Welcome to my walk down memory lane!

This BigInt code was part of a utility library written by DejaVu
Software, Inc. - a small software company that wrote software for
early handheld computers. DejaVu's founding members (Scott Harker and
me, Jorj Bauer) released products for the Apple Newton, Palm Pilot,
Windows CE 2.x through 6.x, and Apple iOS.

BigInt was used for our shareware registration codes but served a
larger purpose in PockeTTY, a terminal emulator with SSH and SSL
support. The SSH and SSL implementations used this BigInt code as the
basis of our self-implemented encryption support.

DejaVu Software, Inc. closed its doors in 2015 and we are releasing
this code under the MIT license.

BigInt was originally written and debugged on 32-bit Linux machines
and then ported to Windows CE. Because of the diversity of Windows CE
platforms and various idiosyncrasies in the compilers, we wrote this
code to support multiple data storage bit-sizes. It currently supports
16-bit, 32-bit, and 64-bit storage and, as I'm distributing it,
expects to be compiled on a 32-bit architecture.

To change the architecture, change this line in BigInt.h:

   #define BIGINT_PRIMITIVE_SIZE 32

Supported values are 64, 32, and 16. (Actually, anything that's not 64
or 32 will wind up compiling for 16.)

There was a time when compiling for larger bit-sizes meant a
performance boost. Minor testing with LLVM 6.1.0 shows almost no
difference between the 64 and 32 bit storage engines, presumably due
to modern compiler optimizations.

Casual testing shows the Lua-wrapped implementation to be about the
same speed as the original C++ code. In practical situations, it's
only half that speed because of dynamic type conversion and having to
create new BigInt arguments on the fly.

Since the core library is in C++, this may be tricky to install on
your system; C++ compilation/linking is not LuaRocks' forte. As-is,
I've tested this on:

  - MacOS 10.11 through 10.13 (Homebrew; Lua 5.3.4, luarocks 2.4.4)
  - RHEL 7.5 (Lua 5.1.4, luarocks 2.3.0; gcc-c++ package installed)

Other platforms (and versions of Lua) may differ.

Some suggestions on how you might get your platform to compile this:

1. If you're using Lua 5.1, you can probably use the
   luarocks-build-cpp subsystem.

  $ luarocks install luarocks-build-cpp
  $ luarocks install https://jorj.org/bigint-cpp/bigint-1.0.3-1.rockspec

  (This works for me on Debian 8. It does not work with Lua 5.3 though.)

2. luarocks configuration override. In theory, ~/.luarocks/config.lua
   should set overrides on various luarocks configuration variables. In
   practice, the naming and location of the file differ depending on the
   version of luarocks (e.g. config.lua or config-5.3.lua) and whether or
   not you're using 'sudo' to install (i.e. /root/.luarocks or
   ~/.luarocks). Put this in your config.lua, and then the normal 
   'luarocks install bigint' should link correctly with libstdc++.

       variables = { LD = "gcc -lstdc++" }

   (This also works for me on Debian 8 and 9, regardless of Lua version.)

   At some point in the future I hope the cpp subsystem will be native to
   luarocks (and work well on all platforms). Until then, it's a bit of a 
   crapshoot.