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[set-digest] Comment

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Behdad Esfahbod 2023-04-28 12:22:11 -06:00
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src/hb-set-digest.hh
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@ -45,10 +45,16 @@
* a lookup's or subtable's Coverage table(s), and then when we
* want to apply the lookup or subtable to a glyph, before trying
* to apply, we ask the filter if the glyph may be covered. If it's
* not, we return early.
* not, we return early. We can also match a digest against another
* digest.
*
* We use these filters both at the lookup-level, and then again,
* at the subtable-level. Both have performance win.
* We use these filters at three levels:
* - If the digest for all the glyphs in the buffer as a whole
* does not match the digest for the lookup, skip the lookup.
* - For each glyph, if it doesn't match the lookup digest,
* skip it.
* - For each glyph, if it doesn't match the subtable digest,
* skip it.
*
* The main filter we use is a combination of three bits-pattern
* filters. A bits-pattern filter checks a number of bits (5 or 6)