# `zstd`
[🔗](https://github.com/erlang/otp/blob/master/lib/stdlib/src/zstd.erl#L23)

Zstandard compression interface.

This module provides an API for the Zstandard library
([www.zstd.net](http://www.zstd.net)). It is used to compress and decompress data
and offers the same compression ratio as `m:zlib` but at a lower CPU cost.

Example:

```
1> Data = ~"my data to be compressed".
2> Compressed = zstd:compress(Data).
3> zstd:decompress(Compressed).
[~"my data to be compressed"]
```

If you are compressing or decompressing possibly large amounts of data,
it is also possible to do streamed compression/decompression.

Example:

```erlang
1> Compress = fun F(Ctx, D) ->
                      case file:read(D, 5) of
                          {ok, Data} ->
                              {continue, C} = zstd:stream(Ctx, Data),
                              [C|F(Ctx, D)];
                          eof ->
                              {done, C} = zstd:finish(Ctx, ""),
                              C
                      end
              end.
2> {ok, Ctx} = zstd:context(compress).
3> {ok, D} = file:open(File,[read,binary]).
4> Compressed = iolist_to_binary(Compress(Ctx, D)).
<<40,181,47,253,0,88,89,0,0,108,111,114,101,109,32,105,112,115,117,109>>
5> zstd:decompress(Compressed).
[~"lorem ipsum"]
```

In all functions errors can be thrown, where `Reason` describes the error.

Typical `Reason`s:

- **`badarg`** - Bad argument.
- **`zstd_error`** - An error generated by the Zstandard library.
- **`not_on_controlling_process`** - The context was used by a process that
did not create it.

# `compress_parameters`
*since OTP 28.0* 

```erlang
-type compress_parameters() ::
          #{dictionary => binary() | dict(),
            pledgedSrcSize => non_neg_integer(),
            compressionLevel => compressionLevel(),
            windowLog => non_neg_integer(),
            hashLog => non_neg_integer(),
            chainLog => non_neg_integer(),
            searchLog => non_neg_integer(),
            minMatch => non_neg_integer(),
            targetLength => non_neg_integer(),
            strategy => strategy(),
            targetCBlockSize => non_neg_integer(),
            enableLongDistanceMatching => boolean(),
            ldmHashLog => non_neg_integer(),
            ldmMinMatch => non_neg_integer(),
            ldmBucketSizeLog => non_neg_integer(),
            ldmHashRateLog => non_neg_integer(),
            contentSizeFlag => boolean(),
            checksumFlag => boolean(),
            dictIDFlag => boolean()}.
```

Compression parameters.

Zstandard has many parameters that can be tuned. Setting some parameters will
fail when set to an incorrect value, while others will be silently adjusted to
the closest valid value.

* `dictionary` - Sets the compression dictionary for the context. The dictionary
  can be either a `t:binary()` representing the dictionary or a compression `t:dict/0`.
  When a `t:dict/0` is attached to a context it will be kept alive until
  either the context is closed or it is replaced by another dictionary.

  To reset the context to not use any dictionary use the empty dictionary, that is `<<>>`.
* `pledgedSrcSize` - When using `stream/2` to do streaming compression, the decompressed
  size is not known when the header of the Zstandard frame is emitted. Setting this
  parameter on the context lets the compressor know the expected size of the data to
  compress. If the size is not correct when `finish/2` is called an exception will be
  generated. Using [`compress/1,2`](`compress/2`) will automatically set this value.
* `compressionLevel` - Sets the `t:compressionLevel/0`.
* `windowLog` | `hashLog` | `chainLog` | `searchLog` | `minMatch` |
  `targetLength` | `targetCBlockSize` - Set the corresponding parameter.
  See the Zstandard documentation for more details.
* `strategy` - Sets the compression `t:strategy/0`.
* `enableLongDistanceMatching` - Whether to enable Long Distance Matching or not.
  LDM is useful when compressing large datasets and is enabled by using a high
  `compressionLevel`.
* `ldmHashLog` | `ldmMinMatch` | `ldmBucketSizeLog` | `ldmHashRateLog` -
  Set the corresponding LDM parameter.
  See the Zstandard documentation for more details.
* `contentSizeFlag` - Whether to include the contentSize or not.
* `checksumFlag` - Whether to include the checksum or not.
* `dictIDFlag` - Whether to include the dictionary ID or not.

