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State machines generation using ragel

HarfBuzz uses ragel to generate state machines for some shapers. The produced C code is rather low-level and relies on goto a lot, therefore converting it to Rust by hand is rather problematic. And this is the code that updates relatively often.

Can we use ragel to generate Rust code directly? Not using the stable ragel. But the latest development branch does have a native Rust support. Therefore to use ragel directly, we have to use the latest devel version and modify ragel scripts a bit.

Build ragel

ragel is a C++ project that uses the dreadful autotools. Good luck building it on Windows. But on Unix-like OSes it should be relatively easy.

It also requires a 3rdparty dependency of colm, which we have to build first.

In case of macOS, we would need:

brew install automake autoconf libtool

And now we can build ragel:

# build `colm` first
git clone https://github.com/adrian-thurston/colm
cd colm
./autogen.sh
./configure --prefix=/path/to/colm/install # prefer a custom path to /usr/local
make
make install

cd ..
git clone https://github.com/adrian-thurston/ragel
./autogen.sh
# --with-colm takes the same path we used above
./configure --prefix=/path/to/ragel/install --with-colm=/path/to/colm/install
make
make install

Running ragel

Now we can convert our ragel scripts using:

/path/to/ragel/install/bin/ragel-rust -e -F1 src/hb/ot_shape_complex_indic_machine.rl # or any other .rl file

That's it!

PS: ragel will create temporary *.ri files. They can be safely removed.

Code format

ragel uses strange code formatting for the Rust output, therefore running cargo fmt is required.

The Universal state machine

The universal_machine.rl is special since we have to modify its output manually because we cannot express our needs via ragel directly.

The change is simple. After the universal_machine.rs is generated, Rust will complain about about some variables set to 0. Like ts = 0;. In all those cases 0 should simply be replaced with p0. There are no better solution for now...