Internally, HarfBuzz uses a structure called a shape plan to track its decisions about how to shape the contents of a buffer. The hb_shape() function builds up the shape plan by examining segment properties and by inspecting the contents of the font.

This process can involve some decision-making and trade-offs — for example, HarfBuzz inspects the GSUB and GPOS lookups for the script and language tags set on the segment properties, but it falls back on the lookups under the DFLT tag (and sometimes other common tags) if there are actually no lookups for the tag requested.

HarfBuzz also includes some work-arounds for handling well-known older font conventions that do not follow OpenType or Unicode specifications, for buggy system fonts, and for peculiarities of Microsoft Uniscribe. All of that means that a shape plan, while not something that you should edit directly in client code, still might be an object that you want to inspect. Furthermore, if resources are tight, you might want to cache the shape plan that HarfBuzz builds for your buffer and font, so that you do not have to rebuild it for every shaping call.

You can create a cacheable shape plan with hb_shape_plan_create_cached(face, props, user_features, num_user_features, shaper_list), where face is a face object (not a font object, notably), props is an hb_segment_properties_t, user_features is an array of hb_feature_ts (with length num_user_features), and shaper_list is a list of shapers to try.

Shape plans are objects in HarfBuzz, so there are reference-counting functions and user-data attachment functions you can use. hb_shape_plan_reference(shape_plan) increases the reference count on a shape plan, while hb_shape_plan_destroy(shape_plan) decreases the reference count, destroying the shape plan when the last reference is dropped.

You can attach user data to a shaper (with a key) using the hb_shape_plan_set_user_data(shape_plan,key,data,destroy,replace) function, optionally supplying a destroy callback to use. You can then fetch the user data attached to a shape plan with hb_shape_plan_get_user_data(shape_plan, key).