doc/README

dirstate: speed up repeated missing directory checks

In a mozilla repo with tip at bb3ff09f52fe,

  hg update tip~1000 && time hg revert -nq -r tip .

displays ~4:20 minutes. With tip~100, it runs in ~11 s. With revision
100000, it did not finish in 12 minutes.

Revert calls dirstate.status() with a matcher that matches each file
in the target revision. The main problem [1] lies in
dirstate._walkexplicit(), which looks for matching deleted directories
by checking whether each path is prefix of any path in the
dirstate. With m files in the dirstate and n files in the target
revision that are not in the dirstate, this is clearly O(m*n). Let's
improve by keeping a lazily initialized set of all the directories in
the dirstate, so the time becomes O(m+n).

After this patch, the 4:20 minutes become 5.5 s, while for a single
missing path, it slows down from 1.092 s to 1.150 s (best of 4). The
>12 min case becomes 5.8 s.

 [1] A narrower optimization would be to make revert take the fast
     path for '.' and '--all'.

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