1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
|
#
# initially generated by c2l
#
implement Perms;
include "draw.m";
Perms: module
{
init: fn(nil: ref Draw->Context, argl: list of string);
};
include "sys.m";
sys: Sys;
init(nil: ref Draw->Context, argl: list of string)
{
sys = load Sys Sys->PATH;
main(len argl, argl);
}
#
# * generate permutations of N elements
# * from ``On Programming, an interim report on the SETL project'',
# * Jacob T Schwartz (ed), New York University
#
Seq: adt{
nel: int;
el: array of int;
};
origin: int = 1;
main(argc: int, argv: list of string): int
{
n: int;
if(argc > 1 && (as := hd tl argv)[0] == '-'){
origin = int (as[1: ]);
argc--;
argv = tl argv;
}
if(argc != 2){
sys->fprint(sys->fildes(2), "Usage: perms #elements\n");
exit;
}
n = int hd tl argv;
if(n > 0)
perms(n);
exit;
}
perms(n: int)
{
seq: ref Seq;
seq = newseq(n);
do
putseq(seq);
while(eperm(seq) != nil);
}
putseq(seq: ref Seq)
{
k: int;
for(k = 0; k < seq.nel; k++)
sys->print(" %d", seq.el[k]+origin);
sys->print("\n");
}
eperm(seq: ref Seq): ref Seq
{
j, k, n: int;
n = seq.nel;
# if sequence is monotone decreasing, there are no more
# permutations. Otherwise, find last point of increase
hit := 0;
for(j = n-2; j >= 0; j--)
if(seq.el[j] < seq.el[j+1]){
hit = 1;
break;
}
if(!hit)
return nil;
# then find the last seq[k] which exceeds seq[j] and swop
for(k = seq.nel-1; k > j; k--)
if(seq.el[k] > seq.el[j]){
{
t: int;
t = seq.el[k];
seq.el[k] = seq.el[j];
seq.el[j] = t;
}
;
break;
}
# then re-arrange all the elements from seq[j+1] into
# increasing order
for(k = j+1; k < (n+j+1)/2; k++){
kk: int;
kk = n-k+j;
{
t: int;
t = seq.el[k];
seq.el[k] = seq.el[kk];
seq.el[kk] = t;
}
;
}
return seq;
}
newseq(n: int): ref Seq
{
seq: ref Seq;
k: int;
seq = ref Seq;
seq.nel = n;
seq.el = array[n] of int;
for(k = 0; k < n; k++)
seq.el[k] = k;
return seq;
}
|
