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// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
Package codec implements the general-purpose part of an encoder for Go
values. It relies on code generation rather than reflection so it is
significantly faster than reflection-based encoders like gob. It also
preserves sharing among struct pointers (but not other forms of sharing, like
other pointer types or sub-slices). These features are sufficient for
encoding the structures of the go/ast package, which is its sole purpose.
# Encoding Scheme
Every encoded value begins with a single byte that describes what (if
anything) follows. There is enough information to skip over the value, since
the decoder must be able to do that if it encounters a struct field it
doesn't know.
Most of the values of that initial byte can be devoted to small unsigned
integers. For example, the number 17 is represented by the single byte 17.
Only a few byte values have special meaning.
The nil code indicates that the value is nil. We don't absolutely need this:
we could always represent the nil value for a type as something that couldn't
be mistaken for an encoded value of that type. For instance, we could use 0
for nil in the case of slices (which always begin with the nValues code), and
for pointers to numbers like *int, we could use something like "nBytes 0".
But it is simpler to have a reserved value for nil.
The nBytes code indicates that an unsigned integer N is encoded next,
followed by N bytes of data. This is used to represent strings and byte
slices, as well numbers bigger than can fit into the initial byte. For
example, the string "hi" is represented as:
nBytes 2 'h' 'i'
Unsigned integers that can't fit into the initial byte are encoded as byte
sequences of length 4 or 8, holding little-endian uint32 or uint64 values. We
use uint32s where possible to save space. We could have saved more space by
also considering 16-byte numbers, or using a variable-length encoding like
varints or gob's representation, but it didn't seem worth the additional
The nValues code is for sequences of values whose size is known beforehand,
like a Go slice or array. The slice []string{"hi", "bye"} is encoded as
nValues 2 nBytes 2 'h' 'i' nBytes 3 'b' 'y' 'e'
The ref code is used to refer to an earlier encoded value. It is followed by
a uint denoting the index data of the value to use.
The start and end codes delimit a value whose length is unknown beforehand.
It is used for structs.
package codec