contains history information that is used for “backreferences” during decompressing. Storing the history information in the same buffer requires it to be an in out parameter that must not be changed between invocations of the Update procedure. While this slightly undercuts encapsulation, it can have a notable performance impact: As all data from the history has been an output at some time, it makes sense to try storing it only once i.e. as output without keeping a separate copy as “history”. As a result, fewer copy operations are necessary, yielding better overall performance.

No Final

There is no need for a “Final” procedure: For non-legacy frames, LZ4 clearly indicates when processing has reached the end of the frame. For legacy frames, the library reports the end of block as Maybe end of frame.

This makes Final sort of an optional check to see if the end of input data correlates whith LZ4’s understanding of the end of frame. The recommended way to perform this check in applications that include support for legacy frames is as follows:

if LZ4Ada."="(Ctx.Is_End_Of_Frame, LZ4Ada.No) then
    raise Constraint_Error with "Input ended mid-frame.";
end if;

As an alternative to Constraint_Error, other exception types might be suitable, e.g. the library-supplied Data_Corruption is also a sensible choice here.

Multi-Frame Handling

The library supports a “simple” usage where it internally detects multiple concatenated frames and processes them in sequence. This is the behaviour of the commandline unlz4 command and probably well-suited for a wide range of LZ4 decompression problems.

As LZ4 frames can contain additional information in “skipppable frames” it also makes sense to provide a manes for the application to detect the end of frames and probably process some special variants by itself. To enable this, the Init_With_Header API can be supplied a memory reservation Single_Frame to enable single-frame mode of operation. Then, only one LZ4 frame is processed using the context and data beyond that can be processed differently by the API user or a new context can be allocated for the next frame. This mode can also be used to limit the memory allocation to what is needed for the specific, currently processed frame.

As the use of this API is more complicated (compare the example unde tool_unlz4ada), the more conveniently usable Init (without header) API is provided, too.

Functions and Procedures

A detailed description of the API functions follows after the overview excerpt from lz4ada.ads.

function Init(Min_Buffer_Size:   out    Stream_Element_Offset;
        Reservation:     in     Memory_Reservation := For_All)
        return Decompressor;
function Init(Min_Buffer_Size:   out    Integer;
        Reservation:     in     Memory_Reservation := For_All)
        return Decompressor;

function Init_With_Header(Input: in     Octets;
        Num_Consumed:    out    Integer;
        Min_Buffer_Size: out    Integer;
        Reservation:     in     Flexible_Memory_Reservation
                            := Single_Frame)
        return Decompressor with Pre => Input'Length >= 7;

procedure Update(Ctx:            in out Decompressor;
        Input:           in     Stream_Element_Array;
        Num_Consumed:    out    Stream_Element_Offset;
        Buffer:          in out Stream_Element_Array;
        Output_First:    out    Stream_Element_Offset;
        Output_Last:     out    Stream_Element_Offset);
procedure Update(Ctx:            in out Decompressor;
        Input:           in     Octets;
        Num_Consumed:    out    Integer;
        Buffer:          in out Octets;
        Output_First:    out    Integer;
        Output_Last:     out    Integer)
        with Pre => (Buffer'First = 0);

function Is_End_Of_Frame(Ctx: in Decompressor) return End_Of_Frame;

function Init(Min_Buffer_Size: out; Reservation: in) return Decompressor

This function initializes a new Decompressor without having to provide initial header.

This Decompressor accepts multiple concatenated frames in sequence such as long as they fit the Memory_Reservation. For the meaning of the different memory reservations, check the documentation for the Memory_Reservation data type above.

This is a convenient API for cases where either memory consumption does not matter much (e. g. on Desktop OSes) or where the upper bound for the maximum block size that is going to be used is known in advance.

This function can be called with either Integer as number type or Stream_Element_Offset to directly allow passing the Min_Buffer_Size to an array declaration.

function Init_With_Header(Input: in; Num_Consumed: out; Min_Buffer_Size: out; Reservation: in)

This function creates an LZ4 decompression context. It requires the begin of the frame to be supplied as Input and returns the decompression context.

Additionally, it outputs as Num_Consumed how many of the supplied bytes it processed and as Min_Buffer_Size the suggested buffer size to be used for Update calls. The bytes consumed by Init must not be sent to subsequent Update calls, i. e. if there is still some data left in the input buffer, then the first Update call is expected to take Input(Num_Consumed .. Input'Last) rather than Input directly.

