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When the two chosen pivots happen to be the smallest and largest
elements of the array, three partitions will be created, two of
size 0 and one of size n-2. If this happens on each recursive call
to dual_pivot_quick_sort, the stack depth will reach approximately n/2.
To avoid the stack from deepening, iteration can be used for the
largest of the three partitions. This ensures the stack depth
will only increase for partitions of size n/2 or smaller, which
results in a maximum stack depth of log(n).
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Picking the first and last elements as pivots makes it so that
a sorted array is the worst-case input for the algorithm.
This change instead picks pivots at approximately 1/3 and 2/3 in
the array. This results in desired performance for sorted arrays.
Of course this only changes which inputs result in worst-case
performance, but hopefully those inputs occur less frequently than
already sorted arrays.
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SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
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- The change to quick_sort requires SimpleIterator to support
assignment.
- Rename quick_sort to single_pivot_quick_sort to make it easier
to choose a specific implementation (not based on signature).
- Ensure single_pivot_quick_sort does not copy the pivots
- Expand sorts_without_copy test case to cover both single and dual
pivot implementations.
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Also add a test that would fail to compile if quick_sort tries to copy
anything :P
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Now it actually defaults to "a < b" comparison, instead of forcing you
to provide a trivial less-than comparator. Also you can pass in any
collection type that has .begin() and .end() and we'll sort it for you.
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As suggested by Joshua, this commit adds the 2-clause BSD license as a
comment block to the top of every source file.
For the first pass, I've just added myself for simplicity. I encourage
everyone to add themselves as copyright holders of any file they've
added or modified in some significant way. If I've added myself in
error somewhere, feel free to replace it with the appropriate copyright
holder instead.
Going forward, all new source files should include a license header.
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Also run it across the whole tree to get everything using the One True Style.
We don't yet run this in an automated fashion as it's a little slow, but
there is a snippet to do so in makeall.sh.
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It was depressing not being able to capture anything when passing a lambda
to qsort_r() so let's just have our own QuickSort. I was gonna have to do
this eventually anyway. :^)
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