vim-patch:9.2.1099: diff mode redraws slowly with many changes (#41906)

Problem:  Redrawing or folding a diff with many changes is slow because for
          every screen line the list of diff blocks is scanned from the
          start.
Solution: Remember the last examined diff block and resume the scan from
          there while the line number does not move backwards
          (Julien Voisin).

This change is similar to v9.2.1042 (5feaa6c6c201d4d05b833a7230d07da44bf2504b)
and v9.2.1036 (f1b454912996d6417acd0d738de0eb7b60902c56) quadratic
behaviour replaced with a linear one by keeping track of previous
ordered insertions.

diff_check_with_linestatus() and diff_infold() are called for every line
during redraw and fold updates, with a line number that usually
increases monotonically, yet each restarted its scan at tp_first_diff. A
cursor into the block list is now kept and cleared whenever a block is
freed, allocated or has its line numbers shifted, so the next call
continues where the previous one stopped.

Scanning the diff blocks during a scroll of a 20000-line diff drops from
4.12G to 1.27G instructions with ~3300 changes, and scales with the
number of changes.

Tests were added as the change is non-trivial.

closes: vim/vim#21256

https://github.com/vim/vim/commit/2ec2a612c7ac9315a17d70f6a2a5ea89ddaea854

Co-authored-by: Julien Voisin <julien.voisin@dustri.org>

0 files changed, 0 insertions, 0 deletions