The Zstandard documentation contains more details about each parameter.

# `compressionLevel`
*not exported* *since OTP 28.0* 

```erlang
-type compressionLevel() :: -22..22.
```

The compression level.

Higher values mean better compression ratio at the sacrifice of
performance. A negative value sacrifices compression ratio in
favor of performance.

0 is a special value which represents the default compression level.

# `context`
*since OTP 28.0* 

```erlang
-opaque context()
```

A compression or decompression context that can be used
for streaming compression or decompression.

Only the process that created the context can use it.

# `decompress_parameters`
*since OTP 28.0* 

```erlang
-type decompress_parameters() :: #{dictionary => binary() | dict(), windowLogMax => non_neg_integer()}.
```

Decompression parameters.

Zstandard has many parameters that can be tuned. Setting some parameters will
fail when set to an incorrect value, while others will be silently adjusted to
the closest valid value.

* `dictionary` - Sets the decompression dictionary for the context. The dictionary
  can be either a `t:binary()` representing the dictionary or a decompression `t:dict/0`.
  When a `t:dict/0` is attached to a context it will be kept alive until
  either the context is closed or it is replaced by another dictionary.

To reset the context to not use any dictionary use the empty dictionary, that is `<<>>`.
* `windowLogMax` - Set the corresponding parameter. See the Zstandard documentation for more details.

# `dict`
*since OTP 28.0* 

```erlang
-opaque dict()
```

A compression or decompression dictionary.

# `strategy`
*not exported* *since OTP 28.0* 

```erlang
-type strategy() ::
          default | fast | dfast | greedy | lazy | lazy2 | btlazy2 | btopt | btultra | btultra2.
```

The compression strategy.

The strategies are listed depending on which compression ratio they
give, that is the `fast` strategy is the fastest but also has the
worst compression ratio, while `btultra2` is the slowest but has
the best compression ratio.

`default` is a special strategy representing the current default strategy.

See the Zstandard documentation for details on each strategy.

# `close`
*since OTP 28.0* 

```erlang
-spec close(Ctx :: context()) -> ok.
```

Close a `t:context/0`, releasing all referenced resources. After a `t:context/0`
is closed it is no longer possible to use it.

A `t:context/0` is automatically closed when GC:ed, so the only reason to call
this function is to make the resources attached to the context be released
before the next GC.

# `compress`
*since OTP 28.0* 

```erlang
-spec compress(iodata()) -> iodata().
```

# `compress`
*since OTP 28.0* 

```erlang
-spec compress(Data :: iodata(), Options :: compress_parameters()) -> iodata();
              (Data :: iodata(), Ctx :: context()) -> iodata().
```

Compress `Data` using the given `t:compress_parameters/0` or the `t:context/0`.

Example:

```
1> zstd:compress("abc").
2> zstd:compress("abc", #{ compressionLevel => 20 }).
```

# `context`
*since OTP 28.0* 

```erlang
-spec context(compress | decompress) -> {ok, context()}.
```

# `context`
*since OTP 28.0* 

```erlang
-spec context(compress, Options :: compress_parameters()) -> {ok, context()};
             (decompress, Options :: decompress_parameters()) -> {ok, context()}.
```

Create a compression or decompression context.

A context can be used to do streaming compression/decompression and allows
re-using parameters for multiple compressions/decompressions.

