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bb_min(+Goal, ?Cost, ?Options)

Find a minimal solution using the branch-and-bound method
Goal
The (nondeterministic) search goal
Cost
A (usually numeric domain) variable representing the cost
Options
A bb_options structure or variable

Description

A solution of the goal Goal is found that minimizes the value of Cost. Cost should be a variable that is affected, and eventually instantiated, by Goal. Usually, Goal is the search procedure of a constraint problem and Cost is the variable representing the cost. The solution is found using the branch and bound method: as soon as a solution is found, it gets remembered and the search is continued or restarted with an additional constraint on the Cost variable which requires the next solution to be better than the previous one. Iterating this process yields an optimal solution in the end.

The possible options are

strategy:
continue (default)
after finding a solution, continue search with the newly found bound imposed on Cost
restart
after finding a solution, restart the whole search with the newly found bound imposed on Cost
step
a synonym for 'restart'
dichotomic
after finding a solution, split the remaining cost range and restart search to find a solution in the lower sub-range. If that fails, assume the upper sub-range as the remaining cost range and split again.
The new bound or the split point, respectively, are computed from the current best solution, while taking into account the parameters delta and factor.
from:
number - an initial lower bound for the cost (default -1.0Inf)
to:
number - an initial upper bound for the cost (default +1.0Inf)
delta:
number - minimal absolute improvement required for each step (default 1.0), applies to all strategies
factor:
number - minimal improvement ratio (with respect to the lower cost bound) for strategies 'continue' and 'restart' (default 1.0), or split factor for strategy 'dichotomic' (default 0.5)
timeout:
number - maximum seconds of cpu time to spend (default: no limit)
report_success:
GoalPrefix - an atom (predicate name) or structure (goal prefix), specifying a goal to be invoked whenever the branch-and-bound process finds a better solution. The invoked goal is constructed by adding three arguments (Cost, Handle, Module) to GoalPrefix. Cost is a float number representing the cost of the solution found, Handle is a handle as accepted by bb_cost/2 or bb_solution/2, and Module is the context module of the minimisation. The default handler prints a message.
report_failure:
GoalPrefix - an atom (predicate name) or structure (goal prefix), specifying a goal to be invoked whenever the branch-and-bound process cannot find a solution in a cost range. The invoked goal is constructed by adding three arguments (Cost, Handle, Module) to GoalPrefix. Cost is a From..To structure representing the range of cost in which no solution could be found, Handle is a handle as accepted by bb_cost/2 or bb_solution/2, and Module is the context module of the minimisation. The default handler prints a message.
The default options can be selected by passing a free variable as the Options-argument. To specify other options, pass a bb_options- structure in struct-syntax, e.g.
	bb_options{strategy:dichotomic, timeout:60}
	
In order to maximize instead of minimizing, introduce a negated cost variable in your model and minimize that instead.

Compatibility note: For backward compatibility, the report_success and report_failure options also accept Name/Arity specifications with maximum arity 3 for the handler goals. The three optional arguments are then Cost, Handle, and Module.

Modules

This predicate is sensitive to its module context (tool predicate, see @/2).

Examples

?- bb_min(member(X,[9,6,8,4,7,2,4,7]), X, O).
Found a solution with cost 9
Found a solution with cost 6
Found a solution with cost 4
Found a solution with cost 2
Found no solution with cost -1.0Inf .. 1
X = 2
O = bb_options(continue, -1.0Inf, 1.0Inf, 1, 1, 0, 0, _, _)
yes.

[eclipse 6]: bb_min(member(X,[9,6,8,4,7,2,4,7]), X, bb_options{delta:4}).
Found a solution with cost 9
Found a solution with cost 4
Found no solution with cost -1.0Inf .. 0
X = 4
yes.

[eclipse 10]: bb_min(member(X,[99,60,80,40,70,30,70]), X,
	bb_options{strategy:dichotomic, from:0}).
Found a solution with cost 99
Found a solution with cost 40
Found no solution with cost 0.0 .. 20.0
Found a solution with cost 30
Found no solution with cost 20.0 .. 25.0
Found no solution with cost 25.0 .. 27.5
Found no solution with cost 27.5 .. 28.75
Found no solution with cost 28.75 .. 29.0

X = 30
yes.

See Also

bb_min / 6