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bdiff: don't check border condition in loop...
bdiff: don't check border condition in loop `plast = a + len - 1`. So, this "for" loop iterates from "a" to "plast", inclusive. So, `p == plast` can only be true on the final iteration of the loop. So checking for it on every loop iteration is wasteful. This patch simply decreases the upper bound of the loop by 1 and adds an explicit check after iteration for the `p == plast` case. We can't simply add 1 to the initial value for "i" because that doesn't do the correct thing on empty input strings. `perfbdiff -m 3041e4d59df2` on the Firefox repo becomes significantly faster: ! wall 0.072763 comb 0.070000 user 0.070000 sys 0.000000 (best of 100) ! wall 0.053221 comb 0.060000 user 0.060000 sys 0.000000 (best of 100) For the curious, this code has its origins in 8b067bde6679, which is the changeset that introduced bdiff.c in 2005. Also, GNU diffutils is able to perform a similar line-based diff in under 20ms. So there's likely more perf wins to be found in this code. One of them is the hashing algorithm. But it looks like mpm spent some time testing hash collisions in d0c48891dd4a. I'd like to do the same before switching away from lyhash, just to be on the safe side.

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requirements.txt
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Repositories contain a file (``.hg/requires``) containing a list of
features/capabilities that are *required* for clients to interface
with the repository. This file has been present in Mercurial since
version 0.9.2 (released December 2006).
One of the first things clients do when opening a repository is read
``.hg/requires`` and verify that all listed requirements are supported,
aborting if not. Requirements are therefore a strong mechanism to
prevent incompatible clients from reading from unknown repository
formats or even corrupting them by writing to them.
Extensions may add requirements. When they do this, clients not running
an extension will be unable to read from repositories.
The following sections describe the requirements defined by the
Mercurial core distribution.
revlogv1
========
When present, revlogs are version 1 (RevlogNG). RevlogNG was introduced
in 2006. The ``revlogv1`` requirement has been enabled by default
since the ``requires`` file was introduced in Mercurial 0.9.2.
If this requirement is not present, version 0 revlogs are assumed.
store
=====
The *store* repository layout should be used.
This requirement has been enabled by default since the ``requires`` file
was introduced in Mercurial 0.9.2.
fncache
=======
The *fncache* repository layout should be used.
The *fncache* layout hash encodes filenames with long paths and
encodes reserved filenames.
This requirement is enabled by default when the *store* requirement is
enabled (which is the default behavior). It was introduced in Mercurial
1.1 (released December 2008).
shared
======
Denotes that the store for a repository is shared from another location
(defined by the ``.hg/sharedpath`` file).
This requirement is set when a repository is created via :hg:`share`.
The requirement was added in Mercurial 1.3 (released July 2009).
dotencode
=========
The *dotencode* repository layout should be used.
The *dotencode* layout encodes the first period or space in filenames
to prevent issues on OS X and Windows.
This requirement is enabled by default when the *store* requirement
is enabled (which is the default behavior). It was introduced in
Mercurial 1.7 (released November 2010).
parentdelta
===========
Denotes a revlog delta encoding format that was experimental and
replaced by *generaldelta*. It should not be seen in the wild because
it was never enabled by default.
This requirement was added in Mercurial 1.7 and removed in Mercurial
1.9.
generaldelta
============
Revlogs should be created with the *generaldelta* flag enabled. The
generaldelta flag will cause deltas to be encoded against a parent
revision instead of the previous revision in the revlog.
Support for this requirement was added in Mercurial 1.9 (released
July 2011). The requirement was disabled on new repositories by
default until Mercurial 3.7 (released February 2016).
manifestv2
==========
Denotes that version 2 of manifests are being used.
Support for this requirement was added in Mercurial 3.4 (released
May 2015). The requirement is currently experimental and is disabled
by default.
treemanifest
============
Denotes that tree manifests are being used. Tree manifests are
one manifest per directory (as opposed to a single flat manifest).
Support for this requirement was added in Mercurial 3.4 (released
August 2015). The requirement is currently experimental and is
disabled by default.