gitea/vendor/github.com/mailru/easyjson/README.md

easyjson

Package easyjson provides a fast and easy way to marshal/unmarshal Go structs
to/from JSON without the use of reflection. In performance tests, easyjson
outperforms the standard encoding/json package by a factor of 4-5x, and other
JSON encoding packages by a factor of 2-3x.

easyjson aims to keep generated Go code simple enough so that it can be easily
optimized or fixed. Another goal is to provide users with the ability to
customize the generated code by providing options not available with the
standard encoding/json package, such as generating "snake_case" names or
enabling omitempty behavior by default.

Usage

# install
go get -u github.com/mailru/easyjson/...

# run
easyjson -all <file>.go

The above will generate <file>_easyjson.go containing the appropriate marshaler and
unmarshaler funcs for all structs contained in <file>.go.

Please note that easyjson requires a full Go build environment and the GOPATH
environment variable to be set. This is because easyjson code generation
invokes go run on a temporary file (an approach to code generation borrowed
from ffjson).

Options

Usage of easyjson:
  -all
    	generate marshaler/unmarshalers for all structs in a file
  -build_tags string
        build tags to add to generated file
  -gen_build_flags string
        build flags when running the generator while bootstrapping
  -byte
        use simple bytes instead of Base64Bytes for slice of bytes
  -leave_temps
    	do not delete temporary files
  -no_std_marshalers
    	don't generate MarshalJSON/UnmarshalJSON funcs
  -noformat
    	do not run 'gofmt -w' on output file
  -omit_empty
    	omit empty fields by default
  -output_filename string
    	specify the filename of the output
  -pkg
    	process the whole package instead of just the given file
  -snake_case
    	use snake_case names instead of CamelCase by default
  -lower_camel_case
        use lowerCamelCase instead of CamelCase by default
  -stubs
    	only generate stubs for marshaler/unmarshaler funcs
  -disallow_unknown_fields
        return error if some unknown field in json appeared
  -disable_members_unescape
        disable unescaping of \uXXXX string sequences in member names

Using -all will generate marshalers/unmarshalers for all Go structs in the
file excluding those structs whose preceding comment starts with easyjson:skip.
For example:

//easyjson:skip
type A struct {}

If -all is not provided, then only those structs whose preceding
comment starts with easyjson:json will have marshalers/unmarshalers
generated. For example:

//easyjson:json
type A struct {}

Additional option notes:

• -snake_case tells easyjson to generate snake_case field names by default
  (unless overridden by a field tag). The CamelCase to snake_case conversion
  algorithm should work in most cases (ie, HTTPVersion will be converted to
  "http_version").

• -build_tags will add the specified build tags to generated Go sources.

• -gen_build_flags will execute the easyjson bootstapping code to launch the
  actual generator command with provided flags. Multiple arguments should be
  separated by space e.g. -gen_build_flags="-mod=mod -x".

Structure json tag options

Besides standart json tag options like 'omitempty' the following are supported:

• 'nocopy' - disables allocation and copying of string values, making them
  refer to original json buffer memory. This works great for short lived
  objects which are not hold in memory after decoding and immediate usage.
  Note if string requires unescaping it will be processed as normally.
• 'intern' - string "interning" (deduplication) to save memory when the very
  same string dictionary values are often met all over the structure.
  See below for more details.

Generated Marshaler/Unmarshaler Funcs

For Go struct types, easyjson generates the funcs MarshalEasyJSON /
UnmarshalEasyJSON for marshaling/unmarshaling JSON. In turn, these satisfy
the easyjson.Marshaler and easyjson.Unmarshaler interfaces and when used in
conjunction with easyjson.Marshal / easyjson.Unmarshal avoid unnecessary
reflection / type assertions during marshaling/unmarshaling to/from JSON for Go
structs.

easyjson also generates MarshalJSON and UnmarshalJSON funcs for Go struct
types compatible with the standard json.Marshaler and json.Unmarshaler
interfaces. Please be aware that using the standard json.Marshal /
json.Unmarshal for marshaling/unmarshaling will incur a significant
performance penalty when compared to using easyjson.Marshal /
easyjson.Unmarshal.

Additionally, easyjson exposes utility funcs that use the MarshalEasyJSON and
UnmarshalEasyJSON for marshaling/unmarshaling to and from standard readers
and writers. For example, easyjson provides easyjson.MarshalToHTTPResponseWriter
which marshals to the standard http.ResponseWriter. Please see the GoDoc
listing for the full listing of
utility funcs that are available.

