I'd like to point out that in your solution you are already using
Math::Pari for "factorint", so you could also use "divisors" to get the complete powerset in sorted order.
Update: adding code at Limbic~Region's request.
#! /usr/bin/perl
use strict;
use Math::Pari qw(:int factorint divisors);
for (@ARGV) {
my $N = $_;
my $a = factorint($N);
my $div = divisors($a);
print "sorted list of divisors of $N is ",$div,"\n";
}
For 12345, this prints:
perl ddd.pl 12345
sorted list of divisors of 12345 is [1,3,5,15,823,2469,4115,12345]
However, I see you are looking for a lazy evaluation method that will produce the whole list in sorted order, one at at time. I saw an example once in Haskell of this sort of thing.
Suppose N = (p1^a1)*(p2^a2)*...(pr^ar) for p1..pr primes in sorted order.
Form a1+1 lists: the first where p1 is not a factor, the second where p1 is a factor only once, ... up to the last list where p1 is a factor a1 times. Mergesort these lists, in each case taking the lowest item available. That is your answer.
These lists can be created recursively, by taking the next highest factor to all possible powers and merging in the same way.
Update: I have written some Haskell code to do the recursive ennumeration of the factor list. Since Haskell does lazy evaluation, this code should compute the "factor square root" without multiplying out all the factors:
module Powerset where
{ Expand muliplies of two list of numbers.
For example:
mult_expand [1, 2, 4] [1, 5]
Produces:
[[1,5],[2,10],[4,20]]
}
mult_expand p [] = p:[]
mult_expand p lst =
map (\x > map (*x) lst) p
{ expand the powers of x from 0 to exp }
pows x exp = map (\e > x^e) [0..exp]
{ expand the powers the first item in a pair
from 0 to the second item in the pair.
}
powerlst p = pows (fst p) (snd p)
{ One stage of a merge sort }
merge [] ys = ys
merge xs [] = xs
merge (x:xs) (y:ys) = if x <= y
then x : merge xs (y:ys)
else y : merge (x:xs) ys
{ Do multiple merges to get an ordered list. }
bigmerge lsts =
foldl merge [] lsts
{ The complete power set of a list of factor/exponent pairs. For exa
+mple:
powerset [(2,3),(5,1)]
Produces:
[1,2,4,5,8,10,20,40]
}
powerset [] = []
powerset (x:xs) =
bigmerge (mult_expand (powerlst x) (powerset xs))
{ Helper function for factor_sqrt.
Finds the last list value less than or equal to the
given limit.
}
fsqrt lim lastgood [] = lastgood
fsqrt lim lastgood (x:xs) =
if (x > lim) then lastgood
else fsqrt lim x xs
{ Find the last divisor less than the square root,
given the number and its factors
}
factor_sqrt n factors =
fsqrt (sqrt n) 0 (powerset factors)
For example, under Hugs:
Prelude> :l Powerset.hs
Powerset> factor_sqrt 24777695232 [(2,13),(3,8),(461,1)]
149364.0
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