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doc: improve zlib docs
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General improvements to zlib doc copy

PR-URL: #6746
Reviewed-By: Anna Henningsen <anna@addaleax.net>
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@jasnell
jasnell committed May 16, 2016
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Stability: 2 - Stable

You can access this module with:
The `zlib` module provides compression functionality implemented using Gzip and
Deflate/Inflate. It can be accessed using:

const zlib = require('zlib');

This provides bindings to Gzip/Gunzip, Deflate/Inflate, and
DeflateRaw/InflateRaw classes. Each class takes the same options, and
is a readable/writable Stream.

## Examples
```js
const zlib = require('zlib');
```

Compressing or decompressing a file can be done by piping an
fs.ReadStream into a zlib stream, then into an fs.WriteStream.
Compressing or decompressing a stream (such as a file) can be accomplished by
piping the source stream data through a `zlib` stream into a destination stream:

```js
const gzip = zlib.createGzip();
Expand All @@ -24,8 +21,7 @@ const out = fs.createWriteStream('input.txt.gz');
inp.pipe(gzip).pipe(out);
```

Compressing or decompressing data in one step can be done by using
the convenience methods.
It is also possible to compress or decompress data in a single step:

```js
const input = '.................................';
Expand All @@ -47,25 +43,33 @@ zlib.unzip(buffer, (err, buffer) => {
});
```

To use this module in an HTTP client or server, use the [accept-encoding][]
on requests, and the [content-encoding][] header on responses.
## Compressing HTTP requests and responses

The `zlib` module can be used to implement support for the `gzip` and `deflate`
content-encoding mechanisms defined by
[HTTP](https://tools.ietf.org/html/rfc7230#section-4.2).

The HTTP [`Accept-Encoding`][] header is used within an http request to identify
the compression encodings accepted by the client. The [`Content-Encoding`][]
header is used to identify the compression encodings actually applied to a
message.

**Note: these examples are drastically simplified to show
the basic concept.** Zlib encoding can be expensive, and the results
**Note: the examples given below are drastically simplified to show
the basic concept.** Using `zlib` encoding can be expensive, and the results
ought to be cached. See [Memory Usage Tuning][] for more information
on the speed/memory/compression tradeoffs involved in zlib usage.
on the speed/memory/compression tradeoffs involved in `zlib` usage.

```js
// client request example
const zlib = require('zlib');
const http = require('http');
const fs = require('fs');
const request = http.get({ host: 'izs.me',
const request = http.get({ host: 'example.com',
path: '/',
port: 80,
headers: { 'accept-encoding': 'gzip,deflate' } });
headers: { 'Accept-Encoding': 'gzip,deflate' } });
request.on('response', (response) => {
var output = fs.createWriteStream('izs.me_index.html');
var output = fs.createWriteStream('example.com_index.html');

switch (response.headers['content-encoding']) {
// or, just use zlib.createUnzip() to handle both cases
Expand Down Expand Up @@ -97,10 +101,10 @@ http.createServer((request, response) => {
// Note: this is not a conformant accept-encoding parser.
// See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
if (acceptEncoding.match(/\bdeflate\b/)) {
response.writeHead(200, { 'content-encoding': 'deflate' });
response.writeHead(200, { 'Content-Encoding': 'deflate' });
raw.pipe(zlib.createDeflate()).pipe(response);
} else if (acceptEncoding.match(/\bgzip\b/)) {
response.writeHead(200, { 'content-encoding': 'gzip' });
response.writeHead(200, { 'Content-Encoding': 'gzip' });
raw.pipe(zlib.createGzip()).pipe(response);
} else {
response.writeHead(200, {});
Expand All @@ -109,7 +113,7 @@ http.createServer((request, response) => {
}).listen(1337);
```

By default, the zlib methods with throw an error when decompressing
By default, the `zlib` methods with throw an error when decompressing
truncated data. However, if it is known that the data is incomplete, or
the desire is to inspect only the beginning of a compressed file, it is
possible to suppress the default error handling by changing the flushing
Expand Down Expand Up @@ -146,43 +150,43 @@ The memory requirements for deflate are (in bytes):
(1 << (windowBits+2)) + (1 << (memLevel+9))
```

that is: 128K for windowBits=15 + 128K for memLevel = 8
That is: 128K for windowBits=15 + 128K for memLevel = 8
(default values) plus a few kilobytes for small objects.

For example, if you want to reduce
the default memory requirements from 256K to 128K, set the options to:
For example, to reduce the default memory requirements from 256K to 128K, the
options shoud be set to:

```
{ windowBits: 14, memLevel: 7 }
```

Of course this will generally degrade compression (there's no free lunch).
This will, however, generally degrade compression.

The memory requirements for inflate are (in bytes)

