@Override
    public FieldMatchMetrics clone() {
        try {
            FieldMatchMetrics clone = (FieldMatchMetrics)super.clone();
            clone.segmentStarts = new ArrayList<>(segmentStarts);
            return clone;
        }
        catch (CloneNotSupportedException e) {
            throw new RuntimeException("Programming error",e);
        }
    }
Called when a new segment is started

@param previousJ the end of the previous segment, or -1 if this is the first segment
searchlib/src/main/java/com/yahoo/searchlib/ranking/features/fieldmatch/FieldMatchMetrics.java