### Re^2: Powerset short-circuit optimization

by Limbic~Region (Chancellor)
 on Oct 03, 2006 at 22:13 UTC ( #576200=note: print w/replies, xml ) Need Help??

in reply to Re: Powerset short-circuit optimization

ikegami,
This is the approach I previously discussed in the CB with jdporter and blokhead. The undesireable aspect is that _r() is called 29 times to yield 15 items for A,B,C,D instead of just 15. I am not sure there is a way around this but that's why I posted it.

Cheers - L~R

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Re^3: Powerset short-circuit optimization
by ikegami (Pope) on Oct 03, 2006 at 23:29 UTC

The following only calls _v 15 times. ;) Of course, splice (or its optimized equivalent) and %seen are still called 29 times, but that's a far cry less than the possible 41. Also, even with the original code, the (presumably expensive) callback is only called 15 times.

local *_r = sub { my @v = @_; return if not \$cb->(@v); return if @v == 1; for (0..\$#v) { my @v_ = @v; splice(@v_, \$#v-\$_, 1); _r(@v_) if not \$seen{join \$;, @v_}++; } };

It breaks the rule "A,B,C should be generated before A,C", but the following might allow you to break down the problem such that your requirements can be loosened:

my (\$u_set1, \$u_set2, \$common) = extract_common(\$set1, \$set2); my \$psetc = powerset(\$common); my \$pset1 = product(\$psetc, powerset(\$u_set1)); my \$pset2 = product(\$psetc, powerset(\$u_set2)); my \$pset = union(\$pset1, \$pset2);

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