Customizing Unicode functions: HarfBuzz Manual

## Customizing Unicode functions

HarfBuzz requires some simple functions for accessing information from the Unicode Character Database (such as the `General_Category` (gc) and `Script` (sc) properties) that is useful for shaping, as well as some useful operations like composing and decomposing code points.

HarfBuzz includes its own internal, lightweight set of Unicode functions. At build time, it is also possible to compile support for some other options, such as the Unicode functions provided by GLib or the International Components for Unicode (ICU) library. Generally, this option is only of interest for client programs that have specific integration requirements or that do a significant amount of customization.

If your program has access to other Unicode functions, however, such as through a system library or application framework, you might prefer to use those instead of the built-in options. HarfBuzz supports this by implementing its Unicode functions as a set of virtual methods that you can replace — without otherwise affecting HarfBuzz's functionality.

The Unicode functions are specified in a structure called `unicode_funcs` which is attached to each buffer. But even though `unicode_funcs` is associated with a hb_buffer_t, the functions themselves are called by other HarfBuzz APIs that access buffers, so it would be unwise for you to hook different functions into different buffers.

In addition, you can mark your `unicode_funcs` as immutable by calling `hb_unicode_funcs_make_immutable (ufuncs)`. This is especially useful if your code is a library or framework that will have its own client programs. By marking your Unicode function choices as immutable, you prevent your own client programs from changing the `unicode_funcs` configuration and introducing inconsistencies and errors downstream.

You can retrieve the Unicode-functions configuration for your buffer by calling `hb_buffer_get_unicode_funcs()`:

    hb_unicode_funcs_t *ufunctions;
    ufunctions = hb_buffer_get_unicode_funcs(buf);

The current version of `unicode_funcs` uses six functions:

- `hb_unicode_combining_class_func_t`: returns the Canonical Combining Class of a code point.

- `hb_unicode_general_category_func_t`: returns the General Category (gc) of a code point.

- `hb_unicode_mirroring_func_t`: returns the Mirroring Glyph code point (for bi-directional replacement) of a code point.

- `hb_unicode_script_func_t`: returns the Script (sc) property of a code point.

- `hb_unicode_compose_func_t`: returns the canonical composition of a sequence of two code points.

- `hb_unicode_decompose_func_t`: returns the canonical decomposition of a code point.

Note, however, that future HarfBuzz releases may alter this set.

Each Unicode function has a corresponding setter, with which you can assign a callback to your replacement function. For example, to replace `hb_unicode_general_category_func_t`, you can call

    hb_unicode_funcs_set_general_category_func (*ufuncs, func, *user_data, destroy)

Virtualizing this set of Unicode functions is primarily intended to improve portability. There is no need for every client program to make the effort to replace the default options, so if you are unsure, do not feel any pressure to customize `unicode_funcs`.