This is Gorsium, a mean and lean implementation of the rsync algorithm in Go. The most important thing about it that its name is not just a kind of combination of "go" and "rsync", but also the name of an old Roman settlement located in current Hungary and you can actually visit the ruins.

Other most important thing is that it's based on the following resources: Lars Wirzenius' sketch of the rsync algorithm in Python and description of the rolling (aka. weak) sum in Tridge's thesis. Lars' writeup is just ingenious beyond words' expressive capability -- there are a bunch of sketches of rsync in Python but all are lost in technical details and funky control flow; while Lars' code is a clear demonstration of the alleged "executable pseudocode" character of Python. Only thing missing bit is the rolling sum, as that involves a kind of bit-arithmetics with which Python is pathetic (Lars falls back to another kind of weak sum which is algorithmically worse than the rolling sum, but available as a C-coded library module in Python, so it's still faster than if the rolling sum were coded in pure Python) -- so to pick up that I reached out to Tridge's original publication.

Now Go... is also quite close to executable pseudocode. Unlike Python, it's compiled and it's realistic to do bit arithmetics in it. I'm amazed of the succinctness of the result. Apart for some general goodies of the language which can be discovered on the homepage, I made a particular good use of the following features:

• a readily available map (dictionary) type
• general serizalization capabilites
• an excellent rpc module based on the above referred serialization format as the wire protocol

Also, if this experient will be continued, I might see a good use of go's built-in concurrency support.

Compilation

go install <reponame> does not yet work. You can build locally by

mkdir GOROOT
cd GOROOT
ln -s .. src
GOPATH=$PWD go build gorsium

Trying out

A basic summary is avaliable by gorsium -h. Note that the command line interface is optimized for machines, designed in a way which is easy both to generate and parse/validate; so you as a human might find it a bit weird. Therefore, instead of explaining it in detail, let's see how can you accomplish something with it that you now already how to do with rsync.

• rsync: rsync /foo/a /foo/b example.com:/baz
• gorsium: gorsium ssh example.com gorsium RECEIVE /baz -- a b SEND /foo

(contrary to rsync, we should not assume gorsium is in the $PATH, so actually you would pass the full path of the gorsium binaries, so that first one is the location of the local binary, and second one is the location of the remote binary (ie. the copy on example.com)).