= S-Expressions parser and generator library in C\++ (SEXP in C++)

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== Purpose

This is a C++ library for working with S-Expressions.

This implementation is derived from the reference SEXP C library developed by
Prof. Ronald Rivest and Prof. Butler Lampson of MIT LCS (now CSAIL).

This library differs from the original C implementation in the following ways:

* It aims to be reuseable in C++ implementations and is importable via CMake.
* It includes a test suite for correctness testing and tests against malformed
  S-Expressions.
* It supports, and is tested against, all major platforms, including:
** Ubuntu, Debian, Fedora, CentOS
** macOS
** Windows
** msys
* It implements additional interface to work with S-Expressions wrapped by GnuPG
  2.3+ extended format
  (https://github.com/gpg/gnupg/blob/master/agent/keyformat.txt[specification]).


The original C library was available at (but no longer accessible):

* http://people.csail.mit.edu/rivest/sexp.html


== Background

S-Expressions are a data structure for representing complex data as a variation
on https://en.wikipedia.org/wiki/Lisp_(programming_language)[LISP] S-Expressions.

S-Expressions were originally adopted for use in
http://theory.lcs.mit.edu/~cis/sdsi.html[SDSI] and
http://world.std.com/~cme/html/spki.html[SPKI].

SDSI has been developed by Prof.
https://people.csail.mit.edu/rivest/index.html[Ronald L. Rivest] and
Prof. Butler Lampson of
http://www.lcs.mit.edu/[MIT's Laboratory for Computer Science],
members of
http://theory.lcs.mit.edu/~cis[LCS's Cryptography and Information Security]
research group.

NOTE: SDSI research has been supported by DARPA contract DABT63-96-C-0018,
"Security for Distributed Computer Systems".

NOTE: SPKI has been developed by
http://www.clark.net/pub/cme/home.html[Carl Ellison] and others in the IETF SPKI
working group.


== S-Expressions specification

* https://datatracker.ietf.org/doc/draft-rivest-sexp/

NOTE: The "SEXP 1.0 guide" used to be at
https://people.csail.mit.edu/rivest/Sexp.txt.


== Code

The library is a deep rework to C++ of the original
https://people.csail.mit.edu/rivest/sexp.html[SEXP library] that maintains full
support of original specification.

While most applications will not need anything but the simple canonical and
transport formats; however, the code here is considerably more complex because
it also supports the advanced format, both for input and for output.


== Building and installation

[source,sh]
----
$ mkdir build
$ cd build
$ cmake ..
$ cmake --build .
$ ctest
$ cmake --install .
----


== CMake script options

`BUILD_SHARED_LIBS:BOOL`::
(default: `OFF`)
build shared library

`WITH_SEXP_TESTS:BOOL`::
(default: `ON`)
build tests

`DOWNLOAD_GTEST`::
(default: `ON`)
if tests are built, automatically download googletest from GitHub;
when set to `OFF`, the googletest binary package needs to be available for SEXP
tests.

`WITH_SEXP_CLI:BOOL`::
(default: `ON`) build the `sexp` command-line utility

`WITH_SANITIZERS:BOOL`::
(default: `OFF`)
build with address and other sanitizers (requires clang compiler)



== SEXPP command-line utility

The `sexpp` command-line utility is reference parser and generator of
S-Expressions. It can read, parse and print out SEXP in all defined formats.

=== Switches

.`sexpp` switches
[options="header"]
|===
| Switch          | Description                                    | Default

3+| Input
| `-i <filename>` | input file name                                | read input from console (stdin)
| `-p`            | prompt input if reading from console           | disabled
| `-s`            | treat input as a single SEXP string            | disabled, input is treated as an S-Expression

3+| Output
| `-o <filename>` | output file name:                              | write output to console (stdout)
| `-a`            | generate advanced transport format             | enabled if no format is specified
| `-b`            | generate base-64 transport format              | disabled
| `-c`            | generate canonical format                      | disabled
| `-l`            | suppress linefeeds after output                | disabled
| `-w <width>`    | set output line width (0 implies no constraint)| 75

3+| Miscellaneous
| `-x`            | execute repeatedly until EOF                   | process single S-Expression then exit
| `-h`            | print help message and exit                    |

|===

Running without switches implies: `-p -a -b -c -x`.

=== Usage examples

Prompt for S-Expressions input from console, parse and output it to
`certificate.dat` in base64 transport format.

[source]
----
$ sexpp -o certificate.dat -p -b

> Input:
> (aa bb (cc dd))
>
> Writing base64 (of canonical) output to 'certificate.dat'
----

Parse all S-Expressions from `certificate.dat`, output them to console in
advanced transport format with no prompts:

[source,sh]
----
$ sexpp -i certificate.dat -x

> (2:aa2:bb(2:cc2:dd))
----

Parse S-Expressions from `certificate.dat`, output it to console in canonical,
base64 and advanced format with prompts and no width limitation:

[source,sh]
----
$ sexpp -i certificate.dat -a -b -c -p -w 0

> Reading input from certificate.dat
>
> Canonical output:
> (2:aa2:bb(2:cc2:dd))
> Base64 (of canonical) output:
> {KDI6YWEyOmJiKDI6Y2MyOmRkKSk=}
> Advanced transport output:
> (aa bb (cc dd))
----

Repeatedly prompt for S-Expressions input from console, parse and output it
console in advanced, base64 and canonical formats:

[source,sh]
----
$ sexpp -p -a -b -c -x
----

or just

[source,sh]
----
$ sexpp

> Input:
> (abc def (ghi jkl))
>
> Canonical output:
> (3:abc3:def(3:ghi3:jkl))
> Base64 (of canonical) output:
> {KDM6YWJjMzpkZWYoMzpnaGkzOmprbCkp}
> Advanced transport output:
> (abc def (ghi jkl))
>
> Input:
> ^C
----

== License

Copyright Ribose.

The code is made available as open-source software under the MIT License.