Controlling easyjson Marshaling and Unmarshaling Behavior

Go types can provide their own MarshalEasyJSON and UnmarshalEasyJSON funcs
that satisfy the easyjson.Marshaler / easyjson.Unmarshaler interfaces.
These will be used by easyjson.Marshal and easyjson.Unmarshal when defined
for a Go type.

Go types can also satisfy the easyjson.Optional interface, which allows the
type to define its own omitempty logic.

Type Wrappers

easyjson provides additional type wrappers defined in the easyjson/opt
package. These wrap the standard Go primitives and in turn satisfy the
easyjson interfaces.

The easyjson/opt type wrappers are useful when needing to distinguish between
a missing value and/or when needing to specifying a default value. Type
wrappers allow easyjson to avoid additional pointers and heap allocations and
can significantly increase performance when used properly.

Memory Pooling

easyjson uses a buffer pool that allocates data in increasing chunks from 128
to 32768 bytes. Chunks of 512 bytes and larger will be reused with the help of
sync.Pool. The maximum size of a chunk is bounded to reduce redundant memory
allocation and to allow larger reusable buffers.

easyjson's custom allocation buffer pool is defined in the easyjson/buffer
package, and the default behavior pool behavior can be modified (if necessary)
through a call to buffer.Init() prior to any marshaling or unmarshaling.
Please see the GoDoc listing
for more information.

String interning

During unmarshaling, string field values can be optionally
interned to reduce memory
allocations and usage by deduplicating strings in memory, at the expense of slightly
increased CPU usage.

This will work effectively only for string fields being decoded that have frequently
the same value (e.g. if you have a string field that can only assume a small number
of possible values).

To enable string interning, add the intern keyword tag to your json tag on string
fields, e.g.:

type Foo struct {
  UUID  string `json:"uuid"`         // will not be interned during unmarshaling
  State string `json:"state,intern"` // will be interned during unmarshaling
}

Issues, Notes, and Limitations

• easyjson is still early in its development. As such, there are likely to be
  bugs and missing features when compared to encoding/json. In the case of a
  missing feature or bug, please create a GitHub issue. Pull requests are
  welcome!

• Unlike encoding/json, object keys are case-sensitive. Case-insensitive
  matching is not currently provided due to the significant performance hit
  when doing case-insensitive key matching. In the future, case-insensitive
  object key matching may be provided via an option to the generator.

• easyjson makes use of unsafe, which simplifies the code and
  provides significant performance benefits by allowing no-copy
  conversion from []byte to string. That said, unsafe is used
  only when unmarshaling and parsing JSON, and any unsafe operations
  / memory allocations done will be safely deallocated by
  easyjson. Set the build tag easyjson_nounsafe to compile it
  without unsafe.

• easyjson is compatible with Google App Engine. The appengine build
  tag (set by App Engine's environment) will automatically disable the
  use of unsafe, which is not allowed in App Engine's Standard
  Environment. Note that the use with App Engine is still experimental.

• Floats are formatted using the default precision from Go's strconv package.
  As such, easyjson will not correctly handle high precision floats when
  marshaling/unmarshaling JSON. Note, however, that there are very few/limited
  uses where this behavior is not sufficient for general use. That said, a
  different package may be needed if precise marshaling/unmarshaling of high
  precision floats to/from JSON is required.

• While unmarshaling, the JSON parser does the minimal amount of work needed to
  skip over unmatching parens, and as such full validation is not done for the
  entire JSON value being unmarshaled/parsed.

• Currently there is no true streaming support for encoding/decoding as
  typically for many uses/protocols the final, marshaled length of the JSON
  needs to be known prior to sending the data. Currently this is not possible
  with easyjson's architecture.

• easyjson parser and codegen based on reflection, so it won't work on package main
  files, because they cant be imported by parser.

Benchmarks

Most benchmarks were done using the example
13kB example JSON
(9k after eliminating whitespace). This example is similar to real-world data,
is well-structured, and contains a healthy variety of different types, making
it ideal for JSON serialization benchmarks.

Note:

• For small request benchmarks, an 80 byte portion of the above example was
  used.

• For large request marshaling benchmarks, a struct containing 50 regular
  samples was used, making a ~500kB output JSON.

