encoding: fix trim() to be O(n) instead of O(n^2)
`encoding.trim()` iterated over the possible lengths smaller than the
input and created a slice for each. It then calculated the column
width of the result, which is of course O(n), so the overall algorithm
was O(n). This patch rewrites it to iterate over the unicode
characters, keeping track of the length so far. Also, the old
algorithm started from the end of the string, which made it much worse
when the input is large and the limit is small (such as the typical 72
we pass to it).
You can time it by running something like this:
```
time python3 -c 'from mercurial.utils import stringutil; print(stringutil.ellipsis(b"0123456789" * 1000, 5))'
```
That drops from 4.05 s to 83 ms with this patch (and most of that is
of course startup time).
Differential Revision: https://phab.mercurial-scm.org/D12089
$ hg init
$ touch a
$ hg add a
$ hg ci -m "a"
$ echo 123 > b
$ hg add b
$ hg diff --nodates
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
$ hg diff --nodates -r tip
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
$ echo foo > a
$ hg diff --nodates
diff -r 3903775176ed a
--- a/a
+++ b/a
@@ -0,0 +1,1 @@
+foo
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
$ hg diff -r ""
hg: parse error: empty query
[10]
$ hg diff -r tip -r ""
hg: parse error: empty query
[10]
Remove a file that was added via merge. Since the file is not in parent 1,
it should not be in the diff.
$ hg ci -m 'a=foo' a
$ hg co -Cq null
$ echo 123 > b
$ hg add b
$ hg ci -m "b"
created new head
$ hg merge 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg rm -f a
$ hg diff --nodates
Rename a file that was added via merge. Since the rename source is not in
parent 1, the diff should be relative to /dev/null
$ hg co -Cq 2
$ hg merge 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg mv a a2
$ hg diff --nodates
diff -r cf44b38435e5 a2
--- /dev/null
+++ b/a2
@@ -0,0 +1,1 @@
+foo
$ hg diff --nodates --git
diff --git a/a2 b/a2
new file mode 100644
--- /dev/null
+++ b/a2
@@ -0,0 +1,1 @@
+foo