This function can only be called using Octets as data type and Integer as number type because it is a low-level API that is expected to be only needed in rare cases.

If too little input data is supplied to process the entire header, exception Too_Few_Header_Bytes is raised.

procedure Update(Ctx: in out; Input: in, Num_Consumed: out, Buffer: in out; Output_First: out; Output_Last: out)

This procedure can be called to decompress data.

Ctx and Buffer together form the context that is expected to be provided each time data from the same LZ4 stream is to be decompressed.

Input must always point to previously unprocessed data. Check the value of Num_Consumed after each invocation to find out how many of the input bytes must be skipped (e. g. by using slice notation) when invoking Update again on the same Input buffer.

Buffer must not be modified between invocations of Update since it is used to hold “history” information about previously produced output that is integral to the decompression process.

Output_First marks the index of the first octet of the decompressed data in the Buffer (inclusive).

Output_Last marks the index of the last octet of the decompressed data in the output buffer (inclusive).

If no output was generated, then Output_Last is smaller than Output_First.

The procedure exists in two variants: One with Octets and Integer and one with Stream_Element_Array and Stream_Element_Offset types for ease of integration with the Standard Stream APIs.

function Is_End_Of_Frame(Ctx: in) return End_Of_Frame;

This function returns the “end-of-frame” state of the decompressor. See the End_Of_Frame type’s description for the meanings of the values.

Applications are expected to check this value, but depending on implementation and intended formats to support, this check can happen at different times:

XXHash32 API

The subpackage LZ4Ada.XXHash32 provides access to the XXHash32 hash function which is used internally by LZ4 but might be interesting to be used in other contexts, too. Since I expect such use cases to be niche, no API using the Stream_Element_Array types is provided here.

package XXHash32 is
    type Hasher is tagged limited private;
    function  Hash(Input: in Octets) return U32;
    function  Init(Seed: in U32 := 0) return Hasher;
    procedure Reset(Ctx: in out Hasher; Seed: in U32 := 0);
    procedure Update(Ctx: in out Hasher; Input: in Octets);
    function  Final(Ctx: in Hasher) return U32;
    -- ...
end XXHash32;

Type Hasher represents the internal computation context of the hash function, suitable for computing a single XXHash32. There is currently no API to reset the hasher after use, hence API users are suggested to create new Hasher contexts whenever they need to compute the hash of different data.

Functions and Procedures

The hashing API functions closely resemble the standard pattern of Init/Update/Final with an additional convenience function that can be used to perform the entire computation in a single step:

function Hash(Input: in) return U32;

As a convenient means to compute the hash over input data without having to call any of the Init/Update/Final routines, function Hash can be used to compute the hash over the given Input data in a single step.

In case the input data is large, it might be better to design using the Init/Update/Final set of functions since those allow processing arbitrarily long data whereas Hash expects all of the input to be present in memory at once.

function Init(Seed: in) return Hasher;

This function creates and returns a Hasher instance using the supplied seed value (default: 0).

procedure Reset(Ctx: in out; Seed: in);

Resets a Hasher to the state just like an Init. This allows re-creating a hasher despite it being a limited record.

procedure Update(Ctx: in out; Input: in);

Use this procedure to supply the data that is considered input into the hashing function.

function Final(Ctx: in) return U32;

This function outputs the 32-bit hash corresponding to the concatenation of all data supplied with Update to the provided Hasher context and returns it,

It is possible to call Update on the same context again afterwards to append input data and then invoke Final again to obtain the hash of the concatenation of all preceding plus the newly added input data.


On my system (Intel Xeon W-2295, inside a test VM), the following decompression speeds are observed when running the test_benchmark.sh script:

$ ./test_benchmark.sh -h
benchmark zeroes
Benchmark 1: ./tool_unlz4ada/unlz4ada < /tmp/zeroes.lz4
  Time (mean ± σ):     978.1 ms ±  37.7 ms    [User: 971.0 ms, System: 6.7 ms]
  Range (min … max):   939.5 ms … 1143.0 ms    50 runs

