isomorphic-git/src/utils/pkt-line-encoder.js

Node library for interacting with git repositories, circa 2017

// @flow
/**
pkt-line Format
---------------

Much (but not all) of the payload is described around pkt-lines.

A pkt-line is a variable length binary string.  The first four bytes
of the line, the pkt-len, indicates the total length of the line,
in hexadecimal.  The pkt-len includes the 4 bytes used to contain
the length's hexadecimal representation.

A pkt-line MAY contain binary data, so implementors MUST ensure
pkt-line parsing/formatting routines are 8-bit clean.

A non-binary line SHOULD BE terminated by an LF, which if present
MUST be included in the total length. Receivers MUST treat pkt-lines
with non-binary data the same whether or not they contain the trailing
LF (stripping the LF if present, and not complaining when it is
missing).

The maximum length of a pkt-line's data component is 65516 bytes.
Implementations MUST NOT send pkt-line whose length exceeds 65520
(65516 bytes of payload + 4 bytes of length data).

Implementations SHOULD NOT send an empty pkt-line ("0004").

A pkt-line with a length field of 0 ("0000"), called a flush-pkt,
is a special case and MUST be handled differently than an empty
pkt-line ("0004").

----
  pkt-line     =  data-pkt / flush-pkt

  data-pkt     =  pkt-len pkt-payload
  pkt-len      =  4*(HEXDIG)
  pkt-payload  =  (pkt-len - 4)*(OCTET)

  flush-pkt    = "0000"
----

Examples (as C-style strings):

----
  pkt-line          actual value
  ---------------------------------
  "0006a\n"         "a\n"
  "0005a"           "a"
  "000bfoobar\n"    "foobar\n"
  "0004"            ""
----
*/
import { Buffer } from 'buffer'
import pad from 'pad'

export function flush () {
  return Buffer.from('0000', 'utf8')
}

export function encode (line /*: string|Buffer */) /*: Buffer */ {
  if (typeof line === 'string') {
    line = Buffer.from(line)
  }
  let length = line.length + 4
  let hexlength = pad(4, length.toString(16), '0')
  return Buffer.concat([Buffer.from(hexlength, 'utf8'), line])
}