# `decompress`
*since OTP 28.0* 

```erlang
-spec decompress(iodata()) -> iodata().
```

# `decompress`
*since OTP 28.0* 

```erlang
-spec decompress(Data :: iodata(), Options :: decompress_parameters()) -> iodata();
                (Data :: iodata(), Ctx :: context()) -> iodata().
```

Decompress `Data` using the given `t:decompress_parameters/0` or the `t:context/0`.

Example:

```
1> Compressed = zstd:compress("abc").
2> zstd:decompress(Compressed).
[~"abc"]
```

# `dict`
*since OTP 28.0* 

```erlang
-spec dict(Mode :: compress | decompress, Dict :: binary()) -> {ok, dict()}.
```

# `dict`
*since OTP 28.0* 

```erlang
-spec dict(compress, Dict :: binary(), #{compressionLevel => compressionLevel()}) -> {ok, dict()};
          (decompress, Dict :: binary(), #{}) -> {ok, dict()}.
```

Create a compression or decompression dictionary.

A compression dictionary can be used as a `t:compress_parameters/0` to use
a dictionary for compression. Dictionaries allow good compression ratios
even for small amounts of data.

A decompression dictionary can be used as a `t:decompress_parameters/0` to use
a dictionary for decompression. The same dictionary has to be used for
compression as decompression. To verify that the same dictionary is used
you can use `get_dict_id/1` on the dictionary and compressed data, or just
try to decompress as decompression will raise and exception if an incorrect
dictionary is given.

The `compressionLevel` set on a dictionary will override the `compressionLevel`
set in the `t:context/0`.

Example:

```
1> {ok, CDict} = zstd:dict(compress, Dict).
2> Data = lists:duplicate(100, 1).
[1, 1, 1 | _]
3> iolist_size(zstd:compress(Data)).
17
4> iolist_size(zstd:compress(Data, #{ dictionary => CDict, dictIDFlag => false })).
16
```

As loading a dictionary can be a heavy operations, it is possible to create
only a single `t:dict/0` and provide it to multiple `t:context/0`.

There is no API exposed in `m:zstd` to create a dictionary, instead use the
`zstd` command line tool.

# `finish`
*since OTP 28.0* 

```erlang
-spec finish(Ctx :: context(), Data :: iodata()) -> Result when Result :: {done, erlang:iovec()}.
```

Finish compressing/decompressing data.

This flushes all output buffers and resets the `t:context/0` so
that it can be used for compressing/decompressing again.

Example:

```
1> {ok, DCtx} = zstd:context(decompress).
2> {continue, D1} = zstd:stream(DCtx, <<40,181,47,253,32>>).
3> {done, D2} = zstd:finish(DCtx, <<2,17,0,0,97,98>>).
4> iolist_to_binary([D1,D2]).
<<"ab">>
```

# `flush`
*since OTP 29.0* 

```erlang
-spec flush(Ctx :: context()) -> Result when Result :: {continue, Output :: binary()}.
```

Flush the compression stream.

This flushes all pending compressed data without ending the frame,
allowing the compressed data to be read immediately while keeping
the context open for further compression.

Example:

```
1> {ok, CCtx} = zstd:context(compress).
2> {continue, C1} = zstd:stream(CCtx, ~"hello").
3> {continue, C2} = zstd:flush(CCtx).
4> zstd:decompress([C1, C2]).
[<<"hello">>]
```

# `get_dict_id`
*since OTP 28.0* 

```erlang
-spec get_dict_id(DictOrFrame :: dict() | binary()) -> non_neg_integer().
```

Get the dictionary ID of a dictionary or a frame.

The dictionary ID 0 represents no dictionary.

Example:

```
1> {ok, CDict} = zstd:dict(compress, Dict).
2> zstd:get_dict_id(CDict).
1850243626
3> zstd:get_dict_id(zstd:compress("abc")).
0
```

# `get_frame_header`
*since OTP 28.0* 

```erlang
-spec get_frame_header(Frame :: iodata()) ->
                          {ok,
                           #{blockSizeMax => non_neg_integer(),
                             checksumFlag => boolean(),
                             dictID => non_neg_integer(),
                             frameContentSize => non_neg_integer(),
                             frameType => 'ZSTD_frame' | 'ZSTD_skippableFrame',
                             headerSize => non_neg_integer(),
                             windowSize => non_neg_integer()}} |
                          {error, unicode:chardata()}.
```

Get header of a Zstandard compressed frame.

