Algorithms and Data Structures

N.Wirth. Algorithms and Data Structures. Oberon version 91
from all sources with d i = 0, and d i is decremented for all other sequences, indicating that one dummy run
was taken off. We denote the number of input sequences involved in a merge by k.
4. It is impossible to derive termination of a phase by the end-of status of the N-1'st sequence, because
more merges might be necessary involving dummy runs from that source. Instead, the theoretically
necessary number of runs is determined from the coefficients a i . The coefficients a i were computed during
the distribution phase; they can now be recomputed backward.
The main part of the Polyphase Sort can now be formulated according to these rules, assuming that all
N-1 sequences with initial runs are set to be read, and that the tape map is initially set to t i = i.
REPEAT (*merge from t[0] ... t[N-2] to t[N-1]*)
z := a[N-2]; d[N-1] := 0;
REPEAT (*merge one run*)
k := 0;
(*determine no. of active sequences*)
FOR i := 0 TO N-2 DO
IF d[i] > 0 THEN
DEC(d[i])
ELSE
ta[k] := t[i]; INC(k)
END
END;
IF k = 0 THEN
INC(d[N-1])
ELSE merge one real run from t[0] ... t[k-1] to t[N-1]
END;
DEC(z)
UNTIL z = 0;
Runs.Set(r[t[N-1]], f[t[N-1]]);
rotate sequences in map t;
compute a[i] for next level;
DEC(level)
UNTIL level = 0
(*sorted output is f[t[0]]*)
The actual merge operation is almost identical with that of the N-way merge sort, the only difference
being that the sequence elimination algorithm is somewhat simpler. The rotation of the sequence index map
and the corresponding counts d i (and the down-level recomputation of the coefficients a i ) is
straightforward and can be inspected in the following program that represents the Polyphase algorithm in its
entirety.
PROCEDURE Polyphase (src: Files.File): Files.File; (* ADenS24_MergeSorts *)
VAR i, j, mx, tn: INTEGER;
k, dn, z, level: INTEGER;
x, min: INTEGER;
a, d: ARRAY N OF INTEGER;
t, ta: ARRAY N OF INTEGER; (*index maps*)
R: Runs.Rider; (*source*)
f: ARRAY N OF Files.File;
r: ARRAY N OF Runs.Rider;
PROCEDURE select;