benchmark reference zeroes
Benchmark 1: unlz4 < /tmp/zeroes.lz4
  Time (mean ± σ):     739.3 ms ±  23.9 ms    [User: 723.5 ms, System: 15.5 ms]
  Range (min … max):   696.5 ms … 799.2 ms    50 runs
benchmark random
Benchmark 1: ./tool_unlz4ada/unlz4ada < /tmp/random.lz4
  Time (mean ± σ):      1.847 s ±  0.038 s    [User: 1.505 s, System: 0.342 s]
  Range (min … max):    1.788 s …  1.961 s    50 runs
benchmark reference random
Benchmark 1: unlz4 < /tmp/random.lz4
  Time (mean ± σ):     649.2 ms ±  19.9 ms    [User: 378.6 ms, System: 270.2 ms]
  Range (min … max):   616.1 ms … 701.8 ms    50 runs
benchmark text
Benchmark 1: ./tool_unlz4ada/unlz4ada < /tmp/text.lz4
  Time (mean ± σ):      1.863 s ±  0.043 s    [User: 1.523 s, System: 0.339 s]
  Range (min … max):    1.791 s …  1.994 s    50 runs
benchmark reference text
Benchmark 1: unlz4 < /tmp/text.lz4
  Time (mean ± σ):     644.6 ms ±  20.2 ms    [User: 372.1 ms, System: 272.2 ms]
  Range (min … max):   604.5 ms … 689.2 ms    50 runs

$ ./test_benchmark.sh
benchmark zeroes
0+32768 records in
0+32768 records out
2147483648 bytes (2.1 GB, 2.0 GiB) copied, 1.4438 s, 1.5 GB/s

benchmark reference zeroes
0+32897 records in
0+32897 records out
2147483648 bytes (2.1 GB, 2.0 GiB) copied, 1.52586 s, 1.4 GB/s

benchmark random
0+32768 records in
0+32768 records out
2147483648 bytes (2.1 GB, 2.0 GiB) copied, 3.07091 s, 699 MB/s

benchmark reference random
0+65303 records in
0+65303 records out
2147483648 bytes (2.1 GB, 2.0 GiB) copied, 1.02479 s, 2.1 GB/s

benchmark text
0+32768 records in
0+32768 records out
2147483648 bytes (2.1 GB, 2.0 GiB) copied, 3.20152 s, 671 MB/s

benchmark reference text
0+33049 records in
0+33049 records out
2147483648 bytes (2.1 GB, 2.0 GiB) copied, 0.973975 s, 2.2 GB/s

Computing the speed from the measures yields the following results:

Case Ref/Ada DD [MiB/s] Avg [MiB/s] Min/6σ [MiB/s] Max/6σ [MiB/s] PercOfRef [%]
Zero Ada 1418 2094 1701 2723 76
Zero Ref. 1342 2770 2320 3437 106
Random Ada 667 1109 987 1265 35
Random Ref. 1998 3155 2665 3866 33
Text Ada 639 1099 966 1276 30
Text Ref. 2103 3177 2674 3913 35

Raw Computation

“Zero” Row

“Random” Row

“Text” Row

Short Summary: The Ada implementation seems to attain about one third of the speed of the Debian-supplied unlz4 command. In absolute figures this is still around 1000 MiB/s (for the bad cases) which can be expected to be enough for plenty of use cases. Measuring with dd consistently yields smaller throughputs compared to the hyperfine approach. This could be explained by the fact that dd needs to perform a copy whereas hyperfine just discards the extracted output right away.

Performance vs. Safety – Use of pragma Suppress

During optimization, some areas in the library that are performance crtical turned out to be hugely slowed down by compiler-generated length and overflow checks. In order to balance safety and performance, some checks are currently disabled for the Write_Output procedure in the library. All other checks were left in place.

If you need maximum safety even accepting strong performance penalties, feel free to comment-out the following pragma directives in procedure Write_Output in file lz4ada.adb:

pragma Suppress(Length_Check);
pragma Suppress(Overflow_Check);
pragma Suppress(Index_Check);
pragma Suppress(Range_Check);

Rationale and Usage Recommendation

This library was created out of the need to process data from a Rust program that encodes data in LZ4. Of course, if decoding is enough for your purposes, you may as well use this library in new designs. I encourage you to consider alternatively choosing a more widely supported compression format such as e.g. LZMA. This way, better-maintained implementations and also the compression side might be available in Ada.

Interesting Links to alternative Compression/Decompression implementations in Ada:


Feel free to send patches with bugfixes or missing functionality directly to . Include a note to confirm that you are OK with these patches being included under Expat license and add your preferred copyright line to the patch or e-mail.

Please note that API breaks are only accepted if very strong reasons exist to motivate them.

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Created: 2022/12/14 20:49:20 | Tags: lz4, decompress, ada, library | Version: 1.0.0 | SRC (Pandoc MD) | GPL

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