A compressed Zstandard stream can consist of multiple frames. This
function will read metadata from the first frame. This information
can be useful when debugging corrupted Zstandard streams.

Example:

```
1> Compressed = zstd:compress(~"abc").
2> zstd:get_frame_header(Compressed).
{ok,#{frameContentSize => 3,windowSize => 3,blockSizeMax => 3,
      frameType => 'ZSTD_frame',headerSize => 6,
      dictID => 0, checksumFlag => false}}
```

# `get_parameter`
*since OTP 28.0* 

```erlang
-spec get_parameter(Ctx :: context(), Key :: term()) -> Value :: term().
```

Get a parameter from a `t:context/0`.

See `t:compress_parameters/0` and `t:decompress_parameters/0` for details on
which parameters are available and what each parameter does.

Note that it is not possible to get the `dictionary` and `pledgedSrcSize`
parameters using this API. Instead you can use `get_dict_id/1` on the
`t:context/0` to get the id of the dictionary used. There is no way to
get the `pledgedSrcSize`.

Returns `ok` on success, raises an error on failure.

Example:

```
1> {ok, CCtx} = zstd:context(compress).
{ok, _}
2> zstd:get_parameter(CCtx, compressionLevel).
3
3> zstd:set_parameter(CCtx, compressionLevel, 15).
ok
4> zstd:get_parameter(CCtx, compressionLevel).
15
```

# `reset`
*since OTP 28.0* 

```erlang
-spec reset(Ctx :: context()) -> ok.
```

Reset a context while streaming data, returning it to its original state
but keeping all parameters set.

By resetting the state, the context can be re-used for other operations even
if it is in the middle of a (de)compression stream.

Example:

```
1> {ok, CCtx} = zstd:context(compress).
2> zstd:stream(CCtx, "a").
{continue, _}
3> zstd:reset(CCtx).
ok
4> {done, Compressed} = zstd:finish(CCtx, "b").
5> zstd:decompress(Compressed).
[~"b"]
```

# `set_parameter`
*since OTP 28.0* 

```erlang
-spec set_parameter(Ctx :: context(), Key :: term(), Value :: term()) -> ok.
```

Set a parameter on a `context/0`.

See `t:compress_parameters/0` and `t:decompress_parameters/0` for details on
which parameters are available and what each parameter does.

Returns `ok` on success, raises an error on failure.

Example:

```
1> {ok, CCtx} = zstd:context(compress).
{ok, _}
2> ok = zstd:set_parameter(CCtx, compressionLevel, 15).
ok
3> zstd:stream(CCtx, "abc").
{continue, _}
4> catch zstd:set_parameter(CCtx, dictionary, "abc").
{'EXIT', {{zstd_error, <<"Operation not authorized at current processing stage">>}, _}}
```

# `stream`
*since OTP 28.0* 

```erlang
-spec stream(Ctx :: context(), Data :: iodata()) -> Result
                when
                    Result ::
                        {continue, Remainder :: erlang:iovec(), Output :: binary()} |
                        {continue, Output :: binary()}.
```

Compress or decompress a stream of data. The last stream of data should be called
with `finish/2` to complete the compression/decompression.

Example:

```
1> {ok, CCtx} = zstd:context(compress).
2> {continue, C1} = zstd:stream(CCtx, ~"a").
3> {done, C2} = zstd:finish(CCtx, ~"b").
4> Compressed = iolist_to_binary([C1, C2]).
<<40,181,47,253,0,88,17,0,0,97,98>>
5> zstd:decompress(Compressed).
[<<"ab">>]
```

---

*Consult [api-reference.md](api-reference.md) for complete listing*
