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Goal priorities

When you add more than one goal, Rebalancer needs to know their relative importance. The objective is a tuple of buckets compared in order (lexicographically), and within each bucket the goals are combined into a weighted sum. By default every goal goes into a single bucket (tuple position 0), so the objective is just one weighted sum, set with weights. To give some goals strict priority over others, split them across buckets using tuples.

Weights​

solver.addGoal(spec, weight) scales each goal by its weight and sums them into a single objective to minimize. A larger weight means the goal matters more.

solver.addGoal(balanceSpec, 1.0);
solver.addGoal(minimizeMovementSpec, 10.0); // 10x as important

A practical approach: start with equal weights, solve, inspect the result, and scale a goal up or down until the trade-off looks right.

Weights cannot express a strict priority---even with a large weight gap, one goal still trades off against another. For a strict order, use tuples.

Tuples (strict priority)​

A goal tuple is an ordered list of buckets compared lexicographically: Rebalancer first minimizes bucket 0; among all solutions that tie on bucket 0 it minimizes bucket 1; and so on.

Build a tuple by separating buckets with addGoalBoundary(). Weights still apply within each bucket:

solver.addGoal(goalA, 2.0);  // bucket 0
solver.addGoal(goalB, 1.0);  // bucket 0
solver.addGoalBoundary();    // end bucket 0, start bucket 1
solver.addGoal(goalC, 1.0);  // bucket 1 (strictly lower priority)

This builds the objective tuple (2*goalA + goalB, goalC): Rebalancer first minimizes the weighted sum 2*goalA + goalB, and only among the assignments that tie on it does it minimize goalC.

Or target a bucket directly with the tuple-position argument:

solver.addGoal(goalA, 1.0, /*tuplePos=*/0);
solver.addGoal(goalC, 1.0, /*tuplePos=*/1);

Lexicographic order means tuples are compared left to right, moving on only when earlier buckets tie:

(a, b, c) is smaller than (A, B, C) when
    a is smaller than A, or
    a == A and b is smaller than B, or
    a == A and b == B and c is smaller than C

Source​