Quirks are configuration options that let Wuffs more closely match other codec implementations.
Some codecs and file formats are more tightly specified than others. When a spec doesn‘t fully dictate how to treat malformed files, different codec implementations have disagreed on whether to accept it (e.g. whether they follow Postel’s Principle of “be liberal in what you accept”) and if so, how to interpret it. Some implementations also raise those decisions to the application level, not the library level: “provide mechanism, not policy”.
For example, in an animated GIF image, the first frame might not fill out the entire image, and different implementations choose a different default color for the outside pixels: opaque black, transparent black, or something else.
Wuffs, out of the box, makes particular choices (typically mimicking the de facto canonical implementation), but enabling various quirks result in different choices. In particular, quirks are useful in regression testing that Wuffs and another implementation produce the same output for the same input, even for malformed input.
Each quirk is assigned a uint32_t value, packed using the base38 namespace convention. Decoders and encoders can have a set_quirk_enabled!(quirk base.u32, enabled base.bool) method whose first argument is this uint32_t value.
For example, the base38 encoding of "gif " is 0xF8586, so that the GIF-specific quirks have a uint32_t value of ((0xF8586 << 10) | n), for some small integer n. The generated C language file defines human-readable names for those constant values.