changeset 48671:f1ed5c304f45

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
author Martin von Zweigbergk <martinvonz@google.com>
date Wed, 26 Jan 2022 10:11:01 -0800
parents 7ee07e1a25c0
children 657e490756e6
files mercurial/encoding.py
diffstat 1 files changed, 14 insertions(+), 10 deletions(-) [+]
line wrap: on
line diff
--- a/mercurial/encoding.py	Tue Jan 11 21:40:08 2022 +0100
+++ b/mercurial/encoding.py	Wed Jan 26 10:11:01 2022 -0800
@@ -511,17 +511,21 @@
     if width <= 0:  # no enough room even for ellipsis
         return ellipsis[: width + len(ellipsis)]
 
+    chars = list(u)
     if leftside:
-        uslice = lambda i: u[i:]
-        concat = lambda s: ellipsis + s
-    else:
-        uslice = lambda i: u[:-i]
-        concat = lambda s: s + ellipsis
-    for i in pycompat.xrange(1, len(u)):
-        usub = uslice(i)
-        if ucolwidth(usub) <= width:
-            return concat(usub.encode(_sysstr(encoding)))
-    return ellipsis  # no enough room for multi-column characters
+        chars.reverse()
+    width_so_far = 0
+    for i, c in enumerate(chars):
+        width_so_far += ucolwidth(c)
+        if width_so_far > width:
+            break
+    chars = chars[:i]
+    if leftside:
+        chars.reverse()
+    u = u''.join(chars).encode(_sysstr(encoding))
+    if leftside:
+        return ellipsis + u
+    return u + ellipsis
 
 
 class normcasespecs(object):