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/* PL/I procedure for Shell Sort. */
/* Copyright (c) 1996 by R. A. Vowels, from "Introduction to PL/I, Algorithms, and */
/* Structured Programming". All rights reserved. */
/* This subroutine sorts an array by sorting continually-changing sub-groups. */
SHELL_SORT:
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PROCEDURE (A) OPTIONS (REORDER);
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/* INCOMING: A = an array whose elements are to be sorted. */
/* OUTGOING: A = the sorted array. Elements are in ascending order. */
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DECLARE A(*) FIXED BINARY;
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DECLARE Temp FIXED BINARY;
DECLARE (J, K, N) FIXED BINARY;
DECLARE Gap FIXED BINARY UNSIGNED;
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N = HBOUND(A, 1); /* The number of elements in the array A. */
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Gap = N/2; /* The initial gap is half the number of elements. */
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DO WHILE (Gap >= 1); /* We are done after the gap is 1, when all */
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/* elements are examined in the next loop: */
/* Perform a Ranking Sort on selected elements. */
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DO K = 1 TO N-Gap;
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IF A(K) > A(K+Gap) THEN /* Select elements every Gap positions apart . . */
DO;
Temp = A(K+Gap);
DO J = K TO 1 BY -Gap WHILE (A(J) > Temp);
A(J+Gap) = A(J); /* Leapfrog by Gap locations. */
END;
A(J+Gap) = Temp;
END;
END;
Gap = Gap/2; /* Halve the gap for the next Ranking Sort. */
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END;
END SHELL_SORT;