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Stupid Columnsort Tricks
Geeta Chaudhry, Thomas H. Cormen
Dartmouth TR2003-444

Abstract: Leighton's columnsort algorithm sorts on an $r \times s$ mesh, subject to the restrictions that $s$ is a divisor of~$r$ and that $r \geq 2s^2$ (so that the mesh is tall and thin). We show how to mitigate both of these restrictions. One result is that the requirement that $s$ is a divisor of~$r$ is unnecessary; columnsort sorts correctly whether or not $s$ divides~$r$. We present two algorithms that, as long as $s$ is a perfect square, relax the restriction that $r \geq 2s^2$; both reduce the exponent of~$s$ to~$3/2$. One algorithm requires $r \geq 4s^{3/2}$ if $s$ divides~$r$ and $r \geq 6s^{3/2}$ if $s$ does not divide~$r$. The other algorithm requires $r \geq 4^{3/2}$, and it requires $s$ to be a divisor of~$r$. Both algorithms have applications in increasing the maximum problem size in out-of-core sorting programs.

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   Geeta Chaudhry and Thomas H. Cormen, "Stupid Columnsort Tricks." Dartmouth Computer Science Technical Report TR2003-444, April 2003.

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