The purpose of this new `:nth-ancestor(n)` operator is to
lookup the nth ancestor relative to the currently selected
node.
It is essentially equivalent to `:xpath(..)`, where
ancestor distance is expressed as a number rather than a
sequence of slash-separated `..`.
The rationale to introduce this new procedural selector
is to have a low overhead way to accomplish ancestor
selection.
With the new PSL implementation, benchmarks do not show benefit
from caching the domain extracted from a hostname for later
reuse -- the caching seems to even add an overhead instead with
the new publicSuffixList implementation.
The motivation is to address the higher peak memory usage at launch
time with 3rd-gen HNTrie when a selfie was present.
The selfie generation prior to this change was to collect all
filtering data into a single data structure, and then to serialize
that whole structure at once into storage (using JSON.stringify).
However, HNTrie serialization requires that a large UintArray32 be
converted into a plain JS array, which itslef would be indirectly
converted into a JSON string. This was the main reason why peak
memory usage would be higher at launch from selfie, since the JSON
string would need to be wholly unserialized into JS objects, which
themselves would need to be converted into more specialized data
structures (like that Uint32Array one).
The solution to lower peak memory usage at launch is to refactor
selfie generation to allow a more piecemeal approach: each filtering
component is given the ability to serialize itself rather than to be
forced to be embedded in the master selfie. With this approach, the
HNTrie buffer can now serialize to its own storage by converting the
buffer data directly into a string which can be directly sent to
storage. This avoiding expensive intermediate steps such as
converting into a JS array and then to a JSON string.
As part of the refactoring, there was also opportunistic code
upgrade to ES6 and Promise (eventually all of uBO's code will be
proper ES6).
Additionally, the polyfill to bring getBytesInUse() to Firefox has
been revisited to replace the rather expensive previous
implementation with an implementation with virtually no overhead.
Related issue:
- https://github.com/gorhill/uBlock/issues/3683
This commit further increases uBO's procedural cosmetic filters
Adguard's cosmetic filter syntax -- specifically those procedural
cosmetic filters where plain CSS selectors appeared following
a procedural oeprator (this was rejected as invalid by uBO).
Also, experimental support for `:watch-attrs` procedural
operator, as discussed in <https://github.com/uBlockOrigin/uBlock-issues/issues/341#issuecomment-449765525>.
Support may be dropped before next release depending on whether
a better solution is suggested.
Additionally, the usual opportunistic refactoring toward ES6
syntax.
A new filtering class has been created: "static extended filtering".
This new class is an umbrella class for more specialized filtering
engines:
- Cosmetic filtering
- Scriptlet filtering
- HTML filtering
HTML filtering is available only on platforms which support modifying
the response body on the fly, so only Firefox 57+ at the moment.
With the ability to modify the response body, HTML filtering has
been introduced: removing elements from the DOM before the source
data has been parsed by the browser.
A consequence of HTML filtering ability is to bring back script tag
filtering feature.