```
1 << windowBits
```

that is, 32K for windowBits=15 (default value) plus a few kilobytes
That is, 32K for windowBits=15 (default value) plus a few kilobytes
for small objects.

This is in addition to a single internal output slab buffer of size
`chunkSize`, which defaults to 16K.

The speed of zlib compression is affected most dramatically by the
The speed of `zlib` compression is affected most dramatically by the
`level` setting. A higher level will result in better compression, but
will take longer to complete. A lower level will result in less
compression, but will be much faster.

In general, greater memory usage options will mean that node.js has to make
fewer calls to zlib, since it'll be able to process more data in a
single `write` operation. So, this is another factor that affects the
In general, greater memory usage options will mean that Node.js has to make
fewer calls to `zlib` because it will be able to process more data on
each `write` operation. So, this is another factor that affects the
speed, at the cost of memory usage.

## Flushing

Calling [`.flush()`][] on a compression stream will make zlib return as much
Calling [`.flush()`][] on a compression stream will make `zlib` return as much
output as currently possible. This may come at the cost of degraded compression
quality, but can be useful when data needs to be available as soon as possible.

Expand Down Expand Up @@ -214,13 +218,11 @@ http.createServer((request, response) => {

<!--type=misc-->

All of the constants defined in zlib.h are also defined on
`require('zlib')`.
In the normal course of operations, you will not need to ever set any of
these. They are documented here so that their presence is not
surprising. This section is taken almost directly from the
[zlib documentation][]. See <http://zlib.net/manual.html#Constants> for more
details.
All of the constants defined in `zlib.h` are also defined on `require('zlib')`.
In the normal course of operations, it will not be necessary to use these
constants. They are documented so that their presence is not surprising. This
section is taken almost directly from the [zlib documentation][]. See
<http://zlib.net/manual.html#Constants> for more details.

Allowed flush values.

Expand Down Expand Up @@ -280,19 +282,19 @@ For initializing zalloc, zfree, opaque.

<!--type=misc-->

Each class takes an options object. All options are optional.
Each class takes an `options` object. All options are optional.

Note that some options are only relevant when compressing, and are
ignored by the decompression classes.

* flush (default: `zlib.Z_NO_FLUSH`)
* finishFlush (default: `zlib.Z_FINISH`)
* chunkSize (default: 16*1024)
* windowBits
* level (compression only)
* memLevel (compression only)
* strategy (compression only)
* dictionary (deflate/inflate only, empty dictionary by default)
* `flush` (default: `zlib.Z_NO_FLUSH`)
* `finishFlush` (default: `zlib.Z_FINISH`)
* `chunkSize` (default: 16*1024)
* `windowBits`
* `level` (compression only)
* `memLevel` (compression only)
* `strategy` (compression only)
* `dictionary` (deflate/inflate only, empty dictionary by default)

See the description of `deflateInit2` and `inflateInit2` at
<http://zlib.net/manual.html#Advanced> for more information on these.
Expand All @@ -303,7 +305,7 @@ Compress data using deflate.

## Class: zlib.DeflateRaw

Compress data using deflate, and do not append a zlib header.
Compress data using deflate, and do not append a `zlib` header.

## Class: zlib.Gunzip

Expand Down Expand Up @@ -338,7 +340,7 @@ class of the compressor/decompressor classes.
Flush pending data. Don't call this frivolously, premature flushes negatively
impact the effectiveness of the compression algorithm.

Calling this only flushes data from the internal zlib state, and does not
Calling this only flushes data from the internal `zlib` state, and does not
perform flushing of any kind on the streams level. Rather, it behaves like a
normal call to `.write()`, i.e. it will be queued up behind other pending
writes and will only produce output when data is being read from the stream.
Expand Down Expand Up @@ -385,9 +387,9 @@ Returns a new [Unzip][] object with an [options][].

<!--type=misc-->

All of these take a [Buffer][] or string as the first argument, an optional second
argument to supply options to the zlib classes and will call the supplied
callback with `callback(error, result)`.
All of these take a [Buffer][] or string as the first argument, an optional
second argument to supply options to the `zlib` classes and will call the
supplied callback with `callback(error, result)`.

Every method has a `*Sync` counterpart, which accept the same arguments, but
without a callback.
Expand Down Expand Up @@ -427,8 +429,8 @@ Decompress a Buffer or string with InflateRaw.

Decompress a Buffer or string with Unzip.

[accept-encoding]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
[content-encoding]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.11
[`Accept-Encoding`]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.3
[`Content-Encoding`]: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.11
[Memory Usage Tuning]: #zlib_memory_usage_tuning
[zlib documentation]: http://zlib.net/manual.html#Constants
[options]: #zlib_class_options
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