private void init(int collapseSize, double extraFactor) {
        this.defaultCollapseSize = collapseSize;
        this.extraFactor = extraFactor;
        if (extraFactor < 1.0) {
            throw new IllegalArgumentException("FieldCollapsingSearcher: extraFactor " + extraFactor + " should be >= 1.0");
        }
    }
Creates a collapser

@param collapseSize the maximum number of hits to keep per
       field the default max number of hits in each collapsed group
@param extraFactor the percentage by which to scale up the
       requested number of hits, to allow some hits to be removed
       without refetching
container-search/src/main/java/com/yahoo/prelude/searcher/FieldCollapsingSearcher.java