• Benchmarks are showing the results of easyjson's default behaviour,
  which makes use of unsafe.

Benchmarks are available in the repository and can be run by invoking make.

easyjson vs. encoding/json

easyjson is roughly 5-6 times faster than the standard encoding/json for
unmarshaling, and 3-4 times faster for non-concurrent marshaling. Concurrent
marshaling is 6-7x faster if marshaling to a writer.

easyjson vs. ffjson

easyjson uses the same approach for JSON marshaling as
ffjson, but takes a significantly
different approach to lexing and parsing JSON during unmarshaling. This means
easyjson is roughly 2-3x faster for unmarshaling and 1.5-2x faster for
non-concurrent unmarshaling.

As of this writing, ffjson seems to have issues when used concurrently:
specifically, large request pooling hurts ffjson's performance and causes
scalability issues. These issues with ffjson can likely be fixed, but as of
writing remain outstanding/known issues with ffjson.

easyjson and ffjson have similar performance for small requests, however
easyjson outperforms ffjson by roughly 2-5x times for large requests when
used with a writer.

easyjson vs. go/codec

go/codec provides
compile-time helpers for JSON generation. In this case, helpers do not work
like marshalers as they are encoding-independent.

easyjson is generally 2x faster than go/codec for non-concurrent benchmarks
and about 3x faster for concurrent encoding (without marshaling to a writer).

In an attempt to measure marshaling performance of go/codec (as opposed to
allocations/memcpy/writer interface invocations), a benchmark was done with
resetting length of a byte slice rather than resetting the whole slice to nil.
However, the optimization in this exact form may not be applicable in practice,
since the memory is not freed between marshaling operations.

easyjson vs 'ujson' python module

ujson is using C code for parsing, so it
is interesting to see how plain golang compares to that. It is important to note
that the resulting object for python is slower to access, since the library
parses JSON object into dictionaries.

easyjson is slightly faster for unmarshaling and 2-3x faster than ujson for
marshaling.

Benchmark Results

ffjson results are from February 4th, 2016, using the latest ffjson and go1.6.
go/codec results are from March 4th, 2016, using the latest go/codec and go1.6.

Unmarshaling

lib	json size	MB/s	allocs/op	B/op
standard	regular	22	218	10229
standard	small	9.7	14	720
easyjson	regular	125	128	9794
easyjson	small	67	3	128
ffjson	regular	66	141	9985
ffjson	small	17.6	10	488
codec	regular	55	434	19299
codec	small	29	7	336
ujson	regular	103	N/A	N/A

Marshaling, one goroutine.

lib	json size	MB/s	allocs/op	B/op
standard	regular	75	9	23256
standard	small	32	3	328
standard	large	80	17	1.2M
easyjson	regular	213	9	10260
easyjson*	regular	263	8	742
easyjson	small	125	1	128
easyjson	large	212	33	490k
easyjson*	large	262	25	2879
ffjson	regular	122	153	21340
ffjson**	regular	146	152	4897
ffjson	small	36	5	384
ffjson**	small	64	4	128
ffjson	large	134	7317	818k
ffjson**	large	125	7320	827k
codec	regular	80	17	33601
codec***	regular	108	9	1153
codec	small	42	3	304
codec***	small	56	1	48
codec	large	73	483	2.5M
codec***	large	103	451	66007
ujson	regular	92	N/A	N/A

* marshaling to a writer,
** using ffjson.Pool(),
*** reusing output slice instead of resetting it to nil

Marshaling, concurrent.

lib	json size	MB/s	allocs/op	B/op
standard	regular	252	9	23257
standard	small	124	3	328
standard	large	289	17	1.2M
easyjson	regular	792	9	10597
easyjson*	regular	1748	8	779
easyjson	small	333	1	128
easyjson	large	718	36	548k
easyjson*	large	2134	25	4957
ffjson	regular	301	153	21629
ffjson**	regular	707	152	5148
ffjson	small	62	5	384
ffjson**	small	282	4	128
ffjson	large	438	7330	1.0M
ffjson**	large	131	7319	820k
codec	regular	183	17	33603
codec***	regular	671	9	1157
codec	small	147	3	304
codec***	small	299	1	48
codec	large	190	483	2.5M
codec***	large	752	451	77574

* marshaling to a writer,
** using ffjson.Pool(),
*** reusing output slice instead of resetting it to nil