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This is an automatically generated e-mail. To reply, visit:
<a href="http://git.reviewboard.kde.org/r/110262/">http://git.reviewboard.kde.org/r/110262/</a>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">This review has been submitted with commit 183e59b8e9fe938c209493aa7aa98382ccd08e71 by Frank Reininghaus to branch master.</pre>
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<p>- Commit</p>
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<p>On May 1st, 2013, 8:34 p.m. UTC, Frank Reininghaus wrote:</p>
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<div>Review request for kdelibs.</div>
<div>By Frank Reininghaus.</div>
<p style="color: grey;"><i>Updated May 1, 2013, 8:34 p.m.</i></p>
<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Description </h1>
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<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">The current algorithm that is used to shuffle lists is rather inefficient. It works by removing the first item of the list repeatedly and inserting it at a random position in a new list, which is finally used to replace the original list. Unfortunately, this results in O(N^2) run time complexity because inserting into a list, which is done N itmes, is O(N).
I propose to replace this algorithm by the Fisher-Yates algorithm, which works by swapping items N - 1 times. One could modify the entire thing further, like providing randomization also for other containers and not only QList, but that would probably be frameworks material. </pre>
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Testing </h1>
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<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">I wrote a small benchmark: http://paste.kde.org/735914/
I got the following results with the existing algorithm:
RESULT : Benchmark::randomSequenceBenchmark():"n=0":
0.000015 msecs per iteration (total: 66, iterations: 4194304)
RESULT : Benchmark::randomSequenceBenchmark():"n=1":
0.000192 msecs per iteration (total: 101, iterations: 524288)
RESULT : Benchmark::randomSequenceBenchmark():"n=3":
0.00070 msecs per iteration (total: 93, iterations: 131072)
RESULT : Benchmark::randomSequenceBenchmark():"n=10":
0.0025 msecs per iteration (total: 83, iterations: 32768)
RESULT : Benchmark::randomSequenceBenchmark():"n=30":
0.0070 msecs per iteration (total: 58, iterations: 8192)
RESULT : Benchmark::randomSequenceBenchmark():"n=100":
0.023 msecs per iteration (total: 97, iterations: 4096)
RESULT : Benchmark::randomSequenceBenchmark():"n=300":
0.077 msecs per iteration (total: 79, iterations: 1024)
RESULT : Benchmark::randomSequenceBenchmark():"n=1000":
0.35 msecs per iteration (total: 90, iterations: 256)
RESULT : Benchmark::randomSequenceBenchmark():"n=3000":
1.8 msecs per iteration (total: 58, iterations: 32)
RESULT : Benchmark::randomSequenceBenchmark():"n=10000":
18 msecs per iteration (total: 72, iterations: 4)
RESULT : Benchmark::randomSequenceBenchmark():"n=30000":
283 msecs per iteration (total: 283, iterations: 1)
RESULT : Benchmark::randomSequenceBenchmark():"n=100000":
3,823 msecs per iteration (total: 3,823, iterations: 1)
Here are the numbers for the proposed new algorithm:
RESULT : Benchmark::randomSequenceBenchmark():"n=0":
0.000015 msecs per iteration (total: 65, iterations: 4194304)
RESULT : Benchmark::randomSequenceBenchmark():"n=1":
0.000015 msecs per iteration (total: 65, iterations: 4194304)
RESULT : Benchmark::randomSequenceBenchmark():"n=3":
0.00018 msecs per iteration (total: 98, iterations: 524288)
RESULT : Benchmark::randomSequenceBenchmark():"n=10":
0.00079 msecs per iteration (total: 52, iterations: 65536)
RESULT : Benchmark::randomSequenceBenchmark():"n=30":
0.0025 msecs per iteration (total: 83, iterations: 32768)
RESULT : Benchmark::randomSequenceBenchmark():"n=100":
0.0084 msecs per iteration (total: 69, iterations: 8192)
RESULT : Benchmark::randomSequenceBenchmark():"n=300":
0.025 msecs per iteration (total: 52, iterations: 2048)
RESULT : Benchmark::randomSequenceBenchmark():"n=1000":
0.085 msecs per iteration (total: 88, iterations: 1024)
RESULT : Benchmark::randomSequenceBenchmark():"n=3000":
0.25 msecs per iteration (total: 66, iterations: 256)
RESULT : Benchmark::randomSequenceBenchmark():"n=10000":
0.85 msecs per iteration (total: 55, iterations: 64)
RESULT : Benchmark::randomSequenceBenchmark():"n=30000":
2.6 msecs per iteration (total: 86, iterations: 32)
RESULT : Benchmark::randomSequenceBenchmark():"n=100000":
10 msecs per iteration (total: 81, iterations: 8)
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Diffs</b> </h1>
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<li>kdecore/util/krandomsequence.h <span style="color: grey">(46949b4)</span></li>
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<p><a href="http://git.reviewboard.kde.org/r/110262/diff/" style="margin-left: 3em;">View Diff</a></p>
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