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Avoid O(n²) in removeDuplicateGradient

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The original implementation of removeDuplicateGradient does O(n²)
search over all gradients to remove duplicates.  In images with many
gradients (such as [MediaWiki_logo_1.svg]), this becomes a significant
overhead as that logo has over 900 duplicated gradients.

We solve this by creating a key for each gradient based on the
attributes we use for duplication detection.  This key is generated
such that if two gradients have the same key, they are duplicates (for
our purpose) and the keys are different then the gradients are
guaranteed to be different as well.  With such a key, we can rely on a
dict to handle the duplication detection (which it does very well).

This change improves the runtime performance on [MediaWiki_logo_1.svg]
by about 25% (8m51s -> 1m56s on 5 runs).

Original:
    $ time for I in $(seq 1 5) ; do \
        PYTHONPATH=. python3 -m scour.scour MediaWiki_logo_1.svg out.svg ; \
      done
    Scour processed file "heavy.svg" in 105042 ms: 1582746/4989544 bytes new/orig -> 31.7%
    Scour processed file "heavy.svg" in 103412 ms: 1582746/4989544 bytes new/orig -> 31.7%
    Scour processed file "heavy.svg" in 105334 ms: 1582746/4989544 bytes new/orig -> 31.7%
    Scour processed file "heavy.svg" in 107902 ms: 1582746/4989544 bytes new/orig -> 31.7%
    Scour processed file "heavy.svg" in 108161 ms: 1582746/4989544 bytes new/orig -> 31.7%

    8m51.855s
    ...

Optimized:
    $ time for I in $(seq 1 5) ; do \
        PYTHONPATH=. python3 -m scour.scour MediaWiki_logo_1.svg out.svg ; \
      done
    Scour processed file "heavy.svg" in 21559 ms: 1582746/4989544 bytes new/orig -> 31.7%
    Scour processed file "heavy.svg" in 21936 ms: 1582746/4989544 bytes new/orig -> 31.7%
    Scour processed file "heavy.svg" in 21540 ms: 1582746/4989544 bytes new/orig -> 31.7%
    Scour processed file "heavy.svg" in 21518 ms: 1582746/4989544 bytes new/orig -> 31.7%
    Scour processed file "heavy.svg" in 21664 ms: 1582746/4989544 bytes new/orig -> 31.7%

    real    1m56.400s
    ...

[MediaWiki_logo_1.svg]: https://upload.wikimedia.org/wikipedia/commons/archive/5/54/20120822053933%21MediaWiki_logo_1.svg

Signed-off-by: Niels Thykier <niels@thykier.net>
This commit is contained in:
Niels Thykier 2018-03-10 08:37:39 +00:00
parent 0776d32179
commit 69471b8e8d
1 changed files with 41 additions and 50 deletions
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91
scour/scour.py
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@ -57,7 +57,7 @@ import sys
import time
import xml.dom.minidom
from xml.dom import Node, NotFoundErr
from collections import namedtuple
from collections import namedtuple, defaultdict
from decimal import Context, Decimal, InvalidOperation, getcontext
import six
@ -1355,6 +1355,32 @@ def collapseSinglyReferencedGradients(doc):
return num
def computeGradientBucketKey(grad):
# Compute a key (hashable opaque value; here a string) from each
# gradient such that "key(grad1) == key(grad2)" is the same as
# saying that grad1 is a duplicate of grad2.
gradBucketAttr = ['gradientUnits', 'spreadMethod', 'gradientTransform',
'x1', 'y1', 'x2', 'y2', 'cx', 'cy', 'fx', 'fy', 'r']
gradStopBucketsAttr = ['offset', 'stop-color', 'stop-opacity', 'style']
# A linearGradient can never be a duplicate of a
# radialGradient (and vice versa)
subKeys = [grad.getAttribute(a) for a in gradBucketAttr]
subKeys.append(grad.getAttributeNS(NS['XLINK'], 'href'))
stops = grad.getElementsByTagName('stop')
if stops.length:
for i in range(stops.length):
stop = stops.item(i)
for attr in gradStopBucketsAttr:
stopKey = stop.getAttribute(attr)
subKeys.append(stopKey)
# Use a raw ASCII "record seperator" control character as it is
# not likely to be used in any of these values (without having to
# be escaped).
return "\x1e".join(subKeys)
def removeDuplicateGradients(doc):
global _num_elements_removed
num = 0
@ -1364,58 +1390,23 @@ def removeDuplicateGradients(doc):
for gradType in ['linearGradient', 'radialGradient']:
grads = doc.getElementsByTagName(gradType)
gradBuckets = defaultdict(list)
for grad in grads:
# TODO: should slice grads from 'grad' here to optimize
for ograd in grads:
# do not compare gradient to itself
if grad == ograd:
key = computeGradientBucketKey(grad)
gradBuckets[key].append(grad)
for bucket in six.itervalues(gradBuckets):
if len(bucket) < 2:
# The gradient must be unique if it is the only one in
# this bucket.
continue
master = bucket[0]
duplicates = bucket[1:]
# compare grad to ograd (all properties, then all stops)
# if attributes do not match, go to next gradient
someGradAttrsDoNotMatch = False
for attr in ['gradientUnits', 'spreadMethod', 'gradientTransform',
'x1', 'y1', 'x2', 'y2', 'cx', 'cy', 'fx', 'fy', 'r']:
if grad.getAttribute(attr) != ograd.getAttribute(attr):
someGradAttrsDoNotMatch = True
break
if someGradAttrsDoNotMatch:
continue
# compare xlink:href values too
if grad.getAttributeNS(NS['XLINK'], 'href') != ograd.getAttributeNS(NS['XLINK'], 'href'):
continue
# all gradient properties match, now time to compare stops
stops = grad.getElementsByTagName('stop')
ostops = ograd.getElementsByTagName('stop')
if stops.length != ostops.length:
continue
# now compare stops
stopsNotEqual = False
for i in range(stops.length):
if stopsNotEqual:
break
stop = stops.item(i)
ostop = ostops.item(i)
for attr in ['offset', 'stop-color', 'stop-opacity', 'style']:
if stop.getAttribute(attr) != ostop.getAttribute(attr):
stopsNotEqual = True
break
if stopsNotEqual:
continue
# ograd is a duplicate of grad, we schedule it to be removed UNLESS
# ograd is ALREADY considered a 'master' element
if ograd not in gradientsToRemove:
if ograd not in duplicateToMaster:
if grad not in gradientsToRemove:
gradientsToRemove[grad] = []
gradientsToRemove[grad].append(ograd)
duplicateToMaster[ograd] = grad
gradientsToRemove[master] = duplicates
for ograd in duplicates:
duplicateToMaster[ograd] = master
# get a collection of all elements that are referenced and their referencing elements
referencedIDs = findReferencedElements(doc.documentElement)