AverageLong.java

import java.math.BigDecimal;
import java.math.BigInteger;
import java.math.MathContext;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;
import java.util.stream.Collector;

final class AverageLong {
    long hi, lo, cnt;

    public void accept(long val) {
        cnt++;
        int cmp = Long.compareUnsigned(lo, lo += val);
        if (val > 0) {
            if (cmp > 0)
                hi++;
        } else if (cmp < 0)
            hi--;
    }

    public AverageLong combine(AverageLong other) {
        cnt += other.cnt;
        hi += other.hi;
        if (Long.compareUnsigned(lo, lo += other.lo) > 0) {
            hi++;
        }
        return this;
    }

    public double result() {
        if (cnt == 0)
            return 0.0d;
        if (hi == 0 && lo >= 0 || hi == -1 && lo < 0) {
            return ((double) lo) / cnt;
        }
        return new BigDecimal(new BigInteger(ByteBuffer.allocate(16)
                .order(ByteOrder.BIG_ENDIAN).putLong(hi).putLong(lo).array()))
                .divide(BigDecimal.valueOf(cnt), MathContext.DECIMAL64)
                .doubleValue();
    }

    static <T> Collector<T, ?, Double> averagingInt(
            ToIntFunction<? super T> mapper) {
        return Collector.of(AverageLong::new,
                (al, n) -> al.accept(mapper.applyAsInt(n)),
                AverageLong::combine, AverageLong::result);
    }

    static <T> Collector<T, ?, Double> averagingLong(
            ToLongFunction<? super T> mapper) {
        return Collector.of(AverageLong::new,
                (al, n) -> al.accept(mapper.applyAsLong(n)),
                AverageLong::combine, AverageLong::result);
    }
}