Method groupSumBy

forge-core/src/main/java/forge/util/Aggregates.java:237–239

(Iterable<Entry<T, Integer>> source, Function<T, U> fnGetField)

    }

    public static <T, U> Iterable<Entry<U, Integer>> groupSumBy(Iterable<Entry<T, Integer>> source, Function<T, U> fnGetField) {
        return StreamUtil.stream(source).collect(Collectors.groupingBy(kv -> fnGetField.apply(kv.getKey()), Collectors.summingInt(e -> e.getValue()))).entrySet();
    }
}