An implementation of Hamilton's method for Smallest-Remainder rounding of percentages.

Hamilton.java

package chris.impl;

import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.function.Function;
import java.util.stream.IntStream;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

import static java.util.stream.Collectors.toList;

import static chris.impl.Hamilton.IndexWrapper.zipWithIndex;

public class Hamilton {

    public static <T> List<T> normalisePercentages(
            List<T> inputs,
            Function<T, Double> getNumber,
            Function<Double, Function<T, T>> newValue) {

        double total = inputs
                .stream()
                .map(getNumber)
                .mapToDouble(Double::doubleValue)
                .sum();

        double multiple = 100 / total;
        Function<T, T> normalise = t -> newValue.apply(multiple * getNumber.apply(t)).apply(t);

        return inputs.stream().map(normalise).collect(toList());

    }

 
    public static <T> List<T> normalisePercentagesWithRounding(
            List<T> unNormalisedInputs,
            Function<T, Double> getNumber,
            Function<Double, Function<T, T>> newValue) {

        final Function<T, T> roundUp = t -> getNumber.andThen(Math::ceil).andThen(newValue).apply(t).apply(t);
        final Function<T, T> roundDown = t -> getNumber.andThen(Math::floor).andThen(newValue).apply(t).apply(t);

        final Comparator<IndexWrapper<T>> byRemainderAsc = IndexWrapper.compareContentsWith(t -> getNumber.apply(t) % 1);

        final List<T> inputs = normalisePercentages(unNormalisedInputs, getNumber, newValue);

        final int difference = 100 - inputs.stream()
                .map(roundDown)
                .map(getNumber)
                .mapToInt(Double::intValue)
                .sum();

        final List<IndexWrapper<T>> sortables = zipWithIndex(inputs.stream())
                .sorted(byRemainderAsc.reversed())
                .collect(toList());

        final List<IndexWrapper<T>> toCeiling = sortables.subList(0, difference);
        final List<IndexWrapper<T>> toFloor = sortables.subList(difference, sortables.size());

        return Stream.concat(
                toCeiling.stream().map(s -> s.map(roundUp)),
                toFloor.stream().map(s -> s.map(roundDown)))
                .sorted()
                .map(IndexWrapper::getValue)
                .collect(toList());

    }

    public static void main(String[] args) {

        Function<HasNumeric, Double> value = s -> s.num;
        Function<Double, Function<HasNumeric, HasNumeric>> newValue = d -> s -> new HasNumeric(s.name, d);

        List<HasNumeric> values = List.of(s("a", 17), s("b", 25), s("c", 12), s("d", 5));

        List<HasNumeric> hasNumerics = normalisePercentages(values, value, newValue);

        System.out.println(hasNumerics.stream().map(HasNumeric::toString).collect(toList()));
        List<HasNumeric> fixed = normalisePercentagesWithRounding(values, value, newValue);
        System.out.println(fixed.stream().map(HasNumeric::toString).collect(toList()));

    }

    private static HasNumeric s(String name, double num) {
        return new HasNumeric(name, num);
    }

    static class IndexWrapper<T> implements Comparable<IndexWrapper<T>> {
        final T value;
        final int index;

        IndexWrapper(T value, int index) {
            this.value = value;
            this.index = index;
        }

        T getValue() {
            return value;
        }

        <R> IndexWrapper<R> map(Function<T, R> replacer) {
            return new IndexWrapper<>(replacer.apply(value), index);
        }

        static <T> Stream<IndexWrapper<T>> zipWithIndex(Stream<T> in) {
            Iterator<T> ts = in.iterator();
            Iterator<Integer> indexes = IntStream.iterate(0, i -> i + 1).iterator();
            Iterable<IndexWrapper<T>> wrapped = () -> new Iterator<>() {
                @Override public boolean hasNext() {
                    return ts.hasNext();
                }

                @Override public IndexWrapper<T> next() {
                    return new IndexWrapper<T>(ts.next(), indexes.next());
                }
            };
            return StreamSupport.stream(wrapped.spliterator(), false);
        }

        static <T, U extends Comparable<? super U>> Comparator<IndexWrapper<T>> compareContentsWith(Function<T, U> contentsComparator) {
            return Comparator.comparing(w -> contentsComparator.apply(w.value));
        }

        @Override public int compareTo(Hamilton.IndexWrapper<T> o) {
            Comparator<IndexWrapper<T>> byIndex = Comparator.comparing(w -> w.index);
            return byIndex.compare(this, o);
        }
    }

    static class HasNumeric {
        final double num;
        final String name;

        HasNumeric(String name, double num) {
            this.num = num;
            this.name = name;
        }

        @Override public String toString() {
            return name + " " + num;
        }
    }
}